Role of the nitroacidium ion, H2
+
ONO
in atmospheric cryochemistry
,
a
Paul O’Driscoll, Stig Hellebust, Eoin Riordan, David Healy, Nick Minogue, Daniel O’Sullivan
b
c
db
and John Sodeau
c
Department of Chemistry and Environmental Research Institute, University College Cork, Ireland
Introduction
[email protected]
Water-ice and the cryosphere
Why study cryochemistry?
The cryosphere is defined
as the portions of the Earth
System where water is in a
solid form and includes sea-,
lake- and river-ice, snow
cover, frost flowers, glaciers,
cold clouds, and long-term
frozen ground (permafrost).
Polar atmospheres suffer from air
pollution episodes that are more
commonly experienced in urban
environments. NOx, HONO and
H2CO can be emitted from snowpacks. “Sudden” depletion events
for ozone and mercury also occur
(ODE & MDE). Does snow or ice
play a part in the chemistry?
a
UV spectroscopic study for aqueous
nitrite ion solutions
Ices, halogens and H2ONO+
Hg
IO BrCl
H2CO
•ODEs (and MDEs) show associations with BrO
production. Acidity of ice is possibly important.
•NOx and HONO production ascribed to nitrate ion
photolysis. Acidity of ice is again possibly important.
•Is there a connection?
Optimization modelling study of nitrous acid
speciation as a function of pH
Computer model simulation of speciation
for the two-step equilibrium process
1.6
1.4
LOW pH
HONO
pH =
1.5
2.6
1.2
5
3.1
4
two-step
pH
4.1
Absorbance
one-step
5.1
experimental
3
0.8
HIGH pH
2
0.6
0.4
HONO
0.6
NO2H2ONO+
0.4
HONO + H3O+ ↔ H2ONO+ + H2O
1
Nitroacidium Ion,
(H2ONO+)
0.8
mole fraction
3.8
1
0.2
0
0.2
0
250
0
NO2-
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0
[HCl]
1.5
300
350
400
2
2.5
3
450
Wavelength / nm
3.5
4
4.5
5
pH
A Newton-Gauss method was used to solve a set of
non-linear equations defining the speciation for
HONO. A model based on one-step protonation
agreed well with experiment in pH range 3-6. A
second (protonation) step was required at pH <3 to
explain the results. The NITROACIDIUM ION is the
likely product in this regime.
-0.2
The model can be configured to display equilibrium
speciation of each component in the system. Clearly
protonated HONO becomes increasingly important below
pH 3. These levels are found in environmental samples and
the question posed in reference 1 was: can halides react
with such a species? Indeed what happens to it on ice?
Release of NO and I2 to the atmosphere from
freezing sea-salt aerosol components
What is water-ice?
Freezing halide ion solutions and the
release of interhalogens to the atmosphere
•The effects of freezing on a variety of acidified and neutral nitrite
ion and halide-containing mixtures was investigated using UV-Vis
spectroscopy. (See reference 3).
•Several TRIHALIDE ions were formed and monitored:
[I-I-Cl]-, [I-I-Br]- (no freezing) and [Cl-I-Cl]- , [Br-I-Br]- ( freezing)
•The NITROACIDIUM ion and all the components of the I-/I2/I3equilibrium appear to be integral to the mechanism.
•The room-temperature, solution-phase reaction between
•The chemistry could occur in micropockets or the QLL.
ACIDIFIED nitrite and iodide ions (pH<5.5) releases NO and I2 • The production of [Cl-I-Cl]- or [Br-I-Br]- is potentially important
T>200K
The mechanism is thought to involve the nitroacidium ion.
to the polar atmosphere because release of ICl or IBr may result
•The experiments performed here have shown that the release
Concentration of Species Present Vs. pH
process
is
accelerated
for
NEUTRAL
solutions
when
FROZEN.
The frozen-state between the freezing point and
•Reactant/Product
monitoring
was
performed
by:
(i)
UV-Vis
eutectic point for a multi-component system
- and I -); (ii) chemiluminescence for NO
spectroscopy
(for
NO
2
3
x
includes liquid “micropockets”. Such environments
•pH
changes
(to
form
more
acidic
media)
are
commonplace
promote the freeze-concentration of ions.
when salt/water solutions are frozen, the Workman-Reynolds
effect), and freeze-concentration of reactant ions into the
micropockets is also promoted. (See reference 2).
