Abstract
Abstract. Accurate information on gas-to-particle partitioning is needed to model secondary organic aerosol formation. However, determining reliable saturation vapor pressures of atmospherically relevant multifunctional organic compounds is extremely difficult. We estimated saturation vapor pressures of α-pinene-ozonolysis-derived secondary organic aerosol constituents using Filter Inlet for Gases and AEROsols (FIGAERO)–chemical ionization mass spectrometer (CIMS) experiments and conductor-like screening model for real solvents (COSMO-RS). We found a good agreement between experimental and computational saturation vapor pressures for molecules with molar masses around 190 g mol−1 and higher, most within a factor of 3 comparing the average of the experimental vapor pressures and the COSMO-RS estimate of the isomer closest to the experiments. Smaller molecules likely have saturation vapor pressures that are too high to be measured using our experimental setup. The molecules with molar masses below 190 g mol−1 that have differences of several orders of magnitude between the computational and experimental saturation vapor pressures observed in our experiments are likely products of thermal decomposition occurring during thermal desorption. For example, dehydration and decarboxylation reactions are able to explain some of the discrepancies between experimental and computational saturation vapor pressures. Based on our estimates, FIGAERO–CIMS can best be used to determine saturation vapor pressures of compounds with low and extremely low volatilities at least down to 10−10 Pa in saturation vapor pressure.
Highlights
Secondary organic aerosol (SOA) is formed in the gas phase by the condensing of organic molecules with low volatilities
The measured α-pinene ozonolysis monomer products selected from our SOA sample are mainly low-volatility organic compounds (LVOCs), and dimers are mainly extremely low volatility organic compounds (ELVOCs)
The smaller monomers (Mw < 190 g mol−1) with the highest saturation vapor pressures (IVOCs) were likely not present in the sample aerosol collected from the chamber; instead, they are likely products of thermal decomposition formed from larger compounds during the experiment
Summary
Secondary organic aerosol (SOA) is formed in the gas phase by the condensing of organic molecules with low volatilities. ULVOCs can nucleate to initiate SOA formation (Kirkby et al, 2016; Bianchi et al, 2016), while ELVOCs, LVOCs and SVOCs can condense on existing particles to contribute to the growth of SOA (Ehn et al, 2014). A large source of organic compounds in the atmosphere comprises biogenic volatile organic compounds (BVOCs) emitted by plants (Jimenez et al, 2009; Hallquist et al, 2009). These BVOCs are oxidized in the gas phase by oxidants, such as OH, O3 and NO3, to form less volatile compounds through addition of oxygen-containing functional groups. In order to determine the role of different oxidation products in SOA formation, it is essential to have reliable methods to estimate the volatility of complex organic molecules formed in the atmosphere
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