Ozonolysis of primary biomass burning organic aerosol particles: insights into reactivity and phase state

Abstract. Biomass burning organic aerosol (BBOA) particles are a major contributor to atmospheric particulate matter, with various effects on climate and public health. Quantifying these effects is limited by our understanding of the BBOA particles' evolving chemical composition during atmospheric aging, driven by their exposure to atmospheric oxidants. This study explores the role of ozone (O3) as an atmospheric oxidant in processing primary BBOA particles. We exposed particulate emissions from beech, spruce, and pine wood fires to O3 in an oxidative flow reactor, monitoring their chemical evolution using high-resolution time-of-flight aerosol mass spectrometry (HR-ToF-AMS) and extractive electrospray ionization time-of-flight mass spectrometry (EESI-ToF-MS). We found that the oxidative state of the particles increased with O3 exposure, as shown by the consistent, albeit minor, rise in O/C ratios. Analysis of the EESI-ToF-MS data revealed specific molecular groups containing 18 and 20 carbon atoms, likely mainly abietic, linoleic, and oleic acids, as highly reactive towards O3 and driving the increase in oxidative state. At higher relative humidity, increased oxidation and loss of reactive species indicate that enhanced O3 diffusion into particles allows the ozonolysis to progress further, highlighting humidity's role in overcoming diffusion barriers that limit ozonolysis in dry conditions. This study provides qualitative insights into the oxidative processing of primary BBOA particles in different phase states, presenting O3 as a selective oxidant. Further research could focus on quantifying the progression of the ozonolysis, in particular, the change in diffusion rates depending on relative humidity conditions or particle sizes.