0.5
Nitroacidium
0.45
Dibromoiodide
0.4
Concentration (mM)
Release of halogens and NOx from Water-ice
1
3.7
By varying the pH, accurate determinations of
e values for both NO2- and HONO were
made. These were used as input to the
Henderson-Hasselbach equation and a
pKa value of 2.8 ± 0.1 was calculated.
0.35
Dichloroiodide
0.3
0.25
0.2
0.15
0.1
0.05
0
0
Summary Mechanism
H2ONO+ + I- ↔ INO + H2O
I- + INO → I2- + NO
I2- + H2ONO+ → NO + I2 + H2O
Photochemical release of HONO
and NOx from ice/snow surfaces
Summary Mechanism
1. NO3- + hn→ NO2 + O2a. NO3- + hn→ NO2- + O.
2b. NO2- + H+ → HONO
NO+
The Authors wish to thank the following bodies for their assistance:
 Enterprise Ireland
 IRCSET
 EU Marie Curie Programme
NO3-
16000
773
3
pH
4
5
6
7
≡
1.
i.e. solvated nitrosonium ion OR protonated nitrous acid
18000
1312 1283
2
HONO release from water-ice via the
nitroacidium ion
Thermal Desorption profiles subsequent to photolysis of (NO2)2 on water-ice
20000
m/z =18 (H2O)
Absorbance Units
0.2
0.3
14000
1882 1763 1738
12000
1592
1423
0.1
2218
2200
2000
1800
1600
1400
Wavenumber cm-1
1200
1000
800
600
C:\Data Files\OPUS\sample.509
sample
on gold
29/07/2005
C:\Data Files\OPUS\sample.512
sample
on gold
29/07/2005
•The total quantum yield for pathways 1 and 2 is
0.01 with NO2 as the major product
•Direct nitrite ion production is very improbable
with F ~ 0.001. Hence pathway 2b is unlikely
•Production of NO2 on/in a cold surface solid
leads to efficient D2h-N2O4 (dimer) formation
Acknowledgements
Thermal Desorption profiles showing HONO
release upon annealing post-photolysis
surface
Post-photolysis (1hr xenon arc):
FT-RAIRS
0.4
•Elevated NOx levels over snowpacks have
been measured
•Photolysis of acidified NITRATE IONS in snow
identified as a probable mechanism
1
Trihalide ([X-I-X]-) production
a-d shows how single ice after freezing does not occur
crystals can aggregate to until pH 3 (below which the
form “micropockets”
nitroacidium ion forms).
FT-RAIRS/Mass Spectrometry/Photolysis experiment
investigating (NO2)2 on a thin-film water-ice surface
0.0
The Nitroacidium Ion on Ice
HONO + H2O ↔ H3O+ + NO2-
6
2
The Nitroacidium Ion
Possible reaction with Cl-, Br- or I- to
release halogens to atmosphere?
1.2
7
UHV chamber coupled to FT-RAIRS
(for identifying surface species) and
QMS detection (for gas-phase
release)
c/s
m/z = 30 (NOx/HONO)
8000
H3O+ + HONO
6000
4000
m/z = 46 (NO2/HONO) m/z = 17 (HONO)
m/z = 47 (HONO)
2000
0
130
180
230
Temp/K
FTIR results consistent with following mechanism:
hn
2. H2O
hn
+
280
330
•The solvolysis/hydrolysis processes shown
above might occur in micropockets or in the QLL
•The initiating step for HONO release is nitrate
ion photolysis leading to NO2 build up as a result
of freeze-concentration effects. (See reference 4)
Airborne
Pollutants
water-ice
References
1.
2.
3.
4.
Journal of Physical Chemistry A 109 779-786 (2005)
Journal of Physical Chemistry A 110 4615-4618 (2006)
Journal of Physical Chemistry A 112 1677-1682 (2008)
Journal of Physical Chemistry A 111 1167-1171 (2007)
Quasi Liquid Layer
(QLL)
“Micropockets”
Further Information:
More information regarding Atmospheric Chemistry activities in the CRAC
Laboratory at UCC can be found at http://crac.ucc.ie