Observation and simulation of ethane at 23 FTIR sites

Ethane is the most abundant non-methane hydrocarbon (NMHC) in the Earthatmosphere. Its main sources are of anthropogenic origin, with globally62% from leakage during production and transport of natural gas, 20%from biofuel combustion and 18% from biomass burning. In the Southernhemisphere, anthropogenic emissions are lower which makes biomassburning emissions a more significant source. The main removal process isoxidation by the hydroxyl radical (OH), leading to a mean atmosphericlifetime of 2 months. Until recently, a prolonged decrease of itsabundance has been documented, at rates of -1 to -2.7%/yr, with globalemissions dropping from 14 to 11 Tg/yr over 1984-2010 owing tosuccessful measures reducing fugitive emissions from its fossil fuelsources. However, subsequent investigations have reported on an upturnin the ethane trend, characterized by a sharp rise from about 2009onwards. The ethane increase is attributed to the oil and natural gasproduction boom in North America, although significant changes in OHcould also be at play. In the present contribution, we report the trendof ethane at 23 ground-based Fourier Transform Infrared (FTIR) sitesspanning the 80oN to 79oS latitude range. Over 2010-2015, asignificant ethane rise of 3-5%/yr is determined for most sites in theNorthern Hemisphere, while for the Southern hemisphere the rates ofchanges are not significant at the 2-sigma uncertainty level . Dedicatedmodel simulations by EMAC (ECHAM5/MESSy Atmospheric Chemistry;1.8×1.8 degrees) implementing various emission scenarios areincluded in order to support data interpretation. The usualunderestimation of the NMHCs emissions in the main inventories isconfirmed here for RCP85 (Representative Concentration Pathway Databasev8.5). Scaling them by 1.5 is needed to capture the background levels ofatmospheric ethane. Moreover, additional and significant emissions ( 7Tg over 2009-2015) are needed to capture the ethane rise in the Northernhemisphere. Attributing them to the oil and gas sector and locating themin North America allows EMAC to produce adequate trends in the Northernhemisphere, but not in the Southern hemisphere, where they areoverestimated. Possible causes for this difference are discussed.