Diurnal and seasonal variations of ultrafine particle formation in anthropogenic SO2 plumes.

The cloud condensation nuclei concentrations predicted by global aerosol models are sensitive to how new particle formation in subgrid anthropogenic SO(2) plumes is parameterized. Using a state-of-the-art kinetic nucleation model, we carried out two case studies to investigate the large difference in the number concentrations of ultrafine particles observed in the plumes from the Horne smelter: one in the summer and the other in the winter. Our model predicted that particle number concentrations are in good agreement with observations for both cases, showing that particle formation in the Horne smelter plumes is dominated by binary homogeneous nucleation (BHN) in the winter case and by ion-mediated nucleation (IMN) in the summer case. Further sensitivity studies reveal significant diurnal and seasonal variations of sulfate particle formation in the anthropogenic SO(2) plume, mainly associated with corresponding variations of two key parameters: hydroxyl radical concentration ([OH]) and temperature. Nucleation in the plume is negligible at night because of very low [OH]. BHN is significant when [OH] is relatively high or temperature is relatively low, and it is generally limited to the fresh plumes (within approximately 15 km from source), but it can generate very high concentrations of ultrafine particles (peak values as high as 10(5)-10(6) cm(-3)) under favorable conditions. IMN generally dominates nucleation in the plume when [OH] is relatively low or temperature is relatively high, and it extends from fresh plume to more aged plume and produces 2-3 x 10(4) cm(-3) of nucleated particles. The implications of the results are discussed.