Measurements of Speciated Terpenoids in Chicago, Illinois: An Outsized Role for Anthropogenic Emissions of Monoterpenes

Terpenoids are highly reactive volatile organic compounds (VOC) with important implications for urban air quality. Terpenoids are commonly thought to be sourced primarily from biogenic emissions; however, recent work has shown that in urban environments, emissions from volatile chemical products can outpace biogenic emissions. Here, we present speciated measurements of isoprene, monoterpenes, and oxygenated monoterpenes, alongside a host of ancillary VOC measurements made in downtown Chicago, Illinois during the summer of 2023. Despite lower mixing ratios of terpenoids (101 ± 88 ppt) compared to aromatics (936 ± 739 ppt), the sum of isoprene and monoterpenes had much larger reactivities with key atmospheric oxidants. We use two independent approaches to estimate that 46–59% of monoterpenes in downtown Chicago are of anthropogenic origin, while the vast majority of isoprene is controlled by biogenic emissions when accounting for fragmentation and dehydration reactions in commonly used mass spectrometric techniques. A comparison of the monoterpene speciation measured in Chicago with similar measurements in other urban locations highlights that anthropogenic terpenoids are an essential component of VOC reactivity in densely populated cities.