Abstract
Abstract. The relationship between synoptic circulation patterns over the western North Atlantic Ocean in spring (March, April, and May) and tropospheric O3 and CO was investigated using retrievals from the Tropospheric Emission Spectrometer (TES) for 2005 and 2006. Seasonal composites of TES retrievals reprocessed to remove the artificial geographic structure added from the a priori revealed a channel of slightly elevated O3 (>55 ppbv) and CO (>115 ppbv) at the 681 hPa retrieval level between 30° N and 45° N extending from North America out over the Atlantic Ocean. Ozone and CO in this region were correlated at r=0.22 with a slope value of 0.13 mol mol−1 indicative of the overall impact of photochemical chemical processes in North American continental export. Composites of TES retrievals for the six predominant circulation patterns identified as map types from sea level pressure fields of the NCEP FNL analyses showed large variability in the distribution of tropospheric O3. Map types MAM2 and MAM3 featuring cyclones near the US east coast produced the greatest export to the lower free troposphere with O3>65 ppbv and a relatively well-defined O3-CO correlation (slope values near 0.20 mol mol−1). The ensembles of HYSPLIT backward trajectories indicated that the high O3 levels were possibly a result of pollutants lofted to the free troposphere by the warm conveyor belt (WCB) of a cyclone. An important finding was that pollutant export occurred in the main WCB branch to the east of the cyclone and in a secondary branch circling to the back of the cyclone center. Conversely, a map type featuring a large anticyclone dominating the flow over the US east coast (MAM6) restricted export with O3 levels generally <55 ppbv and CO levels generally <110 ppbv. There was also evidence of stratospheric intrusions particularly to the north of 45° N in the 316 hPa composites predominately for MAM1 which featured a large cyclone near Newfoundland. However, the concurrence of these intrusions with pollutant export, specifically in the southwestern North Atlantic Ocean, made it difficult to delineate their respective contributions to the 681 hPa O3 composites.
Highlights
An important and ongoing area of research is the quantification of air pollutant outflow from North America and identification of the meteorological processes that govern the continental export
The highest O3 and carbon monoxide (CO) levels in the lower free troposphere of the western North Atlantic Ocean were associated with map types MAM2-MAM3 featuring cyclones near the east coast of the US
We hypothesize that these elevated levels were due to lofting of anthropogenic pollutants from the continental boundary layer via the warm conveyor belt (WCB) of cyclones
Summary
An important and ongoing area of research is the quantification of air pollutant outflow from North America and identification of the meteorological processes that govern the continental export. A number of field missions (e.g., NARE, NEAQS2002, and INTEXA/ICARTT2004) have been dedicated to understanding the composition of North American outflow (e.g., Parrish et al, 1993, 1998; Banic et al 1996; Berkowitz et al, 1996; Cooper et al, 2001, 2002, 2005; Fehsenfeld et al, 2006; Singh et al, 2006; Mao et al, 2006) In many of these studies critical vertical information was obtained via highly coordinated measurement programs involving ozonesonde networks, ships and aircraft. TES is a Fourier transform infrared spectrometer designed to measure global distributions of tropospheric O3 and its precursors such as carbon monoxide (CO) (Beer et al, 2001) These measurements may prove to be crucial for the study of many global air quality problems, including the estimation of intercontinental transport (ICT) of pollutants exported from North America. In this study we focused on the spring, i.e., March, April, and May (MAM), and classified the TES retrievals for the 2005 and 2006 seasons by circulation type over a domain covering the eastern US, southeastern Canada and the adjacent western North Atlantic Ocean
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