Influences of El Nino-Southern Oscillation on summertime ozone pollution over central-eastern China during 1950-2014

Summertime ozone pollution has become increasingly severe over many parts of China in recent years. Due to lack of historical ozone observations, few studies have analyzed the linkage between natural climate variability and ozone levels for a long time series. This study uses the simulation datasets from CMIP6 to explore the effects of El Nino-Southern Oscillation (ENSO) on summertime (June/July/August) surface ozone concentrations in central-eastern China (CEC; 20 degrees N-42 degrees N, 100 degrees E-123 degrees E) during the period of 1950-2014. Our results show that, after excluding the emission-related trend, the detrended summertime daily mean surface ozone concentrations averaged over CEC in El Nino years (30.69 ppb) are higher than those in La Nina events (29.34 ppb). Compared to the summertime mean ozone of 1950-2014 (30.25 ppb), the maximum anomalies in CMIP6 are 2.88 ppb (9.52% higher) and - 5.52 ppb (18.25% lower) in El Nino and La Nina years, respectively. In addition, the summertime MDA8 ozone of CEC is significantly correlated with the central-eastern equatorial Pacific SST (5 degrees N-5 degrees S, 170 degrees W-120 degrees W) (R = 0.29, P-value = 0.02). Such ozone increases/declines in El Nino/La Nina years are also found in satellite observations of OMI ozone. The results show that the ENSO affects the large-scale circulations over central-eastern China, which regulate the regional atmospheric stability and meteorological conditions (including horizontal wind fields, geopotential height, vertical velocity, surface air temperature, and precipitation) to influence the efficiency of ozone photochemical formation and transport. Our study makes better estimation and attribution of future surface ozone pollution in China.