Emission factors and optical properties of black and brown carbon emitted at a mixed-conifer forest prescribed burn

Prescribed burning is a fuel management practice employed globally that emits carbonaceous aerosols that affect human health and perturb the global climate system. Fuel-based black and brown carbon (BC and BrC) emission factors were calculated from ground and aloft smoke during prescribed burns at a mixed-conifer, montane forest site in the Sierra Nevada in California. BC emission factors were 0.52 ± 0.42 and 1.0 ± 0.48 g kg^-1 for the smoldering and flaming combustion phases. Modified combustion efficiency is a poor predictor of BC emission factor, in this study and published literature. We discuss limitations of using generalized BC to PM_2.5 mass emission ratios to generate emission inventories; using BC emission factors measured in this study, we recommend BC to PM_2.5 ratios of 0.7 % and 9.5 % for the smoldering and flaming combustion in mixed conifer prescribed burns. We apportioned the measured aerosol spectral absorption between BrC and BC and calculated absorption Ångström exponents (AAE) of 6.26 and 0.67, respectively. Using a BrC-specific absorption cross-section, we estimated BC concentrations and a smoldering combustion BrC emission factor of 7.0 ± 2.7 g kg^-1, nearly 14 and 7 times greater than the smoldering and flaming BC emission factors. Furthermore, we estimate that BrC would account for 23 % and 82 %, respectively, of the solar radiation absorbed by the smoldering smoke in the atmosphere integrated over the solar spectrum (300–2500 nm) and in the UV spectrum (300–400 nm), indicating that BrC affects tropospheric photochemistry in addition to atmospheric warming.