Observing the Transition From NOx-Limited to NOx-Saturated O3 Production
J. A. Thornton1, P. J. Wooldridge1, R. C. Cohen1, M. Martinez2, H. Harder2, W. H. Brune2, E. J. Williams3, S. R. Hall4, R. E. Shetter4, B. P. Wert4, B. Henry4, A. Fried4, F. E. Fehsenfeld3
Department of Chemistry; University of California, Berkeley; Berkeley, CA 94720; 2 Department of Meteorology; Pennsylvania State University; 3 Aeronomy Laboratory, NOAA; Boulder, CO; 4 Atmospheric Chemistry Division, NCAR; Boulder CO
V. The Dependence of PO3 on Primary Radical Production and NO
HOx-HOx
Method
NO2 + OH + M
HOx-NOx
M
RO2 + NO
HHLoss  2kOHHO2 OHHO2   2kHO2HO2 HO2   2kHO2RO2 HO2 RO2 
2
d[OH] d[HO 2 ] d[RO 2 ]


0
dt
dt
dt
HNO3
RONO2
RO + NO2 not terminating
Eqn (3)
VS
NHLoss  k OHNO2 OHNO 2   αk RO2NO RO 2 NO
Eqn (4)
LHOx = HHLoss + NHLoss
Eqn (5)
1.50
1.50
1.25
1.25
1.25
1.00
1.00
1.00
0.75
0.50
Chemiluminescence
10%
O3
UV Absorbance
<10%
OH
Laser-Induced Fluorescence
~20%
HO2
Titration to OH by NO Followed by
Laser-Induced Fluorescence
H2CO
Tunable Diode Laser Absorption
Spectroscopy
Solar Actinic Flux
Scanning Actinic Flux Spectral
Radiometer
Relative Humidity and
Temperature
~5%
10%
Commercial Probe
PHOx
5%
k 7 H 2 O
 2J O3 O 3 
 2J H2CO H 2 CO
k 6 N 2  O 2 
Eqn (6)
NO HOx Cycle
NO
RO
HO2
O2
RO
NO
NOx Cycle
PO3
O2
RO2
HO2
O
O2
O3
6
8
10
12
14
16
18
0
0.5<PHOx<0.7 ppt/s
0.2<PHOx<0.3 ppt/s
0.03<PHOx<0.07 ppt/s
h
10
5
0
JNO2 is the photolysis rate constant for NO2 derived from solar actinic flux
measurements. Reaction rate constants are taken from DeMore, et al, 2000,
DeMore, et al., 1997, and Atkinson, 1994.
1000
1500
2000
2500
-5
176.0
176.5
177.0
177.5
178.0
178.5
179.0
Figure 2 (left) shows PO3
for three consecutive
days. While there is a
clear diurnal trend with
rates peaking near 12pm,
there is evidence of
significant day-to-day
variation and variation on
the time scale of minutes
to hours as well.
Day of Year
IV. The Photo-Stationary State Assumption
Is the atmosphere in steady state?: The closest large NOx
Is the PSS-calculated PO3 accurate and precise?: The
sources were ~ 15 minutes away for typical wind speeds of 4-5 m/s.
This is several e-folds in the NOx intra-conversion lifetime (~ 100 sec).
The effect of potential surface emissions of NO on the stationary state
were estimated by subtracting typical nighttime values of NO (~20-50
ppt) from the observed daytime concentrations. NO emissions lead to
a potential bias of at most 5-10% in the PO3 calculated from the steady
state assumption.
experiment at CFA provided some of the most accurate measurements of
NO2. The reported total uncertainty of the measurements combine to give
a total uncertainty in PO3 of  34%. However, we note that for examining
trends PO3 and our model of the crossover point, precision is more
important than the total uncertainty or accuracy, and the precision of the
calculated PO3 is approximately 10%.
6
4
1.00
0.75
0.000
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0.025
1.0
0.050
0.075
0.100
0.125
0.150
0.175
0.200
0.25
0.0
0.2
0.4
0.6
0.8
HHLoss/LHOx
NHLoss/LHOx
P(O3)/7
1
1.00
1.0
0.8
0.6
0.4
0.2
0.0
Implications for RO2+HO2 Chemistry,
any combination of:
0
200
400
600
800
1000
1200
1400
1600
0.75
A RO2 + HO2
ROOH + O2 kHO2+RO2 10x’s Smaller
0.50
B RO2 + HO2
ROOH + O2
C RO2 + HO2
0.00
0
250
500
750
1000
NO (ppt)
1250
1500
1750
2000
0
200
400
600
800
1000
1200
1400
1600
1800
ROOH + h
2000
ROOH + O2
RO + OH
3
Figure 3 The upper panels, illustrate the predictive behavior of this chemical system in a simple model. The
Left panel shows PO3 versus NO for three different regimes of PHOx The right panel shows PO3 (red), and
the rates of HHLoss (blue) and NHLoss relative to the total plotted versus NO. A wide range of parameters
and conditions were used in the model. The point where the two fractions HHLoss/LHOx and NHLoss/LHOx
were equal always occurred at an NO concentration that was ~ 25% less than that where the peak in PO3
occurred, however, the model is not expected to reproduce the observations in an absolute sense.
Questions for the Future
Is Nashville comparable to other urban areas?
Isoprene-type ROOH
NOT Scavenged
The lower panels show the same quantities as the upper panels but now derived from the observations. PO3
derived from observations exhibits, qualitatively, the expected behavior as a function of PHOx and NO (lower
left). However, the two fractions HHLoss/LHOx and NHLoss/LHOx (lower right) are equal at approximately
900-1000 ppt NO, a higher concentration than where PO3 peaks. This conflicts with the expected behavior
shown in the upper right panel.
These conflicting results imply that the rate of peroxide formation (HHLoss) is either too
fast or the rate of nitrate formation (NHLoss) is too slow, or some combination of the two.
Noon
VOC/NOx
RO2/OH
HOx-HOx
vs.
HOx-NOx
Do RO2 + HO2 conclusions depend on identity of R?
[O3] are the integral of PO3 (and LO3)
old
2000
10%
NO (ppt)
2
1800
NO (ppt)
0.25
0
1.0
k OH  NO 2 OH NO2 

