The plot above displays the sum of all measured VOC concentrations in the weekly integrated canister samples. Measured methane concentrations are not included in the sum. Each bar represents one weekly sample. VOC concentrations in each bar are separated by chemical group: light alkanes, heavy alkanes, alkenes, BTEX (the sum of aromatic compounds benzene, toluene, ethylbenzene, and xylenes), other aromatic compounds, halocarbons, acetylene, acetonitrile, and isoprene. Light alkanes are by far the most abundant chemical group in each sample. 

The plot above displays the concentration of select Hazardous Air Pollutants (HAPs) in each weekly integrated canister sample. Concentrations are displayed on a logarithmic scale. Hexane is the most abundant HAP in each sample.  Concentrations are typically low (below 1 part per billion by volume) because the sampling duration is extended over an entire week.  

The bottom panel in the plot above displays the concentration of select Hazardous Air Pollutants (HAPs) in each triggered canister sample.  Concentrations are displayed on a logarithmic scale. The top panel shows the ratio of i-pentane to n-pentane (red lines, scale on the left axis) and average wind direction (dashed black lines, scale on the right axis) for each triggered canister sample.  

Oil and gas activities emit both i-pentane and n-pentane. These light alkanes are also present in vehicle exhaust, although oil and gas activities typically emit more i-pentane relative to n-pentane compared to vehicle emissions. This means that the ratio of the two compounds can be used to indicate the source of emissions. Oil and gas emissions are associated with a ratio of less than 1, while vehicle or urban emissions typically have a ratio greater than 1. 

Most triggered canister samples had i-pentane to n-pentane ratios that were less than 1, indicating influence from oil and gas activities. Samples with these low ratios were also triggered during southeasterly winds, which is the direction of the closest active oil and gas well pad relative to our measurement site.  Several triggered canister samples had higher i-pentane to n-pentane ratios and more different wind directions, suggesting possible influence from a mixture of emission sources. 

Concentrations of HAPs are much higher in the triggered samples compared to the integrated samples because the sampling duration is much shorter: each triggered sample was collected for approximately 2 minutes.  Samples were also triggered when our measurement site was impacted by plumes with elevated total VOC concentrations (as detected by the SPOD).  Hexane was typically the most abundant HAP, however,  several samples also had elevated BTEX compounds. The two samples collected in the fall of 2023 had elevated halocarbons, namely trichloroethylene and tetrachloroethylene.