Sulphur and nitrogen compounds in variable marine/continental air masses at the southwest European coast

Measurements of atmospheric sulphur and nitrogen species were made on the west coast of Portugal, within the framework of BMCAPE project. Four research intensive campaigns took place between November 1993 and August 1994 and revealed pronounced seasonal cycles for DMS and MSA. Surprisingly high levels of DMS were detected during autumn (mean concentration: 776ngSm−3), possibly as a consequence of a later algae bloom or local marsh emissions. COS displayed a slight seasonal variation with mean concentrations ranging from 741 ng S m−3 in autumn up to 903 ng S m−3 in summer. On the basis of backward air mass trajectories and local meteorological data, samples were assigned into different classes of air masses. The lowest concentrations of SO2, nssSO42−, NH3, NH4+, HNO3, N03− and NO2 were found in air masses transported over the Atlantic Ocean and were about 3–10 times lower than those recorded in air masses from continental origin. Nevertheless, for many of the species, the mean concentrations in maritime air were still significantly higher than those previously recorded in remote marine sites, presumably owing to long-range transport of pollutants or contamination induced by mesoscale wind circulation.