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, T_max, 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) have been used to quantitatively connect FIGAERO measurement results (in particular T_max) to volatilities (i.e., calibrate). However, available literature values of saturation pressure (P_sat) or saturation mass concentration (C*) for these compounds only extend to PEG 9, which exhibits T_max values around ~90 °C, whereas T_max values of OA constituents routinely reach up to 150 °C. 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 T_max values of up to ~150 °C. We then set out to estimate the hitherto unknown P_sat (C*) values of PEGs 10–15 by utilizing suite of different P_sat 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 log₁₀(C* (µg m^–3)) values ranging from 0.51 ± 0.07 (PEG 6) to –9.2 ± 1.6 (PEG 14), while agreeing with previous literature results on PEGs <10. Our results broadly continue the 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 log₁₀(C* (µg m^–3)) and measured T_max is not linear. We explore the consequences of that 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 now-known 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.