GAS-TO-PARTICLE CONVERSION OF SULFUR AND NITROGEN-COMPOUNDS AS STUDIED AT MARINE STATIONS IN NORTHERN EUROPE

The present work provides insights into the N- and S-pollution behavio ur at rural and remote sites in Northern Europe. Formation of SO42- an d NO3- from their precursor gases receives special attention in order to understand the significance of chemical conversion in shaping the d istributions of concentration and deposition monitored at our sites: S aby-on the Swedish west coast, Lista-on the Norwegian south coast and South Uist-on the Hebride Islands in Northern Scotland. Owing mainly t o the complexity of the problem, the approach is semiquantitative. Con centration patterns of S- and N-compounds for air mass transport betwe en the sites were investigated for 10 periods (13 d), within continent al (6 d) and oceanic (7 d) air masses. The observations spanned over b oth winter and summer days and conclusions could be drawn in some case s 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 indi cators 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 rat io of gaseous HNO3-N over particulate NO3--N. S-ratios at Saby and Lis ta agreed very well in those cases when no or insignificant precipitat ion occurred. Higher S-ratios and lower N-ratios were typical for cond itions of high relative humidity. It is conceivable that condensed pha se chemistry was of major importance in the SO2 oxidation and that HNO 3 was absorbed onto the wet aerosol particles. Stagnation of the air m asses over source regions of high emission densities and rates resulte d in accumulation of pollution, the S-ratios in such conditions being high due to the longer transport time. The S-ratios for marine air flo w at Saby 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 i n the case of maritime air masses due to unexpectedly high NO3- concen trations. It is conceivable that the high load of NO3- is due to sea s pray of surface water. More efficient SO2 deposition over land than ov er sea has been observed in the westerly airflow by the relative loss of SO2 while the Bow swept over land. Copyright (C) 1996 Elsevier Scie nce Ltd