Trends in bromide wet deposition concentrations in the contiguous United States, 2001-2016

摘要

AbstractBromide (Br−) and other solute concentration data from wet deposition samples collected and analyzed by the National Atmospheric Deposition Program (NADP) from 2001 to 2016, were statistically analyzed for trends both geographically and temporally by precipitation type. Analysis was limited to NADP sites in the contiguous 48 United States. The Br−concentrations for this time period had a high number of values censored at the detection limits with greater than 86 percent of sample concentrations below analytical detection. Bromide was more frequently detected at NADP sites in coastal regions. Analysis using specialized statistical techniques for censored data revealed that Br−concentrations varied by precipitation type with higher concentrations usually observed in liquid versus precipitation containing snow. Negative temporal trends in Br−wet deposition concentrations were observed at a majority of NADP sites; approximately 25 percent of these trend values were statistically significant at less than 0.05 to 0.10 significance levels. Potential causes for the negative trends were explored, including annual and seasonal changes in precipitation depth, reduced emissions of methyl bromide (CH3Br) from coastal wetlands, and declining industrial use of bromine compounds. The results indicate that Br−in non-coastal wet-deposition comes mainly from long-range transport, not local sources. Correlations between Br−, chloride, and nitrate concentrations also were evaluated.Graphical abstractDisplay OmittedHighlights•Trends in bromide concentrations in U.S. atmospheric wet deposition generally are decreasing.•Trends in precipitation depth indicate dilution of bromide in atmospheric wet deposition.•Decreasing wet deposition bromide is consistent with declining methyl bromide emissions.Trends in bromide atmospheric wet deposition in the contiguous 48 U.S. for 2001–2016 reflect changes in brominated hydrocarbon emissions and precipitation depth.