An observational study on precipitation chemistry data as a function of surface wind direction

Abstract

Precipitation chemistry data were collected at 8 sites in the western part of the Netherlands over a period of 151/2 yr. Using specially-designed wind direction-dependent rain collectors, it is demonstrated that levels of ion constituents in rain water vary with wind direction, which can be assigned to different sources of contamination. The strongest variation was measured for Na+. As expected, trajectories over sea contributed most. Continental sources slightly reduced the variation for Cl− and Mg2+. For both NH inf4 sup+ and excess SO inf4 sup2− , maxima were found in southeasterly rainfall. It is assumed that polluted air from the large industrial Ruhr Area crosses a region with strong NH3 emissions in the Southeast of the Netherlands. The presence of ammonium sulfate was suggested. Minor variations were measured for NO inf3 sup− (due to diffusely-spread emissions and possible sampling artefact) and for H+ and Ca2+ (dry-deposition effects). Over the period 1973–1987 excess-SO inf4 sup2− levels decreased significantly (−3.3% yr−1, p<0.01) which is consistent with reduced S emission in Western Europe. Over the last decade an upward trend in NO inf3 sup− levels has been observed (3.2% yr−1, p<0.05) which is possibly related to the increased usage of cars.