Molecular characterization of the formation and aging of biomass burning-derived organic aerosols

Secondary organic aerosols (SOA) have significant effects on visibility1, human health2, and climate3. They are formed from the oxidation of volatile organic compounds (VOCs) in the atmosphere, leading to less volatile oxidation products that can subsequently partition into, or react with existing, aerosol particles.4-5 Biomass burning (BB) is estimated to be the second-largest source of VOCs and the largest source of fine OA globally.6 Extreme fires have been estimated to increase by 30% by 2050, which will greatly increase the concentration of BB VOCs and OA in the atmosphere. While photochemistry and humidity are known to influence SOA formation and aging,7–10 their impacts on BB-SOA remain poorly constrained and should be addressed to better capture the evolution of BB-SOA in the atmosphere. In this work, an oxygenated aromatic BB-marker, i.e., o-cresol (C7H8O), and two types of fuels (South African grass and chaparral from California) were used to study the chemical processes leading to the formation and aging of BB-SOA. The experiments were conducted in simulation chambers at PSI and LISA, respectively. Various oxidants (OH, O3, NO3) and humidity levels were used for these experiments, to explore gas- and particle-oxidation processes. A fast-switching chemical-ionization Orbitrap mass spectrometer, and a Vocus proton-transfer-reaction mass spectrometer were used to characterize gaseous species, while BB-SOA were characterized using an extractive electrospray ionization mass spectrometer, and a newly developed Vocus wall-less aerosol load - evaporator (WALL-E) AIM mass spectrometer.   1 Finlayson-Pitts, B. J. et al. Chemistry of the upper and lower atmosphere: theory, experiments, and applications; Academic Press: San Diego, 2000.2 Nel, A. Science 2005, 308, 804–805.3 Boucher, O. et al. IPCC Report 2013, 571–657.4 Ziemann, P. J. et al. Chem. Soc. Rev. 2012, 41, 6582.5 Srivastava, D. et al. NPJ Clim. Atmos. Sci. 2022, 5, 22.6Akagi, S. K. et al. Atmos. Chem. Phys. 2011, 11, 4039–4072.7 McNeill, V. F. Environ. Sci. & Technol. 2015, 49, 1237–1244.8 Xu, W. et al. Environ. Sci. & Technol. 2017, 51, 762–7709 Kuang, Y. et al. Environ. Sci. & Technol. 2020, 54, 3849–3860.10 Wang, J. et al. Proc. Natl. Acad. Sci. (PNAS), 2021, 118.