New York City greenhouse gas emissions estimated with inverse modeling of aircraft measurements

Cities are greenhouse gas emission hot spots, making them targets for emission reduction policies. Effective emission reduction policies must be supported by accurate and transparent emissions accounting. Top-down approaches to emissions estimation, based on atmospheric greenhouse gas measurements, are an important and complementary tool to assess, improve, and update the emission inventories on which policy decisions are based and assessed. In this study, we present results from 9 research flights measuring CO2 and CH4 around New York City during the nongrowing seasons of 2018-2020. We used an ensemble of dispersion model runs in a Bayesian inverse modeling framework to derive campaign-average posterior emission estimates for the New York-Newark, NJ, urban area of (125 +/- 39) kmol CO2 s(-1) and (0.62 +/- 0.19) kmol CH4 s(-1) (reported as mean +/- 1 sigma variability across the nine flights). We also derived emission estimates of (45 +/- 18) kmol CO2 s(-1) and (0.20 +/- 0.07) kmol CH4 s(-1) for the 5 boroughs of New York City. These emission rates, among the first top-down estimates for New York City, are consistent with inventory estimates for CO2 but are 2.4 times larger than the gridded EPA CH4 inventory, consistent with previous work suggesting CH4 emissions from cities throughout the northeast United States are currently underestimated.