Secondary Effects of Oil and Gas End-Use on Summertime Air Pollutants in the Eastern US

Oil and gas account for more than two-thirds of energy consumed in the US. The high-temperature combustion from this use yields large quantities of nitrogen oxides (NO_x) that modulate the oxidative fate of isoprene, a precursor of health-hazardous air pollutants ozone, formaldehyde, and fine particulate matter (PM_2.5). The COVID-19 pandemic and resulting lockdowns provided a unique opportunity to examine changes in ozone and PM_2.5 linked to dramatic reduction in vehicle emissions. These occurred mostly in early spring when photochemistry is weak, biogenic emissions of isoprene are nascent, and ozone is titrated by vehicular nitric oxide (NO) emissions. Here, we use the 3D chemical transport model GEOS-Chem nested over contiguous US at a spatial resolution of 0.25º × 0.3125º (~28 km latitude × ~27 km longitude) to examine the complex influence of all oil and gas consumption or end-use activities on summertime (June-August) air pollutants in the eastern US where large cities, roadways and seasonal isoprene emission hotspots coincide. The model is driven with air pollutant precursor emissions for end-use activities from the US EPA National Emissions Inventory (NEI) for non-mobile sources and from the Fuel-based Inventory for Vehicular Emissions (FIVE) for mobile sources. We find that in the eastern US, end-use activities account for most NO_x (59% of NO and 57% of nitrogen dioxide, NO₂) and most (63% or 0.28 µg m^-3) aerosol-phase nitrate. As ammonia, predominantly from agricultural activity, buffers aerosol acidity, end-use activities also indirectly contribute to 21% (0.10 µg m^-3) of aerosol-phase ammonium. The influence on aerosol sulfate is negligible. NO from oil and gas end-use activities also modulates the proportion of isoprene that oxidizes via the NO and HO₂ pathways that in turn affects yields of formaldehyde and other reactive oxygenated volatile organic compounds (VOCs) as well as isoprene secondary organic aerosol (SOA) precursors. NO_x from oil and gas end-use activities enhances formaldehyde abundance by 0.3 ppb by increasing the proportion of isoprene reacting via the NO oxidation pathway that yields formaldehyde (and other oxygenated VOCs) promptly and in higher yields than the competing HO₂ (low-NO_x) oxidation pathway. This influence on reactive VOCs also adds 8 ppb of maximum daily mean 8-hour ozone, the metric used to assess the impact of ozone on health. Suppression of the HO₂ oxidation pathway and isoprene SOA precursors only decreases SOA by 0.02 µg m^-3. The net contribution of oil and gas end-use activities to PM_2.5 is 1.2 µg m^-3 or 12% of eastern US summertime mean PM_2.5. Our results suggest multiple, substantial improvements to summertime air quality by ending reliance on oil and gas.