On the calibration of FIGAERO-ToF-CIMS: importance and impact of calibrant delivery for the particle-phase calibration

The Filter Inlet for Gases and AEROsols (FI-GAERO) coupled with a time-of-flight chemical ionization mass spectrometer (ToF-CIMS) enables online measurements of both gas-phase and particle-phase chemical constituents of ambient aerosols. When properly calibrated, the incorporated particle filter collection and subsequent thermal desorption enable the direct measurement of volatility of said constituents. Previously published volatility calibration results however differ from each other significantly. In this study we investigate the reason for this discrepancy. We found a major source of error in the widely used syringe deposition calibration method that can lead to an overestimation of saturation vapour pressures by several orders of magnitude. We propose a new method for volatility calibration by using atomized calibration compounds that more accurately captures the evaporation of chemical constituents from ambient aerosol particles. For example, we found a difference of similar to 15 degrees C in observed T-max values between the atomizer method and syringe method using the lowest solution concentration (0.003 g L-1). This difference translates into a difference of up to 3 orders of magnitude in saturation concentration C* space. We justify our claim with evaporation modelling and direct scanning electron microscopy imaging, while also presenting possible error sources of the atomizer method. We finally present how typical calibration parameters derived with both methods impact the volatility basis set (VBS) derived from measurements of secondary organic aerosols (SOAs).

Automated summary

This study identified a major source of error in the widely used syringe deposition calibration method that may lead to an overestimation of saturation vapour pressures. A new volatility calibration method using atomized calibration compounds was proposed, which showed significant differences in observed T-max values compared to the syringe method at the lowest solution concentration. The results highlight the importance of accurate calibration methods when deriving the volatility basis set from measurements of secondary organic aerosols.