Black carbon emission factors and size distribution from biomass burning in California

Black carbon (BC) aerosol plays an important role in global and regional climate, and has been described as the second most important radiative forcing agent. Biomass burning accounts for a significant fraction (40%) of global BC emissions, both from wildfires and human activities such as slash-and-burn deforestation, crop-residue burning and prescribed fires used for land management. Trace gas and aerosol emission factors, including BC, change with different combustion conditions and thus the amount of BC in smoke shows considerable variation. Freshly emitted smoke changes with time to become a regional haze, a process not well understood. There are large uncertainties regarding the processes through which BC becomes mixed with primary and secondary aerosol species and the extent to which mixing state affects the radiative properties of BC. Further understanding of these areas will lead to improvements in the treatment of biomass burning BC in global climate modelling and more accurate predictions of its impacts on climate and visibility. Properties of biomass burning aerosols were investigated using airborne sampling with a Single Particle Soot Photometer (SP2) (Droplet Measurement Technologies, Boulder, Colorado). During the field study, the SP2 was mounted on the US Forest Service (USFS) Twin Otter aircraft and flown through smoke plumes from prescribed fires as part of the SLOBB (San Luis Obispo Biomass Burning experiment) campaign in California in November 2009. Plumes were sampled on scales ranging from near-source to 60 km downwind. We present BC size distributions and emission factors. An analysis of particle aging behaviour combining the SP2 measurements with aerosol composition measurements made simultaneously by an Aerodyne compact Time-of-Flight Aerosol Mass Spectrometer (AMS) is also presented. Data collected from an Airborne Fourier-Transform Infrared spectrometer (AFTIR) system provided information regarding several important trace gas species emitted from fires and allowed for a better determination of plume aging processes.