Urban Air Quality and Greenhouse Gases from Recent Airborne Field Campaigns in the United States

The NOAA Chemical Sciences Laboratory has led a series of recent airborne campaigns in the U.S. to investigate urban air quality, emissions and greenhouse gases.  The Atmospheric Emissions and Reactivity Observed from Megacities to Marine Areas (AEROMMA) flew the three largest U.S. cities (New York, Chicago and Los Angeles) in 2023 on the NASA DC-8 with a comprehensive suite of trace gases, aerosols, radiation and meteorology.  The Airborne and Remote Sensing Methane and Air Pollutant Surveys (AiRMAPS) is a series of campaigns sampling urban areas and oil and gas basins over 3 years.  Airborne measurements during AEROMMA probed nonlinear O₃ photochemistry in New York, Chicago and Los Angeles. The mean ozone production efficiency (OPE), the ratio of O_x (O₃ + NO₂) to NO_x oxidation product enhancements, were 9 ± 4 (1 s), 6 ± 3 and 6 ± 3 ppbv ppbv^-1, respectively. OPE exhibited a nonlinear, inverse dependence on total reactive nitrogen (NO_y, a proxy for initial NO_x) and a positive correlation with ∆VOC/∆NO_y. A zero-dimensional photochemical model supports these observed OPE dependences on NO_x and VOCs and shows that OPE is a distinct metric from total O₃ production that may be informative to the development of O₃ pollution control strategies.  AEROMMA flights also quantified the magnitude and sources of urban methane (CH₄) emission from in-situ measurements of CH₄, CO­­₂, CO, and C₂-C₅ alkanes in Los Angeles. Using the CA Air Resources Board CO emissions inventory alongside CH4/CO enhancement ratios, the analysis determines summertime CH4 and C2–C5 alkanes emissions.  Roughly half of Los Angeles CH₄emissions are from natural gas sources and half from sources such as landfills and dairies.   Comparison to historical aircraft campaigns from 2010-2023 shows declining CH₄ but increasing ethane emissions, with the latter due to changes in pipeline natural gas ethane content.