Seasonality and Source Apportionment of Volatile Organic Compounds in Northern Colorado

Colorado's Northern Front Range Metropolitan Area (NFRMA), which stretches from Denver to Fort Collins, has unique air pollution challenges due to its diverse combination of pollution sources. Regional sources include volatile organic compounds (VOCs) from urban activities (i.e., traffic and industrial activities), oil and natural gas (O&NG) exploration and production, biogenic emissions, and wildfire smoke. The NFRMA has been repeatedly designated as a nonattainment area for ozone (O₃) under the U.S. National Ambient Air Quality Standard (NAAQS), making reductions in VOC and nitrogen oxides emissions a priority. Concerns regarding exposure to hazardous air pollutants (HAPs) have also increased as population growth and O&NG production have risen together, frequently creating intersections of O&NG operations with residential neighborhoods.

While several studies have characterized VOCs south of Longmont, few have focused on the northern portion of the NFRMA. Using spatially arrayed weekly integrated canisters collected between 2015-2016 and 2022-2024, we characterize more than 50 VOC species, including HAPs such as benzene, toluene, n-hexane, 2,2,4-trimethylpentane, n-nonane, and xylenes, from Loveland and Greeley to Fort Collins and north toward the Wyoming border. This analysis includes measurements near O&NG operations, in suburban areas, in remote grasslands, and at a large gas station. We identify regional gradients of seasonally averaged alkanes, alkenes, and aromatics, with differences as large as three orders of magnitude between sites for some species. To characterize the emission source profiles, we use characteristic VOC ratios (e.g., i/n-pentane, i/n-butane, and toluene/benzene) along with multivariate regression analysis with O&NG (propane) and tailpipe (ethyne) markers to quantify relative source contributions from vehicle combustion and O&NG. These results reveal that both sources have a large-scale regional impact on ambient air quality and HAPs concentrations with seasonal and spatial variability.