Changes in Atmospheric Butanes and Pentanes and Their Isomeric Ratios in the Continental United States

Abstract

Nonmethane hydrocarbons have been used as tracers in research on emissions and atmospheric oxidation chemistry. This research investigates source region mixing ratio trends of the nonmethane hydrocarbons i‐butane, n‐butane, i‐pentane, and n‐pentane, and the (i/n) isomeric ratios of these compounds between 2001 and 2015. Data collected at Photochemical Assessment Monitoring Stations, mandated by the U.S. Environmental Protection Agency in ozone nonattainment areas, and data collected at Global Greenhouse Gas Reference Network sites within the National Oceanic and Atmospheric Administration network, and analyzed at the Institute of Arctic and Alpine Research at the University of Colorado‐Boulder, were examined. Among all considered species, linear regression analyses on concentration time series had negative slopes at 81% of sites, indicating predominantly declining butane and pentane atmospheric concentrations. Mostly negative slopes (78% of sites) were found for the (i/n) butane and pentane isomeric ratios, including all six and seven statistically significant (i/n) butane and pentane trends, respectively. Over the ~15 year investigation period and averaged over all sites, total relative changes were ~30 and 45% for the (i/n) ratios of butanes and pentanes, respectively, with a relative increase in the prominence of the n‐isomers. Most likely causes include changing isomeric ratios in gasoline sector emissions, and increasing influence of oil and natural gas industry emissions. Changes in concentrations and isomeric ratios depend on proximity of contributing emission sources to measurement sites.

Impact of U.S. Oil and Natural Gas Emission Increases on Surface Ozone Is Most Pronounced in the Central United States

“…For Southern Great Plains, similar magnitude growth rates have also been reported for the butane and pentane isomers. 12 A number of recent publications have developed estimates of the tropospheric ethane increase based on column Fourier transform infrared (FTIR) spectroscopy measurements. 10 , 43 − 45 These observations are primarily from remote and/or high elevation sites and are therefore most representative of the ethane growth in the background atmosphere.…”

“… 1 , 3 , 7 Measurements within and downwind of O&NG basins have shown at times highly elevated atmospheric levels of VOCs. 8 − 12 Ground-based monitoring and remote sensing observations have also revealed increased NO x . 13 , 14 While elevated VOC levels have been attributed mostly to fugitive emissions from oil and gas drilling, increases in NO x can be linked to flaring, on-site power generation, and heavy trucking associated to drilling and establishing of the production infrastructure.…”

Non Methane Hydrocarbon (C2–C8) sources and sinks around the Arabian Peninsula

“…The atmospheric ratio of i-butane to n-butane has been frequently reported to be between 0.3 and 0.8 for a wide range of environmental conditions (e.g. Baker et al, 2016;Rossabi and Helmig, 2018). For the AQABA campaign, the isomeric ratio was on average 0.63 ± 0.44 (median = 0.56) with 82.4 % of the samples to between 0.3 and 0.8.…”

Non-methane hydrocarbon (C<sub>2</sub>–C<sub>8</sub>) sources and sinks around the Arabian Peninsula

“…The atmospheric ratio of i-butane to n-butane has been frequently reported to be between 0.3 and 0.8 for a wide range of environmental conditions (e.g. Baker et al, 2016;Rossabi and Helmig, 2018). For the AQABA campaign, the isomeric ratio was on average 0.63±0.44 (median = 0.56), with 82.4 % of the samples between 0.3 and 0.8.…”