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Sources of atmospheric nitrous acid: State of the
science, current research needs, and future prospects
a
Francesca Spataro & Antonietta Ianniello
a
a
CNR-Institute of Atmospheric Pollution Research, Rome, Italy
Accepted author version posted online: 25 Aug 2014.Published online: 20 Oct 2014.
To cite this article: Francesca Spataro & Antonietta Ianniello (2014) Sources of atmospheric nitrous acid: State of the
science, current research needs, and future prospects, Journal of the Air & Waste Management Association, 64:11, 1232-1250,
DOI: 10.1080/10962247.2014.952846
To link to this article: http://dx.doi.org/10.1080/10962247.2014.952846
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REVIEW PAPER
Sources of atmospheric nitrous acid: State of the science, current
research needs, and future prospects
Francesca Spataro⁄ and Antonietta Ianniello
Downloaded by [Consiglio Nazionale delle Ricerche] at 11:23 22 October 2014
CNR-Institute of Atmospheric Pollution Research, Rome, Italy
⁄Please address correspondence to: Francesca Spataro, CNR-Institute of Atmospheric Pollution Research, Via Salaria Km 29.300, 00015
Monterotondo S., Rome, Italy; e-mail: [email protected]
Nitrous acid (HONO) plays a key role in tropospheric photochemistry, primarily due to its role as a source of hydroxyl (OH) radicals
via its rapid photolysis. OH radicals are involved in photooxidation processes, such as the formation of tropospheric O3 and other
secondary atmospheric pollutants (peroxyacetyl nitrate [PAN] and secondary particles). Recent field and modeling studies have
postulated the occurrence of a strong and unknown daytime HONO source, but there are still many significant uncertainties
concerning the identification and formation mechanisms of these unknown sources. Up to now, five HONO formation pathways are
known: direct emission, homogeneous gas-phase reactions, heterogeneous reactions, surface photolysis, and biological processes. In this
review paper, the HONO sources proposed to explain the observed HONO budget, especially during daytime, are discussed, highlighting
the knowledge gaps that need further investigation. In this framework, it is crucial to have available accurate and reliable measurements of
atmospheric HONO concentrations; thus, a short description of HONO measurement techniques currently available is also reported. The
techniques are divided into three basic categories: spectroscopic techniques, wet chemical techniques, and off-line methods.
Implications: As important OH radical precursor, HONO plays a key role in the atmosphere. OH radicals are involved in
photo-oxidation processes, resulting in the formation of tropospheric O3 and secondary pollutants (PAN and secondary particles).
The identification of species affecting the oxidation capacity of the atmosphere is crucial for the understanding of the tropospheric
chemistry and in determining effective pollution control strategies. Current models underestimate HONO, especially during daytime,
since sources are lacking. Although significant uncertainties still exist, some additional sources contributing to HONO budget have
been discussed. Advantages and limitations of the currently HONO measurement techniques are also reported.
Introduction
Since the late 1970s, nitrous acid (HONO) has been identified
as atmospheric key species due to its role of direct source of
hydroxyl (OH) radicals (Perner and Platt, 1979). The OH radicals are involved in the formation of ozone (O3) and peroxyacetylnitrates (PANs), causing the so-called “photochemical smog”
in polluted regions and contribute to the oxidation of volatile
organic compounds (VOCs), forming secondary oxygenated
gaseous and particulate species. In the past, the HONO photolysis (R1; 300 < l < 405 nm) was considered a dominant source
of OH radicals in the early morning, when other primary HOx
(OH þ HO2) sources, such as the photolysis of O3 and formaldehyde (HCHO), are still weak.
JðHONOÞ
HONO þ hn ! NO þ OH
(R1)
Recent field and modeling studies have shown that R1 contributes significantly to the OH budget not only in the early morning, but also during daytime, accounting on average up to 60% of
OH production in the boundary layer in some cases (Alicke et al.,
2002; Kleffmann et al., 2005; Kleffmann, 2007; Elshorbany
et al., 2009). HONO concentrations show a vertical gradient
(depending on its sources); thus, HONO contribution to OH
production is affected by altitude. Modeling studies indicated
that HONO contribution to HOx production is more relevant near
the surfaces (where HONO is mainly formed), whereas O3 contribution is higher at elevated levels. (Czader et al., 2013;
Rappenglück et al., 2014).
Alvarez et al. (2013) showed that the HONO photolysis may
be an important indoor OH source under certain conditions (such
as direct solar irradiation inside the room), causing indoor OH
concentrations similar to the urban outdoor OH levels.
HONO is also proposed as a toxic and harmful pollutant.
Short-term exposures to elevated HONO concentrations may
damage mucous membranes, the respiratory system of asthmatics (Rasmussen et al., 1995). HONO is also precursor of
the mutagenic and carcinogenic nitrosamines by its reaction
with secondary and tertiary amines (Pitts et al., 1978; Sleiman
et al., 2010).
In the last few decades, field experiments have been carried
out at remote, rural, and urban locations, reporting atmospheric
HONO concentrations to range from a few ppt in remote and
clean areas (Beine et al., 2001; Zhou et al., 2001; Honrath et al.,
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Journal of the Air & Waste Management Association, 64(11):1232–1250, 2014. Copyright © 2014 A&WMA. ISSN: 1096-2247 print
DOI: 10.1080/10962247.2014.952846 Submitted March 13, 2014; final version submitted August 1, 2014; accepted August 1, 2014.
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Spataro and Ianniello / Journal of the Air & Waste Management Association 64 (2014) 1232–1250
2002; Liao et al., 2006; Villena et al., 2011b) up to 15 ppb in a
polluted urban site (Elshorbany et al., 2009, 2010; Spataro et al.,
2013). Recent studies highlighted a daytime unknown HONO
source, having strength in the order of 90, 500–600, and 2 ppb
hr1 in polar, rural, and urban environments, respectively
(Kleffmann et al., 2005; Acker et al., 2006; Villena et al.,
2011b; Wong et al., 2011; Li et al., 2012; Spataro et al., 2013).
Rappenglück et al. (2014) daytime HONO source strength
reaching up to 10 ppb hr1 near snow surface at relatively low
altitudes and under extreme conditions.
The formation mechanisms of HONO in the atmosphere are
still under discussions. Kleffmann (2007) reported that HONO is
mainly produced by heterogeneous processes, such as the NO2
conversion on different surfaces. However, the HONO sources,
especially during daytime, are still elusive.
This article reviews the recent progresses in understanding of
HONO formation and sink processes in the troposphere, especially during daytime. The contribution of different reactions to
the HONO budget is discussed, evaluating the cognitive gaps
related to this pollutant. Finally, recommendations of further
researches needed to better understand and define the HONO
budget are also reported.
HONO Formation Rate and
Photostationary State (PPS)
The atmospheric HONO concentrations are affected by several processes, which can cause its formation or removal in the
troposphere. The HONO formation rate can be expressed by the
following equation, including HONO source/production (Pi) and
losses (Li) processes (Spataro et al., 2013):
k2
HONO þ OH ! NO2 þ H2 O
(R2)
R1 and R2 affect the daytime HONO lifetime being in the order
of 10–20 min under typical conditions.
Ldep can be estimated by dividing the dry deposition velocity
(Harrison et al., 1996) with the boundary layer height. TV can be
estimated as in Dillon et al. (2002). During daytime, when more
turbulence occurs in the atmosphere and Lphoto is high, both Ldep
and TV are small compared with Lphoto and they can be neglected
(Su et al., 2008b, Sörgel et al., 2011, Spataro et al., 2013).
During nighttime, the boundary layer is stable and Lphoto is
small. Both Ldep and TV cannot be neglected, because this
would result in the Punknown underestimation (Sörgel et al.,
2011). Su et al. (2008b) reported that Th can be ignored assuming
weak horizontal transport effects of HONO. This last simplification is reliable in a relatively homogeneous atmosphere with low
wind speeds (Su et al., 2008b, Sörgel et al., 2011).
Assuming that the daytime HONO concentrations reach an
instantaneous photoequilibrium around midday, as a photostationary state (PSS approach; d[HONO]/dt 0), the HONO
production rate related to the unknown source (Punknown) can
be determined as follows:
X
Punknown ¼ Lphoto þ LHONOþOH Pi;known
(E2)
i
The missing HONO concentration ([HONO]unknown) can be
calculated by the following equation (E3):
½HONOunknown ¼
Punknown
JHONO þ k 2 ½OH
(E3)
JHONO is the photolysis frequency of HONO, k2 (6.0 1012
cm3 sec1) is the rate constant of R2 at 298 K and pressure ¼
X
X
dHONO
¼
P
L
i
i
1010 hPa (Atkinson et al., 2004), and [OH] is the atmospheric
dt
i
i
X
OH concentration.
¼ð
Pi;known þ Punknown Þ Lphoto þ LHONOþOH þ Ldep
Great care must be exercised using PSS method to quantify
i
the unexplained secondary HONO source strength. Since the
þ ðTV þ Th Þ
model input data can vary significantly on short time scale, the
(E1) PSS approach might not be correct (Kleffmann et al., 2005; Lee
et al., 2013).
P
E1 includes three terms. The first one,
Pi;known þPunknown Þ,
i
indicates the HONO sources including the known HONO
sources (Pi, known) and the missing daytime HONO sources
(Punknown). The second one, Lphoto þ LHONOþOH þ Ldep , indicates the HONO losses including three processes: the R1 (Lphoto)
and R2 (LHONOþOH) reactions and the dry HONO deposition
(Ldep). The third term, ðT V þ Th Þ, includes the vertical (TV) and
horizontal (Th) transport processes (Su et al., 2008b, Sörgel et al.,
2011, Spataro et al., 2013). According to the modeling analysis
by Czader et al. (2012), Tv and Th can reach appreciable
amounts. TV and Th can be sources or sinks depending on the
HONO concentration in advected air compared with the HONO
concentration at the measurement site (and height) (Sörgel et al.,
2011). If HONO is formed at the ground or near surface aerosol,
TV means a sink because vertical mixing dilutes HONO formed
near the ground.
