“…This was due primarily to regulatory measures such as the NOx State Implementation Plan (SIP) Call of 2002 (Aburn et al, 2015) that lead to regional reductions of anthropogenic NOx (nitrogen oxides), an ozone precursor (Gégo, et al, 2007), and a decline of the annual average number of exceedance days from 52 during [2000][2001][2002] Wildfires are known sources of both primary and secondary pollutants, including carbon monoxide (CO), fine particles (PM 2.5 ), NOx, volatile organic compounds (VOCs; Hu et al, 2008), and O 3 (Andreae and Merlet, 2001;McKeen et al, 2002;Bytnerowicz, et al, 2010). Similar to the study presented here, Canadian wildfires have increased O 3 concentrations in Houston, TX (Morris et al, 2006), and as far away as Europe (Spichtinger et al, 2001). Evidence of Canadian wildfire smoke and biomass burning affecting the MA's particulate matter (PM) air quality was also previously reported (Adam et al, 2004;Colarco et al, 2004;Sapkota et al, 2005), but wildfire smoke has also been recognized in high-O 3 events on the East Coast (Fiore et al, 2014).…”