PNNL

Abstract

The effect of relative humidity (RH) on the yield and chemical composition of secondary organic aerosol (SOA) formed from toluene oxidation was investigated. Toluene SOA was prepared by photooxidation in an aerosol smog chamber under either low or high NOx conditions with a range of RHs from 0% to 90%. Particle mass concentrations were measured throughout each experiment with a Scanning Mobility Particle Sizer (SMPS), while toluene concentrations were measured with a Proton Transfer Reaction - Time of Flight Mass Spectrometer (PTR-TOF-MS). These concentrations were used to calculate the aerosol yield for each sample. The yield decreased by an order of magnitude when RH increased from 0% to 75-90% for low NOx toluene SOA. In contrast, the yield decreased much less with an increase of RH for high NOx toluene SOA. Low-NOx SOA samples prepared at 0% RH and 75% RH were collected on Teflon filters and analyzed with nanospray desorption electrospray ionization high-resolution mass spectrometry (nano-DESI-HRMS) to investigate the effect of RH on the molecular composition. Measurements revealed a significant reduction in the fraction of oligomers present in the SOA generated at 75% RH compared to SOA generated under dry conditions. These results suggest that the presence of water vapor in the smog chamber suppresses oligomer formation in low NOx toluene SOA and reduces the aerosol yield.