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| Titel |
Long-term trends in aerosol and precipitation composition over the western North Atlantic Ocean at Bermuda |
| VerfasserIn |
W. C. Keene, J. L. Moody, J. N. Galloway, J. M. Prospero, O. R. Cooper, S. Eckhardt, J. R. Maben |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 14, no. 15 ; Nr. 14, no. 15 (2014-08-13), S.8119-8135 |
| Datensatznummer |
250118944
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| Publikation (Nr.) |
 copernicus.org/acp-14-8119-2014.pdf |
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| Zusammenfassung |
| 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. |
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