Effects of Relative Humidity on Secondary Organic Aerosol and Brown Carbon Formation from Nighttime Oxidation of Furan and Pyrrole

KUNPENG CHEN, Caitlin Hamilton, Bradley Ries, Michael Lum, Raphael Mayorga, Linhui Tian, Roya Bahreini, Haofei Zhang, Ying-Hsuan Lin, University of California, Riverside

     Abstract Number: 155
     Working Group: Aerosol Chemistry

Abstract
Wildfires are an important source of volatile organic compound precursors for secondary organic aerosol (SOA) formation, and the resulting light-absorbing SOA (also known as secondary brown carbon, BrC) has the potential to perturb the Earth’s energy budget and contribute to climate change. The chemical composition and optical properties of SOA are significantly influenced by environmental factors such as pre-existing particles and relative humidity (RH). Specifically, an increase in RH may change gas-phase reactions or introduce aerosol liquid water (ALW) on the pre-existing particles, but the effects of RH on SOA composition and secondary BrC’s light absorption are not fully constrained in current climate models. An in-depth understanding of the correlation between environmental factors and secondary BrC’s light absorption is hence required for more accurate parameterization. Here, we conducted chamber experiments with a suite of online and offline instruments to examine the influence of RH (<20% and ~50%) on SOA and secondary BrC formation from nighttime oxidation of furan and pyrrole, two typical heterocyclic VOCs emitted in wildfires. Our results showed that an increased RH can facilitate particulate growth in furan SOA while inducing new particle formation with impeded particulate growth in pyrrole SOA. Yet, the presence of RH decreased the mass absorption coefficients (MAC) of both BrC systems. A detailed compositional analysis revealed that RH can increase the fraction of oxygenated products in both SOA systems, in which the non-absorbing, high-molecular-weight products may account for the decreased MAC. Furthermore, hydrolysis driven by ALW might have depleted molecular chromophores and contributed to the change in MAC profiles. Overall, this study demonstrated that RH may be important in modulating SOA composition and the optical properties of secondary BrC in the wildfire plumes.