Temporal and spatial variations of airborne Mg, Cl, Na, Ca and K in rural areas of Norway
The Norwegian air chemistry monitoring network includes measurements of gaseous and particulate compounds in air at 12 rural sites. The sampling method is designed to determine concentration levels of sulphur and nitrogen compounds and uses a three-stage filter pack sampler. Concentration levels of base-cations and sea salts may also be determined from the chemical analysis of the filter extracts. In this study, concentration levels of water soluble Mg, Cl, Na, Ca and K in air have been investigated in terms of temporal, seasonal and spatial trends during the period 1986-1996. The results have been combined with precipitation chemistry data to estimate total deposition, and to evaluate the relative importance of dry deposition. While Na, Cl and Mg are of marine origin, which is clearly reflected in concentration ratios and spatial gradients, Ca and K are of a mixed origin. Air trajectory analyses indicate that air masses originating in Eastern Europe result in significantly higher airborne concentrations of non-sea salt Ca and K, than air from other sectors. Major sources of airborne Ca and K in Norway are expected to be of anthropogenic origin from combustion plants and industrial processes in Eastern Europe, while aeolian dust from agricultural areas within Europe or from Sahara is of limited importance. Concentration levels of non-sea salt base-cations in air were 20-30% lower during the 1990s, compared to 1986-1989. Precipitation chemistry data available since the early 1980s indicate even larger reductions, and in the order of 50%. Total deposition of non-sea salt base-cations during 1993-1996 was generally below 10 mmol(c) m-2 year-1, whereas in coastal areas with large precipitation amounts, inputs exceeding 15 mmol(c) m-2 year-1 occurred. Deposition of base-cations may thus counteract up to 25% of the strong acid anion input, but more typically in the order of 5-10%. Dry deposition of non-sea salt base-cations of southern Norway is comparatively small, typically contributing 10-30% to the total deposition in the inland areas, and relatively less in other regions.
Torseth
K
Norwegian Institute for Air Research, Kjeller, Norway. kt@nilu.no
Hanssen
JE
Semb
A