Why Is the Copious Aerosol Observed in the Boundary Layer in June at Ascension Island So Highly Absorbing of Sunlight?

AMIE DOBRACKI, Paquita Zuidema, Arthur J. Sedlacek, Maria Zawadowicz, University of Miami

     Abstract Number: 371
     Working Group: Remote and Regional Atmospheric Aerosol

Abstract
Ascension Island, located at the northwest corner of the southeast Atlantic Ocean stratocumulus deck (-7.95° N, -14.36° E), receives biomass burning aerosol at the surface during much of the year despite its remote location. Zuidema et al., 2018 use in situ data from the DOE’s Layered Atlantic Smoke Interactions with Clouds (LASIC) campaign to show that the month of June has the highest absorption per unit mass of black carbon, the lowest single scattering albedo, the highest absorption angstrom exponent, and the highest hygroscopicity, of the June-October biomass-burning season. This occurs despite the highest smoke loadings being measured in August, not June, at Ascension Island. Here we further investigate the chemical and physical properties alongside the optical properties of biomass burning aerosol during June 2017, towards explaining this annual phenomenon. Data from an aerosol chemical speciation monitor indicates that the chemical composition of the smoky time periods in the boundary layer contain only slightly more black carbon than in August (12% versus 11%), but that overall this is approximately twice the fractional percentage of free-tropospheric smoke plumes. Fractionally more sulfates and less organic aerosol are measured than in August-September. The meteorology is similar between smoky and clean time periods in June, and this is exploited to conclude that most of the sulfates during the smoky time periods are associated with biomass-burning aerosol, not sea spray. We hypothesize the properties of the June smoke aerosol reflects black carbon that is more thickly coated than in other months, and will use this presentation to assess this hypothesis, and counterhypotheses based on variations in fire source and transport pathways to Ascension.