Increased UV–Visible Particle Absorption via Evaporation and Drying of Aqueous Droplets Containing Catechol/HONO Solutions

The role of cloud droplet evaporation in brown carbon (BrC) formation is poorly established with previous studies focusing on carbonyl/(NH₄)₂SO₄ solutions and secondary organic aerosols (SOA). Here, by mixing dry air into an aerosol flow reactor, we examine whether droplet evaporation may affect wildfire BrC by studying the aqueous nitration of catechol by HONO and darkening of the soluble component of wood smoke. Using online aethalometry and offline UV–vis analysis of particle filter extracts, we observe for the catechol/HONO system that droplet evaporation leads to significantly increased mass absorption coefficients and enhanced ratios of visible to UV absorption compared to bulk solutions, consistent with offline mass spectrometric analysis of dried particles that indicates aromatic nitration and oligomer formation. Differences between the aethalometer and UV–vis filter measurements are attributed to reactions on the dry filter that enhance BrC formation. Evaporation of wood smoke extract droplets also leads to darkening reactions but much less significantly than with catechol/HONO, as expected if only a select fraction of the wood smoke molecules react via concentration enhancement. These findings underscore droplet evaporation/drying as important to BrC atmospheric evolution and suggest that dryer usage may affect particle composition in field measurements.