Formation Pathways of HONO
Despite the importance of HONO for atmospheric chemistry
and its potential effects on the human health, the HONO sources
are still not completely identified. There are five HONO formation pathways actually known: direct emission, homogeneous
gas-phase reactions, heterogeneous reactions, surface photolysis, and biological processes (Kleffmann, 2007; Su et al., 2011).
The major HONO source and loss pathways are reported in
Figure 1. In the following, a short description and discussion of
the presently established formation pathways is reported.
Direct emission
HONO can be directly emitted into the troposphere by combustion processes such as biomass burning, vehicle exhaust,
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Spataro and Ianniello / Journal of the Air & Waste Management Association 64 (2014) 1232–1250
SOURCES
SINKS
Only Daytime
Photolysis surface
reactions (nitroPhotoenhanced NO2
organic species,
conversion on
HNO3/NO3organic surfaces
adsorbed
Only Daytime
HONO photolysis
to form
OH + NO
Maily during daytime
Mainly during daytime
HONO
BUDGET
Homogeneous gas
phase reaction
OH + NO
Daytime and Nighttime
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Daytime and Nighttime
Direct
emission
Non-photolytic
heterogeneous
reactions
Reaction between
HONO and OH to
formNO2
Biological
Processes
Heterogeneous
loss on surfaces
Deposition
Figure 1. A schematic sketch over the chemical processes acting as sources and sink for nitrous acid (HONO) in the lower atmosphere. The processes occurring only
in presence of solar radiation are reported in the yellow section, whereas the processes producing and removing HONO mainly during daytime are showed in the blue
section. Finally, the gray section includes the sources and sinks of HONO operating similarly both during daytime and nighttime.
domestic heating, and industrial burn. The quantitative determination of direct HONO emission was mainly focused to quantify
the contribution related to vehicle tailpipes.
Pitts et al. (1984) found that cars with three-way catalyst
emitted lower HONO and NO levels than station wagons
equipped with leaded gasoline engine. The HONO to nitrogen
oxides (NOx) ratios (HONO/NOx) ranged between 1 103 and
8 103, with higher ratios associated with older light-duty
motor vehicles without emission control devices.
Kichstetter et al. (1996) determined HONO/NOx ratios ranging between 2.4 103 and 3.4 103 at the Caldecott Tunnel
(San Francisco Bay). These values were comparable to the
HONO/NOx ratios reported by Pitts et al. (1984). Similarly,
Kurtenbach et al. (2001) measured simultaneously HONO, nitrogen dioxide (NO2), nitrogen monoxide (NO), and carbon dioxide
(CO2) in the Wuppertal Kiesbergtunnel (Germany), determining
the direct HONO emission from both single vehicles (such as a
truck, a diesel passenger car, or a gasoline passenger car) and a
traffic fleet. Higher HONO concentrations were measured during higher density traffic and were related to the using conditions
of the single vehicles. The heterogeneous HONO formation on
tunnel walls was investigated and determined to be strongly
dependent on the residence time of an air parcel and on the
surface-to-volume (S/V) ratio in the tunnel. The authors concluded that during high traffic density, direct HONO emission
was more important than heterogeneous NO2 into HONO conversion on tunnel walls. In contrast, during low traffic density,
the HONO concentration was dominated by heterogeneous NO2
conversion on tunnel walls. The calculated average HONO/NOx
ratio of (8 1) 103 was higher than that obtained by
Kirchstetter et al. (1996), (2.9 0.3) 103. The difference
can be related to the type of vehicles, which in the
Kiesbergtunnel was mainly composed by diesel-fueled vehicles.
These traffic tunnel studies showed that on average only
0.3–0.8% of the total traffic-induced NOx (NO þ NO2) can be
attributed to direct emitted HONO, suggesting that direct traffic
emission can contribute significantly to the tropospheric HONO
budget only in areas with high traffic volume. The atmospheric
HONO/NOx ratios observed in the atmosphere are usually
higher than those calculated for direct emission, indicating that
HONO is mostly secondarily formed. Rappenglück et al. (2013,
2014) reported that current mobile emission models underestimate the HONO/NOx ratios and found HONO traffic emissions
up 1.7% of the total traffic NOx emissions, likely due to heavyduty diesel (HDD) vehicles, which previous tunnel studies may
not have included, as there might have been some restrictions for
HDD vehicles in those tunnels.
Homogeneous gas-phase reactions
Several gas-phase reactions have been proposed as HONO
formation mechanisms, but kinetic studies indicated them to be
not important HONO sources due to their small rate constants
and competing daytime photolysis (Stockwell and Calvert, 1983;
Tyndall et al., 1995; Mochida and Finlayson-Pitts, 2000;
Finlayson-Pitts et al., 2003).
Up to now, only few homogeneous gas-phase mechanisms
have been accounted for the atmospheric HONO concentrations.
The most important homogeneous gas-phase reaction producing
HONO is the reaction R3:
k3
NO þ OH ðþMÞ ! HONO ðþMÞ
(R3)
R3 contributes mainly to the HONO budget during the daytime,
when OH and NO concentrations are both high (Pagsberg et al.,
1997; Alicke et al., 2002; Qin et al., 2009; Li et al., 2012), but it
competes with HONO photolysis (R1).
Lee et al. (2013) used the PSS to explain the observed HONO,
NO, and NO2 concentrations at Houston and showed that the
comparison between the modeled and measured concentrations
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Spataro and Ianniello / Journal of the Air & Waste Management Association 64 (2014) 1232–1250
might not be reliable. The PSS approach assumes that HONO
concentrations reach an instantaneous photoequilibrium; however, the chemistry and dilution of boundary layer can affect the
assumptions necessary to reach PSS. If the transport time for the
pollutants from the source to the measurement site is lower than
the time required to reach PSS, discrepancies between measured
and modeled values can be recognized (Lee et al., 2013). The
HONO concentration associated with this PSS can be determined
by the following equation (E4), assuming d[HONO]/dt ~ 0:
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dHONO
¼ Sources Loss
dt
¼ POHþNO Lphoto þ LHONOþOH
(E4)
E4 assumes R3 (POHþNO) as the only daytime HONO source,
whereas R1 (Lphoto) and R2 (LHONOþOH) are HONO loss processes. Assuming the PSS approach, the HONO concentration at
PSS ([HONO]PSS) can be calculated (E5):
½HONOPSS ¼
k 3 ½OH½NO
J HONO þ k 2 ½OH
(E5)
[OH] and [NO] are the atmospheric OH and NO concentrations,
respectively, JHONO is the photolysis frequency of HONO,
whereas k2 (6.0 1012 cm3 sec1) and k3 (9.8 1012 cm3
sec1) are the rate constants of R2 and R3, respectively, at 298 K
and pressure ¼ 1010 hPa (Atkinson et al., 2004). Kleffmann
(2007) calculated the theoretical [HONO]PSS of low ppt (<10
ppt) during the daytime. Field experiments reported daytime
HONO concentrations in urban and rural areas to be substantially higher than the theoretical [HONO]PSS (Kleffmann et al.,
2005; Acker et al., 2006; Li et al., 2012; Spataro et al., 2013).
Since this discrepancy could not be related to measurement and/
or model errors, R3 can explain partially the observed daytime
HONO and that other HONO sources, not included in E5, occur
in the atmosphere.
Very high OH levels have been observed in Chinese environments, with daily maxima ranging between 15 106 molecules
cm3 (0.610 ppt) and 26 106 molecules cm3 (1.057 ppt), and
nighttime values ranging between 1 106 molecules cm3
(0.041 ppt) and 5 106 molecules cm3 (0.203 ppt) during
the summer period. Therefore, in highly polluted Chinese environments, with significant amount of nighttime OH and NO
concentrations, the contribution of R3 to the nocturnal HONO
formation cannot be negligible (Qin et al., 2006; Hofzumahaus
et al., 2009; Li et al., 2012; Lu et al., 2012, 2013; Zhang et al.,
2012; Spataro et al., 2013).
Bejan et al. (2006) studied the HONO formation by the
photolysis of different gaseous nitrophenols using two glass
flow-tube reactors with significantly different surface-to-volume
(S/V) ratios and volumes. HONO formation (at ppb levels) was
observed when mixtures of nitrophenols (at ppm levels) were
irradiated by six ultraviolet-visible (UV-Vis) lamps (300–500
nm). This HONO production was linearly correlated with the
light intensity, nitrophenol concentration, and photolysis time,
indicating that any intermolecular reaction between two nitrophenol molecules occurred (because a quadratic concentration
dependency should result). Any dependence of HONO
formation from S/V ratio was recognized, supporting that the
process proceeded in the gas phase. Assuming that the orthonitrophenol concentration of 1 ppb was representative of an
urban site, Bejan et al. (2006) estimated the HONO production
rate of 100 ppt hr1. Thus, the photolysis of ortho-nitrophenols
can contribute to the daytime HONO budget, but its strength
source is not enough to explain the observed HONO concentrations. These results were obtained by extrapolating the laboratory data; thus, further field investigations should be needed to
verify the reliability of this estimate.
