Influences on long-term trends in ion concentration and deposition at Holme Moss

Long-term monitoring of rain and cloud water has been carried out at a high level site within the urban plume downwind of Manchester in north-west England. Samples have been taken weekly at Holme Moss. The site experiences significant orographic cloud cover, resulting in enhancement of rainfall volume via the seeder-feeder process. Samples are analysed for major ions, with the results being interpreted in the light of the effects of the seeder-feeder enhancement and reductions in UK emissions of pollutants. Rainfall volume enhancement is dependent on the spatial and temporal extent of orographic cloud at the site, with resultant consequences on ionic strength in the samples. Large rainfall amounts are associated with longer cloud duration and lower cloud base. Rain passing through the cloud scavenges proportionately more larger droplets, resulting in relatively lower ionic concentration. Coupled with increased washout upwind, this limits the amount of deposition, with some ions showing a decrease in deposition at high rainfall amounts. Clear downward trends are seen in sulphate concentration in rain and cloud water, with both emissions reduction and the dilution effect of increasing rainfall being responsible. Less significant downward trends are observed for nitrate, although this is in line with national patterns of deposition. Deposition at Holme Moss for all ions is dominated by the prevailing, predominantly maritime, southwesterly airflow, although for non-marine ions there are significant contributions from the industrial areas of Yorkshire to the east, and to a lesser extent the Birmingham conurbation to the south.