Monitoring Studies of Precipitation and Cap Cloud Chemistry at Holme Moss in the Southern Pennines

Abstract

Weekly collections of samples of precipitation and hill cloudwater have been made at Holme Moss (530 m.a.s.l.) in the southernPennines covering a six-year period (1994–1999). In addition continuous meteorological measurements have been conducted at thesite for a five year period (1995–1999). The concentrations of major ions in the samples have been determined by ion chromatography. Analysis of ion concentrations as a function ofwind direction reveals that the ions with anthropogenic sources (SO4 −, NO3 −, NH4 +, H+) exhibit higher concentrations during easterly wind directions whilst the ions with predominantly marine origins (Na+, Cl−, Ca++, Mg++, K+) have concentrations thatare not significantly dependant on wind direction. Precipitation and cloud deposition are strongly correlated to south-westerly wind directions with a secondary peak occurring for north-easterly directions. Fifty nine per cent of ion deposition by rain was found to occur during wind from the SSW to W sector.The average concentrations of ions in cloud water were found tobe much higher than those in rain (by factors of between 2.5 and4.2). It is thought that the high precipitation and annual deposition of ions by precipitation at Holme Moss is due in partto the enrichment of ion concentrations in precipitation by thescavenging of more concentrated cap cloud droplets (the `seeder-feeder effect'). Comparison with data from a nearby lower level site shows encouraging agreement with the scheme currently used in the U.K. deposition mapping procedure to incorporate the influence of orography on deposition by precipitation.