Abstract
Abstract. Since the 1980s, emissions of SO2 and NOx (NO + NO2) from anthropogenic sources in the United States (US), Canada, and Europe have decreased significantly suggesting that the export of oxidized S and N compounds from surrounding continents to the atmosphere overlying the North Atlantic Ocean (NAO) has also decreased. The chemical compositions of aerosols and precipitation sampled daily on Bermuda (32.27° N, 64.87° W) from 1989 to 1997 and from 2006 to 2009 were evaluated to quantify the magnitudes, significance, and implications of associated tends in atmospheric composition. The chemical data were stratified based on FLEXPART (FLEXible PARTicle dispersion model) retroplumes into four discrete transport regimes: westerly flow from eastern North America (NEUS/SEUS); easterly trade-wind flow from northern Africa and the subtropical NAO (Africa); long, open-ocean, anticyclonic flow around the Bermuda High (Oceanic); and transitional flow from the relatively clean open ocean to the polluted eastern North America (North). Based on all data, annual average concentrations of non-sea-salt (nss) SO42– associated with aerosols and annual volume-weighted-average (VWA) concentrations in precipitation decreased significantly (by 22% and 49%, respectively) whereas annual VWA concentrations of NH4+ in precipitation increased significantly (by 70%). Corresponding trends in aerosol and precipitation NO3– and of aerosol NH4+ were insignificant. Nss SO42– in precipitation under NEUS/SEUS and Oceanic flow decreased significantly (61% each) whereas corresponding trends in particulate nss SO42– under both flow regimes were insignificant. Trends in precipitation composition were driven in part by decreasing emissions of SO2 over upwind continents and associated decreases in anthropogenic contributions to nss SO42– concentrations. Under NEUS/SEUS and Oceanic flow, the ratio of anthropogenic to biogenic contributions to nss SO42– in the column scavenged by precipitation were relatively greater than those in near surface aerosol, which implies that, for these flow regimes, precipitation is a better indicator of overall anthropogenic impacts on the lower troposphere. Particulate nss SO42– under African flow also decreased significantly (34%) whereas the corresponding decrease in nss SO42– associated with precipitation was insignificant. We infer that these trends were driven in part by reductions in the emissions and transport of oxidized S compounds from Europe. The lack of significant trends in NO3– associated with aerosols and precipitation under NEUS/SEUS flow is notable in light of the large decrease (37%) in NOx emissions in the US and Canada over the period of record. Rapid chemical processing of oxidized N in marine air contributed to this lack of correspondence. Decreasing ratios of nss SO42– to NH4+ and the significant decreasing trend in precipitation acidity (37%) indicate that the total amount of acidity in the multiphase gas–aerosol system in the western NAO troposphere decreased over the period of record. Decreasing aerosol acidities would have shifted the phase partitioning of total NH3 (NH3 + particulate NH4+ towards the gas phase thereby decreasing the atmospheric lifetime of total NH3 against wet plus dry deposition. The trend of increasing NH4+ in precipitation at Bermuda over the period of record suggests that NH3 emissions from surrounding continents also increased. Decreasing particulate nss SO42– in near-surface air under NEUS/SEUS flow over the period of record implies that the corresponding shortwave scattering and absorption by nss S and associated aerosols constituents also decreased. These changes in radiative transfer suggest a corresponding lower limit for net warming over the period in the range of 0.1–0.3 W m–2.
Highlights
It has been recognized for many years that material emitted to the atmosphere from continental sources can be transported long distances over adjacent oceans
Under northeastern United States (NEUS)/southeastern US (SEUS) and Oceanic flow, the ratio of anthropogenic to biogenic contributions to nss SO24− in the column scavenged by precipitation were relatively greater than those in near surface aerosol, which implies that, for these flow regimes, precipitation is a better indicator of overall anthropogenic impacts on the lower troposphere
Trends in CH3SO−3 for both aerosols and precipitation were insignificant, which indicates that long-term temporal variability in nss SO24− associated with both aerosols and precipitation was driven primarily by differential variability in anthropogenic contributions. These results indicate that relative contributions of anthropogenic sources to VWA nss SO24− in precipitation at Bermuda have decreased more rapidly that those to mean nss SO24− associated with aerosols (Fig. 4)
Summary
It has been recognized for many years that material emitted to the atmosphere from continental sources can be transported long distances over adjacent oceans. Subsequent, studies have attempted to quantitatively differentiate relative contributions from major source types (continental versus marine; anthropogenic versus biogenic versus crustal) and major source regions based on analysis of elemental and molecular tracers (e.g., Arimoto et al, 1995; Savioe et al, 2002), radionuclides (e.g., Arimoto et al, 1999), isotopic composition (e.g, Turekian et al, 2001, 2003; Hastings et al, 2003; Lin et al, 2012), air-mass transport history (e.g., Moody and Galloway, 1988; Galloway et al, 1989; Moody et al, 1995, 2014), and satellite remote sensing (e.g., Zhao et al, 2008; Zhang and Reid, 2010; Hsu et al, 2012; Prospero et al, 2012) These and other studies indicate that the transport of anthropogenic emissions and reaction products from North America is the dominant source of pollutants associated with aerosols and precipitation over the western NAO at Bermuda. Anthropogenic emissions from surrounding continents have changed significantly over the past quarter century suggesting that relative impacts of anthropogenic sources to air quality over the western NAO have changed
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