3 Dec 2004IUP Heidelberg Reading Group Air-Snow Interactions and Atmospheric Chemistry Florent Domine and Paul B. Shepson, Science, 297, 1506 (2002). Reviewed for reading group by Bill Simpson

3 Dec 2004IUP Heidelberg Reading Group 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 ( x 10 6 km 2 ) 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.

3 Dec 2004IUP Heidelberg Reading Group 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

3 Dec 2004IUP Heidelberg Reading Group 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 –NO 3 - (snow) + hv  NO 2 + O - O - + H +  OH –NO 3 - (snow) + hv  NO O H + + NO 2 -  HONO First pathway produces OH in snow (a critical oxidant) Second pathway produces HONO that is then photolysed to OH

3 Dec 2004IUP Heidelberg Reading Group 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 HO 2 )

3 Dec 2004IUP Heidelberg Reading Group Impact of snow on HOx chemistry Generally, OH is produced by ozone photolysis followed by reaction with water –O 3 + hv  O( 1 D) + O 2 O( 1 D) + H 2 O  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

3 Dec 2004IUP Heidelberg Reading Group 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 Hg 0 to reactive gaseous Hg that then deposits.

3 Dec 2004IUP Heidelberg Reading Group 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. CO 2 or water isotopes) For reactive compounds or compounds deposited by reactive compounds, these impacts of snow chemistry affect ice core inversions. –NO 3 - lost from snowpack by photochemistry –OH produced in snowpack may remove organic matter and produce small molecules (e.g. aldehydes) –Some CO 2 and CO may be produced in snowpack from OH chemistry – could affect CO 2 records in Greenland cores (where there are more organics) –In air there may be feedbacks of the highly oxidative snow environment

3 Dec 2004IUP Heidelberg Reading Group 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

3 Dec 2004IUP Heidelberg Reading Group 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.

3 Dec 2004IUP Heidelberg Reading Group 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.

3 Dec 2004IUP Heidelberg Reading Group 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.