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Water-Ice and the Cryosphere Why Study Cryochemistry? Ices

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Water-Ice and the Cryosphere Why Study Cryochemistry? Ices + Role of the nitroacidium ion, H2ONO , in atmospheric cryochemistry a Paul O’Driscoll, Stig Hellebust, Eoin Riordan, David Healy, Nick Minogue, Daniel O’Sullivan b c c and John Sodeau d b Department of Chemistry and Environmental Research Institute, University College Cork, Ireland [email protected] + Water-ice and the cryosphere Why study cryochemistry? Ices, halogens and H2ONO The cryosphere is defined Polar atmospheres suffer from air as the portions of the Earth pollution episodes that are more System where water is in a Hg IO BrCl commonly experienced in urban H2CO a solid form and includes sea-, environments. NOx, HONO and lake- and river-ice, snow H2CO can be emitted from snow- •ODEs (and MDEs) show associations with BrO troduction cover, frost flowers, glaciers, packs. “Sudden” depletion events production. Acidity of ice is possibly important. In cold clouds, and long-term for ozone and mercury also occur •NOx and HONO production ascribed to nitrate ion frozen ground (permafrost). (ODE & MDE). Does snow or ice photolysis. Acidity of ice is again possibly important. play a part in the chemistry? •Is there a connection? UV spectroscopic study for aqueous Optimization modelling study of nitrous acid Computer model simulation of speciation nitrite ion solutions speciation as a function of pH for the two-step equilibrium process 1.6 LOW pH 7 1.2 Possible reaction with Cl-, Br- or I- to 1.4 HONO pH = 1.5 6 release halogens to atmosphere? 2 + - 1 1.2 2.6 HONO + H2O ↔ H3O + NO2 3.1 5 Nitroacidium Ion, 3.7 0.8 1 3.8 one-step + Ion 4 (H ONO ) 4.1 two-step 2 HONO 5.1 pH experimental 0.8 3 0.6 - NO2 + mole fraction mole H ONO HIGH pH 2 2 0.6 0.4 Absorbance 1 + ↔ + 0.4 HONO + H3O H2ONO + H2O 0.2 0 0.2 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 NO - 0 2 [HCl] 0 1.5 2 2.5 3 3.5 4 4.5 5 Nitroacidium 250 300 350 400 450 pH -0.2 A Newton-Gauss method was used to solve a set of Wavelength / nm The non-linear equations defining the speciation for The model can be configured to display equilibrium By varying the pH, accurate determinations of HONO. A model based on one-step protonation speciation of each component in the system. Clearly - e values for both NO2 and HONO were agreed well with experiment in pH range 3-6. A protonated HONO becomes increasingly important below made. These were used as input to the second (protonation) step was required at pH <3 to pH 3. These levels are found in environmental samples and Henderson-Hasselbach equation and a explain the results. The NITROACIDIUM ION is the the question posed in reference 1 was: can halides react pKa value of 2.8 ± 0.1 was calculated. likely product in this regime. with such a species? Indeed what happens to it on ice? What is water-ice? Release of NO and I2 to the atmosphere from Freezing halide ion solutions and the freezing sea-salt aerosol components release of interhalogens to the atmosphere •The effects of freezing on a variety of acidified and neutral nitrite ice ion and halide-containing mixtures was investigated using UV-Vis - spectroscopy. (See reference 3). •Several TRIHALIDE ions were formed and monitored: Water [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 from equilibrium appear to be integral to the mechanism. x •The room-temperature, solution-phase reaction between •The chemistry could occur in micropockets or the QLL. NO ACIDIFIED nitrite and iodide ions (pH<5.5) releases NO and - - T>200K I2 • The production of [Cl-I-Cl] or [Br-I-Br] is potentially important 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 0.5 Concentration of Species Present Vs. pHNitroacidium process is accelerated for NEUTRAL solutions when FROZEN. 