Effects of Relative Humidity on Time-Resolved Molecular Characterization of Secondary Organic Aerosols from the OH-Initiated Oxidation of Cresol in the Presence of NOx.

While biomass burning (BB) is the largest source of fine particles in the atmosphere, the influence of relative humidity (RH) and photochemistry on BB secondary organic aerosol (BB-SOA) formation and aging remains poorly constrained. These effects need to be addressed to better capture and comprehend the evolution of BB-SOA in the atmosphere. Cresol (C7H8O) is used as a BB proxy to investigate these effects. It is emitted directly from BB and has been identified as a significant SOA precursor from residential wood-burning emissions. The gas- and particle-phase signal intensities are investigated using online mass spectrometers. An increase in the SOA mass yield of 7% is observed when the RH rises from 0.5-20 to 70-87%. At elevated RH, a significant increase in the formation of nitrogen-containing compounds is observed due to particle-phase processes. This is linked to a net decrease in the SOA viscosity, enabling these compounds to be formed to a greater extent at elevated RH in the presence of nitrogen oxides. These results highlight the importance of the particle water content for the molecular formation and aging of BB-SOA compounds.