Quantification of Oxygenated Volatile Organic Compounds using Collision-Induced-Dissociation during the AEROMMA Campaign

A current bottleneck in accurately predicting the impacts of urban emissions on secondary pollution, including ozone and secondary organic aerosol, is the quantification of oxygenated volatile organic compounds (OVOCs). In this work, a voltage scanning (VS) method for quantifying OVOCs, utilizing collision-induced dissociation, is developed using the VOCUS chemical ionization mass spectrometer operated with ammonium as reagent ions. The method is optimized in laboratory studies and tested in the most challenging environment aboard a scientific aircraft during the AEROMMA 2023 campaign to quantify OVOCs in plumes over the Chicago metropolitan area. Voltage scans are optimized to produce for the first-time outcomes down to within a 5-second time resolution. Several OVOCs are quantified that originate from unconventional emerging pollution sources in urban air including cooking and daily household chemicals, in particular solvents and fragrances. Furthermore, the VS method is used to successfully quantify oxidation products within these emissions, notably organic nitrates, traditionally difficult to calibrate. Importantly, we determine the sensitivity of a prevalent organic nitrate in urban air, laying the foundation for refining chemical transport models. This study therefore demonstrates the voltage scanning method’s versatility and effectiveness in quantifying complex compounds during field measurements, particularly in urban environments.

Figure 1: In the left time series of C₆H₆O₂ ionized by NH₄⁺, the data points used for the VS (Voltage Scanning) fitting are indicated. The ∆E⁵⁰_kin results for the respective VS data section are displayed on the second y-axis. ∆E⁵⁰_kin is the kinetic energy of the cluster at half signal strength derived from a VS measurement, where an increase in electric field strength in a scanning region results in a reduction of signal due to collision induced dissociation. The size and opacity of the ∆E⁵⁰_kin markers are adjusted based on the r² of the fit, with the number of the VS labeled in circles for easier identification. The Violin plot on the right illustrates the sensitivities corresponding to these ∆E⁵⁰_kin values together with the measured sensitivity to Ethylene Glycol in the lab (orange). In the violin plot, the median and values for the upper and lower quartiles are presented.