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

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 SO₄^2- 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 SO₄^2- 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.