| Summary: | We utilize a multiphase model, CON-AIR ( Con densed Phase to Air Transfer Model), to show that the photochemistry of nitrate (NO 3 − ) in and on ice and snow surfaces, specifically the quasi-liquid layer (QLL), can account for NO x volume fluxes, concentrations, and [NO]/[NO 2 ] (γ=[NO]/[NO 2 ]) measured just above the Arctic and coastal Antarctic snowpack. Maximum gas phase NO x volume fluxes, concentrations and γ simulated for spring and summer range from 5.0×10 4 to 6.4×10 5 molecules cm −3 s −1 , 5.7×10 8 to 4.8×10 9 molecules cm −3 , and ~0.8 to 2.2, respectively, which are comparable to gas phase NO x volume fluxes, concentrations and γ measured in the field. The model incorporates the appropriate actinic solar spectrum, thereby properly weighting the different rates of photolysis of NO 3 − and NO 2 − . This is important since the immediate precursor for NO, for example, NO 2 − , absorbs at wavelengths longer than nitrate itself. Finally, one-dimensional model simulations indicate that both gas phase boundary layer NO and NO 2 exhibit a negative concentration gradient as a function of height although [NO]/[NO 2 ] are approximately constant. This gradient is primarily attributed to gas phase reactions of NO x with halogens oxides (i.e. as BrO and IO), HO x , and hydrocarbons, such as CH 3 O 2 .
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