Abstract
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.
Highlights
Continental outflow plays an important role in influencing the chemical environment of the remote troposphere through long-range transport of natural and anthropogenic trace gases and aerosols
INTEX-NA was performed over North America and the adjacent North Atlantic Ocean using the NASA DC-8 aircraft to examine the large-scale distribution of trace gases and aerosols associated with the North America continent (Singh et al, 2006)
This study focused on flight 13 which was conducted on 28 July 2004 with one of the main objectives being sampling of United States (US) continental outflow as described in Methven et al (2006) and Arnold et al (2007)
Summary
Continental outflow plays an important role in influencing the chemical environment of the remote troposphere through long-range transport of natural and anthropogenic trace gases and aerosols. A case of deep convective lofting was obtained during TRACE-A, where a NASA DC-8 flight in the vicinity of a meso-scale complex moving across burning Brazilian savannah measured high levels of biomass combustion products in the middleto-upper troposphere (Bartlett et al, 1996; Pickering et al, 1996). These scenarios are highly conducive to long-range transport of pollutants due to faster zonal winds aloft and reduced photochemical reactivity. We utilized data obtained primarily on flight 13 of the NASA DC-8 during the Intercontinental Chemical Transport Experiment – North America (INTEX-NA) component of ICARTT (Singh et al, 2006)
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