VOC Measurements in Modern Urban Areas: PTR-MS Interferences for Commonly Observed VOCs

Urban emissions have changed in the past few decades as well-studied sources, such as motor vehicles, decline in an abundance while lesser-studied sources, such as volatile chemical products (VCPs) and cooking, become the dominant source of urban volatile organic compounds (VOCs). Ambient VOC mixing ratios are commonly quantified by mass spectrometry. Proton-transfer-reaction mass spectrometry (PTR-MS) is a technique that is routinely used in atmospheric research to measure a wide spectrum of VOCs, such as small oxygenates, aromatics, and biogenic species like isoprene. The products from the chemical ionization in PTR-MS not only reflect the parent ion mass, but also the sum of VOC isomers and fragments produced from reactions within the drift tube reactor. Historically, the assignment of PTR-MS ions to specific VOCs in urban areas has been conducted based on detailed analysis and instrument intercomparisons during a time when mobile sources dominated the VOC mixture.

Here, we present urban VOC measurements conducted using a VOCUS reactor proton-transfer-reaction time-of-flight mass spectrometer (PTR-ToF-MS). We evaluate the contribution of isomers and fragments to VOCs commonly reported by PTR-ToF-MS using gas-chromatography pre-separation. In urban regions such as Las Vegas and Los Angeles, we observe significant interferences to isoprene due to the fragmentation of C5-C9 aldehydes emitted from cooking. Cooking also emits crotonaldehyde, which contributes to isomers such as MVK + methacrolein. We also observe significant interferences to small aromatics, such as benzene, from the fragmentation of alkyl aromatics. Alkyl aromatics are common ingredients in hydrocarbon solvents, and the increased contribution to the ion assigned to benzene likely reflects a higher proportion of VCPs in urban air. We quantify these interferences, compare PT-ToF-MS measurements to those reported from co-located instruments, and present methods to correct for these interferences.