The influence of meteorology and atmospheric transport patterns on the chemical composition of rainfall in south-east England

Rainwater composition was examined at event temporal resolution, over a 6 month period, at a site in southeast England. The data were used to assess the overall levels of acidic deposition at the site, and to identify functional relationships between wet deposition and causal meteorological processes. Rainfall-weighted average concentrations were close to those estimated from the U.K. national acid deposition network, but deposition levels were below those suggested by network data because of the unusually dry summer in 1989. The rainfall chemistry data were related to locally recorded wind direction, and to back trajectories calculated with analysed wind field data from U.K. Meteorological Office numerical weather prediction models. The influence of local wind direction during rainfall was significant in terms of observed concentrations and deposition. However back trajectory analysis was a better indicator of the Lagrangian history and pollutant loading of the air masses reaching the site, with clear differences in rainfall composition noted between different transport patterns. A power-law relationship existed between wet deposition and rainfall amount, although the data exhibited considerable scatter around the functional relationship because of the influence of transport pattern. Proportional ionic composition was also influenced by transport pattern with enhanced chloride levels for maritime events. The nitrate : sulphate ratio was inversely related to the time of travel from major anthropogenic source regions.