PHYSICOCHEMICAL PROPERTIES OF AEROSOLS OVER THE NORTHEAST ATLANTIC - EVIDENCE FOR WIND-SPEED-RELATED SUBMICRON SEA-SALT AEROSOL PRODUCTION

Physicochemical characteristics of submicron aerosol particles over the Northeast Atlantic (63-degrees-N, 8-degrees-W) during October/November 1989 have been examined using a thermal analytical technique and are classified according to air mass origin. Aerosol associated with anthropogenically influenced air masses contained typically 80% sulphate particles by number, the remainder being soot carbon and sea salt. For Arctic air masses the contribution of sulphate to the total aerosol was reduced to around 65%, due to low concentrations relative to sea salt which is dependent on wind speed. In situations with clean maritime air and high wind speeds, sulphate aerosol accounted for less than 25% of the total accumulation mode particles, the remainder consisting predominantly of sea salt. Arctic air masses and clean maritime air during periods of high winds were consistently acidic with inferred molar ratios of NH4+/SO4 = near 0.2. The continental and modified maritime aerosol encountered was found to have molar ratios of about 0.8. Soot carbon was present in all air masses to a similar degree (5-13%). In clean air masses, submicron sea salt aerosol concentrations showed a strong exponential increase with wind speed (correlation coefficients cc greater-than-or-equal-to 0.8), down to a dry particle radius of 0.05 mum. Under these ''clean air'' conditions and high winds the sea salt aerosol dominated all particle sizes for r > 0.05 mum and accounted for approximately 75% of the total concentration, suggesting that under these conditions, sea salt aerosol would comprise the primary source of cloud condensation nuclei (CCN) in stratiform clouds.