LHOx
2(0.08)k HO 2 RO 2 HO2 RO2 

LHOx
RO + OH + O2 90%
2
1.0
0.50
VIII. Conclusions and Implications
HHLoss/LHOx
NHLoss/LHOx
P(O3)/7
15
P(O3) (ppt/sec)
PO3  JNO2 NO 2   k NOO3 NOO3  Eqn (2)
500
1.25
8
k
NO  J NO   k




HO

k
RO

 HO2NO
2
(RO2NO)i
2 i
NO2
2
NOO3 NOO3 
i


Eqn (1)
0.50
0.75
NHLoss/LHOx plotted versus NO where we have reduced the organic
peroxide formation rate by a factor of ~ 10, and assumed a 3% organic
nitrate yield. As opposed to the results shown in the lower right panel of
Figure 3, the new model suggests a crossover at approximately 500600 ppt NO. This result is more consistent with the PO3 derived from
observations.
NO
20
June 25 - 27, 1999
0.75
1.00
1.2
NO (ppt)
Hour of Day (CST)
NO2
1.00
P(O3)
HHLoss/LHOx
NHLoss/LHOx
0.00
-30
0.8
ROOH formation by a factor of 10 and including 3% organic nitrate formation improves the radical budget and removes most of the trends of
PHOx/LHOx versus chemical coordinates observed in Figure 4. We use this improved model to calculate the crossover point between NOxlimited and NOx-saturated O3 production below in Figure 6.
0.25
10
4
1.25
Figure 6 (right) shows the two fractions, HHLoss/LHOx and
1-minute averaged PO3
calcuated using Eqn. 2
plotted versus Hour of
Day. All of the data
obtained at CFA is
shown.
-20
1.25
0.1
Figure 1 (left) shows
-10
1.25
0.00
0.75
P(O3) (ppt/s)
H2CO + h
P(O3) (ppt/sec)
RO2
0.6
Figure 5 The panels show PHOx/LHOx with a new model of HOx loss plotted versus the same chemical coordinates as in Figure 4. Reducing
RH
OH
0.4
kOH  NO 2 OH NO2 
LHOx
1.50
PHOX (ppt/s)
0.50
0
0.2
2k HO 2 RO 2 HO 2 RO2 
LHOx
1.50
new
Noon
?? isoprene-RO2 ??
How to extrapolate to larger spatial scales?
P(O3)
H2O
1.0
1.50
1.25
10
0.9
0.25
1.00
20
III. Photochemical O3 Production
O3 + h
PHOx
0.8
0.00
0.0
1.00
0.25
VI. NOx-limited versus NOx-saturated O3 Production at CFA
The extensive suite of measurements made at Cornelia Fort Airpark (CFA) over the period June 15 – July 15, 1999
as part of the Southern Oxidant Study (SOS 99) provides one of the most detailed characterizations of an urban
environment to date. CFA is located 8 km, NE of downtown Nashville, TN in the flood plain of the Cumberland River.
The measured species used here, the methods used to measure them, and the reported uncertainties are shown in
the table above. For the purposes of this study, all species were averaged to 1-minute intervals and none of the
measurements required interpolation to this time base.
0.7
0.75
0.50
We calculate PHOx using observations and Equation 6.
* Includes both accuracy and precision
0.6
0.50
and the fraction of HOx loss due to HNO3 formation. In this initial model, we assume alkyl nitrate formation is negligible and set  = 0. From this
analysis, it is apparent that the chemistry describing ROOH formation is in error by nearly a factor of 10.
HHLoss
NHLoss
~10%
0.5
0.25
