Spatial and temporal variation of rural nitrogen dioxide concentrations across the United Kingdom

The distribution of nitrogen dioxide across the United Kingdom between June 1987 and May 1990 was investigated using passive diffusion tube samplers. Sampling sites were situated in rural locations collocated with the U.K. secondary national acid deposition monitoring network. The spatial distribution of nitrogen dioxide is described and mapped for the annual, winter and summer periods spanning April 1987–June 1988 from 57 sites. Data collected in subsequent years from a reduced network were used to investigate the temporal variability. An analysis of the temporal variation across all the sites established a distinct seasonal cycle, but showed that the cycle was not uniform in magnitude across the network. The winter maxima were attributed to some seasonality in emissions, and meteorological conditions allowing concentrations to build up coupled with a lower rate of daytime conversion by reaction with OH. The summer minima were thought to be principally the result of the greater atmospheric mixing, a higher rate of atmospheric conversion by reaction with OH during the daytime and, to a lesser extent, enhanced dry deposition by stomatal uptake. Data from the survey were compared with the results from a simple Lagrangian trajectory model which reproduced the spatial pattern of the measurements well, but underestimated the concentrations by an average factor of 2.3. It is believed that this underestimation is the result of the modelʹs simplified meteorology, particularly regarding instantaneous mixing within a boundary layer of fixed :height together with the non-seasonal chemical parameterisation. A simple budget model indicated that wet deposition exceeds that of dry by a factor of approximately 3 and that the airborne reservoir of NOy is primarily NO2 rather than HNO3.