Secondary organic aerosol (SOA) is a large fraction of particulate matter (PM) worldwide and remains a large source of uncertainty in global models that aim to predict the radiative forcing contribution of PM. The properties of SOA are susceptible to change during transport in the atmosphere in response to different environmental conditions, such as sunlight, temperature, relative humidity (RH), and interaction with other atmospheric species. My research investigated the effects on the physical properties and chemical composition of SOA resulting from the exposure of SOA to common environmental species, such as ammonia, amines, and water vapor mixing ratios.
My first project examined the effect of RH on the chemical composition of SOA formed from low-NOx toluene oxidation. We found that the particle mass loading decreased by nearly an order of magnitude when RH increased from < 2 to 75–90 % for low-NOx toluene SOA, but this effect was much smaller for high-NOx toluene SOA. Mass spectrometry revealed a significant reduction in the fraction of oligomers present in the SOA generated at 75 % RH compared to SOA generated under dry conditions in the low-NOx toluene SOA. The observed increase in the oligomer fraction and particle mass loading under dry conditions were attributed to the enhancement of condensation reactions which produce water and oligomers from smaller compounds in low-NOx toluene SOA. These results indicate the mass yield of toluene SOA in the atmosphere depends on NOx concentrations and RH.
Another focus of my research was to explore the reactive uptake of reduced nitrogen compounds by SOA. Currently, inorganic nitrogen contribution to PM is represented in air quality models, but the contribution of nitrogen organic compounds (NOC) to PM is missing. We found that the reactive uptake of NH3 or dimethylamine (DMA) by low-NOx SOA (toluene, cedrene, or limonene) did not change SOA particle mass but did change particle composition and color due to the formation of NOC. Air quality model simulations showed that inclusion of this new chemistry significantly reduces gas-phase NH3 and can therefore affect particle pH and reduce the formation of inorganic PM.
An additional study exploring the formation of NOC in SOA consisted of generating SOA from a N-containing biogenic precursor (indole). This study found that indole oxidation by OH, O3 or NO3 efficiently produces brown SOA. Overall, this PhD work highlights the effects that RH can have on SOA formation, as well as the novel chemistry and important properties of NOC in the atmospheric environment.