Li et al. (2008a) suggested the reaction between electronically
excited NO2 (l > 420 nm) and water vapor (H2O) to form HONO
and OH (R4–R6):
NO2 þ hn ! NO2
(R4)
NO2 þ M ! NO2 þ M
(R5)
NO2 þ H2 O ! OH þ HONO
(R6)
When NO2 absorbs radiation at wavelengths longer than ~420
nm (l > 420 nm), it forms electronically excited NO2 (NO2*)
(R4), having fluorescence lifetimes of 40–60 msec. Most of this
NO2* will be quenched by collisions with any nonreactive
molecule (M) that causes NO2* to relax to a lower energy state
(R5). The molecule M can be N2 or O2 or water vapor (H2O) for
which the quenching rate constants are known to be 2.7 1011,
3.0 1011, and ~1.7 1010 cm3 molecule1 sec1, respectively. Since the atmosphere is mainly composed by N2 and O2,
most of the NO2* will be quenched by collisions with these
species. However, H2O is a relatively abundant trace gas in the
troposphere and some of NO2* can collide with it. NO2* can also
react directly with H2O producing HONO and OH (R6). Li et al.
(2008a) determined a reaction rate for R6 of 1.7 1013 cm3
molecule1 sec1. The HONO production rate, which is the
same as OH production rate, can be expressed by the
equation E6:
NO
NO
2
POH2 ¼ PHONO
¼
J 4 ½NO2 1 þ k k6 ½5H½M2 O (E6)
where J4 is rate constant for no dissociative excitation of
NO2, k5 is the total collisional quenching rate constant of
NO2* by air, and k6 is the rate constant for R6. Modeling
studies, based on the rate constants reported by Li et al.
(2008a), concluded that NO2* chemistry can enhance the
formation of oxidants in the urban areas (Sarwar et al.,
2009; Ensberg et al., 2010).
Three- and one-dimensional modeling studies (Li et al., 2010,
2011; Czander et al., 2012, 2013; Gonçalves et al., 2012; Zhang
et al., 2012) confirmed that R3 cannot explain the observed
daytime HONO levels and that the comparison between measured and simulated HONO concentrations improved when
additional HONO sources (direct emission, heterogeneous NO2
conversion, and the reactions R4–R6) were included into the
model. The results indicated that the HONO budget was only
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Spataro and Ianniello / Journal of the Air & Waste Management Association 64 (2014) 1232–1250
slightly affected by NO2*, suggesting that it was not an important HONO source.
In conclusion, homogeneous gas-phase processes, such as R3
and the reaction between NO2* and water vapor, act as atmospheric HONO sources, but they cannot explain the observed
HONO concentrations. Laboratory investigations focused on the
determination of the rate constant k6 should be needed. Finally,
the real contribution of photolysis of nitrophenols to the daytime
HONO budget should be quantified.
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Heterogeneous reactions
It is currently accepted that heterogeneous processes involving conversion of NO2 on humid surfaces (R7) are the predominant formation pathways of nocturnal HONO:
2NO2ðgÞ þ H2 OðadsÞ ! HONOðgÞ þ HNO3ðadsÞ
(R7)
Laboratory studies indicate that the kinetics of R7 is likely to be
first order in NO2 and depends on various parameters, including
the gaseous NO2 concentration, the surface adsorbed water,
relative humidity, surface-to-volume ratio (S/V), and surface
properties (Jenkin et al., 1988; Kleffmann et al., 1998a;
Finlayson-Pitts et al., 2003, and reference therein; Stutz et al.,
2002, 2004). The surfaces available for reaction include airborne
particles, mineral dust, soils, and urban surfaces such as glass,
concrete, and foliage. These laboratory observations of NO2
hydrolysis have in common the formation of three gas-phase
products: HONO (the main gas-phase product), NO, and nitrogen protoxide (N2O, in small amounts) (Finlayson-Pitts et al.,
2003, and references therein). Nevertheless, the investigations
on R7, its detailed reaction mechanism, and its dependence on
surface water content, chemical properties (such as chemical
composition and pH), and type of surface are still under discussions. Table 1 reports the main reaction mechanisms that have
been proposed for R7.
Jenkin et al. (1988) suggested that the reaction between NO2
and water vapor, both adsorbed on the surface, formed the water
complex NO2H2O. NO2H2O reacted with gaseous NO2, producing gaseous HONO and adsorbed HNO3. However, this
mechanism was not consistent with other experimental results.
Finlayson-Pitts et al. (2003) proposed the formation of the dimer
of nitrogen dioxide (N2O4) on the reactive surface. N2O4 isomerizes to the asymmetric form, ONONO2, which either can
autoionize to generate the complex NOþNO3, or can react with
gaseous NO2 to form symmetric N2O4. The complex
NOþNO3, reacts with H2O, producing HNO3 and
HONO. HNO3 on the surface generates NO2þ, which reacts
with HONO, forming NO. HONO either can be released by the
surface or can undergo secondary reactions to produce NO, NO2,
and small amounts of nitrous oxide (N2O). Studies performed by
Gustafsson et al. (2009) indicated that the HONO formation acid
on mineral surfaces through reaction R7 does not involve an
N2O4 intermediate. Ramazan et al. (2004) investigated the
HONO formation from NO2 hydrolysis in a borosilicate glass
cell in the presence and absence of UV radiation (320–400 nm).
They proposed a similar mechanism to Finlayson-Pitts et al.
(2003), eliminating the NO2 adsorption on the surface (not
observed) and including two more reactions: the competitive
adsorption between water vapor and HONO on the surface,
which caused HONO desorption from the surface, and the reaction between HONO and HNO3, producing NO2. The results
reported by Ramazan et al. (2004) highlighted that R7 is not
photoenhanced; thus, it proceeds through the same mechanism
in light and dark conditions. Both Finalyson-Pitts et al. (2003)
and Ramazan et al. (2004) performed their experiments at high
gaseous NO2 concentrations (40–100 ppm), but their proposed
mechanism could not explain the observations at low NO2 concentrations (Kleffmann et al., 1998a; Kleffamann, 2007).
Kleffmann et al. (1998a) investigated the heterogeneous NO2
conversion on different humid surfaces and measured the
observed uptake coefficient (g), which is defined as the net
Table 1. Main reaction mechanisms proposed for the heterogeneous NO2 hydrolysis
Surface
Pyrex glass
Borosilicate glass
Reaction Mechanism
H2 OðgÞ $ H2 OðadsÞ
NO2ðgÞ $ NO2ðadsÞ
NO2ðadsÞ þ H2 OðadsÞ ! NO2 H2 OðadsÞ
NO2ðadsÞ þ NO2 H2 OðadsÞ ! HONOðgÞ þ HNO3ðadsÞ
2NO2ðgÞ $ N2 O4ðgÞ $ N2 O4ðadsÞ
H2 O
N2 O4ðadsÞ ! ONONO2ðadsÞ ! NOþ NO
3ðadsÞ
NOþ NO
3 þ H2 O4ðadsÞ ! HONOðg;adsÞ þ HNO3ðadsÞ
Reference
Jenkin et al., 1988
Finlyson-Pitts et al., 2003
HONO
þ
HNO3ðadsÞ ! NOþ
2 ! 2NOðgÞ þ O2 ðgÞ þ H ðadsÞ
NO
3
Mineral dust
Borosilicate glass
HONOðadsÞ þ HNO3ðadsÞ ! NOþ ! N2 O4 ; other?
H2 OðgÞ $ H2 OðadsÞ
H2 OðadsÞ $ Hþ
ðadsÞ þ OHðadsÞ
þ
NO2ðadsÞ þ HðadsÞ ! HONOðgÞ ! HONOðadsÞ
NO2ðadsÞ þ OH
ðadsÞ ! HNO3ðgÞ ! HNO3ðadsÞ
NO2ðgÞ þ H2 OðgÞ ! HONOðgÞ
NO2ðgÞ þ H2 OðgÞ ! HNO3ðadsÞ
HONOðadsÞ þ H2 OðgÞ ! HONOðgÞ þ H2 OðadsÞ
HONOðgÞ þ HNO3ðadsÞ ! NO2ðgÞ þ H2 OðgÞ
Gustafsson et al., 2009
Ramazan et al., 2004
Spataro and Ianniello / Journal of the Air & Waste Management Association 64 (2014) 1232–1250
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probability that a molecule undergoing a gas-kinetic collision
with a surface is actually taken up at the surface. The g value is
function of the mass accommodation, gas- and liquid-phase
diffusion, Henry’s law solubility, and liquid and surface chemistry (Crowley et al., 2010). Kleffmann et al. (1998a, 1998b)
determined g values for NO2 of about 106 and 107 for surface
with adsorbed pure water and acidic solutions, respectively.
In order to account R7 for the HONO budget, the average
nighttime conversion frequency from NO2 into HONO (CHONO)
can be estimated by the following equation (E7), where ½NO2 is
the average NO2 concentration in the time interval of (t2 t1)
(Alicke et al., 2002, 2003; Su et al., 2008a; Li et al., 2012):
C HONO ¼
½HONOt2 ½HONOt1
ðt 2 t 1 Þ½NO2 (E7)
Assuming that the conversion frequency of NO2 into HONO is
the same in the night, the daytime HONO production rate
(expressed as ppb hr1) can be determined by the equation E8:
PHONO;day ¼ C HONO ½NO2 ðt Þ
(E8)
Su et al. (2008a) suggested combinations of scaling methods in
deriving the CHONO to reduce the uncertainties in emissions and
diffusion processes. Modeling studies, including homogeneous
gas-phase reactions, direct emission, and heterogeneous reactions at the aerosol and ground surfaces (R7) as HONO sources,
showed that R7 and direct emission could explain the observed
nighttime HONO concentration, but other HONO sources prevailed during daytime (Vogel et al., 2003; Wong et al., 2011).
There are also discussions about the prevalence of aerosol or
ground surfaces in the heterogeneous HONO production.
