Temporary pause in the growth of atmospheric ethane and propane in 2015–2018

Abstract. Atmospheric non-methane hydrocarbons (NMHCs) play an important role in the formation of secondary organic aerosols and ozone. After a multidecade global decline in atmospheric mole fractions of ethane and propane – the most abundant atmospheric NMHCs – previous work has shown a reversal of this trend with increasing atmospheric abundances from 2009 to 2015 in the Northern Hemisphere. These concentration increases were attributed to the unprecedented growth in oil and natural gas (ON 2012–2020) high-resolution (~3-hour) record of ambient air C2-C7 NMHCs from in-situ measurements at the Greenland Environmental Observatory at Summit station (GEOSummit, 72.58° N, 38.48° W, 3210 m above sea level). We confirm previous findings that the ethane mole fraction significantly increased by +69.0 [+47.4, +73.2; 95 % confidence interval] ppt per year from January 2010 to December 2014. Subsequent measurements, however, reveal a significant decrease by −58.4 [−64.1, −48.9] ppt per year from January 2015 to December 2018. A similar reversal is found for propane. The upturn observed after 2019 suggests, however, that the pause in the growth of atmospheric ethane and propane might only have been temporary. The analysis of 2012–2019 air mass back-trajectories shows that this pause in mole fraction increases can neither be attributed to changes in atmospheric transport nor to changes in regional emissions. Discrete samples collected at other northern-hemisphere baseline sites under the umbrella of the NOAA cooperative global air sampling network show a similar decrease in 2015–2018 and suggest a hemispheric pattern. Here, we further discuss the potential contribution of biomass burning and O&NG emissions, the main sources of ethane and propane, and we conclude that O&NG activities likely played a role in these recent changes. This study, however, highlights the crucial need for better constrained emission inventories.