Seasonality and Source Apportionment of Nonmethane Volatile Organic Compounds at Boulder Reservoir, Colorado, Between 2017 and 2019

Multiyear observations of 13 nonmethane volatile organic compounds (NMVOCs) were collected at the Boulder Reservoir in the Colorado Northern Front Range Metropolitan Area (NFRMA). We separate abundances of NMVOCs in the 2017–2019 data into source contributions using two approaches that have been applied to prior NFRMA datasets. Positive matrix factorization analysis identifies five NMVOC factors in winter, spring, and fall that correspond to long‐lived and short‐lived NMVOCs from regional oil and natural gas (O&NG) production, traffic, local shorter‐lived alkenes, and regional anthropogenic background. In summer, there is an additional biogenic NMVOC factor dominated by isoprene. The PMF model indicates that 79 ± 1% of C2‐C5 alkanes in winter and 84 ± 20% in summer are attributed to O&NG activities. Ethyne is largely from traffic with contributions ranging from 45 ± 6% in winter to 87 ± 32% in summer. Ethene and propene are associated with a potentially separate source of shorter‐lived alkenes that we cannot identify. The largest contributing sectors to the observed hazardous air pollutants (HAPs) differ substantially by species and season. Benzene is attributed to O&NG production, traffic and other industrial activities. Toluene is predominantly attributed to regional anthropogenic activities in all seasons. Of the HAPs quantified in this dataset, hexane stands out as largely attributed to O&NG production. Consistent with prior analyses, this work shows that the NFRMA is more strongly influenced by O&NG sources than many other U.S. urban regions. Plain Language Summary Colorado's Northern Front Range Metropolitan Area (NFRMA), which includes the cities of Denver, Boulder, Longmont, Loveland, Greeley, and Fort Collins, has unique air pollution challenges. Sources of air pollutants in this region include both typical urban activities as well as intense oil and natural gas production and large animal feeding operations. The wind often facilitates the mixing of emissions from these sources. We separate a continuous multiyear dataset of 13 nonmethane volatile organic compounds (NMVOCs) from the Boulder Reservoir, located in the Colorado NFRMA into source contributions. We identify five NMVOC source factors in the winter, spring, and fall, corresponding to long‐lived and short‐lived NMVOCs from regional oil and gas production, traffic, local shorter‐lived alkenes, and regional anthropogenic background. In summer, there is an additional biogenic factor. Key Poin