Size distributions of ionic aerosols measured at Waliguan Observatory: Implication for nitrate gas-to-particle transfer processes in the free troposphere

[1] Waliguan Observatory (WO) is a land-based Global AtmosphereWatch baseline station on the Tibetan Plateau. Size-resolved ionic aerosols (NH4 , Na, K, Ca, Mg, SO4 2 , Cl , NO3 CO3 2 , formate, acetate and oxalate), organic aerosols, black carbon and gaseous HNO3 and SO2 were measured during an intensive fall-winter field experiment. The observational data were analyzed with a focus on the partitioning of nitrate between the gas and particle phases. Nitrate was found to exist mainly in the particle phase with a typical particulate-to-total nitrate ratio, i.e., NO3 (p)/(NO3 (p) + HNO3(g)), of about 0.9. It was also found that the size distribution pattern of particulate nitrate at WO varied in different samples and the amount of particulate nitrate residing in the fine mode (Dp < 2.0 mm) was typically larger than or comparable with that in the coarse mode. A gas-particle chemical equilibrium model was used to predict these particulate nitrate size distributions. The size distributions of particulate nitrate were reasonably reproduced with the model within the uncertainties caused by the detection limits. The chemical pathways for the formation of particulate nitrate at WO were analyzed with the size distributions of measured ionic aerosols. It was demonstrated that fine nitrate particles may have been produced by the reaction of gaseous nitric acid with gaseous ammonia, while coarse nitrate particles may have been generated via the condensation of nitric acid on the surface of mineral aerosols. The signature of biomass burning at WO was found to be associated with black carbon as well as the accumulation of potassium and oxalate in the fine particles.