Effect of biomass burning organic aerosol on the structure of black carbon at the wildland-urban interface varies with evaporative aging

Biomass burning organic aerosol (BBOA) and black carbon (BC) play significant roles in climate, via their interactions with light and clouds, as well as public health. BC in the urban atmosphere, largely emitted by diesel vehicles, is initially highly branched and fractal, but it can be restructured upon coating. Urban BC aggregates can be exposed to the components of BBOA when wildfire plumes cross the wildland-urban interface (WUI), which may partition to coat the BC. Here, we investigate the effects of laboratory-generated BBOA from pine on BC from an inverted diffusion burner. Initially 200 nm BC aggregates were selected using a differential mobility analyzer (DMA) and exposed to different volatility fractions of BBOA. These fractions were prepared by evaporative aging in the laboratory, which mimics evaporative aging with transport in the atmosphere, to give fresh, moderately aged, and most aged BBOA. The resulting mobility diameters with and without removal of the coating were determined using a second DMA. We find that an exceedingly small coating volume of fresh (i.e., higher volatility) BBOA causes complete restructuring while a significantly higher coating volume of most aged (i.e., lower volatility) BBOA is necessary for this same extent of restructuring. Our results highlight the importance of evaporative age of BBOA as wildfire plumes are transported across the WUI into urban environments.