Measurements of CO and O3 at Shemya, Alaska

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Abstract
In situ measurements of surface CO were conducted from June 1994 to May 1995, and surface ozone was measured from February to May, 1995, at Shemya, Alaska (52o44'N, 174o06'E) using nondispersive infrared‐gas filter correlation (GFC) and UV absorption spectroscopy, respectively. Over the same period, air samples were collected in flasks for analysis of CO and other trace gases as part of the NOAA‐CMDL cooperative air sampling network. We compared the continuous GFC measurements with the National Oceanic and Atmospheric Administration Climate Monitoring and Diagnostics Laboratory (NOAA‐CMDL) flask data. Over this 1‐year period, CO mixing ratios varied between 60 and 250 parts per billion by volume (ppbv). Within this range, the comparison between the two measurements is quite good, with an overall R2 of 0.953 and an average difference of 3.1%. A seasonal cycle is apparent in the CO data, with a springtime maximum and a summer minimum. Synoptic influences on the data include transport from the lower‐latitude Pacific, bringing air with very low CO mixing ratios to Shemya, and, occasionally, transport from industrial areas in east Asia, including northern China, Japan, and eastern Russia. A scatterplot of CO versus O3 using all the data shows essentially no relationship at this remote site. This result implies that the primary sources and sinks for these trace gases are different at this location. However, during a few 12‐ to 48‐hour periods in fall and late spring, enhancements in CO or in both CO and O3 were observed. During these periods, isentropic back trajectories indicate transport from the west and southwest. However, the trajectories are often difficult to interpret due to looping and are of short duration due to impact at the surface. These complexities are associated with the presence of cyclonic systems in the region. The local meteorological data suggest that the enhanced concentrations occurred around the time when these cyclonic systems passed near the sampling site. Synoptic surface pressure maps indicate that these cyclonic systems formed or crossed industrial regions in east Asia and passed near Shemya on their way to the Bering Sea. This process suggests a role for transport of anthropogenic pollutants by cyclonic systems moving northward along the western edge of the Pacific Ocean.

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