Trends in NADP bromide wet deposition concentrations, 2001-2016

Gregory Wetherbee1, Christopher M.B. Lehmann, Ph.D.2, Lee A. Green3, Mark F. Rhodes4, Brian M. Kerschner5 and Amy S. Ludtke6

Bromide (Br-) and other solute concentration data from wet deposition samples collected by the National Atmospheric Deposition Program (NADP) from 2001 to 2016 (study period), were analyzed for statistical trends both geographically and temporally and by precipitation type.  Bromide was more frequently detected at NADP sites in coastal regions of the continental United States. The Br- concentration data set was characterized by greater than 86 percent of sample concentrations below analytical detection.  Analysis revealed that Br- concentrations were higher in rain (only) versus precipitation containing snow.  Decreasing Br- wet deposition concentrations were observed at a majority of NADP sites over the study period; approximately 25 percent of the trend slopes were statistically significant at the alpha=0.10 level.  Correlations between Br-, Cl-, and NO3- concentrations were evaluated to investigate sea-salt as a Br- source and potential Br- oxidation of atmospheric nitrite (NO2(g)) to NO3-(aq).

Potential causes for decreasing Br- concentrations 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.  Trend analyses indicated that precipitation depth generally increased during the study period at the majority of NADP sites, which diluted Br- below analytical detection in many samples.  Declining Br- concentrations also are consistent with declining industrial CH3Br emissions mandated by the Montreal Protocol. Chemical processes that deplete Br- from sea-salt aerosols and inundation of saltmarsh habitat suggest changing marine conditions were identified as possible influences on temporal trends in Br- concentrations.  There are no known ecological consequences of changing Br- concentrations in wet deposition at this time, but the trends may help to explain the chemistry of pollutants like CH3Br and the changing chemical climate of the atmosphere.


1U.S. Geological Survey,
2Univ. Illinois, Prairie Research Institute, ISWS,
3Univ. Illinois, Prairie Research Institute, ISWS,
4Univ. Illinois, Prairie Research Institute, ISWS,
5Univ. Illinois, Prairie Research Institute, ISWS,
6U.S. Geological Survey,