Greenhouse gas emission rate estimates from airborne remote sensing in the short-wave infrared

Abstract

The quantification of emissions of the greenhouse gases carbon dioxide (CO2) and methane (CH4) is essential for attributing the roles of anthropogenic activity and natural phenomena in global climate change. The current measurement systems and networks, whilst having improved during the last decades, are deficient in many respects. For example, the emissions from localised and point sources such as fossil fuel exploration sites are not readily assessed. A tool developed to better understand point sources of CO2 and CH4 is the optical remote sensing instrument MAMAP, operated from aircraft. With a ground scene size of the order of 50m and a relative accuracy of the column-averaged dry air mole fractions of about 0.3% for XCO2 and less than 0.4% for XCH4, MAMAP can make a significant contribution in this respect. Detailed sensitivity studies showed that the modified WFM-DOAS retrieval algorithm used for MAMAP has an approximate accuracy of about 0.24% for XCH4 and XCO2 in typical atmospheric conditions. At the example of CO2 plumes from two different power plants and CH4 plumes from coal mine ventilation shafts, two inversion approaches to obtain emission rates were developed and tested. One is based on an optimal estimation scheme to fit Gaussian plume models from multiple sources to the data and the other is based on a simple Gaussian integral method. Compared to CO2 emission estimates as reported by the power plants' operator within the framework of emission databases (24 and 13 MtCO2/yr), the results of the individual inversion techniques were within plus/minus 10% with uncertainties of plus/minus 20-30% mainly due to insufficient wind information and non-stationary atmospheric conditions. Measurements at the coal mine included on-site wind observations by an aircraft turbulence probe that could be utilised to calibrate the wind model. In this case, the inversion results have a bias of less than 1% compared to the reported CH4 emissions (50 ktCO2/yr) with an uncertainty of approximately plus/minus 13.5%. In cases where no elevated CO2 or CH4 is observed, MAMAP data are useful to provide upper limit constraints as was shown for a marine gas seep. The inversion techniques developed in this work have the potential to provide the basis for quantification and independent validation of anthropogenic and natural point source emission rates. These concepts are not restricted to airborne applications and are of particular value also for future satellite remote sensing missions.