Evolution of volatility and composition in sesquiterpene-mixed and α-pinene secondary organic aerosol particles during isothermal evaporation

Abstract

Abstract. Efforts have been spent on investigating the isothermal evaporation of α-pinene SOA particles at ranges of conditions and decoupling the impacts of viscosity and volatility on evaporation. However, little is known about the evaporation behavior of SOA particles from biogenic organic compounds other than α-pinene. In this study, we investigated the isothermal evaporation behaviors of α-pinene (αpin) and sesquiterpene mixture (SQTmix) SOA particles under a series of relative humidity (RH) conditions. With a set of in-situ instruments, we monitored the evolution of particle size, volatility, and composition during evaporation. Our finding demonstrates that the SQTmix SOA particles evaporated slower than the αpin ones at any set of RH (expressed with the volume fraction remaining (VFR)), which is primarily due to their lower volatility and possibly aided by higher viscosity under dry conditions. We further applied positive matrix factorization (PMF) to thermal desorption data containing volatility and composition information. Analyzing the net change ratios (NCRs) of each PMF-resolved factor, we can quantitatively compare how each sample factor evolves with increasing evaporation time/RH. When sufficient particulate water content was present in either SOA system, the most volatile sample factor was primarily lost via evaporation and changes in other sample factors were mainly governed by aqueous-phase processes. The evolution of each sample factor of SQTmix SOA particles was controlled by a single type of process, whereas for αpin SOA particles it was regulated by multiple processes. As indicated by the coevolution of VFR and NCR, the effect of aqueous-phase processes could vary from one to another according to particle type, sample factors and evaporation timescale.

Isothermal evaporation of α-pinene secondary organic aerosol particles formed under low-NOx and high-NOx conditions

“…1), and a larger fraction of compounds was removed during isothermal evaporation. As previous particle evaporation studies have suggested (Vaden et al, 2011;Yli-Juuti et al, 2017;Buchholz et al, 2019;Li et al, , 2021Zaveri et al, 2020), considerable kinetic limitations exist in SOA particles at low RH, and thus the 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 and therefore enhances particle evaporation.…”

“…The experimental conditions and results of SOA generation are summarized in Table S1 in the Supplement. The operation procedure of this OFR has been described previously (Buchholz et al, 2019;Li et al, 2021). Briefly, αpinene was continuously injected into a heated flow of N 2 with a syringe pump (Kari et al, 2018) and then mixed with a humidified flow of N 2 , O 2 , O 3 , and N 2 O before entering into the OFR.…”

“…Positive matrix factorization (PMF; Paatero and Tapper, 1994) was deployed to analyze the mass spectra data collected by the FIGAERO-CIMS (Buchholz et al, 2020;Li et al, 2021). Organic compounds which have similar temporal behavior (i.e., similar thermal desorption behavior and/or aqueous-phase process) during isothermal evaporation were grouped into a single factor.…”

“…On the one hand, particulate water content acts a plasticizer accelerating the bulk-surface diffusion (Yli-Juuti et al, 2017;. On the other hand, it behaves as a catalyst to induce aqueous-phase processes which form organic compounds of low volatility, especially for SOA particles with high oxidation levels (Buchholz et al, 2019;Li et al, 2021). All of these findings primarily advance our knowledge about the evaporation processes of biogenic SOA particles but are limited to those in remote forest areas lacking in anthropogenic influence.…”

“…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 forest areas (Vaden et al, 2011;Yli-Juuti et al, 2017;Buchholz et al, 2019;Li et al, , 2021; hence, 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 volatile organic com-Z.…”