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

J. L. Moody, W. C. Keene, O. R. Cooper, Kenneth Voss, R. Aryal, S. Eckhardt, B. Holben, J. R. Maben, M. A. Izaguirre, J. N. Galloway

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Abstract

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) SO4 2-, 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 SO4 2- was anthropogenic in origin, while only 25(±9)% of the AFR nss SO 4 2- 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 SO 4 2- 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 SO4 2- 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.

Original languageEnglish (US)
Pages (from-to)691-717
Number of pages27
JournalAtmospheric Chemistry and Physics
Volume14
Issue number2
DOIs
StatePublished - Jan 22 2014

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physicochemical property
climatology
aerosol
salt
scattering
cooling
top of atmosphere
aerosol property
radiative forcing
North Atlantic Ocean
Africa
chemical property
observatory
physical property
combustion
particle size
sulfate
wavelength
coast
sampling

ASJC Scopus subject areas

  • Atmospheric Science

Cite this

Flow climatology for physicochemical properties of dichotomous aerosol over the western North Atlantic Ocean at Bermuda. / Moody, J. L.; Keene, W. C.; Cooper, O. R.; Voss, Kenneth; Aryal, R.; Eckhardt, S.; Holben, B.; Maben, J. R.; Izaguirre, M. A.; Galloway, J. N.

In: Atmospheric Chemistry and Physics, Vol. 14, No. 2, 22.01.2014, p. 691-717.

Research output: Contribution to journalArticle

Moody, JL, Keene, WC, Cooper, OR, Voss, K, Aryal, R, Eckhardt, S, Holben, B, Maben, JR, Izaguirre, MA & Galloway, JN 2014, 'Flow climatology for physicochemical properties of dichotomous aerosol over the western North Atlantic Ocean at Bermuda', Atmospheric Chemistry and Physics, vol. 14, no. 2, pp. 691-717. https://doi.org/10.5194/acp-14-691-2014
Moody, J. L. ; Keene, W. C. ; Cooper, O. R. ; Voss, Kenneth ; Aryal, R. ; Eckhardt, S. ; Holben, B. ; Maben, J. R. ; Izaguirre, M. A. ; Galloway, J. N. / Flow climatology for physicochemical properties of dichotomous aerosol over the western North Atlantic Ocean at Bermuda. In: Atmospheric Chemistry and Physics. 2014 ; Vol. 14, No. 2. pp. 691-717.
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AU - Keene, W. C.

AU - Cooper, O. R.

AU - Voss, Kenneth

AU - Aryal, R.

AU - Eckhardt, S.

AU - Holben, B.

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N2 - 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) SO4 2-, 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 SO4 2- was anthropogenic in origin, while only 25(±9)% of the AFR nss SO 4 2- 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 SO 4 2- 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 SO4 2- 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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