Although the g values cannot be transferred directly from laboratory to other conditions, Kleffmann et al. (1998a) used g 106
to estimate the heterogeneous HONO formation from NO2 conversion on atmospheric aerosol, by the equation E9:
%HONO 1 %NO2
¼
hr
2 hr
1
S
¼ vNO2 3600 100
2
V
Paerosol
heter ¼
(E9)
where S/V is the surface-to-volume ratio (9 106 cm2 cm3 for
normally polluted areas and 3 104 cm2 cm3 for highly
polluted areas) and vNO2 is the mean NO2 molecular velocity.
aerosol
The HONO formation rate (Pheter
) was calculated as half (considering the stoichiometry of R7) of NO2 conversion rate (%NO2/
hr), resulting of 0.5% hr1 and 1.5%/hr1 for normally and highly
polluted areas, respectively. Thus, in the case of high aerosol load,
significant HONO amount can be formed by heterogeneous NO2
conversion on aerosol surface. Field measurements of vertical
gradients and fluxes of HONO suggested that HONO can be
significantly produced not only close to the ground, but also at
higher altitudes of the boundary layer (Harrison and Kitto, 1994;
Harrison et al., 1996; Reisinger, 2000; Kleffmann et al., 2003;
Stutz et al., 2002; Villena et al., 2011a, 2011b; Wong et al., 2011,
2012; VandenBoer et al., 2013).
1237
The reaction of NO and NO2 on humid surfaces (R8) was also
proposed as HONO source, but laboratory studies showed that
its contribution to the atmospheric HONO budget cannot be
significant (Kleffmann et al., 1998a):
NOðgÞ þ NO2ðgÞ þ H2 OðadsÞ ! 2HONOðgÞ
(R8)
Later, the heterogeneous reaction between HNO3 and NO on
glass surfaces (R9) was suggested as atmospheric HONO source
and as of importance for the renoxification of the boundary
layer:
HNO3ðadsÞ þ NOðgÞ ! HONO þ NO2
(R9)
The HONO and NO2 formations were not observed when
HNO3 and NO concentrations were close to atmospheric
levels (Kleffmann et al., 2004). The upper limit of the g
value for NO resulted to be <4 1011, indicating that R9
cannot be an important HONO source. These results confirmed previous field and laboratory studies indicating that
the heterogeneous HONO formation is unlikely to involve
high NO concentrations (Harrison and Kitto, 1994;
Kleffmann et al., 1998a).
Ammann et al. (1998) proposed the interaction between NO2
and soot particles to explain the observed HONO concentrations
in air masses where combustion sources contribute to soot and
NOx emissions. The authors reported a g value of 3.3 104 for
the first 30 sec of the reaction. Kleffmann et al. (1999) investigated the heterogeneous NO2 conversion on different carbonaceous surfaces (freshly prepared flame soot and commercial soot
treated or not treated with sulfuric acid) and showed that the
mean initial g value of 106 decreased to <108 with NO2
consumption increasing. The interaction between NO2 and the
soot surface caused the decrease of the number of active sites on
the surface; thus, without any recycling mechanism atmospheric
HONO formation on soot surfaces is not of major importance.
The discrepancy between the results obtained by Kleffmann
et al. (1999) and Ammann et al. (1998) was likely related to
the different time scale of the experiments. Kleffmann et al.
(1999) reported also that the HONO formation required the
presence of water, since the g values increased with increasing
water vapor pressure. Aubin and Abbatt (2007) summarized the
literature data for the initial uptake coefficient (g0) of NO2 on
various black carbon substrates, showing that they ranged from
0.1 to 108, but most of them were of the order of 105. The
variance among these measurements can be related to initial NO2
concentration, type of soot, the different time scale of the laboratory experiments, and the different method employed for the
determination of the uptake coefficient. Indeed, the uptake coefficient can be calculated by the following equation (E10), where
r is the radius of the flow reactor, kw is the wall-loss rate constant,
nNO2 is the mean molecular speed of NO2, ASSA is the specific
surface area of the sample, and Ageo is the geometric surface area:
2rk w ASSA 1
0 ¼
nNO2 Ageo
(E10)
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Spataro and Ianniello / Journal of the Air & Waste Management Association 64 (2014) 1232–1250
Many studies accounted only the geometric surface area in the g0
calculation, and since Ageo can be >3 orders of magnitude lower
than ASSA, the term g0 was overestimated.
Han et al. (2013a) investigated the composition of soot aerosol
and its influence on NO2 uptake coefficient and HONO yield,
showing that (1) preheated samples exhibited a greater decrease in
NO2 uptake coefficient and HONO yield than the fresh soot samples,
due to the removal of organic carbon from soot; and (2) ozonized
soot showed a lower reactivity toward NO2 than fresh soot, due to the
increase of oxidation state of the soot surface. When fresh soot is
emitted in the atmosphere, it undergoes aging processes by the
uptake of reactive gases, leading to an increase of soot surface
oxidation and thus to a lower reactivity of soot towards NO2.
The semivolatile and/or water-soluble organic species in diesel exhaust were also suggested to be significantly involved in
secondary HONO formation (R10; Gutzwiller et al., 2002a):
NO2 þ fCHgred ! HONO þ fCgox
(R10)
{C–H}red indicates surface-bound species or class of compounds. Diesel exhaust contains a water-soluble class of compounds, condensing below 100 C and able to reduce NO2 to
nitrite (NO2) in the aqueous phase or to HONO on a surface.
{C–H}red indicates surface-bound species or class of compounds. Diesel exhaust contains a water-soluble class of compounds, condensing below 100 C and able to reduce NO2 to
nitrite (NO2) in the aqueous phase or to HONO on a surface.
Kirchstetter et al., 1996; Kurtenbach et al., 2001). The NO2
reduction in aqueous solutions by organic species (resorcinol,
2,7-dihydroxynaphthalene, guaiacol, syringol, and catechol)
was recognized to be enhanced in alkaline conditions
(Gutzwiller et al., 2002b; Ammann et al., 2005). Although R10
seems to contribute significantly to the atmospheric HONO,
there are questions to be resolved. First, laboratory experiments
focused on the determination of dependence of the reaction rate
from the surface water content is still lacking. Then, field studies
aimed to identify the organic compounds, which are involved in
NO2 reduction and to assess its real contribution of reaction R10
to the HONO budget, should be performed.
Ziemba et al. (2010) used PSS assumption, suggesting heterogeneous HONO production due to the HNO3 depletion on
hydrocarbon-like organic aerosol (HOA) surface. However, Lee
et al. (2013) showed that the PSS assumption might not be valid
and might lead to overestimation of secondary HONO source
strength. Additionally, different air mass, vertical mixing, and
parameters such as HONO/NOx ratio, deposition velocity, and
uptake coefficients could contribute to the uncertainty of PSS
calculation. Rutter et al. (2014) showed that HONO formation
rate, due to the HNO3 reduction into HONO by volatile organic
compounds (VOCs), ranged between 0.1 and 0.5 ppb hr1, but
that it was not affected by the increasing of surface area. These
results indicated that the reaction does not occur on aerosol
surface and that, likely, HNO3 reacts homogeneously with
VOC emitted by the aerosol surface (R11):
HNO3 þ VOC ! HONO þ VOCox
(R11)
The correlation between HONO concentration and aerosol surface, recognized by Ziemba et al. (2010), can be explained by the
simultaneous emission of VOCs and aerosol by vehicle exhaust.
However, more laboratory investigations are required to assess if
the HONO formation observed by Ziemba et al. (2010) was
heterogeneous or homogeneous in nature. Laboratory experiments involving aerosol loading comparable to that observed
by Ziemba et al. (2010) and evaluating the dependence of the
HNO3 conversion from relative humidity and water content of
aerosol surface should be needed. Up to now, it is not possible to
exclude that the HNO3 into HONO conversion might occur also
on the organic aerosol surface.
In conclusion, heterogeneous processes are important atmospheric HONO sources. The ground is mainly involved in the
HONO production, but high aerosol loading in the atmosphere
can affect significantly the HONO budget. R7 can explain the
nighttime HONO concentrations, but more research focused on
the identification of this reaction mechanism as well as its
dependence on surface water content, on chemical properties
(such as chemical composition and pH), and on type of surface
is needed. However, R7 cannot explain the observed daytime
HONO levels. R10 is also a possible HONO source, but the
following items should be evaluated: dependence from the surface water content, the involved organic compounds, and the real
contribution of R10 to the HONO budget. Finally, the homogeneous and/or heterogeneous nature of R11 should be clarified.
Photolysis/photoenhanced surface reactions
Since direct emission and homogeneous gas-phase and heterogeneous reactions could not fully explain the observed daytime HONO concentrations, the scientific efforts have been
focused on the photochemical/photoenhanced processes. In
this framework, several studies have been performed on the
photoenhanced NO2 or HNO3 conversion on organic and inorganic surfaces.
Photoenhanced reactions on soot/organic surfaces.
Heterogeneous reactions on organic aerosol surfaces have been
widely reported in literature and several carbonaceous surfaces,
such as soot, hydrocarbons, diesel soot, and humic acid, have
been studied.
George et al. (2005) investigated the effect of light with 300 <
l <500 nm on the uptake kinetic of NO2 of different organic
films (such as benzophenone, catechol, anthracene, anthrarobin,
and their mixtures) occurring ubiquitously on aerosol and
ground surfaces. The NO2 uptake coefficient determined under
UV light for the different organic films are reported in Table 2
and ranged between 107 and 106, with HONO yields ranging
between 50% and 100% under UV irradiation comparable to the
irradiance in the wavelength interval of 300–420 nm at the Earth
surface under 0 zenith angle.
