Sources of aerosol nitrate and non-sea-salt sulfate in the Iceland region

Abstract

Daily aerosol filter samples were collected on Heimaey, Iceland (63.40° N, 20.30° W), beginning in July 1991. Samples were analyzed for NO3−, non-sea-salt (nss) SO42−, and methanesulfonate (MSA). Along with SO2 and nss-SO42−, MSA is a product of the atmospheric oxidation of dimethyl sulfide (DMS) that is produced by marine organisms. For much of the time, concentrations of aerosol nss-SO42− and NO3− were relatively low. Occasionally, however, concentrations increased sharply, by an order of magnitude or more, often for periods of several days. These concentration peaks were usually associated with the presence of a high-pressure field over western Europe; the large-scale wind fields associated with the high pressure subsequently transported pollutants to the Iceland region. The 2-year mean NO3− aerosol concentration was 0.239 μg/m3, while that for nss-SO42− was 0.642 μg/m3; the median values were, respectively, 0.113 μg/m3 and 0.367 μg/m3. Excluding the high-aerosol events (i.e. about 10% of the samples), the NO3− average was 0.131 μg/m3 and that for nss SO42− was 0.385 μg/m3; these values are similar to those measured in the pristine South Pacific. Thus, although pollution events were relatively infrequent, they had a substantial impact on atmospheric chemistry in this region, in effect doubling the annual mean concentrations. There was a very strong seasonal cycle in MSA concentrations, with a summer maximum of about 500 ng/m3, which decreased to a few ng/m3 in December. The seasonal cycle of MSA matches that ofPhaeocystis pouchetii andEmiliania huxleyi, both of which are strong DMS producers; intense and widespread blooms of these organisms are often found around Iceland in the late spring and summer. During the summer, the nss-SO42−MSA ratio was very low much of the time, suggesting that biogenic DMS was the dominant source of aerosol nss-SO42− in this region in this season.