Contrasting Ambient Brown Carbon from Primary Emissions and Secondary Production

Measurements of particulate light absorption in wintertime Fresno, CA indicate a substantial contribution of light absorbing organic carbon (aka brown carbon) to the total ambient absorption. Two key brown carbon sources are identified and characterized in terms of their spectral absorption properties: residential combustion of wood and secondary production, most likely related to fog processing. The two brown carbon sources exhibit distinct diurnal profiles, indicating differences in their local and regional climate impacts. The derived wavelength-dependent mass absorption coefficients indicate that the primary wood-combustion derived brown carbon is more absorbing than the secondary brown carbon, by about a factor of two, while the secondary has a stronger wavelength dependence. The ambient observations will be contrasted with results from controlled laboratory burns of various biofuels from the FIREX study. This study provides constraints on the atmospheric sources and properties of brown carbon and their contribution to total absorption relative to black carbon.