Continental outflow from the US to the upper troposphere over the North Atlantic during the NASA INTEX-NA Airborne Campaign

Abstract

A case of continental outflow from the United States (US) was examined using airborne measurements from NASA DC-8 flight 13 during the Intercontinental Chemical Transport Experiment – North America (INTEX-NA). Mixing ratios of methane (CH₄) and carbon monoxide (CO) at 8–11 km altitude over the North Atlantic were elevated to 1843 ppbv and 134 ppbv respectively, while those of carbon dioxide (CO₂) and carbonyl sulfide (COS) were reduced to 372.4 ppmv and 411 pptv respectively. In this region, urban and industrial influences were evidenced by elevated mixing ratios and good linear relationships between urban and industrial tracers compared to North Atlantic background air. Moreover, low mixing ratios and a good correlation between COS and CO₂ showed a fingerprint of terrestrial uptake and minimal dilution during rapid transport over a 1–2 day time period. Analysis of synoptic conditions, backward trajectories, and photochemical aging estimates based on C₃H₈/C₂H₆ strongly suggested that elevated anthropogenic tracers in the upper troposphere of the flight region were the result of transport via convection and warm conveyor belt (WCB) uplifting of boundary layer air over the southeastern US. This mechanism is supported by the similar slope values of linear correlations between long-lived (months) anthropogenic tracers (e.g., C₂Cl₄ and CHCl₃) from the flight region and the planetary boundary layer in the southeastern US. In addition, the aircraft measurements suggest that outflow from the US augmented the entire tropospheric column at mid-latitudes over the North Atlantic. Overall, the flight 13 data demonstrate a pervasive impact of US anthropogenic emissions on the troposphere over the North Atlantic.