Experimental Investigation on the Effects of Chemical Regimes on α-Pinene Oxidation and Formation of Secondary Organic Aerosol

Abstract

The atmospheric oxidation of volatile organic compounds (VOCs) produces lower volatility products that can lead to the formation of Secondary organic aerosols (SOA). SOA accounts for a significant fraction of the total organic aerosol burden in many regions of the world, affecting climate and having adverse effects on human health. On a global scale, SOA is mainly produced from biogenically emitted VOCs. Among these, monoterpenes, a class of VOCs containing 10 carbon atoms, are the second most abundantly emitted after isoprene, making them a significant contributor to global SOA production. Within the monoterpene family, α-pinene is the most abundant in the atmosphere and therefore one of the most important contributors to SOA. The formation of SOA from VOCs has been extensively studied using chamber experiments. However, the experiments were typically conducted under conditions dominated by organic peroxy radicals (RO2∙), resulting from VOC oxidation, with a lack of hydroperoxy radicals (HO2∙). In the atmosphere, both in urban and rural environments, the concentration of HO2∙ commonly equals or exceeds that of RO2∙. These chamber conditions may therefore overestimate SOA formation potential, as they favor dimerization via RO2∙+RO2∙ reactions leading to the production of high-molecular-weight compounds that can enhance SOA formation. To understand the importance of the chemical regime, we conducted several experiments in which we varied the HO2-to-RO2 ratio from a low value (≈1:100) to a high value (≈1 and above). Furthermore, oxidant levels were kept equal across all conditions to provide equal oxidative environments, allowing for a more controlled investigation of the effect of the changing chemical regime. A low α-pinene concentration of 10 ppb was used to approximate realistic atmospheric conditions. SOA composition was characterized using a Filter Inlet for Gases and AEROsols (FIGAERO) coupled with a high-resolution time-of-flight chemical ionization mass spectrometer (HR-ToF-CIMS). Gas phase highly oxygenated molecules (HOMs) chemistry was analyzed using a ToF-CIMS coupled to a multi-scheme ionization inlet (MION) with NO3- as the reagent ion. The results show that shifting from a low to a high HO2/RO2 regime substantially reduces SOA formation and changes its composition. Under high HO2/RO2 conditions, the overall particle phase signal was reduced by 47%, driven primarily by a 78% suppression of accretion products (C11–20), which are among the least volatile oxidation products formed via RO2∙+RO2∙ reactions. In the gas phase, HOM accretion products were similarly reduced by 60%, and the estimated SOA mass reduction from the total HOM reduction was ≈30%. Both phases showed a pronounced increase in hydroperoxide monomers (C10) of formula C10H18Ox, yielding a product distribution more representative of real atmospheric conditions for a high HO2/RO2 regime. Furthermore, experiments incorporating NOx showed that NOx-driven suppression of SOA is substantially weakened under high HO2/RO2 conditions. Indeed, sustained RO2∙+HO2∙ reactions produce HOM and non-HOM products of lower volatility even in the presence of NO compared to RO2∙+NO reactions alone. These findings demonstrate that the chemical regime, defined by the interplay of HO2∙, RO2∙, and NO, is a critical determinant of SOA formation potential, composition, and volatility, with important implications for atmospheric modelling of biogenic SOA and its effects on air quality and climate.