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 (SO 2 ) and nitrogen dioxide (NO 2 ) play important roles in regional air quality and global climate change. In this study, the long-term trends in SO 2 and NO 2 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 SO 2 showed a substantial decrease over the CDPUA from 2008 to 2018. The cities of Mianyang, Deyang, Ziyang, and Leshan exhibited similar NO 2 variations, with a slight increase from 2008 to 2013, then decrease to 2018. The decrease in SO 2 from 2008 to 2018 in the CDPUA reflects the success of China's SO 2 emissions reduction program after the Chinese national 11th Five-year Plan (2005–2010). The fluctuations in the NO 2 concentrations imply that NO 2 emissions reduction in the CDPUA was not successful during the 11th Five-year Plan, and the non-obvious decrease in NO 2 after the 12th Five-year Plan (2011–2016) also indicates the lack of effective NO X control methods. Routine meteorological parameters also have an influence on air pollutant concentrations, and the influence of the relative humidity shows significant local differences. • The long-term variations of SO 2 and NO 2 in Chengdu Plain urban agglomeration were illustrated. • The annual mean concentrations of SO 2 decreased dramatically from 2008 to 2018. • The fluctuations of NO 2 imply that NO 2 emission reduction was not successful during 2008-2018.. • The NO 2 /SO 2 ratios showed clear rising trend which indicated the success of desulfurization. • The influence of relative humidity on air pollutants has significant local differences.