Abstract. This study analyzed the long-term variations in carbon monoxide (CO) mixing
ratios from January 2006 to December 2017 at the Lin'an regional atmospheric
background station (LAN; 30.3∘ N, 119.73∘ E, 138 m a.s.l.) in China's Yangtze River Delta (YRD) region. The CO mixing ratios
were at their highest (0.69 ± 0.08 ppm) and lowest (0.54 ± 0.06 ppm) in winter and summer, respectively. The average daily variation in CO
exhibited a double-peaked pattern, with peaks in the morning and evening and
a valley in the afternoon. A significant downward trend of −11.3 ppb yr−1 of
CO was observed from 2006 to 2017 at the LAN station, which was in
accordance with the negative trends of the average CO mixing ratios and
total column retrieved from the satellite data (Measurements of
Pollution in the Troposphere, MOPITT) over the YRD region during the same
period. The average annual CO mixing ratio at the LAN station in 2017 was
0.51 ± 0.04 ppm, which was significantly lower than that (0.71 ± 0.12 ppm) in 2006. The decrease in CO levels was largest in autumn (−15.7 ppb yr−1), followed by summer (−11.1 ppb yr−1), spring (−10.8 ppb yr−1), and
winter (−9.7 ppb yr−1). Moreover, the CO levels under relatively polluted
conditions (the annual 95th percentiles) declined even more rapidly (−22.4 ppb yr−1, r=-0.68, p<0.05) from 2006 (0.91 ppm) to 2017 (0.58 ppm), and the CO levels under clean conditions (the annual 5th percentiles)
showed decreasing evidence but not statistically significant (r=-0.41, p=0.19) throughout the years. The long-term decline and
short-term variations in the CO mixing ratios at the LAN station were mainly
attributed to the implementation of the anthropogenic pollution control
measures in the YRD region and to events like the Shanghai Expo in 2010 and
Hangzhou G20 in 2016. The decreased CO level may influence atmospheric
chemistry over the region. The average OH reactivity of CO at the LAN
station is estimated to significantly drop from 4.1 ± 0.7 s−1 in
2006 to 3.0 ± 0.3 s−1 in 2017.