Aging of Refractory Black Carbon in Power Plant Plumes by Sulfate-Driven Chemistry during the Southeast Nexus (SENEX) study

Abstract:

Refractory black carbon (rBC) aerosol is mostly emitted into the atmosphere from biomass burning and incomplete combustion of fossil and biofuel. Emissions of rBC are strongly linked to anthropogenic activity, with potential for short-term mitigation strategies. rBC aerosol is currently the second largest anthropogenic climate forcing agent after CO_2(g) with the total radiative forcing of 0.64 W m^-2. Its climate impacts, which depend in part on vertical distribution and internal mixing state, are not fully understood. Internal mixing of rBC with sulfate was previously predicted by the aerosol-climate model, ECHAM5-HAM, to increase the aerosol absorption optical depth by 10 – 30 % despite decreasing the atmospheric burden of rBC by 10 – 20 % in the Southeastern U.S. The measurements of rBC from Single Particle Soot Photometer (SP2) instruments onboard the NOAA WP-3D research aircraft during the SENEX study constrain chemical aging of rBC by sulfate in coal-fired power plant plumes, and hence, constrain the model predictions. The number fraction of rBC-containing particles that can be identified as coated (CF) and the estimated coating thickness (CT) of non-rBC material internally mixed in them were in many cases higher (1 – 5 %, 1 - 12 nm) in power plant plume intercepts downwind of the power plants than in the background air. However, there was a lack of a clear pattern of increase in CT and CF with an age of a power plant plume, and the respective growth rates were only 3.5 ± 0.8 nm hr^-1 and 1.1 ± 0.5 % hr^-1 at maxima. This is likely the consequence of the condensation of sulfate on non-rBC particles, which are present in higher concentrations in the power plant plumes. The impacts of the observed CT and CF growth rates on the lifetime and radiative properties of rBC in the Southeast U.S. will be further discussed and compared to the model predictions.