Local and seasonal variations in atmospheric nitrogen dioxide levels at Rothamsted, UK, and relationships with meteorological conditions

Abstract As part of the UK Environmental Change Network (ECN), a number of passive nitrogen dioxide (NO 2 ) diffusion tubes were set up at Rothamsted Experimental Station in September 1993. Concentrations of atmospheric NO 2 recorded between September 1993 and April 1997 are described in terms of seasonal and local spatial variations and in relation to different types of background vegetation. At most sites, NO 2 levels fluctuate seasonally: the general pattern is one of winter highs and summer lows, modified by a slight dip in December and January and slower changes in May and August. Smoothing splines with a trigonometric penalty function are used to describe the periodic seasonal variation. An adequate summary of patterns of NO 2 concentration is obtained using a common seasonal component of variation, together with local spatial variations in the mean level and the amplitude of the seasonal fluctuations. The presence of trees reduces the mean NO 2 level and the amplitude of the seasonal variation, while proximity to road traffic and dense vegetation increases the mean level and almost eliminates seasonal variation. The effect of traffic decreases rapidly with distance from the road. Associations between NO 2 levels and local meteorological conditions are also investigated. Levels of NO 2 are generally unrelated to meteorological conditions, suggesting that they are mainly driven by changes in background inputs, such as those associated with the nearby town of Harpenden. A weak negative association was observed between NO 2 concentration and wind speed. This suggests that unseasonably high wind speeds decrease concentrations of NO 2 . Non-linearity in the relationship between NO 2 levels and wind speed is accounted for by the effects of extreme wind speeds on the diffusion lengths of the tubes.

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