Quantification of CO2 and CH4 emissions over Sacramento, California, based on divergence theorem using aircraft measurements

J. Ryoo, , L.T. Iraci, T. Tanaka, J.E. Marrero, E.L. Yates, I. Fung, A.M. Michalak, J. Tadic, W. Gore, T.P. Bui, J.M. Dean-Day and C.S. Chang


SUMMARY

The ability to accurately estimate anthropogenic greenhouse gas emissions has important implications for understanding air quality and climate concerns at local and regional scales. One way to measure greenhouse gas emissions is by using an airborne sensor, but the methods used can impact the emissions estimates obtained. In this study, carbon dioxide and methane measurements were collected in Sacramento, California, using an airborne sensor and the effects of different assumptions and sampling techniques on emission estimates were assessed. The choice of a cylindrical, closed-shape flight profile helped to capture more of the total emissions at the city scale where many sources of emissions are unknown. Assumptions about background conditions and the type of wind data used to calculate emissions had significant impacts on the final emissions estimates. These results highlight how sampling regimes, wind treatment and assumptions about background conditions impact the accuracy of greenhouse gas emissions estimates.

 

Figure: An illustration of airflow (kg m−2 s−1) passing through a cylinder representing the flight pattern over Sacramento, CA. The blue and red colors represents the inflow and outflow of air mass flux (density, kg m−3; multiplied by wind vector, m s−1), respectively. The vertical mass transfer through the top and bottom are referred to as the entrainment and surface flux, respectively. 


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 emission fluxes, we designed flights in a cylindrical pattern and computed the emission fluxes from two flights using a kriging method and Gauss's divergence theorem. Differences in wind treatment and assumptions about background concentrations affect the emission estimates by a factor of 1.5 to 7. The uncertainty is also impacted by meteorological conditions and distance from the emission sources. The vertical layer averaging affects the flux estimate, but the choice of raw wind or mass-balanced wind is more important than the thickness of the vertical averaging for mass-balanced wind for both urban and local scales. The importance of vertical mass transfer for flux estimates is examined, and 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 measurements made in the downwind region. This indicates that a closed-shape flight profile can better contain total emissions relative to a 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 emission estimate, it is important that the sampling 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.

Ryoo, J., L.T. Iraci, T. Tanaka, J.E. Marrero, E.L. Yates, I. Fung, A.M. Michalak, J. Tadic, W. Gore, T.P. Bui, J.M. Dean-Day, C.S. Chang (2019) "Quantification of CO2 and CH4 emissions over Sacramento, California, based on divergence theorem using aircraft measurements