The effect of environmental regulations on air quality: A long-term trend analysis of SO2 and NO2 in the largest urban agglomeration in southwest China

Sulfur dioxide (SO2) and nitrogen dioxide (NO2) play important roles in regional air quality and global climate change. In this study, the long-term trends in SO2 and NO2 concentrations in the Chengdu Plain urban agglomeration (CDPUA) of the Sichuan Basin, Southwest China, were investigated from 2008 to 2018. The annual mean concentrations of SO2 showed a substantial decrease over the CDPUA from 2008 to 2018. The cities of Mianyang, Deyang, Ziyang, and Leshan exhibited similar NO2 variations, with a slight increase from 2008 to 2013, then decrease to 2018. The decrease in SO2 from 2008 to 2018 in the CDPUA reflects the success of China's SO2 emissions reduction program after the Chinese national 11 th Five-year Plan (2005-2010). The fluctuations in the NO2 concentrations imply that NO2 emissions reduction in the CDPUA was not successful during the 11th Five-year Plan, and the non-obvious decrease in NO2 after the 12th Five-year Plan (2011-2016) also indicates the lack of effective NOx control methods. Routine meteorological parameters also have an influence on air pollutant concentrations, and the influence of the relative humidity shows significant local differences.