George et al. (2005) concluded that this process may contribute to the daytime HONO budget and noted that the proposed
mechanism can occur continuously during daytime. Similarly,
Stemmler et al. (2006, 2007) proposed the photosensitized NO2
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Spataro and Ianniello / Journal of the Air & Waste Management Association 64 (2014) 1232–1250
Table 2. NO2 uptake coefficients (gNO2) of different organic surfaces under UV irradiation
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Surface
Humic acid (soil)
Humic acid (coating)
Humic acid (aerosol)
Benzophenone/catechol
Pyrene/KNO3
Benzophenone/catechol
Benzophenone/catechol
Anthracene
Fluoranthene surface
Catechol
Catechol
Catechol/anthracene
Anthracene
Benzophenone
Soot surface
RH
(%)
l
(nm)
[NO2]
(ppb)
gNO2
Reference
20
26
26
76
300–700
400–750
400–750
300–420
300–420
300–420
300–420
300–420
300–420
300–420
300–420
300–420
300–420
300–420
300–420
20
~10
~10
20
30
20
20
20
25
20
20
20
20
20
150
2 105
~1.5 105
~5.5 106
5.1 106
2.7 106
2.5 106
2.4 106
2.3 106
2.0 106
1.8 106
1.4 106
1.3 106
1.2 106
6.5 107
5.0 107
Stemmler et al., 2006
Stemmler et al., 2007
Stemmler et al., 2007
George et al., 2005
Ammar et al., 2010
George et al., 2005
George et al., 2005
George et al., 2005
Cazoir et al., 2014
George et al., 2005
George et al., 2005
George et al., 2005
George et al., 2005
George et al., 2005
Monge et al., 2010a
14
Dry
56
<5
42
Dry
46
Dry
Dry
30
Notes: RH ¼ relative humidity; [NO2] ¼ concentration of NO2; l ¼ wavelength.
reduction on humic acid, representing the complex unsaturated
organic materials ubiquitously present in the environment
(R12–R14):
hn
HA ! Ared þ X
(R12)
Ared þ X ! A0
(R13)
Ared þ NO2 ! A00 þ HONO
(R14)
The mechanism proposed for this process involves the photochemical activation of reductive sites (Ared) on the organic surface (R12). The reductive sites may undergo two different fates:
their deactivation (R13) or the reaction with gaseous NO2 to
form HONO. The X species is an oxidant formed on the organic
surface. However, it is not clear the exact chemical nature of the
reductive species formed on the humic acid surface. The upper
limit of NO2 uptake coefficients (gNO2) for humic acid coating
and aerosol were lower at higher NO2 concentrations, and when
the relative humidity (RH) was lower than 20% and higher than
60% (Stemmler et al., 2007), the gNO2 values for humic acid
coatings were 3 times higher than those observed for humic acid
aerosol, suggesting that HONO formation on ground surface is
higher than those on aerosol surface. The daytime HONO production rate from soils containing humic acid (700 ppt hr1
within a boundary layer of 100 m height) was estimated to be
comparable to the unknown HONO sources (90, 500–600, and 2
ppb hr1 for polar, rural, and urban environments, respectively)
(Stemmler et al., 2006). Monge et al. (2010b) showed the persistence of the photoenhanced NO2 into HONO conversion on soot
over long periods. Thus, soot transported away from a polluted
environment might provide a local photochemical source of NO
and HONO. However, the initial gNO2 coefficients of 5 107
and 5 108 were determined at initial NO2 concentrations of
16 and 120 ppb, respectively. More recently, Han et al. (2013b)
showed that simulated sunlight enhanced the aging process of
soot toward NO2, resulting in the production of nitro compounds
on soot surface. The photolysis of nitro compounds resulted in
carbonyl groups and gaseous NO and HONO. Soot samples with
higher organic carbon concentration exhibited higher photochemical reactivity. This fraction of organic carbon was composed
by polycyclic aromatic hydrocarbons (PAHs) with 3–5 rings and
some unidentified components. However, the low gNO2 coefficients indicated that light-induced NO2 conversion on soot aerosol surface cannot be an important HONO source even if the soot
surface may remain chemically active for long periods. These
laboratory experiments investigated the interaction between NO2
and soot under UV-A radiation, whereas the effect of visible and
UV-B as well as the changes of the chemical and physical
properties of soot surface have not been evaluated yet.
Ammar et al. (2010) performed laboratory experiments on
the heterogeneous NO2 reaction with solid pyrene/KNO3 films
(as a proxy of urban grime), determining lNO2 coefficient of
2.67 106 under near-UV irradiation (300–420 nm) and
estimating the HONO production rate of 130 ppt hr1.
Recently, Cazoir et al. (2014) studied the heterogeneous loss
kinetics of gaseous NO2 on solid fluoranthene films deposited on a
Pyrex substrate under UV-A (range 300–420 nm) radiation. The
uptake coefficients decreased from 2 106 to 7.0 107, for
initial NO2 concentrations of 20 and 150 ppb, respectively, and they
were not affected by temperature and RH. These results suggested
the photoenhanced NO2 reactivity towards fluoranthene and possibly other PAHs. It would be interesting to extend this study to other
PAH compounds to evaluate the real influence of this process on
HONO formation and its atmospheric implications.
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In conclusion, the photoenhanced NO2 conversion on organic
surfaces or humic acids can fully explain the observed daytime
HONO concentrations. However, more research, focused on the
organic chemicals participating to the HONO formation and the
reaction mechanisms, should be performed. The effect of both
UV-A and UV-B radiations on HONO production should be also
assessed. Finally, field experiments evaluating the real contribution of these processes to the HONO budget are needed.
Photolysis reactions on inorganic surfaces. During the last 30
years, several studies have been focused on the photolysis reactions on inorganic surfaces resulting in HONO production. The
photolysis of adsorbed HNO3 or nitrate (NO3) (l ~300 nm) was
identified as a daytime HONO and NOx source in rural environments, and the following mechanism has been identified (reactions R15–R17):
HNO3ðadsÞ þ hn ! ½HNO3 ðadsÞ
(R15)
½HNO3 ðadsÞ ! HONOðadsÞ þ O 3 P ðadsÞ
(R16)
½HNO3 ðadsÞ ! NO2 ðadsÞ þ OHðadsÞ
(R17)
R17 results in the formation of NO2, which in turn can react with
the water adsorbed on the surface forming gaseous HONO and
HNO3 adsorbed on the surface (R7). This process is 1–2 orders
of magnitude faster than that in the gas phase and aqueous phase
(Zhou et al., 2002, 2003).
Zhou et al. (2011) performed direct measurements of HONO
fluxes over a rural forest canopy in Michigan, showing that the
daytime HONO flux was positively correlated with the product
of leaf surface nitrate loading and the rate constant of nitrate
photolysis. They suggested that the photolysis of HNO3 on forest
canopies is a significant daytime HONO source to the troposphere in rural environments and an important pathway for the
remobilization of deposited HNO3. The authors explained that
the remobilization of deposited HNO3 causes the overlying
troposphere to be more photochemically reactive than generally
predicted, and the long-range transport of reactive nitrogen to
impact potentially on greater distances than previously thought.
In contrast to heterogeneous HONO formation that mainly
accelerates morning ozone formation, inclusion of HONO
photochemical sources influences ozone throughout the day,
affecting its peak concentration (Sarwar et al., 2008; Czader
et al., 2012; Rappenglück et al., 2014). Li et al. (2012) showed
that the photolysis of HNO3 adsorbed on ground surface was
related to HONO formation in the Pearl River Delta region,
during the Program of Regional Integrated Experiments of Air
Quality over Pearl River Delta 2006 (PRIDE-PRD2006) campaign. The importance of this mechanism is associated with its
potential role of pathway for the remobilization of deposited
HNO3. A similar mechanism has been recognized at high polar
latitudes during springtime (Zhou et al., 2001; Dibb et al., 2002;
Honrath et al., 2002). This process starts with the deposition and
migration of NO3 ion to the snow pack surface, followed by the
photoreduction of NO3 to form NO, NO2 and nitrite (NO2)
ion, and the NO2 acidification producing HONO. Several studies showed that NOx and HONO released from the snow surface
at high and mid latitudes can alter significantly the chemistry in
the remote boundary layer, because the radical pool is altered
(Ianniello et al., 2002; Beine et al., 2003; Rappenglück et al.,
2014). However, more field experiments, including the analysis
in situ of surface sample (chemical and physical properties) and
vertical fluxes of HONO, NO, and NO2, should be performed in
order to better understand the impact of the reactions R15–R17
on the atmospheric HONO budget.
Research studies highlighted the simultaneous increase of NO3
in PM10 and PM2.5 (particulate matter with aerodynamic diameters
of <2.5 and <10 mm, respectively) and gaseous HONO concentrations during Saharan dust events (Wang et al., 2003; Saliba and
Chamseddine, 2012), suggesting that mineral dust affects the conversion of nitrogen species into HONO in the troposphere.
Laboratory studies suggest that HONO can be formed by the UVirradiated interaction of reactive nitrogen compounds (NO2, NO,
HNO3) with mineral-containing substrates such as titanium dioxide
(TiO2), silicon oxide (SiO2), aluminium oxide (Al2O3), calcium
carbonate (CaCO3), and real Saharan and Asian dust (Saliba et al.,
2001; Usher et al., 2003, Cwiertny et al., 2008, Ndour et al., 2008,
Ma et al., 2013). However, these studies were not conclusive
regarding the understanding of reaction mechanisms and their
dependence on several parameters: surface chemical properties
(acidity, water content, and composition), relative humidity, concentration and type of nitrogen species, and wavelength and intensity of radiation. Ramazan et al. (2004) investigated the effect of
UV radiation on the heterogeneous NO2 hydrolysis on borosilicate
glass surface, and their results showed that R7 is not photoenhanced. Their experiments indicated also that HNO3, resulting
from R7, can exist either as nitric acid–water complexes
(HNO3H2O) or as NO3 ion. HNO3H2O can undergo photolysis
(l < 741 nm), producing adsorbed HONO and hydrogen peroxide
(H2O2). The release of this HONO depends on the water vapor
concentration, which triggers a competitive adsorption process
(Ramazan et al., 2006; Ma et al., 2013):
HNO3 H2 OðadsÞ þ hn ! HONOðadsÞ þ H2 O2ðadsÞ
(R18)
HONOðadsÞ þ H2 OðgÞ ! HONOðgÞ þ H2 OðadsÞ
(R19)
Since SiO2 is the major component of mineral dust, laboratory
and field experiments aimed to verify the heterogeneous NO2
hydrolysis on mineral dust and the following photolysis of
HNO3H2O or NO3 ion are still lacking. The laboratory experiments should evaluate also how gaseous HONO production from
mineral dust surface is affected by RH and surface water content.
