Measurements and Analysis of Light Non‐Methane Hydrocarbon Mole Fractions in 1982–1987 at Six Remote Sites Along the Pacific Rim

Abstract Atmospheric non‐methane hydrocarbons (NMHCs) are important trace species with anthropogenic and natural sources including fugitive emissions from fossil fuel mining and distribution, biomass burning, biofuel combustion, and biogenic emissions from microbes and plants. We present previously unpublished whole air sample measurements of NMHCs collected at approximately weekly intervals from 1982 to 1987 at six stations spanning the Northern and Southern hemispheres, which sampled primarily Pacific Basin air masses as determined by back‐trajectory analysis. The four alkanes in this data set, ethane, propane, i‐ and n‐butane had a mean measured mole fraction of 712 22 ppt, 245 13 ppt, 55.2 4.4 ppt, and 87.6 8.7 ppt, respectively. When compared with measurements at comparable sites two decades later, the global ethane mole fraction decreased by 21% between the mid‐1980s and 2009–2010 with larger decreases in propane and butanes, corroborating a multi‐decadal decline in NMHCs over this period. No discernible Pacific Basin secular trend for the NMHCs was found over the 5‐year period 1982–1987, but significant trends at some individual sites including short‐term increases in the Northern Hemisphere and decreases in the Southern Hemisphere. A large multi‐year enrichment anomaly that is likely associated with the strong El Niño of 1982–83 occurs at all sites and influences trends observed in the Southern Hemisphere. Using the GEOS‐Chem chemical transport model, we estimated global emissions of ethane during this period were 13.4 0.4 Tg/yr based on the globally‐averaged measured mole fractions.