Smoke aerosol from biomass burning in Mexico: Hygroscopic smoke optical model

The May 1998 transport of smoke from fires in Mexico and Central America into the United States is examined. We combine data from ground-based Interagency Monitoring of Protected Visual Environments aerosol chemical sampling sites with in situ airborne and Sun photometer measurements to develop a consistent picture of the transported smoke-impacted aerosol optical and chemical properties. The aerosol observed in Mexico and the southern United States is found to have a higher sulfate mass fraction, higher single-scattering albedo, and larger accumulation mode radius than biomass burning aerosols observed by similar instrumentation in South America and Africa. We postulate that the smoke-impacted aerosol in the 1998 event was more hygroscopic than that observed in the other locations, because of the higher mass fractions of sulfate, and show that a simple model of corresponding changes in aerosol water content yields agreement with the observed variations in refractive index and radii. We further show that the single-scattering albedo cannot be fully explained by hygroscopic growth alone. Modifications to the model invoking variations in aerosol light-absorbing carbon content, which are consistent with differences in observed composition among the various smoke-impacted aerosols, bring the predictions of single-scattering albedo into alignment with our observations. The model demonstrates that the particle size, single-scattering albedo, and real refractive index of smoke-impacted aerosols are not independent but vary in tandem with variations in particle hygroscopicity and with variations in black carbon content. This relationship is an important consideration in the assessment of the effects of biomass burning aerosols, particularly those subject to long-range transport, on radiative forcing and climate.