Modal structure of chemical mass size distribution in the high Arctic aerosol

Chemical mass size distributions of aerosol particles were measured in the remote marine boundary layer over the central Arctic Ocean as part of the A tmospheric Research Program on the Arctic Ocean Expedition 1996 (AOE-96). A n inertial impaction method was used to classify aerosol particles into dif ferent size classes for subsequent chemical analysis. The particle chemical composition was determined by ion chromatography and by the particle-induc ed X-ray emission technique. Continuous particle size spectra were extracte d from the raw data using a data inversion method. Clear and varying modal structures for aerosols consisting of primary sea-salt particles or of seco ndary particles related to dimethyl sulfide emissions were found. Concentra tion levels of all modes decreased rapidly when the distance from open sea increased. In the submicrometer size range the major ions found by ion chro matography were sulfate, methane sulfonate, and ammonium. They had most of the time a clear Aitken mode and one or two accumulation modes, with aerody namic mass median diameters around 0.1 mum, 0.3 mum, and between 0.5-1.0 mu m, respectively. The overall submicron size distributions of these three io ns were quite similar, suggesting that they were internally mixed over most of this size range. The corresponding modal structure was consistent with the mass size distributions derived from the particle number size distribut ions measured with a differential mobility particle sizer. The Aitken to ac cumulation mode mass ratio for nss-sulfate and MSA was substantially higher during clear skies than during cloudy periods. Primary sea-salt particles formed a mode with an aerodynamic mass median diameter around 2 mum. In gen eral, the resulting continuous mass size distributions displayed a clear mo dal structure consistent with our understanding of the two known major sour ce mechanisms. One is the sea-salt aerosol emerging from seawater by bubble bursting. The other is related to dimethylsulfide (DMS) emissions from bio genic processes in seawater, followed by gas-to-particle conversion, format ion of particulate sulfate and methane sulfonate (MSA) and neutralization b y ammonia.