Isothermal evaporation of α-pinene secondary organic aerosol particles formed under low-NO x and high-NO x conditions

. Many recent secondary organic aerosol (SOA) studies mainly focus on biogenic SOA particles formed under low-NO x conditions and thus are applicable to pristine environments with minor anthropogenic influence. Although interactions between biogenic volatile organic compounds and NO x are important in, for instance, suburban areas, there is still a lack of 10 knowledge about volatility and processes controlling the evaporation of biogenic SOA particles formed in the presence of high concentrations of NO x . Here we provide detailed insights into the isothermal evaporation of α-pinene SOA particles that were formed under low-NO x and high-NO x conditions to investigate the evaporation process and the evolution of particle composition during the evaporation in more detail. We coupled Filter Inlet for Gases and AEROsols-Chemical Ionization Mass Spectrometer (FIGAERO-CIMS) measurements of the molecular composition and volatility of the particle phase with 15 isothermal evaporation experiments conducted under a range of relative humidity (RH) conditions from dry to 80 % RH. Very similar changes were observed in particle volatility at any set RH during isothermal evaporation for the α-pinene SOA particles formed under low-NO x and high-NO x conditions. However, there were distinct differences in the initial composition of the two SOA types, possibly due to the influence of NO x on the RO 2 chemistry during SOA formation. Such compositional differences consequently impacted the primary type of aqueous-phase processes in each type of SOA particles in the presence 20 of particulate water. Accurate assessments the radiative forcing biogenic SOA a comprehensive understanding the key processes affecting its atmospheric lifetime. As one of those processes controlling aerosol lifetime, evaporation governs the gas-particle partitioning of organic compounds. The recent findings of evaporation studies are primarily applicable to pristine 30 forest areas 2019; there is still a lack of information about the evaporation behavior of biogenic SOA particles in polluted areas where anthropogenic– biogenic interactions are important. Here, we investigate the isothermal evaporation of biogenic SOA particles which are derived from oxidizing one representative biogenic compound (BVOC) precursor in the presence of nitrogen oxides (NO x ≡ NO+NO 2 ) that are important gaseous pollutants. The results of this study will help redefine the atmospheric lifetime of biogenic evaporation of SOA particles from oxidizing α-pinene under low-NO x and high-NO x conditions at room temperature with a wide range of RH. We examined the volume change as well as the evolution of the molecular composition of α-pinene SOA particles during isothermal evaporation. While most recent laboratory studies focusing on the evaporation process of biogenic SOA particles represent conditions relevant for pristine environments with minor anthropogenic influence, our work would be one of the few providing insights into the SOA evaporation under suburban 75 settings involving anthropogenic-biogenic interactions. RH). Afterwards, the size-selected SOA particles with 80-nm electrical mobility diameters were fed into either bypass lines with varying length or stainless-steel residence time chambers (RTC) with volumes of 25 L or 100 L for isothermal evaporation, allowing residence times (t R ) from seconds to hours. The vapor wall losses in the RTC were sufficiently fast with a vapor wall loss coefficient greater than 10 -2 s - 110 1 , ensuring negligibly low concentrations of gas vapors in the RTC (Yli-Juuti et al., 2017). After each isothermal evaporation experiment, the NanoDMAs, bypass lines and RTCs were flushed for at least 12 h with purified air under the same RH condition as the following experiment. considerable kinetic limitations exist in dry SOA particles and thus evaporation of compounds is slowed down due to the substantially high particle viscosity. With increasing RH, particulate water acts as a plasticizer for the bulk – surface diffusion 190 and therefore enhances particle evaporation. In addition, the almost identical particle evaporation rates between intermediate-https://doi.org/10.5194/acp-2022-285Preprint.Discussionstarted:21April the of aqueous-phase to F3, the evolution of evaporation between the two types at a (i.e., isothermal evaporation), in in the x samples but an increase in its NCR in the high-NO x ones. different evolution trends of the NCR of F5 between the two NO x levels at RH indicate that the competition between production and loss reactions in the aqueous phase with the conditions. NF3 isothermal evaporation As compounds NF2 desorbed in SVOC (Figure 5a ,1 and 2 nd loss due to evaporation is a possible explanation for the behavior of NF2. However, the volatility of NF3 was in the ELVOC range and so little evaporation is expected even under high RH (Li et al., 2019). Thus, the decreasing NCR for NF3 under dry conditions can high NO x concentrations during the oxidation of α-pinene goes beyond the formation of organic nitrates. High NO x concentrations could also suppress the formation of organic compounds (e.g., peroxide-containing compounds) from the RO 2 + HO 2 reaction pathways. Apart from competing with RO 2 + HO 2 reactions, al., 2006). As our findings are limited to the aqueous-phase process in α-pinene SOA particles, future studies on other BVOC systems and even emissions from urban vegetation are needed to estimate the overall importance of these processes and how 450 they may affect other physicochemical properties of SOA particles.