Using airborne remote sensing and in-situ observations to assess emissions of complex CH4 sources

Abstract

Methane (CH4) is the second most important anthropogenic greenhouse gas. Its atmospheric concentration is significantly influenced by human activities and has increased over the past years. The adverse effects of such a greenhouse gas on the climate system has identified need to control its emissions. However, an accurate assessment of the different emission sources by existing observations remains challenging. Consequently, the methane budget still has significant uncertainties, especially for local sources. In this study, an attempt was made to quantify emissions for areal sources and complex source regions (about 1 to 90 km2 in area) using passive remote sensing data and in-situ data. The data set was collected during the COMEX (CO2 and MEthane eXperiment) research campaign in California in 2014. It comprised observations of CH4 by airborne remote sensing non-imaging (Methane Airborne MAPper, MAMAP) and imaging (Airborne Visible / Infrared Imaging Spectrometer - Next Generation, AVIRIS-NG) instruments as well as aircraft in-situ observations of CH4 and carbon dioxide (CO2) with a Picarro greenhouse gas in-situ analyser. The main objective was the quantitative analysis of emissions from prominent CH4 sources such as landfills and oil fields and, if present, also accompanying CO2 emissions. In particular, the unique spectroscopic measurements in the short wave infrared region from the MAMAP remote sensing instrument have successfully been used for this purpose. This was also the first time that CH4 emissions from an entire landfill and an oil field complex were quantitatively estimated from airborne remote sensing data. Elevated CH4 concentrations (or 'CH4 plumes') were detected downwind from landfills and across oil fields by remote sensing aircraft surveys using the MAMAP instrument. Following each remote sensing survey, the detected plumes were sampled within the atmospheric boundary layer by in-situ instruments on the same aircraft for atmospheric parameters such as wind information and dry gas mole fractions of CH4 and CO2. These measurements facilitated an independent assessment and verification of the surface fluxes. During the COMEX campaign, four landfills in the Los Angeles Basin were surveyed, where one landfill repeatedly showed a clear emission plume on four flight days. Additionally, an oil field complex in the San Joaquin Valley was investigated on seven days. Emission rates estimated from the MAMAP remote sensing and Picarro in-situ observations via mass balance approaches vary between 11.6 and 17.8 ktCH4/yr for the landfill, and between 31.0 and 47.1 ktCH4/yr for the oil field complex for several overpasses. Case-dependent relative uncertainties are between 17% to 45%. Furthermore, the in-situ and remote sensing based emission rates agree well within the error bars. The reported inventory value of the landfill of 11.5 ktCH4/yr for 2014 by the US Environmental Protection Agency (EPA) is on average 2.8 ktCH4/yr lower than the top-down estimate from this study. The top-down estimates of the oil field complex are consistent with the latest inventory estimate but can differ significantly if basic assumptions of production rates and emission factors are used yielding only around 6 ktCH4/yr. The imaging capabilities of the AVIRIS-NG instrument aboard a simultaneously flown second aircraft additionally allowed the identification of a possible leak in the landfill cover and the exact source positions of the emitters across the oil field complex.