Role of the nitroacidium ion, H2ONO+, in atmospheric cryochemistry

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Acknowledgements

Enterprise Ireland

IRCSET

EU Marie Curie Programme

The Authors wish to thank the following bodies for their assistance:

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Paul O’Driscoll, Stig Hellebust, Eoin Riordan, David Healy, Nick Minogue, Daniel O’Sullivan

and John Sodeau

Department of Chemistry and Environmental Research Institute, University College Cork, Ireland

j.sodeau@ucc.ie

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References

1. Journal of Physical Chemistry A 109 779-786 (2005)

2. Journal of Physical Chemistry A 110 4615-4618 (2006)

3. Journal of Physical Chemistry A 112 1677-1682 (2008)

4. Journal of Physical Chemistry A 111 1167-1171 (2007)

Further Information:

More information regarding Atmospheric Chemistry activities in the CRAC

Laboratory at UCC can be found at http://crac.ucc.ie

Water-ice and the cryosphere

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).

Why study cryochemistry?

Polar atmospheres suffer from air

pollution episodes that are more

commonly experienced in urban

environments. NOx, HONO and

H2CO can be emitted from snow-

packs. “Sudden” depletion events

for ozone and mercury also occur

(ODE & MDE). Does snow or ice

play a part in the chemistry?

Ices, halogens and H2ONO+

•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?

Hg IO BrClH2CO

pH =

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

250 300 350 400 450

Wavelength / nm

Abs

orba

nce

1.5

2

2.6

3.1

3.7

3.8

4.1

5.1

0

1

2

3

4

5

6

7

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08

[HCl]

pH

one-step

two-step

experimentalHONO

NO2-

H2ONO+

0

0.2

0.4

0.6

0.8

1

1.2

1.5 2 2.5 3 3.5 4 4.5 5

pH

mo

le fr

actio

n

Nitroacidium Ion,

(H2ONO+)

Possible reaction with Cl-, Br- or I- to

release halogens to atmosphere?HONO + H2O ↔ H3O

+ + NO2-

HONO + H3O+ H

2ONO+ + H

2O

HONO

NO2-

LOW pH

HIGH pH

UV spectroscopic study for aqueous

nitrite ion solutions

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.

↔

Optimization modelling study of nitrous acid

speciation as a function of 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.

Computer model simulation of speciation

for the two-step equilibrium process

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?

C:\Data Files\OPUS\sample.509 sample on gold

C:\Data Files\OPUS\sample.512 sample on gold

29/07/2005

29/07/2005

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1423

1882 1763 1738 1592

1312 1283773

6008001000120014001600180020002200

Wavenumber cm-1

0.0

0.1

0.2

0.3

0.4

Absorb

ance U

nits

a

bc

a

bc

d

FT-RAIRS/Mass Spectrometry/Photolysis experiment

investigating (NO2)2 on a thin-film water-ice surfaceHONO release from water-ice via the

nitroacidium ion

Photochemical release of HONO

and NOx from ice/snow surfaces

•Elevated NOx levels over snowpacks have

been measured

•Photolysis of acidified NITRATE IONS in snow

identified as a probable mechanism

Summary Mechanism1. NO3

- + hn→ NO2 + O-

2a. NO3- + hn→ NO2

- + O.

2b. NO2- + H+ → HONO

•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

H3O+ HONOH2O

Thermal Desorption profiles subsequent to photolysis of (NO2)2 on water-ice

0

2000

4000

6000

8000

12000

14000

16000

18000

20000

130 180 230 280 330

Temp/K

c/s

m/z =18 (H2O)

m/z = 30 (NOx/HONO)

m/z = 17 (HONO)

m/z = 47 (HONO)

m/z = 46 (NO2/HONO)

UHV chamber coupled to FT-RAIRS

(for identifying surface species) and

QMS detection (for gas-phase

release)

NO+ NO3-

Post-photolysis (1hr xenon arc):

FT-RAIRS

Thermal Desorption profiles showing HONO

release upon annealing post-photolysis

surface

FTIR results consistent with following mechanism:

hn

+

hn

water-ice

≡i.e. solvated nitrosonium ion OR protonated nitrous acid

1.

2. +

•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 Quasi Liquid Layer

(QLL) “Micropockets”

What is water-ice?

T>200K

The frozen-state between the freezing point and

eutectic point for a multi-component system

includes liquid “micropockets”. Such environments

promote the freeze-concentration of ions.

Release of NO and I2 to the atmosphere from

freezing sea-salt aerosol components

Freezing halide ion solutions and the

release of interhalogens to the atmosphere

•The room-temperature, solution-phase reaction between ACIDIFIED nitrite and iodide ions (pH<5.5) releases NO and I2The mechanism is thought to involve the nitroacidium ion.

•The experiments performed here have shown that the release

process is accelerated for NEUTRAL solutions when FROZEN.

•Reactant/Product monitoring was performed by: (i) UV-Vis spectroscopy (for NO2

- and I3-); (ii) chemiluminescence for NOx

•pH changes (to form more acidic media) are commonplace

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

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0 1 2 3 4 5 6 7

Co

ncen

trati

on

(m

M)

pH

Concentration of Species Present Vs. pHNitroacidium

Dibromoiodide

Dichloroiodide

Summary MechanismH2ONO+ + I- ↔ INO + H2O

I- + INO → I2- + NOI2- + H2ONO+ → NO + I2 + H2O

•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/I3-

equilibrium appear to be integral to the mechanism.

•The chemistry could occur in micropockets or the QLL.• The production of [Cl-I-Cl]- or [Br-I-Br]- is potentially important

to the polar atmosphere because release of ICl or IBr may result

a-d shows how single ice

crystals can aggregate to

form “micropockets”

Trihalide ([X-I-X]-) production

after freezing does not occur

until pH 3 (below which the

nitroacidium ion forms).