Aerosol Absorption from Biomass Burning in the Near and Far Fields: Model Implications (Invited Presentation)

Aerosols emitted from open biomass burning (BB), including wildfires and agricultural burns, are recognized to perturb Earth’s climate through the direct effect (both scattering and absorption of incoming shortwave radiation), the semi-direct effect (evaporation of cloud drops due to absorbing aerosols), and indirect effects (by influencing cloud formation and precipitation). The overall effect of BB emissions on the atmospheric radiation balance, either forcing the atmosphere to heat or cool, depends on the abundance, cloud forming activity, and refractive index (specifically the absorption) of emitted primary particles and secondary aerosol species. Recently, Feng et al., (2013) have highlighted the radiative forcing impact of absorbing organic compounds (“brown” carbon or BrC) through a series of Global Climate Model (GCM) calculations and report that in some locations BrC radiative forcing could be of order +0.25 W/m². BB emission inputs for such model calculations are based upon laboratory experiments and measurements conducted in the field, though the latter are limited. Often these measurements are on fresh (i.e., recently emitted) aerosols, which are dominated by primary particle emissions. A growing body of experimental evidence is beginning to indicate that aerosols generated by BB undergo rapid changes in their chemical, microphysical, and optical properties downwind of the fires. Such rapid change suggests caution in the use of near-field aerosol properties in far field radiative forcing calculations. In this talk, comparisons of laboratory studies together with observations taken in the near/far field will be presented that illustrate the impacts of atmospheric processing on aerosol absorption. The potential implications of such processing on far field radiative forcing calculations will be discussed.

Feng, Y., V. Ramanathan and V. R. Kotamarthi (2013) Brown carbon: a significant atmospheric absorber of solar radiation? Atmos. Chem. Phys., 13, 8607–8621