Precipitation chloride at West Point, NY: Seasonal patterns and possible contributions from non-seawater sources

Abstract

Chloride derived from the atmosphere can be a valuable tracer in ecosystem and watershed processes. For these purposes and other environmental studies, it is important to establish temporal patterns and sources for Cl - in wet deposition. Weekly composite precipitation samples have been analyzed by the National Atmospheric Deposition Program/National Trends Network (NADP/NTN) at West Point, NY during 1981–2003, although systematic contamination of precipitation Na + significantly perturbed data for Na + prior to 1998. Chloride and sodium ion seasonal wet deposition were highest in winter and lowest in summer through most of the record, probably as a result of more frequent marine-trajectory storms in winter. During 1998–2003, the period of highest quality Na + data, the ratio of [ Cl - ] / [ Na + ] was significantly higher than average in summer and lower in winter. Higher summer [ Cl - ] / [ Na + ] occurred consistently throughout the record, often reaching values four times the seawater ratio. Based on the ratio of [ Cl - ] / [ Na + ] in seawater ( 1.16 ) ∼ 16 % of annual wet deposition of Cl - during 1998–2003 was in excess of that for surface seawater. Additionally, a minor terrestrial dust Na + component was approximated, which had the net effect of increasing annual excess Cl - wet deposition to ∼ 22 % ( 2.56 mEq m - 2 or 0.90 kg ha - 1 ) of the mean annual Cl - wet deposition at West Point ( 11.9 mEq m - 2 or 4.2 kg ha - 1 ). Consistent with plausible sources of non-seawater Cl - , we attribute excess Cl - wet deposition to HCl emission from coal fired generating stations, HCl emissions from domestic and industrial waste incineration and to HCl formation in the regional atmosphere from reactions of sea-salt aerosols with S and N acidic gases.