Hygroscopicity and Cloud Condensation Nuclei Activity of Fresh and Aged Biomass Burning Particles

Biomass burning (BB) is a major source of atmospheric particles and trace gases, influencing climate change, air quality, and human health. During the Georgia Wildland-Fire Simulation Experiment, we measured the hygroscopicity (κ) and size-resolved cloud condensation nuclei (CCN) activity of BB particles from controlled burns of fuel beds representative of three ecoregions in Georgia, United States. Primary BB particles were predominantly organic, and photooxidation in an oxidation flow reactor produced secondary organic aerosol (SOA) in a new nucleation mode while transforming primary organic aerosol (POA) into oxidized POA (OPOA) in the pre-existing accumulation mode. We measured hygroscopic growth from 20% to 90% relative humidity using a quartz crystal microbalance and assessed size-resolved CCN activity for particles from 30 to 350 nm at supersaturation between 0.13% and 0.99%. We found that the hygroscopicity parameter of OPOA (κ_OPOA = 0.10-0.19) was higher than that of POA (0.04-0.10), reflecting the influence of heterogeneous oxidation, whereas the hygroscopicity parameter of SOA (κ_SOA = 0.07-0.14) fell between the two. Both fresh and aged BB particles displayed size-dependent κ values and evidence of external mixing, likely because of complex emission characteristics of fuel beds and size-dependent deposition processes. Growth factor-derived and CCN-derived κ values were consistent when accounting for such heterogeneity. A strong positive correlation was found between the mass-averaged κ and O/C ratio, described by the regression κ = 0.31 ± 0.02-(O/C) - 0.05 ± 0.02, which broadly agrees with previous findings for a wide range of laboratory SOA and ambient oxidized organic aerosols. This suggests the potential applicability of a generalized hygroscopicity parameterization across organic aerosols within acceptable uncertainty. Our results highlight the role of BB particles as significant CCN sources during atmospheric aging and emphasize the importance of heterogeneous oxidation in physicochemical evolution of BB particles.