Trends in large-scale VOC concentrations in the Southern Netherlands between 1991 and 1997

Results of C₆–C₁₂ hydrocarbon measurements at three sites in the southern part of The Netherlands, a polluted region in Western Europe, are presented. The measurements were carried out over the period March 1991–February 1997. The concentrations at the sites, with 100–150 km distance between them, are quite similar and they are predominantly determined by large-scale transport. The concentrations in this part of the country are substantially higher than those observed at a coastal site in the north of The Netherlands, but much lower compared to the concentrations in cities and near streets. A distinct difference between the trends of aromatics and aliphatics was observed. The concentrations of the aromatic components display trends that are systematically 4–5% yr^-1 lower than the trends of the aliphatics, which is possibly related to the increased use of catalysts in cars and, partly, to an enhanced atmospheric chemical activity. For the chlorinated species the trends are highly significant. The trend of 1,1,1-trichloroethane is in the order of 8–12% yr^-1 downward while for tetrachloromethane an annual downward trend of 4–6% is found. These downward trends suggest that measures have been taken to fulfil the requirements of the Montreal Protocol to ban the production of these species in a few years time from now.