Abstract
Abstract. The distributions of tropospheric ozone (O3) and carbon monoxide (CO), and the synoptic factors regulating these distributions over the western North Atlantic Ocean during winter and summer were investigated using profile retrievals from the Tropospheric Emission Spectrometer (TES) for 2004–2006. Seasonal composites of TES retrievals, reprocessed to remove the influence of the a priori on geographical and seasonal structure, exhibited strong seasonal differences. At the 681 hPa level during winter months of December, January and February (DJF) the composite O3 mixing ratios were uniformly low (~45 ppbv), but continental export was evident in a channel of enhanced CO (100–110 ppbv) flowing eastward from the US coast. In summer months June, July, and August (JJA) O3 mixing ratios were variable (45–65 ppbv) and generally higher due to increased photochemical production. The summer distribution also featured a channel of enhanced CO (95–105 ppbv) flowing northeastward around an anticyclone and exiting the continent over the Canadian Maritimes around 50° N. Offshore O3-CO slopes were generally 0.15–0.20 mol mol−1 in JJA, indicative of photochemical O3 production. Composites for 4 predominant synoptic patterns or map types in DJF suggested that export to the lower free troposphere (681 hPa level) was enhanced by the warm conveyor belt airstream of mid-latitude cyclones while stratospheric intrusions increased TES O3 levels at 316 hPa. A major finding in the DJF data was that offshore 681 hPa CO mixing ratios behind cold fronts could be enhanced up to >150 ppbv likely by lofting from the surface via shallow convection resulting from rapid destabilization of cold air flowing over much warmer ocean waters. In JJA composites for 3 map types showed that the general export pattern of the seasonal composites was associated with a synoptic pattern featuring the Bermuda High. However, weak cyclones and frontal troughs could enhance offshore 681 hPa CO mixing ratios to >110 ppbv with O3-CO slopes >0.50 mol mol−1 south of 45° N. Intense cyclones, which were not as common in the summer, enhanced export by lofting of boundary layer pollutants from over the US and also provided a possible mechanism for transporting pollutants from boreal fire outflow southward to the US east coast. Overall, for winter and summer the TES retrievals showed substantial evidence of air pollution export to the western North Atlantic Ocean with the most distinct differences in distribution patterns related to strong influences of mid-latitude cyclones in winter and the Bermuda High anticyclone in summer.
Highlights
During recent decades a number of field missions (e.g., NARE, NEAQS2002, and INTEX-NA/ICARTT2004) have been conducted to enhance the understanding of North American pollutant outflow (e.g., Parrish et al, 1993, 1998; Banic et al 1996; Berkowitz et al, 1996; Cooper et al, 2001, 2002a, 2005; Fehsenfeld et al, 2006; Singh et al, 2006; Mao et al, 2006). These studies, whose acronyms along with all others used in this paper are defined in the Appendix A, have indicated that air pollutants such as ozone (O3) and its precursors may be lofted in warm conveyor belts (WCBs) of synoptic-scale cyclones (Eckhardt et al, 2004; Creilson et al, 2003; Stohl et al, 2003) or by smaller-scale convective motions (Angevine et al 2004) from the North American boundary layer to the free troposphere over the western North Atlantic Ocean, and in some cases be transported to Europe within 4–10 days (Trickl et al, 2003; Rodrigues et al, 2004; Huntrieser et al, 2005; Owen et al, 2006)
The O3 seasonal composites indicated a general reduction in mid-latitude tropospheric O3 in winter due to decreased www.atmos-chem-phys.net/10/3723/2010/
Photochemical production while the carbon monoxide (CO) seasonal composites indicated a northward shift of the axis of greatest export from winter to summer corresponding to the northward shift of the storm track
Summary
During recent decades a number of field missions (e.g., NARE, NEAQS2002, and INTEX-NA/ICARTT2004) have been conducted to enhance the understanding of North American pollutant outflow (e.g., Parrish et al, 1993, 1998; Banic et al 1996; Berkowitz et al, 1996; Cooper et al, 2001, 2002a, 2005; Fehsenfeld et al, 2006; Singh et al, 2006; Mao et al, 2006) These studies, whose acronyms along with all others used in this paper are defined in the Appendix A, have indicated that air pollutants such as ozone (O3) and its precursors may be lofted in warm conveyor belts (WCBs) of synoptic-scale cyclones (Eckhardt et al, 2004; Creilson et al, 2003; Stohl et al, 2003) or by smaller-scale convective motions (Angevine et al 2004) from the North American boundary layer to the free troposphere over the western North Atlantic Ocean, and in some cases be transported to Europe within 4–10 days (Trickl et al, 2003; Rodrigues et al, 2004; Huntrieser et al, 2005; Owen et al, 2006). This grouping allowed us to identify the average tropospheric composition associated with each synoptic pattern and to postulate regulating factors
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