Air-Snow Interactions and Atmospheric Chemistry

1. Air-Snow Interactions and Atmospheric Chemistry Florent Domine and Paul B. Shepson, Science, 297, 1506 (2002). Reviewed for reading group by Bill Simpson IUP Heidelberg Reading Group

2. Background • Snow covers up to 50% of landmasses in Northern Hemisphere • Snow is • Porous, gas permeable • High surface area • An intervening phase between surface and atmosphere • Snow clearly impacts Atmospheric chemistry • Ozone depletion episodes • Hg deposition Week of maximum snow extent (52.578 x 106 km2) for the period 1979 to 1995 (image from January 8-14, 1979) Image courtesy of the National Snow and Ice Data Center, University of Colorado, Boulder. IUP Heidelberg Reading Group

3. Sites where we study snow-air interactions • 3 locations in Northern Hemisphere, similar number on Antarctica • Sites generally show similar strong impacts of snow on atmospheric chemistry IUP Heidelberg Reading Group

4. Impact of snow on NOx chemistry • Measurements of NOx inside the snowpack show that the snowpack sources NOx to the atmosphere. Also, HONO is produced • NO3-(snow) + hv  NO2 + O-O- + H+  OH • NO3-(snow) + hv  NO2- + OH+ + NO2-  HONO • First pathway produces OH in snow (a critical oxidant) • Second pathway produces HONO that is then photolysed to OH IUP Heidelberg Reading Group

5. Impact of snow on NOx chemistry II • Both NOx and HONO are >20 times expected levels without snowpack. • These pathways also affect the HOx family (OH and HO2) IUP Heidelberg Reading Group

6. Impact of snow on HOx chemistry • Generally, OH is produced by ozone photolysis followed by reaction with water • O3 + hv  O(1D) + O2 O(1D) + H2O  2OH • This reaction requires UV, which is of low intensity in the Arctic; therefore, models predict low OH levels • Measurements show high OH levels  Snow chemistry affects OH IUP Heidelberg Reading Group

7. Impact of other snow chemistry • Small aldehydes are produced from the snowpack. • Halogens are emitted from snowpack, and they become reactive halogen gases (e.g. BrO, ClO) • These reactive halogens then deplete ozone and convert Hg0 to reactive gaseous Hg that then deposits. IUP Heidelberg Reading Group

8. Impacts on ice core inversions • In some regions, snow accumulates to form ice cores that have a detailed record of atmospheric precipitation. • Some impurities are fairly directly interpreted (e.g. CO2 or water isotopes) • For reactive compounds or compounds deposited by reactive compounds, these impacts of snow chemistry affect ice core inversions. • NO3- lost from snowpack by photochemistry • OH produced in snowpack may remove organic matter and produce small molecules (e.g. aldehydes) • Some CO2 and CO may be produced in snowpack from OH chemistry – could affect CO2 records in Greenland cores (where there are more organics) • In air there may be feedbacks of the highly oxidative snow environment IUP Heidelberg Reading Group

9. Snow-pack scale • Snow is >>99.9% water, but we generally are interested in the impurities – how do they get there • Nucleation of precipitation • Scavenging by precipitation • Adsorption, co-condensation, solid-state diffusion • Once the snow is on the ground, water vapor may remobilize. This water vapor motion is called snow metamorphism • Metamorphism should change the locations of trapped impurities, affecting their chemistry IUP Heidelberg Reading Group

10. Snow-crystal scale • Many of these physical processes on the snow-crystal scale are not well understood • Water ice has a disordered surface layer (often called the quasi-liquid layer, QLL) whose thickness increases with increasing temperature and ionic impurity. • Impurities in snow may preferentially segregate to the QLL, speeding reactions and affording increased interaction with the gas phase. IUP Heidelberg Reading Group

11. Model for chemistry • Shows snow as a processor that uses photochemistry to produce reactive radicals that then oxidize organics in the snow • Oxidation products are smaller gas-phase organics that then again affect the air chemistry • Now we also know that HOOH photolysis is another critical source of OH in snow packs. IUP Heidelberg Reading Group

12. Conclusions • Snow has huge impacts on the overlying atmosphere • This chemistry affects ice cores • This chemistry affects the atmosphere above the snow • [related topic] Ice particles are common in the atmosphere – cirrus, PSCs, a large fraction of rain precipitation formed as ice high in the atmosphere then melted on descent. • Future study recommended: • Surface of ice not well understood • Composition of much of snow (organics, mineral dust, carbon) not understood • Microphysical locations of molecules in snow not understood • Lab studies need to be done • Scale up of process-level understanding to global scale. IUP Heidelberg Reading Group