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

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 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% ( or ) of the mean annual Cl- wet deposition at West Point ( or ). 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.