MODELING OF THE PROCESSING AND REMOVAL OF TRACE GAS AND AEROSOL SPECIES BY ARCTIC RADIATION FOGS AND COMPARISON WITH MEASUREMENTS

A Lagrangian radiation fog model is applied to a fog event at Summit, Greenland. The model simulates the formation and dissipation of fog. I ncluded in the model are detailed gas and aqueous phase chemistry, and deposition of chemical species with fog droplets. Model predictions o f the gas phase concentrations of H2O2, HCOOH, SO2, and HNO3 as well a s the fog fluxes of S(VI), N(V), H2O2, and water are compared with mea surements. The predicted fluxes of S(VI), N(V), H2O2, and fog water ge nerally agree with measured values. Model results show that heterogene ous SO2 oxidation contributes to approximately 40% of the flux of S(VI ) for the modeled fog event, with the other 60% coming from preexistin g sulfate aerosol. The deposition of N(V) with fog includes contributi ons from HNO3 and NO2 initially present in the air mass. HNO3 directly partitions into the aqueous phase to create N(V) and NO2 forms N(V) t hrough reaction with OH and the nighttime chemistry set of reactions w hich involves N2O5 and water vapor. PAN contributes to N(V) by gas pha se decomposition to NO2, and also by direct aqueous phase decompositio n. The quantitative contributions from each path are uncertain since d irect measurements of PAN and NO2 are not available for the fog event. The relative contributions are discussed based on realistic ranges of atmospheric concentrations. Model results suggest that in addition to the aqueous phase partitioning of the initial KNO3 present in the air mass, the gas phase decomposition of PAN and subsequent reactions of NO2 with OH as well as nighttime nitrate chemistry may play significan t roles in depositing N(V) with fog. If a quasi-liquid layer exists on snow crystals, it is possible that the reactions taking place in fog droplets also occur to some extent in clouds as well as at the snow su rface.