Smoke particles generated by the burning of biomass consist mainly of organic aerosol, referred to as biomass burning organic aerosol (BBOA) . For example, smoke sampled from wildfires in western USA was composed of > 90% BBOA . BBOA represents the largest global contributor of organic aerosol emitted directly to the atmosphere (4), and the mass of emitted BBOA is expected to rise in the future due to increased prevalence of forest fires as a result of a warming world . BBOA influences the climate by scattering or absorbing solar radiation or by acting as nuclei of cloud droplets and possibly ice particles . It contains a large amount of organic molecules that absorb UV and visible light, known as brown carbon (BrC), which leads to a warming of the earthatmospheric system . While in the atmosphere, BrC can become oxidized by ozone or other atmospheric oxidants, eventually causing BBOA to become less absorbing with a weaker climatic impact . Despite the importance of BBOA, our understanding of the physical and chemical properties of BBOA particles is limited, leading to uncertainties in its impact on the global radiation budget.