Gas-to-particle conversion of sulphur and nitrogen compounds as studied at marine stations in Northern Europe

The present work provides insights into the N- and S-pollution behaviour at rural and remote sites in Northern Europe. Formation of SO42− and NO3− from their precursor gases receives special attention in order to understand the significance of chemical conversion in shaping the distributions of concentration and deposition monitored at our sites: Säby—on the Swedish west coast, Lista—on the Norwegian south coast and South Uist—on the Hebride Islands in Northern Scotland. Owing mainly to the complexity of the problem, the approach is semiquantitative. Concentration patterns of S- and N-compounds for air mass transport between the sites were investigated for 10 periods (13 d), within continental (6 d) and oceanic (7 d) air masses. The observations spanned over both winter and summer days and conclusions could be drawn in some cases with respect to seasonality. The measurements were performed during the BMCAPE (Background Maritime Contribution to Atmospheric Pollution in Europe) project in 1993 and 1994. S- and N-ratios were used as indicators of air mass age, chemical conversion efficiency and “en-route” deposition. The S-ratio is defined as the ratio of particulate S (as SO42−) to total S (as SO2 and SO42−). The N-ratio is taken as the ratio of gaseous HNO3-N over particulate NO3−-N. S-ratios at Saby and Lista agreed very well in those cases when no or insignificant precipitation occurred. Higher S-ratios and lower N-ratios were typical for conditions of high relative humidity. It is conceivable that condensed phase chemistry was of major importance in the SO2 oxidation and that HNO3 was absorbed onto the wet aerosol particles. Stagnation of the air masses over source regions of high emission densities and rates resulted in accumulation of pollution, the S-ratios in such conditions being high due to the longer transport time. The S-ratios for marine air flow at Säby and Lista no longer agreed (higher at Lista) as in the case of continental air masses. The reason is likely to be the proximity of the Lista station to the sea. Lista was highly affected by sea spray produced by the strong westerly winds. The N-ratio remained very low in the case of maritime air masses due to unexpectedly high NO3− concentrations. It is conceivable that the high load of NO3− is due to sea spray of surface water. More efficient SO2 deposition over land than over sea has been observed in the westerly airflow by the relative loss of SO2 while the flow swept over land.