AEROSOL NUMBER SIZE DISTRIBUTIONS FROM 3 TO 500 NM DIAMETER IN THE ARCTIC MARINE BOUNDARY-LAYER DURING SUMMER AND AUTUMN

Aerosol physics measurements made onboard the Swedish icebreaker Oden in the late Summer and early Autumn of 1991 during the International A rctic Ocean Expedition (IAOE-91) have provided the first data on the s ize distribution of particles in the Arctic marine boundary layer (MEL ) that cover both the number and mass modes of the size range from 3 t o 500 nm diameter. These measurements were made in conjunction with at mospheric gas and condensed phase chemistry measurements in an effort to understand a part of the ocean-atmosphere sulfur cycle. Analysis of the particle physics data showed that there were three distinct numbe r modes in the submicrometric aerosol in the Arctic MEL. These modes h ad geometric mean diameters of around 170 nm, 45 nm and 14 nm referred to as accumulation, Aitken and ultrafine modes, respectively. There w ere clear minima in number concentrations between the modes that appea red at 20 to 30 nm and at 80 to 100 nm. The total number concentration was most frequently between 30 and 60 particles cm(-3) with a mean va lue of around 100 particles cm(-3), but the hourly average concentrati on varied over two to three orders of magnitude during the 70 days of the expedition. On average, the highest concentration was in the accum ulation mode that contained about 45% of the total number, while the A itken mode contained about 40%. The greatest variability was in the ul trafine mode concentration which is indicative of active, nearby sourc es (nucleation from the gas phase) and sinks; the Aitken and accumulat ion mode concentrations were much less variable. The ultrafine mode wa s observed about two thirds of the time and was dominant 10% of the ti me. A detailed description and statistical analysis of the modal aeros ol parameters is presented here.