The role of relative humidity for the formation of oxidized shells on aged wildfire particles

Wildfire smoke strongly affects air quality, human health, climate, and the Earth system. During atmospheric aging, wildfire aerosol particles undergo complex chemical and microphysical transformations that modify their optical properties, radiative effects, and cloud-forming ability. Of particular interest are organic surface coatings, which can enhance light absorption through lensing effects and increase particle hygroscopicity.Here, we present single-particle mass spectrometry measurements from a boreal forest wildfire smoke experiment, resolving the coexistence of hydrophilic compounds and hydrophobic polycyclic aromatic hydrocarbons (brown carbon) within individual particles. We show that glyoxal and methylglyoxal are directly emitted during combustion, contributing to the initial hygroscopicity of freshly emitted particles. During photochemical aging, rapid oxalate formation is observed, accompanied by a moderate increase in hygroscopicity, while PAH signals decrease on a slower timescale. The decay rates of individual PAHs are similar but show a clear dependence on relative humidity, indicating that PAH degradation is controlled by viscosity-dependent radical diffusion into the particles. In contrast, highly oxidized products form on much shorter timescales, suggesting that these reactions are largely confined to the particle surface. At elevated relative humidity, surface oxidation continues, whereas it rapidly ceases under dry conditions. These observations highlight the central role of relative humidity in controlling the microphysical properties, optical effects, and cloud activation potential of aged wildfire smoke.