Many research studies have been focused on the NO2 removal
by titanium oxide (TiO2) surfaces. Although TiO2 is found in
dust particles at mass concentration ranging from 0.1% to 10%,
this species has attracted great interest due its well-known photocatalytic properties. The irradiation of TiO2 molecules by UV
lights cause the photoproduction of excess electrons in the conduction band (ecb) and holes in the valence band (hþvb) (reaction R20). The hole oxidizes water vapor (R21), whereas the
electron reduces the oxygen (R22):
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Spataro and Ianniello / Journal of the Air & Waste Management Association 64 (2014) 1232–1250
TiO2 þ hn ! hþ
vb þ ecb
(R20)
þ
hþ
vb þ H2 O ! OH þ H
(R21)
e
cb þ O2 ! O2
(R22)
NO2 þ OH ! HNO3
(R23)
NO2 þ O
2 ! NO2 þ O2
(R24)
When gas-phase NO2 diffuses on TiO2 surface, it can react with
OH radical to form HNO3, or with the oxygen activated species
(O2) to produce NO2 ion (Ndour et al., 2008; Monge et al.,
2010a). Ndour et al. (2008) performed laboratory studies, exposing mixed TiO2/SiO2 and Saharan and Arizona test dust to
gaseous NO2. TiO2/SiO2 mixtures (with TiO2 and SiO2 contents
of 1 wt% and 5 wt%, respectively) were related to uptake
coefficients ranging from 1.2 107 to 1.9 106. The small
TiO2 amount was enough to make the surface photochemically
active. Higher uptake coefficients were related to increased TiO2
content in the TiO2/SiO2 mixtures (Monge et al., 2010a). These
uptake coefficients ranged from (2.0 0.6) 107 to (1.5 0.4) 106 for TiO2 content of 10 wt% and 100 wt%, respectively. The results showed that the presence of O2 in the carrier
gas affects the HONO production yields in the heterogeneous
reaction between NO2 and TiO2 as well as the products of the
renoxification process. However, more kinetic data should be
collected in order to improve understanding of this process, and
to include it into modeling studies. The influence of parameters
such as RH, surface water content, intensity and wavelength of
radiation, and changes on the substrate have to be considered for
future studies.
Biological processes
Su et al. (2011) proposed that the unknown HONO source
was related to soil biological processes. They reported that the
dominant source of N(III) in soil are biological nitrification and
denitrification processes. The nitrification process converts
ammonium ion (NH4þ) to NO2, which in turn is converted to
NO3 resulting in the accumulation of both NO2 and NO3 in
the soil. During the denitrification, NO3 is reduced to gaseous
NO, which can be released from the soil surface (Su et al., 2011;
Pilegaard, 2013; Oswald et al., 2013). Since NO2 is highly
water soluble, it can be protonated by Hþ (reaction R25) resulting in adsorbed HONO. Then, HONO can undergo the competitive adsorption with water vapor (R26), resulting in the release
of gaseous HONO from the soil.
þ
NO
2ðaqÞ þ HðaqÞ $ HNO2 ðaqÞ
HNO2 ðaqÞ þ H2 OðgÞ $ HONOðgÞ þ H2 OðaqÞ
(R25)
(R26)
The equilibrium gas-phase HONO concentration over an aqueous solution ([HONO]*) can be determined by the following
1241
equation (E11), where [Hþ] is the hydrogen ion concentration,
[N(III)] is the total nitrite concentration ([HNO2]þ[NO2], Ka,
HNO2 is acid dissociation constant for nitrous acid, and HHONO is
Henry’s law coefficient of nitrous acid:
½H þ NO
2
½HONO ¼
K a;HNO2 H HONO
½HNO2 þ NO
2
¼
K
2
1 þ ½a;HNO
H HONO
H þ
½NðIIIÞ
¼
(E11)
K a;HNO2
1 þ ½H þ H HONO
[HONO]* controls the gaseous HONO amount that can be
released to the atmosphere, and its value is affected by pH of
the soil, nitrite concentration in the soil, and temperature (which
affects both Ka,HNO2 and HHONO). If [HONO]* is higher than
gaseous HONO concentration, then HONO will be released
from the soil. Thus, the potential impact of this process is related
to the use of the soil, such as agriculture, fertilization, aquatic
transport, deposition of atmospheric pollutants, waste, etc. Su
et al. (2011) evaluated the functional dependence of [HONO]*
on [N(III)] and pH, suggesting that neutral or alkaline soils (high
pH) were characterized by higher N(III) concentrations, whereas
acid soils had low N(III) concentrations. These results were in
agreement with their proposed mechanism, where acid soils
favor the release of HONO. Su et al. (2011) used the following
equation (E12) to determine the HONO emission fluxes from
soil nitrite. nt is the transfer velocity, affected by meteorological
data and soil conditions:
F ¼ nt ð½HONO ½HONO Þ
(E12)
The obtained fluxes were used to estimate atmospheric HONO
production rate from soil (Psoil):
F ¼ a H Psoil
(E13)
where a is a factor converting ppb units into ng m3 units (a ¼
572 ng m3 ppb1 at 101,325 Pa 132 and 298 K) and H is the
boundary layer height. Their results indicated that even low soil
nitrite concentrations can cause emissive HONO fluxes of ~1 to
1000 ng m2 sec1 for boundary layer heights of ~100 to 1000 m.
Oswald et al. (2013) studied the relation between soil HONO
emissions and biogenic soil NO emissions, using the dynamic
chamber method and investigating soil samples, with a wide
range of soil pH, organic matter, and nutrient contents. The
results showed that maximum HONO (about 13 ng m2 sec1)
and NO (about 18 ng m2 sec1) fluxes from soil samples were
comparable and were affected by water content, pH, and temperature. However, unexpectedly high HONO and NO emissions
were observed for neutral and alkaline soils, which was characterized by higher NH4þ and NO2 concentrations. These results
were in disagreement with the acid-base equilibria proposed by
Su et al. (2011), where acid pH favors release of gaseous
HONO. The authors suggested that beside the acid-base equilibria, ammonia-oxidizing bacteria (AOB) can dominate the
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Figure 2. Schematic sketch of the major components related to the differential optical absorption spectroscopy (DOAS) system.
formation of HONO and NO within the soil. Therefore, HONO
is formed and emitted during nitrification, which is favored at
lower water holding capacity (WHC). Maximum HONO and NO
fluxes were measured within 0–40% of WHC.
HONO emission from soil can be considered as another
potential source of this species in the troposphere, which is
consistent with the observations that ground surfaces are dominant for HONO production. Further studies are recommended to
assess the real impact of HONO release from soil nitrite on in the
biogeochemical cycling of N in both agricultural and natural
environments.
HONO Measurement Techniques
Accurate HONO measurements are difficult to perform.
Certified reference gases for HONO do not exist, because it is
unstable. HONO reactivity and solubility can cause sampling
losses and artifacts. Consequently, intercomparisons between
different techniques may exhibit significant discrepancies.
During the international FIONA (Formal Intercomparisons of
Observations of Nitrous Acid) campaign, a set of experiments
was performed in the EUPHORE (European PHOtoREactor)
chambers, simulating urban and semirural scenarios and intercomparing almost all types of instruments used to detect gaseous
HONO. Good agreement among the techniques was found, but
deviations for some measurements were also revealed. These
differences were related to the difficult adaptation of the instruments to the EUPHORE chambers and to calibrations issues
(Ródenas et al., 2013). However, the FIONA results are only
preliminary and analyses are still ongoing.
Pinto et al. (2014) compared the HONO measurements made
by different instruments during the SHARP (Study of Houston
Atmospheric Radical Precursors) camping. The HONO temporal patterns obtained by these techniques were in good agreement. The largest differences among the measurements were
related to (1) high pollution conditions causing positive and/or
negative interference in in situ techniques; (2) the sampling site,
where heterogeneous reactions can affect the HONO measurement; (3) the position of the instrumental inlet; and (4) other
experiments performed at the sampling site causing differences
among instruments.
In the following sections, an overview of HONO measurement techniques currently available in the literature is reported.
They are divided in three basic categories: spectroscopic techniques, wet chemical techniques, and off-line methods.
Optical spectroscopic methods
The optical spectroscopic techniques measure atmospheric
trace gases without the need for chemical extraction, with
calibrations that are based on known absorption cross-sections
(line strengths) and specificity that can be confirmed by spectral
identification. However, absorption spectroscopic analytical
methods tend to be expensive and to have bulky system components, and for many trace gases the fundamental sensitivity is
relatively low, requiring either long absorption paths or increased
signal averaging time (Kleffmann, 2007).
Differential optical absorption spectroscopy (DOAS). The
DOAS technique, schematically showed in Figure 2, has been
widely used to detect atmospheric HONO in urban, rural, and
semirural environments (Perner and Platt, 1979; Pitts et al.,
1984; Andrés-Hernández et al., 1996; Reisinger, 2000; Stutz
et al., 2002, 2004, 2010; Acker et al., 2006).
