Aircraft-Estimated Emissions of Greenhouse Gases and Mean Vertical Wind

In this presentation we discuss some recent advances in the methodology of aircraft-estimated emissions of greenhouse gases. The aircraft operated by Scientific Aviation, inc. are capable of measuring both wind velocity and trace gases of interest (i.e. CO₂, CH₄, C₂H₆), and when circles are flown around a point source, the flux divergence can be measured at any given altitude. Consecutive circles are flown with altitude intervals of 30-60 m that span from near the surface (~60 m) to the highest altitude where a detectable plume is seen, allowing the emission rate from the point source to be estimated when applying Gauss’s theorem. In appropriate meteorological conditions, the accuracy of these estimates has been shown to be better than 10% with a detection limit of about 5 kg h^-1. Here we will present the results of a field study funded by the Department of Energy and the University of Colorado where over 50 flights to various facilities have been conducted between 2017 and 2018. The aircraft estimates will be compared with that of ground-based techniques involving dispersion models, and the importance of measuring turbulence will be discussed. In particular, the convective heat flux, convective velocity, and entrainment velocity are important turbulence parameters to constrain. Before an Aventech AMMIS-30 gust probe was installed on the aircraft, these parameters were estimated from similarity relationships obtained from additional flight legs aligned with the wind direction in the boundary layer. The installation of the probe then allowed for direct measurements of turbulence, further improving the accuracy of the emissions estimates. Additionally, with the gust probe we test a unique method of measuring mean vertical wind at the top of the boundary layer, which allows for a better estimate of the entrainment velocity. This novel technique combines the gust probe data with GPS data that is post-processed with a precise point positioning service, and estimates of the mean vertical wind are obtained by vector subtraction.