THE PHYSICOCHEMICAL STRUCTURE OF THE GREENLAND SUMMER AEROSOL AND ITSRELATION TO ATMOSPHERIC PROCESSES

Relationships between size-resolved aerosol chemistry and various atmo spheric processes were studied at Summit over the Greenland ice sheet. The Summit summer aerosol displayed an Aitken mode below 0.1 mu m aer odynamic particle diameter, one or two modes in the accumulation size range (0.1-1 mu m), and a supermicron mode. The Aitken mode contained little particulate mass (< 5%) but accounted for most of the particle number concentration. The accumulation size range frequently had two o verlapping modes with a minimum around 0.4 mu m. A potential reason fo r this bimodality is fog processing which occurs frequently over the i ce sheet during summer nights. Most of the particulate sulfate, ammoni um, methane sulfonic acid (MSA), and dicarboxylic were found in the ac cumulation size range, where they displayed quite a similar modal stru cture, suggestive of an internal mixture of these compounds in submicr on particles. The ratio of MSA to sulfate varied with particle size ov er the accumulation size range, so that different deposition pathways are likely to cause different MSA to sulfate ratios in the surface sno w. Less than 20% of particulate sulfate and >95% of particulate nitrat e were usually found in supermicrometer particles. It is likely that t he supermicron sulfate and nitrate are produced when SO2 and HNO3 reac t with particles of mostly crustal origin. Supermicron sulfate centere d at a somewhat smaller size (< 2 mu m) than nitrate (2-3 mu m) The pa rticulate phase contained only minor amounts of semivolatile acidic co mpounds, except when the ice sheet was impacted by apparent biomass bu rning plumes. These plumes are characterized by elevated levels of man y low-volatility compounds, and they may contain submicron ammonium ni trate and ammonium formate.