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Abstract. Emission estimates of carbon dioxide ( CO2 ) and methane ( CH4 ) andthe meteorological factors affecting them are investigated over Sacramento,California, using an aircraft equipped with a cavity ring-down greenhousegas sensor as part of the Alpha Jet Atmospheric eXperiment (AJAX) project.To better constrain the emission fluxes, we designed flights in acylindrical pattern and computed the emission fluxes from two flights usinga kriging method and Gauss's divergence theorem. Differences in wind treatment and assumptions about backgroundconcentrations affect the emission estimates by a factor of 1.5 to 7. Theuncertainty is also impacted by meteorological conditions and distance fromthe emission sources. The vertical layer averaging affects the fluxestimate, but the choice of raw wind or mass-balanced wind is more importantthan the thickness of the vertical averaging for mass-balanced wind for bothurban and local scales. The importance of vertical mass transfer for flux estimates is examined, andthe difference in the total emission estimate with and without vertical masstransfer is found to be small, especially at the local scale. The total fluxestimates accounting for the entire circumference are larger than thosebased solely on measurements made in the downwind region. This indicatesthat a closed-shape flight profile can better contain total emissionsrelative to a one-sided curtain flight because most cities have more thanone point source and wind direction can change with time and altitude. Toreduce the uncertainty of the emission estimate, it is important that thesampling strategy account not only for known source locations but alsopossible unidentified sources around the city. Our results highlight thataircraft-based measurements using a closed-shape flight pattern are anefficient and useful strategy for identifying emission sources andestimating local- and city-scale greenhouse gas emission fluxes.

atmospheric measurement techniques

Quantification of CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; and CH&amp;lt;sub&amp;gt;4&amp;lt;/sub&amp;gt; emissions over Sacramento, California, based on divergence theorem using aircraft measurements