References

ACP

Atmospheric Chemistry and Physics

ACP

Atmos. Chem. Phys.

1680-7324

Copernicus Publications

Göttingen, Germany

10.5194/acp-14-691-2014

Flow climatology for physicochemical properties of dichotomous aerosol over the western North Atlantic Ocean at Bermuda

Moody

J. L.

¹ Keene

W. C.

¹ Cooper

O. R.

² Voss

K. J.

³ Aryal

³ Eckhardt

https://orcid.org/0000-0001-6958-5375

⁴ Holben

⁵ Maben

J. R.

¹ Izaguirre

M. A.

⁶ Galloway

J. N.

Department of Environmental Sciences, University of Virginia, Charlottesville, VA, USA

Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, and NOAA Earth System Research Laboratory, Boulder, CO, USA

Physics Department, University of Miami, Miami, FL, USA

Norwegian Institute for Air Research, Kjeller, Norway

Goddard Space Flight Center, NASA, Greenbelt, MD, USA

Division of Marine and Atmospheric Chemistry, University of Miami, Miami, FL, USA

22 01 2014

14 2 691 717

2014

This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/

This article is available from https://acp.copernicus.org/articles/14/691/2014/acp-14-691-2014.html

The full text article is available as a PDF file from https://acp.copernicus.org/articles/14/691/2014/acp-14-691-2014.pdf

Dichotomous aerosols (nominal super- and sub-μm-diameter size fractions) in sectored on-shore flow were sampled daily from July 2006 through June 2009, at the Tudor Hill Atmospheric Observatory (THAO) on the western coast of Bermuda (32.27° N, 64.87° W) and analyzed for major chemical and physical properties. FLEXPART retroplumes were calculated for each sampling period and aerosol properties were stratified accordingly based on transport from different regions. Transport from the northeastern United States (NEUS) was associated with significantly higher (factors of 2 to 3 based on median values) concentrations of bulk particulate non-sea-salt (nss) SO42-, NO3-, and NH4+ and associated scattering and absorption at 530 nm, relative to transport from Africa (AFR) and the oceanic background. These differences were driven primarily by higher values associated with the sub-μm size fraction under NEUS flow. We estimate that 75(±3)% of the NEUS nss SO42- was anthropogenic in origin, while only 25(±9)% of the AFR nss SO42- was anthropogenic. Integrating over all transport patterns, the contribution of anthropogenic sulfate has dropped 14.6% from the early 1990s. Bulk scattering was highly correlated with bulk nss SO42- in all flow regimes but the corresponding regression slopes varied significantly reflecting differential contributions to total scattering by associated aerosol components. Absorption by super-μm aerosol in transport from the NEUS versus AFR was similar although the super-μm aerosol size fraction accounted for a relatively greater contribution to total absorption in AFR flow. Significantly greater absorption Ångström exponents (AAEs) for AFR flow reflects the wavelength dependence of absorption by mineral aerosols; lower AAEs for NEUS flow is consistent with the dominance of absorption by combustion-derived aerosols. Higher AOD associated with transport from both the NEUS and AFR relative to oceanic background flow results in a top of atmosphere direct radiative forcing on the order of −1.6 to −2.5 W m−2, respectively, showing these aerosols drive cooling. The dominance of transport from the NEUS on an annual basis coupled with the corresponding decreases in anthropogenic nss SO42- aerosols since the early 1990s implies that emission reductions in the US account for a decline in atmospheric cooling over the western North Atlantic Ocean during this period.

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