Baseline carbon monoxide and ozone in the northeast US over 2001–2010

Abstract. Baseline carbon monoxide (CO) and ozone (O₃) were studied at seven rural sites in the northeast US during varying periods over 2001–2010. Interannual and seasonal variations of baseline CO and O₃ were examined for the effects of changes in anthropogenic emissions, stratospheric intrusion, transport pathways and O₃ photochemistry. Baseline CO generally exhibited decreasing trends at most sites, except at Castle Spring (CS), an elevated (~ 400 m a.s.l.) site in rural central New Hampshire. Over April 2001–December 2010, baseline CO at Thompson Farm (TF), Pinnacle State Park (PSP), and Whiteface Mountain (WFM) decreased at rates ranging from −4.3 to −2.5 ppbv yr⁻¹. Baseline CO decreased significantly at a rate of −2.3 ppbv yr⁻¹ at Mt. Washington (MWO) over April 2001–March 2009, and −3.5 ppbv yr⁻¹ at Pack Monadnock (PM) over July 2004–October 2010. Unlike baseline CO, baseline O₃ did not display a significant long term trend at any of the sites, resulting probably from opposite trends in NO_x emissions worldwide and possibly from the overall relatively constant mixing ratios of CH₄ in the 2000s. In looking into long term trends by season, wintertime baseline CO at MWO and WFM, the highest sites, did not exhibit a significant trend, probably due to the competing effects of decreasing CO emissions in the US and increasing emissions in Asia. Springtime and wintertime baseline O₃ at TF increased significantly at a rate of 2.4 and 2.7 ppbv yr⁻¹, respectively, which was likely linked to nitrogen oxides (NO_x) emissions reductions over urban areas and possible resultant increases in O₃ due to less titration by NO in urban plumes. The effects of meteorology on baseline O₃ and CO were investigated. A negative correlation was found between springtime baseline O₃ and the North Atlantic oscillation (NAO) index. It was found that during positive NAO years, lower baseline O₃ in the northeast US was linked to less solar radiation flux, weakened stratospheric intrusion, and intensified continental export. The lowest baseline CO at Appledore Island (AI), PM, TF, PSP, WFM and the lowest baseline O₃ at AI, PM, and PSP in summer 2009 were linked to the negative phase of the Arctic oscillation (AO), when more frequent cyclone activities brought more clean Arctic air to midlatitudes. It was also found that forest fires played a major role in determining baseline CO in the northeast US In summer, ~ 38% of baseline CO variability at AI, CS, MWO, TF, PSP, and WFM could be explained by CO emissions from forest fires in Russia and ~ 22 % by emissions from forest fires in Canada. Long-range transport of O₃ and its precursors from biomass burning contributed to the highest baseline O₃ in summer 2003 at AI, CS, MWO, TF, and WFM. The findings of this study suggested impacts of increasing Asian emissions, NO_x emissions from the Northeast Urban corridor, global biomass burning emissions, and meteorological conditions (e.g. cyclone activity, AO, and NAO) should all be considered when designing strategies for meeting and maintaining National Ambient Air Quality Standards (NAAQS) and evaluating the air quality in the northeast US.