Brown carbon aerosol consists of light-absorbing organic particulate matter with
wavelength-dependent absorption. Aerosol optical extinction, absorption, size distributions, and chemical
composition were measured in rural Alabama during summer 2013. The field site was well located to examine
sources of brown carbon aerosol, with influence by high biogenic organic aerosol concentrations, pollution
from two nearby cities, and biomass burning aerosol. We report the optical closure between measured dry
aerosol extinction at 365nm and calculated extinction from composition and size distribution, showing
agreement within experiment uncertainties. We find that aerosol optical extinction is dominated by
scattering, with single-scattering albedo values of 0.94 ± 0.02. Black carbon aerosol accounts for 91 ± 9%
of the total carbonaceous aerosol absorption at 365 nm, while organic aerosol accounts for 9 ± 9%. The
majority of brown carbon aerosol mass is associated with biomass burning, with smaller contributions from
biogenically derived secondary organic aerosol.