0.45 The frozen-state between the freezing point and 0.4 Dibromoiodide •Reactant/Product monitoring was performed by: (i) UV-Vis 0.35 eutectic point for a multi-component system Dichloroiodide - - 0.3 spectroscopy (for NO2 and I3 ); (ii) chemiluminescence for NOx includes liquid “micropockets”. Such environments 0.25 •pH changes (to form more acidic media) are commonplace promote the freeze-concentration of ions. 0.2 when salt/water solutions are frozen, the Workman-Reynolds Concentration (mM) 0.15 0.1 effect), and freeze-concentration of reactant ions into the 0.05 micropockets is also promoted. (See reference 2). 0 0 1 2 3 pH 4 5 6 7 Release of halogens and halogens of Release Summary Mechanism Trihalide ([X-I-X]-) production + - H2ONO + I ↔ INO + H2O a-d shows how single ice after freezing does not occur - - I + INO → I2 + NO crystals can aggregate to until pH 3 (below which the - + form “micropockets” I2 + H2ONO → NO + I2 + H2O nitroacidium ion forms). Photochemical release of HONO FT-RAIRS/Mass Spectrometry/Photolysis experiment HONO release from water-ice via the and NOx from ice/snow surfaces investigating (NO2)2 on a thin-film water-ice surface nitroacidium ion •Elevated NOx levels over snowpacks have Thermal Desorption profiles showing HONO been measured release upon annealing post-photolysis Post-photolysis (1hr xenon arc): surface 1. •Photolysis of acidified NITRATE IONS in snow FT-RAIRS Thermal Desorption profiles subsequent to photolysis of (NO2)2 on water-ice ≡ identified as a probable mechanism 20000 Ion on Ice on Ion + - 18000 i.e. solvated nitrosonium ion OR protonated nitrous acid NO NO 1312 1283 0.4 3 773 16000 m/z =18 (H2O) 14000 Summary Mechanism 0.3 1882 1763 1738 1592 12000 c/s 2. + - - 0.2 UHV chamber coupled to FT-RAIRS H O 1423 Absorbance Units Absorbance m/z = 30 (NOx/HONO) 2 H3O + HONO 1. NO + hn→ NO + O 2218 8000 3 2 (for identifying surface species) and - - . 0.1 6000 2a. NO + hn→ NO + O QMS detection (for gas-phase 4000 (HONO) 3 2 m/z = 46 (NO2/HONO) m/z = 17 0.0 2000 2200 2000 1800 1600 1400 1200 1000 800 600 release) m/z = 47 (HONO) - + Wavenumber cm-1 → 0 2b. NO + H HONO C:\Data Files\OPUS\sample.509 sample on gold 29/07/2005 2 130 180 230 280 330 •The solvolysis/hydrolysis processes shown C:\Data Files\OPUS\sample.512 sample on gold 29/07/2005 Temp/K Nitroacidium •The total quantum yield for pathways 1 and 2 is above might occur in micropockets or in the QLL FTIR results consistent with following mechanism: •The initiating step for HONO release is nitrate 0.01 with NO2 as the major product The •Direct nitrite ion production is very improbable hn hn ion photolysis leading to NO2 build up as a result of freeze-concentration effects. (See reference 4) with F ~ 0.001. Hence pathway 2b is unlikely + •Production of NO2 on/in a cold surface solid Airborne leads to efficient D -N O (dimer) formation Pollutants Quasi Liquid Layer 2h 2 4 water-ice (QLL) “Micropockets” References Acknowledgements 1. Journal of Physical Chemistry A 109 779-786 (2005) Further Information: The Authors wish to thank the following bodies for their assistance: 2. Journal of Physical Chemistry A 110 4615-4618 (2006) More information regarding Atmospheric Chemistry activities in the CRAC Enterprise Ireland 3. Journal of Physical Chemistry A 112 1677-1682 (2008) IRCSET Laboratory at UCC can be found at http://crac.ucc.ie EU Marie Curie Programme 4. Journal of Physical Chemistry A 111 1167-1171 (2007).
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