Figure 4 The observationally constrained ratio PHOx/LHOx is plotted versus PHOx (left), the fraction of HOx loss due to ROOH formation (center),
PO3
[HOx]
0.4
0.50
0.25
PHOX (ppt/s)
The role of the HOx production rate, PHOx, on PO3 for constant NOx:
PHOx
0.3
PHOx/LHOx
NO
0.2
0.75
P(O3)
10%
0.50
0.00
0.00
0.00
PHOx/LHOx
Laser-Induced Fluorescence
0.75
0.25
0.1
NO2
PHOx/LHOx ~ 1
1.50
0.25
Total Uncertainty*
HOx ~ 30sec
PHOx/LHOx
H2O2 + O2
HO2 + HO2 + M
ROOH + O2
HO2 + RO2
H2O + O2
HO2 + OH
II. Measurements and Site Description:
Species
Examine the balance between PHOx and LHOx
The dependence of PO3 on NOx arises from the competition between two
categories of chain termination reactions:
PHOx/LHOx
Tropospheric O3 concentrations are functions of the chain lengths of NOx (NOx  NO + NO2) and HOx (HOx  OH + HO2 + RO2) radical
catalytic cycles. For a fixed HOx source at low NOx concentrations, kinetic models indicate the rate of O3 production increases linearly with
increases in NOx concentrations (NOx-limited). At higher NOx concentrations, kinetic models predict ozone production rates decrease with
increasing NOx (NOx-saturated). We present observations of NO, NO2, O3, OH, HO2, H2CO, actinic flux, and temperature obtained during the
1999 Southern Oxidant Study from June 15 – July 15, 1999 at Cornelia Fort Airpark, Nashville, TN. The observations are used to evaluate
the instantaneous ozone production rate (PO3) as a function of NO abundances and the primary HOx production rate (PHOx). These
observations provide quantitative evidence for the response of PO3 to NOx. For high PHOx (0.5 < PHOx < 0.7 ppt/s), O3 production at this site
increases linearly with NO to ~ 500 ppt. PO3 levels out in the range 500-1000 ppt NO, and decreases for NO above 1000 ppt. An analysis
along chemical coordinates indicates that models of chemistry controlling peroxy radical abundances, and consequently PO3, have a large
error in the rate or product yield of the RO2 + HO2 reaction for the classes of RO2 that predominate in Nashville. Photochemical models and
our measurements can be forced into agreement if the product of the branching ratio and rate constant for organic peroxide formation, via
RO2 + HO2  ROOH + O2, is reduced by a factor of 3-12. Alternatively, these peroxides could be rapidly photolyzed under atmospheric
conditions making them at best a temporary HOx reservoir. This result implies that O3 production in or near urban areas with similar
hydrocarbon reactivity and HOx production rates may be NOx-saturated more often than current models suggest.
VII. Chemical Coordinate Analysis
PHOx/LHOx
I. Introduction
PHOx/LHOx
1
Sunrise
Sunset
Sunrise
Sunset
NO
NO
Our new model of HOx loss predicts that O3 production will be
NOx-saturated more often than current models predict.
Acknowledgements
NASA Earth Systems Science Graduate Fellowship
NOAA Office of Global Programs