Abstract
Abstract. Efforts have been spent on investigating the isothermal evaporation of α-pinene secondary organic aerosol (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 behavior of the α-pinene 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 α-pinene 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 the 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 or 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 the other sample factors were mainly governed by aqueous-phase processes. The evolution of each sample factor of the SQTmix SOA particles was controlled by a single type of process, whereas for the α-pinene 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.
Highlights
Atmospheric oxidation of volatile organic compounds (VOCs) can lead to a complex mixture of condensable organic vapors spanning ranges of functionalities and structures, and volatilities (Hallquist et al, 2009)
As shown in previous studies (Yli-Juuti et al, 2017; Buchholz et al, 2019; Li et al, 2019; Zaveri et al, 2020), considerable kinetic limitations exist for the evaporation of volatile compounds in this type of dry secondary organic aerosol (SOA) particles due to the substantially high viscosity
The comparable evaporation rates under intermediate- and high-relative humidity (RH) conditions suggest that particle evaporation can be approximated as a liquid-like process for both conditions, but in addition to this plasticizing effect, particulate water content may induce aqueous-phase processes during isothermal evaporation (Buchholz et al, 2019; Petters et al, 2020)
Summary
Atmospheric oxidation of volatile organic compounds (VOCs) can lead to a complex mixture of condensable organic vapors spanning ranges of functionalities and structures, and volatilities (Hallquist et al, 2009). Recent measurements suggest that SOA particles consist of large amounts of low-volatility and extremely low volatility organic compounds (LVOCs and ELVOCs) (Cappa and Jimenez, 2010; Ehn et al, 2014; Mohr et al, 2019) and that particles can adopt viscous semisolid or amorphous solid states (Virtanen et al, 2010; Pajunoja et al, 2013; Zhang et al, 2015). All this emerging evidence challenges the abovementioned assumptions, which underlie the treatment of SOA with the partitioning theory.
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