The DOAS system (Figure 2) includes a light source, emitting
light radiation, which travels across the atmosphere for distances
ranging from few hundred meters to several kilometers (Alicke
et al., 2002; Kleffmann et al., 2006; Stutz et al., 2010; Zhou et al.,
2013). At the end of this path, the radiation is then collected and
detected (Platt and Stutz, 2008). The active DOAS is equipped
with a broadband source, such as a Xenon arc lamp (e.g., longpath DOAS), whereas the passive DOAS uses sunlight or scattered sunlight as source (e.g., multiaxis DOAS). Since the radiation moves through the atmosphere, its intensity is reduced by
absorption of a speci fic trace gas. Simultaneously, the light
intensity can be also absorbed by other trace gases (extinction),
or can be scattered by gaseous and particulate, and these processes cause interferences. The broad bands are related to instrument effects and turbulences, whereas the high-frequency
narrow band cross-section is related to characteristic absorption
lines or bands of trace gases. HONO is detected by its narrowband UV absorption between 340 and 370 nm and is quantified
by Beer-Lambert law. Detection limits (LOD) of 100 and 200 ppt
related to path length of 750 and 6 m, respectively, have been
reported (Kleffmann et al., 2006; Qin et al., 2006). Stutz et al.
(2010) achieved the LOD of 16 ppt related to path length of 4–5
km. DOAS is an accurate and sensitive technique, allowing the
simultaneous detection of different species (Platt and Stutz,
2008; Stutz et al., 2010). Since HONO measurements by
DOAS are artifact-free with respect to the sampling procedures,
DOAS is often used as reference during HONO intercomparison.
However, the simultaneous measurement of different species at
high concentration, especially NO2, can cause large uncertainty
for HONO quantification by DOAS (Kleffmann et al., 2006).
Cavity ring-down spectroscopy (CRDS). The CRDS is a direct
spectroscopic technique (detects directly the HONO molecule),
which measures the rate of decay of the radiation due to the
strong absorption of HONO at the wavelength of 354.2 nm. Two
highly reflective mirrors (R > 99.9%) are used to trap tunable
Spataro and Ianniello / Journal of the Air & Waste Management Association 64 (2014) 1232–1250
laser pulses inside an optical cavity, and the trapped light makes
about 1000 rounds in the cavity. The decay rate (1/t) can be
determined by the following equation (E14), where (1/t0) is the
background decay rate due to the mirror loss and the scattering, a
is the absorption coefficient, n and s are the number density and
absorption cross-section of the absorbing sample, respectively:
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sn ¼
1 1 1
¼a
c t t0
(E14)
The detection limit (LOD) was determined to be in the order
of 5 ppb, with a time resolution of 15 sec, but LOD in the
order of 0.1 ppb could be reached by upgrading optics, a
longer cavity, or other improvements. Absorptions in the
cavity by other trace gases (NO2, SO2, or others) can affect
the HONO determination by CDRS. This problem can be
solved by the use of differential CDRS. However, no field
application of this technique has been reported (Wang and
Zhang, 2000).
Tunable infrared laser absorption spectroscopy (TIRLS).
TIRLS is based on absorption spectroscopy to measure directly
atmospheric HONO without chemical extraction. Ambient air is
sampled into a low-pressure multipass cell, where it interacts
with light from an infrared (IR) laser. The calibrations are based
on known absorption cross-sections (line strengths) and on
spectral identification (Lee et al., 2011). Li et al. (2008b) used
tunable diode laser absorption spectroscopy (TDLS) with a light
absorption path of ~150 m to detect HONO by its strong absorption feature at 1713.511 cm1. The detection limit was of ~200
ppt, with a time resolution of 1 sec. Lee et al. (2011, 2013)
developed a tunable IR laser differential absorption spectrometer
equipped with continuous-wave quantum-cascade lasers. This
instrument was able to measure both HONO and NO2 concentrations. The continuous-wave quantum-cascade lasers allowed
much more mode stability, higher laser power output, and the
ability to operate both lasers and detectors near room temperature for long-term field measurements. Detection limits of 40
and 10 ppt for HONO and NO2, respectively, were reported with
1-hr spectral averaging.
Photofragmentation (PF)/laser-induced fluorescence (LIF).
The determination of HONO by PF/LIF technique consists
in its UV photofragmentation (l ¼ 355 nm) producing NO
and OH, followed by the quantifications of OH by laserinduced fluorescence at an excitation wavelength of 282 nm
and an emission wavelength of 309 nm. The detection limit
is of 2–15 ppt, with a time resolution of 1 min. The excitation wavelength of 282 nm allows minimizing the interferences due to O3. In particular, at high O3 concentrations and
high relative humidity, this interference needs to be quantified and corrected (Kleffmann, 2007). The detection limit is
of 2–15 ppt, with a time resolution of 1 min. Liao et al.
(2006) used PF/LIF to measure HONO at the South Pole
(Liao et al., 2006), but this technique has not been used in
more than 10 years.
1243
Wet chemical methods
The wet chemical techniques operate by collecting gaseous
HONO by several devices (wet effluent denuder, rotated denuder,
bubbler), in which inner walls are wetted with suitable solutions.
These sampling solutions are then analyzed by analytical techniques such as ion chromatography (IC), high-performance liquid
chromatography (HPLC), spectrophotometry, etc. The wet techniques are simpler and less expensive compared with the spectroscopic techniques, providing higher sensitivities with detection
limit in the order of few ppt. Although the wet chemical methods
can be very sensitive, the need to scrub HONO into solution may
introduce chemical interferences caused by, e.g., NO2 and phenol
reaction (Gutzwiller et al., 2002a, 2002b), NO2 and SO2 (Spindler
et al., 2003), NO2 and aromatic amines (Saltzman, 1954), or PAN
hydrolysis (Frenzel et al., 2000). Thus, attention has to be paid to
minimize the chemical interferences when wet chemical techniques are used to measure HONO concentrations.
Wet denuder. A widely used wet denuder for HONO measurement is the wet effluent diffusion denuder (WEDD). It is set up
vertically, in which the purified water (absorbing solution) is
continuously pumped into the tube at the top forming a thin
aqueous film on the inner wall surface. The adsorbing solution
collects gaseous HONO, which is highly soluble in water, converting it into nitrite and analyzing by IC. The detection limit is
of few ppt, with a time resolution of 5–30 min depending on the
run time of IC (Simon and Dasgupta, 1993; Zellweger et al.,
1999; Acker et al., 2004, 2006).
HONO can be also measured by rotating wet annular denuder
(RWAN), set up horizontally. In this case, the absorbing solution
is an alkaline solution, which coats the inner walls of the denuder. HONO is collected as nitrite, and the absorbing solution is
then analyzed by IC. The detection limit is of 12 ppt, with a time
resolution of 5–60 min depending on the run time of IC (Trebs
et al., 2004).
Mist chamber–IC. The mist chamber was used for sampling of
HONO and other water-soluble species. This technique operates
by nebulizing deionized water (scrubbing solution) into fine mist
by the high flow of ambient air drawn into the chamber. The water
droplets collect gaseous HONO, forming nitrite, which is analyzed by IC. The detection limit is of few ppt, with a time
resolution depending on the chromatographic run (5–30 min).
Aerosol particles are removed by a filter at the inlet of the
chamber, to avoid interferences. The inlet is also maintained in
insulated and dark conditions to avoid possible HONO formation
due to heterogeneous and photolytic processes (Kleffmann and
Wiesen, 2008).
HPLC/UV system. HONO can be also determined by reversephase HPLC analysis with UV absorption detection at 309 nm
(Zhou et al., 1999). A schematic sketch of HPLC/UV system is
shown in Figure 3.
Gaseous HONO is trapped quantitatively in a coil sampler
using buffer solution. The scrubbing solution is derivatized with
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Figure 3. Schematic sketch of the semiautomatic HONO instrument based on
reverse-phase HPLC analysis, followed by UV absorption detection.
sulfanilamine (SA)/N-(1-naphtyl)ethylendiamine dihydrochloride
(NED), analyzed by HPLC, and detected by UV-Vis absorption.
The LOD is <0.8 ppt, with a time resolution of about 10 min. This
method suffers from chemical interferences caused by, e.g., NO2
and phenol reaction or by NO2 and SO2, which cannot be quantified. To correct the data, interference from gaseous and particulate
species could be removed operating with a two-channel system.
The first channel collects ambient air directly, whereas the second
channel is equipped with a sodium carbonated (Na2CO3)-coated
denuder to remove interfering compounds. However, during the
last 10 years, HONO measurements have been performed by a
technique based on long-path absorption photometer.
Long-path absorption photometric technique. As HPLC/UV
method, this method operates by collecting HONO by aqueous
coil scrubbing, followed by acid derivatization (SA/NED mixture) of NO2, forming the highly light-absorbing azo dye,
which is the detected by long-path absorption photometer
(Heland et al., 2001; Kleffmann et al., 2002; He et al., 2006;
Kleffmann and Wiesen, 2008, Ren et al., 2011). Up to now, many
variants of this method have been reported. Ren et al. (2011)
used long-path absorption photometric technique with a single
channel to measure HONO concentrations at different heights in
California. Zhou et al. (2002) performed simultaneous HONO
and HNO3 measurements by a two-channel long-path absorption
photometric system. In particular, HONO was measured in the
first channel as described above, whereas HNO3 was measured
by the same technique after first converting it to NO2 by a
hydrazine reduction method. Helland et al. (2001) and
Kleffmann et al. (2002) used long-path absorption photometric
technique operating with two coil samplers in series (Figure 4):
the first coil collects HONO and interfering species (such as NOx
and PAN), whereas the second coil collects the same amount of
interfering species but no HONO.
