PNNL

Abstract

Observations from June through October, 2016, from a surface-based ARM Mobile Facility deployment on Ascension Island (8? S, 14.5? W) indicate that refractory black carbon (rBC) is almost always present within the boundary layer. rBC mass concentrations, light absorption coe?cients, and cloud condensation nuclei concentrations vary in concert and synoptically, peaking in August. Derived mass absorption cross-sections using light absorptioin coe?cients at three wavelengths as a function of rBC mass indirectly indicate the presence of other light-absorbing organic aerosols (e.g., brown car-bon), most pronounced in June. A ?lter-based estimate of single-scattering-albedo increases systematically from August to October, also apparent in 2017. Boundary-layer aerosol loadings are only loosely correlated with total aerosol optical depth, with smoke more likely to be present in the boundary layer earlier in the biomass-burning season, evolving to smoke predominantly present in the free-troposphere in September-October, typically resting upon the cloud-top inversion. The time period with the campaign-maximum near-surface light absorption and column aerosol optical depth, on 13-16 August of 2016, is investigated further. Backtrajectories indicate the boundary layer transport was directly westward from the African continent, which is unusual in August.