Multiphase modeling of nitrate photochemistry in the quasi-liquid layer (QLL): implications for NOx release from the Arctic and coastal Antarctic snowpack

Abstract

We utilize a multiphase model, CON-AIR (<B>Con</B>densed Phase to <B>Air</B> Transfer Model), to show that the photochemistry of nitrate (NO₃^&minus;) in and on ice and snow surfaces, specifically the quasi-liquid layer (QLL), can account for NO_x volume fluxes, concentrations, and [NO]/[NO₂] (γ=[NO]/[NO₂]) 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&times;10⁴ to 6.4&times;10⁵ molecules cm^&minus;3 s^&minus;1, 5.7&times;10⁸ to 4.8&times;10⁹ molecules cm^&minus;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 NO₃^&minus; and NO₂^&minus;. This is important since the immediate precursor for NO, for example, NO₂^&minus;, absorbs at wavelengths longer than nitrate itself. Finally, one-dimensional model simulations indicate that both gas phase boundary layer NO and NO₂ exhibit a negative concentration gradient as a function of height although [NO]/[NO₂] 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 CH₃O₂