Thus, the correct HONO concentration can be determined by
the difference between the signals of the first coil and the second
coil. The detection limit is 1 ppt, with a time resolution of 1.5–4
min. Acidic sampling conditions allows minimizing some
Figure 4. Schematic sketch of the long-path absorption photometric (LOPAP)
technique.
interferences (NO2 and SO2, NO2 and phenols, NO2 and aromatic amines, or peroxyacetyl nitrate [PAN] hydrolysis) (Heland
et al., 2001; Kleffmann et al., 2002). Sampling artifacts (heterogeneous formation on surfaces by different NO2 reactions, by
photolysis, or by condensation of analytes on inlet) can be
avoided by placing the stripping reagent directly in the atmosphere (Kleffmann and Wiesen, 2008).
Off-line methods
Besides the on-line techniques indicated above, HONO concentrations can be determined by off-line methods. They are
based on the HONO sampling by devices such as dry diffusion
denuders (Febo et al., 1993, 1996), filter pack (Sickles and
Hodson, 1989), and passive samplers (Bytnerowicz et al.,
2005), followed by analysis of extracted nitrite. Today, dry
diffusion denuders and passive samplers are still used for
HONO measurement.
The HONO sampling by dry diffusion denuder is performed
by two Na2CO3-coated denuders (1% Na2CO3 þ 1% glycerol in
1:1 ethanol/water solution). Gaseous HONO diffuses and is
absorbed on the Na2CO3-coated surface, forming nitrite, which
is analyzed by IC.
The HONO determination by carbonate-coated denuders can
be affected by both positive interferences from nitrite forming
species, such as NO2 and PAN, and by negative interferences
from ozone (O3) and other oxidants. Corrections for positive
interferences can be accounted for by operating two denuders in
series (absolute differential technique), assuming that NO2 and
PAN are collected with the same efficiencies on the second
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Spataro and Ianniello / Journal of the Air & Waste Management Association 64 (2014) 1232–1250
Na2CO3-coated denuder (Febo et al., 1993, 1996; Ianniello et al.,
2007). Gaseous HNO3 should be selectively removed from the
sampling air by means of sodium fluoride (NaF)- or sodium
chloride (NaCl)-coated denuders in order to avoid the interfering
nitrate amount in the HONO measurements (negative interference). Nitrate ions generated on Na2CO3-coated denuders by the
oxidation of nitrite (from HONO) is indistinguishable from
nitrate ions generated by the HNO3 collection unless this species
is removed from the incoming air stream. Thus, the atmospheric
HONO concentrations are determined by subtracting the sum of
nitrite and nitrate (expressed as nitrite) concentrations in the
second Na2CO3-coated denuder from the sum of nitrite and
nitrate (expressed as nitrite) concentrations in the first
Na2CO3-coated denuder (Febo et al., 1993, 1996; Ianniello
et al., 2007, Spataro et al., 2013). The diffusion denuder technique is accurate and selective but also labor-intensive, and
requires long sampling time. The collection efficiency for
HONO is about 99%, whereas the detection limit is 0.88 ppt,
with a time resolution of 24 hr.
Finally, Bytnerowicz et al. (2005) developed a passive sampler for the simultaneous collection of gaseous HNO3 and
HONO. This device is based on a diffusion of ambient air
through Tefion membrane and absorption of the pollutants on
Nylasorb nylon filter. After the sampling, the passive sampler is
analyzed by IC, and HNO3 and HONO are determined as nitrate
and nitrite, respectively. Passive sampler is simple, easy to make,
inexpensive, resistant to harsh weather conditions and does not
require power supply. It can measure wide ranges of ambient
HONO and HNO3 concentrations for extended periods of time
and can be used for regional-scale monitoring of the pollutants.
The detection limit of 0.2 ppb over the 6-day sampling period is
not suitable to evaluate the diurnal variation of HONO concentrations (Lee et al., 2002).
Conclusions and Future Outlook
This paper reports and discusses the HONO sources proposed
to explain the HONO budget, especially during daytime. Five
HONO formation pathways can be recognized: direct emission,
homogeneous gas-phase reactions, heterogeneous reactions,
photolysis/photoenhanced reactions, and biological processes.
Direct HONO emission (from vehicles) ranges between 0.3%
and 1.7% of the total traffic NOx emission. Vehicular traffic can
be important HONO source only in heavily polluted urban
environment. Since the atmospheric [HONO]/[NOx] ratios are
usually higher than those related to the direct emission, HONO is
mostly secondarily produced. However, more evaluations of
HONO, NO, and NO2 emission are required to quantify the
emitted amount by different vehicles (in particular light- and
heavy-duty diesel vehicles, etc.).
The homogeneous gas-phase R3 and R6 reactions may act as
atmospheric HONO sources, but they cannot explain the
observed daytime HONO levels. Bejan et al. (2006) proposed
the photolysis of different gaseous nitrophenols, estimating the
HONO production rate of 100 ppt hr1 for a typical urban site.
However, there are not yet field studies investigating this process
in the real atmosphere and verifying its estimated HONO production rate.
1245
Heterogeneous reactions, involving the NO2 conversion on
humid surfaces (R7), have been accepted to be the dominant
nighttime HONO source. However, the detailed mechanism of
R7 and its dependence on surface water content, chemical properties (such as chemical composition and pH), and type of surface is still not understood. The ground surface mainly affects the
HONO production, but high aerosol loading in the atmosphere
may increase the heterogeneous HONO production. Thus, more
data, related to the role of aerosol and ground surface into the
heterogeneous HONO production, are needed to clarify this
question. The reactions R8 and R9 and the NO2 conversion on
soot surfaces cannot be important HONO sources. The interaction between NO2 and diesel exhaust can also cause HONO
formation. Ziemba et al. (2010) proposed that VOCs affect the
HNO3 conversion into HONO. Since VOCs are emitted in both
gas and particulate phases by vehicular exhausts, it is not clear
the homogeneous and/or heterogeneous nature of this process as
well as the dependence of the HONO production rate to the
surface water content and to the chemical composition of
substrate.
Photolysis/photoenhanced surface processes were considered
to explain the observed daytime HONO concentrations. Three
processes were identified. The first one, the photolysis of
adsorbed HNO3/NO3 (l ~ 300 nm) results in the daytime
HONO and NOx production and is a possible pathway remobilizing deposited HNO3 in rural and polar environments.
However, there are still few studies quantifying its contribution
to the atmospheric HONO budget. The second one is the photoenhanced NO2 into HONO conversion on mineral dust substrates. The mineral powder (TiO2, SiO2, and CaCO3) are
likely involved in the HONO production, but more studies are
required to clarify the mechanisms and their dependence on
several parameters: surface chemical properties (acidity, water
content, and composition), relative humidity, and concentration
and type of nitrogen species. The last one, the photoenhanced
NO2 conversion on the organic and humic acid surfaces, was
related to the HONO production rate of 700 ppt hr1 within a
boundary layer of 100 m height. However, the organic chemicals
participating in the HONO formation and the reaction mechanism of NO2 with organic materials should be investigated.
Laboratory researches on the photoenhanced HONO production
considered only UV-A radiation, whereas the effect of visible
and UV-B as well as the changes of the chemical and physical
properties of surfaces were not evaluated.
Recently, biological nitrification and denitrification processes in the soil were proposed as the unknown HONO sources.
This mechanism is consistent with the observations that ground
surfaces are dominant for HONO production, but further studies
are recommended to assess the actual impact of HONO release
from soil nitrite on in the biogeochemical cycling of N in both
agricultural and natural environments.
The knowledge of atmospheric HONO sources is crucial
to obtain a more rigorous representation of its sources and
their relative contribution to the HONO budget. Field measurements of HONO, NO, and NO2 concentrations and
fluxes in the lower troposphere are needed. These studies
are crucial to fully characterize air quality and to integrate
the current models.
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Spataro and Ianniello / Journal of the Air & Waste Management Association 64 (2014) 1232–1250
Accurate and fast HONO measurement is the first step to
investigate its sources. During the last 30 years, several techniques have been developed to determine HONO levels, and each
of them is characterized by specific disadvantages and advantages. Wet methods suffer from sampling artifacts and some,
such as NO2 and SO2, NO2 and phenols, NO2 and aromatic
amines, or peroxyacetyl nitrate (PAN) hydrolysis. However,
some versions of the long-path absorption photometric method
claim to have minimized and quantified the known interferences.
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Summary
HONO plays a key role in the tropospheric photochemistry,
acting as a relevant source of OH radicals by its rapid photolysis
(R1). OH radicals are involved in photooxidation processes, such as
the formation of tropospheric O3 and other secondary atmospheric
pollutants (PAN and secondary particles). Recent field and modeling studies showed that HONO photolysis contributes significantly
to the OH production throughout the day and that a strong and
unknown daytime HONO source should exist. Up to now, there are
five HONO formation pathways known: direct emission, homogeneous gas-phase reactions, heterogeneous reactions, surface
photolysis, and biological processes. In this review paper, the
HONO sources proposed to explain the HONO budget, especially
during daytime, have been discussed. Since there are many techniques to detect HONO that do not fully satisfy the need for sensitivity, selectivity, and fast time response, a short description of
HONO measurement techniques currently available in literature is
reported. They are divided in three basic categories: spectroscopic
techniques, wet chemical techniques, and off-line methods.
Acknowledgment
The authors would like to thank all participants of the field
measurements focused on HONO sampling in the troposphere,
and in particular the authors are grateful to Mr. Giulio Esposito
for helpful discussions.
Funding
This work was financially supported by Institute for the
Atmospheric Pollution Research (CNR-IIA).
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About the Authors
Francesca Spataro and Antonietta Ianniello are both researchers at the
CNR-Institute of Atmospheric Pollution Research (CNR-IIA), Rome, Italy.