Winter- and summertime continental influences on tropospheric O₃ and CO observed by TES over the western North Atlantic Ocean

Abstract

Abstract. The distribution of tropospheric O3 and CO and the regulating factors over the western North Atlantic Ocean during winter (December, January, and February, DJF) and summer (June, July, August, JJA) were investigated using retrievals from the Tropospheric Emission Spectrometer (TES) for 2004–2006. Seasonal composites of TES retrievals, reprocessed to remove the artificial geographic and seasonal structure added from the a priori, exhibited strong seasonal differences. At the 681 hPa level during winter composite O3 levels 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 O3 levels were variable (45–65 ppbv) and generally higher due to increased photochemical production. The main export pathway 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 circulation patterns or map types in DJF revealed that export to the lower free troposphere (681 hPa level) was enhanced by the warm conveyor belt (WCB) airstream of cyclones while stratospheric intrusions increased TES O3 levels at 316 hPa. A major finding in the DJF data was that offshore 681 hPa CO levels 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 5 map types showed that the main export pattern of seasonal composites contained the Bermuda High as the dominate feature. However, weak cyclones and frontal troughs could enhance offshore 681 hPa CO levels to greater than 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.