Quantification of CO<sub>2</sub> and CH<sub>4</sub> emissions over Sacramento, California based on divergence theorem using aircraft measurements

Abstract. Emission estimates of carbon dioxide (CO2) and methane (CH4) and the meteorological factors affecting them are investigated over Sacramento, California, using an aircraft equipped with a cavity ring–down greenhouse gas sensor as part of the Alpha Jet Atmospheric eXperiment (AJAX) project. To better constrain the emissions fluxes, we designed flights in a cylindrical pattern and computed the emission fluxes from three flights using a kriging method and Gauss's divergence theorem. The CO2 and CH4 mixing ratios at the downwind side of Sacramento show relatively consistent patterns across the three flights, but the fluxes vary – as a function of different wind patterns on a given flight day. The wind variability, seasonality, and assumptions about background concentrations affect the emissions estimates, by a factor of 1.5 to 8. The uncertainty is also impacted by meteorological conditions and distance from the emissions sources. The largest CH4 mixing ratio was found over a local landfill. The importance of vertical mass transfer for flux estimates is examined, but the difference in the total emission estimate with and without vertical mass transfer is found to be small, especially at the local scale. The total flux estimates accounting for the entire circumference are larger than those based solely on the downwind region. This indicates that a closed-shape flight profile can better contain total emissions relative to one-sided curtain flight because most cities have more than one point source and wind direction can change with time and altitude. To reduce the uncertainty of the emissions estimate, it is important that the sampling and modeling strategy account not only for known source locations but also possible unidentified sources around the city. Our results highlight that aircraft-based measurements using a closed shape flight pattern are an efficient and useful strategy for identifying emission sources and estimating local and city-scale greenhouse gas emission fluxes.