The saturation vapor pressures of higher-order polyethylene glycols and achieving a wide calibration range for volatility measurements by FIGAERO-CIMS

Abstract. The Filter Inlet for Gases and AEROsols coupled with a Chemical Ionization Mass spectrometer (FIGAERO-CIMS) is a widely used method for determining the chemical composition of the molecular constituents of atmospheric organic aerosols (OA). This temperature-programmed desorption technique thermally desorbs OA in a linearly ramped desorption temperature, and the temperature at a detected molecule's peak desorption rate, Tmax⁡, is proportional to the molecule's volatility. Thereby, FIGAERO-CIMS also enables a direct measurement of the volatilities (saturation vapor pressures) of the OA constituents. A series of polyethylene glycols (PEGs) has been used to quantitatively connect FIGAERO measurement results (in particular, Tmax⁡) to volatilities (i.e., calibrate). However, available literature values of saturation pressure (Psat) or saturation mass concentration (C*) for these compounds only extend to PEG 9, which exhibits Tmax⁡ values around ∼ 90 °C, whereas Tmax⁡ values of OA constituents measured from lab-generated or ambient aerosols routinely reach up to 160 °C (Li et al., 2021; Masoud et al., 2022). To extend the region over which we can conveniently calibrate FIGAERO-CIMS, and hypothetically also other thermal desorption-based techniques for investigating OA composition and volatilities, we performed FIGAERO-CIMS calibration experiments using aerosol particles consisting of PEGs 5-15, which yielded Tmax⁡ values of up to ∼ 150 °C. We then set out to estimate the hitherto unknown Psat (C*) values of PEGs 10-15 by utilizing a suite of different Psat estimation methods: both measurement-independent methods (quantum chemistry-based calculations, molecular structure-based group contribution methods, and parametrizations based on molecular sum formulas) and fits of an explicit desorption model to our FIGAERO measurement results with C* and vaporization enthalpies as free parameters. We assess the respective suitability of each method and argue that we obtain the best estimates for PEG volatilities based on the fits to our measurements. We obtained log 10(C* (µg m−3)) values ranging from 0.51 ± 0.07 (PEG 6) to −9.2 ± 1.6 (PEG 14), agreeing with previous literature results on PEGs < 10. Within uncertainties, our results broadly continue the near log-linear relationship of C* with PEG mass for larger PEGs and also agree with some of the independent methods. Contrary to common assumptions in previous literature on FIGAERO results, we find that the relationship between log⁡10(C*(µgm-3)) and measured Tmax⁡ is not linear. We explore the consequences of this finding on the analysis of previously published FIGAERO-CIMS measurements of sesquiterpene-derived OA. Prospects for improving on our results in future work are discussed. We conclude that calibration experiments using aerosol containing PEGs up to ∼ PEG 15, with best-estimated saturation vapor pressures, provide promising opportunities for constraining the volatilities of aerosol constituents, down throughout the range of extremely low-volatility organic compounds (ELVOC, C* < 3 × 10−4 µg m−3), as detected not only via FIGAERO-CIMS but also other (online) temperature-programmed desorption techniques.