Driving factors of the significant increase in surface ozone in the Yangtze River Delta, China, during 2013-2017

Photochemical pollution in the Yangtze River Delta (YRD), China is presently quite serious. It is important to ascertain the impacts of the anthropogenic precursors and the effectiveness of air quality management in the region based on long-term air quality trends independent of meteorological influences. In this study, Kolmogorov-Zurbenko (KZ) filtering, a technique used to separate different scales of motion in a time series, was applied to analyze the time series of the maximum daily 8-h running average for O-3 (MDA8) from 16 cities in the YRD during 2013-2017, and its trends and driving factors were discussed. Results of the KZ filtering revealed that the short-term, seasonal and long-term components of O-3 account for 54.2%, 43.4%, and 2.3% of the total variance, respectively. Long-term component of MDA8 in the YRD is comparable to that of Los Angeles in the 1990s and the most recent 4 years, and is much higher than those of Berlin, Paris and London. The long-term trend in the YRD indicates that MDA8 underwent a large increase during 2013-2017, at average rates of 1.35-8.14 mu gm(-3)/yr for specific cities. Only Taizhou in Zhejiang Province had a decreasing trend of -0.69 mu gm(-3)/yr. This pattern could be attributed to emission changes of O-3 precursors (76.7%), particularly a reduction in nitrogen oxide (NOx) emissions and an increase in the release of volatile organic compounds (VOCs). Large decreases in particulate matter were estimated to be responsible for O-3 increases of 22% on average for cities in the YRD, impling that further decreases in NOx and VOCs emissions are necessary in order to overcome the effect of decreasing PM2.5 causing an increase in ozone.