Abstract
Abstract. Actinic flux, as well as aerosol chemical and optical properties, were measured aboard the NASA DC-8 aircraft during the ARCTAS (Arctic Research of the Composition of the Troposphere from Aircraft and Satellites) mission in Spring and Summer 2008. These measurements were used in a radiative transfer code to retrieve spectral (350–550 nm) aerosol single scattering albedo (SSA) for biomass burning plumes encountered on 17 April and 29 June. Retrieved SSA values were subsequently used to calculate the absorption Angstrom exponent (AAE) over the 350–500 nm range. Both plumes exhibited enhanced spectral absorption with AAE values that exceeded 1 (6.78 ± 0.38 for 17 April and 3.34 ± 0.11 for 29 June). This enhanced absorption was primarily due to organic aerosol (OA) which contributed significantly to total absorption at all wavelengths for both 17 April (57.7%) and 29 June (56.2%). OA contributions to absorption were greater at UV wavelengths than at visible wavelengths for both cases. Differences in AAE values between the two cases were attributed to differences in plume age and thus to differences in the ratio of OA and black carbon (BC) concentrations. However, notable differences between AAE values calculated for the OA (AAEOA) for 17 April (11.15 ± 0.59) and 29 June (4.94 ± 0.19) suggested differences in the plume AAE values might also be due to differences in organic aerosol composition. The 17 April OA was much more oxidized than the 29 June OA as denoted by a higher oxidation state value for 17 April (+0.16 vs. −0.32). Differences in the AAEOA, as well as the overall AAE, were thus also possibly due to oxidation of biomass burning primary organic aerosol in the 17 April plume that resulted in the formation of OA with a greater spectral-dependence of absorption.
Highlights
Aerosols directly scatter and absorb visible (400 nm ≤ λ ≤ 700 nm) solar radiation thereby impacting climate (e.g. Forster et al, 2007) as well as visibility (e.g. McMeeking et al, 2006)
We focus on biomass burning plumes encountered during Aircraft and Satellites (ARCTAS) as biomass burning represents one of the largest sources of organic aerosol (OA) as well as black carbon (BC) (Bond et al, 2004)
single scattering albedo (SSA) retrievals were performed using actinic flux as well as aerosol optical properties measured aboard the NASA DC-8 aircraft during the two main phases of ARCTAS: ARCTAS-A (April 2008) and ARCTAS-B (June–July 2008)
Summary
Aerosols directly scatter and absorb visible (400 nm ≤ λ ≤ 700 nm) solar radiation thereby impacting climate (e.g. Forster et al, 2007) as well as visibility (e.g. McMeeking et al, 2006). Aerosols directly scatter and absorb visible (400 nm ≤ λ ≤ 700 nm) solar radiation thereby impacting climate Aerosol scattering and absorption can significantly impact ultraviolet (UV) radiation (λ < 400 nm) and photochemistry (Dickerson et al, 1997; He and Carmichael, 1999; Lefer et al, 2003; Flynn et al, 2010; Li et al, 2011). Absorbing aerosols act to warm the atmosphere, which reduces the magnitude of the global net cooling due to aerosol scattering of incoming radiation (Forster et al, 2007). Absorbing aerosols impact photochemistry differently than scattering aerosol resulting in a notably larger decrease in photochemical smog. Corr et al.: Spectral absorption of biomass burning aerosol formation compared to dominantly scattering aerosol (e.g. He and Carmichael, 1999)
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