Chemical composition of aerosol during particle formation events in boreal forest

Size-segregated chemical aerosol analysis of a total 5 integrated samples has been performed for the atmospheric aerosol during events of new particle formation. The experiments were conducted during the BIOFOR 3 measurement campaign at a boreal forest site in southern Finland in spring 1999. Aerosol samples collected by a cascade low-pressure impactor were taken selectively to distinguish particle formation event aerosol from non-event aerosol. The division into “event” and “non-event” cases was done “in situ” at field, based on the on-line submicron number size distribution. The results on the chemical ionic composition of the particles show only small differences between the event and non-event sample sets. The event samples show lower concentrations of total sulfate and ammonium as well as light dicarboxylic acids such as oxalate, malonate and succinate. In the event samples, nucleation mode particle MSA (methanesulphonic acid) was found to be present exceeding the concentrations found in the non-event samples, but at larger particle sizes the sample sets contained rather similar concentrations of MSA. The most significant difference between the event and non-event sets was found for dimethylammonium, ionic component of dimethylamine ((CH3)2NH), which seems to be present in the particle phase during the particle formation periods and/or during the subsequent particle growth. The absolute event sample dimethylamine concentrations were more than 30-fold greater than the non-event concentrations in the accumulation mode size range. On the other hand, the non-event back-up filter stage for sub-30 nm particles contained more dimethylamine than the event samples. This fractionation is probably a condensation artifact of the impactor sampling. A simple mass balance estimate is performed to evaluate the quality and consistency of the results for the overall mass concentration.