Regional and Hemispheric Influences on Temporal Variability in Baseline Carbon Monoxide and Ozone over the Northeast US.

Interannual variability in baseline carbon monoxide (CO) and ozone (O), defined as mixing ratios under minimal influence of recent and local emissions, was studied for seven rural sites in the Northeast US over 2001 - 2010. Annual baseline CO exhibited statistically significant decreasing trends (-4.3 - -2.3 ppbv yr), while baseline O did not display trends at any site. In examining the data by season, wintertime and springtime baseline CO at the two highest sites (1.5 km and 2 km asl) did not experience significant trends. Decadal increasing trends (~2.55 ppbv yr) were found in springtime and wintertime baseline O in southern New Hampshire, which was associated with anthropogenic NO emission reductions from the urban corridor. Biomass burning emissions impacted summertime baseline CO with ~38% variability from wildfire emissions in Russia and ~22% from Canada at five sites and impacted baseline O at the two high elevation sites only with ~27% variability from wildfires in both Russia and Canada. The Arctic Oscillation was negatively correlated with summertime baseline O, while the North Atlantic Oscillation was positively correlated with springtime baseline O. This study suggested that anthropogenic and biomass burning emissions, and meteorological conditions were important factors working together to determine baseline O and CO in the Northeast U.S. during the 2000s.