The Exploration of 2015-2019 Surface Background Ozone over China: A Model Study by GEOS-Chem High Performance (GCHP)

Many studies have been conducted to address the surface ozone pollution issue over China, but few model works focus on the background ozone, a metric to represent the best surface ozone levels that can be achieved through emission controls. Here we apply a state-of-art global chemical transport model GEOS-Chem High Performance (GCHP) to understand the sources contributing to Chinese background (CNB) daily maximum 8 h average (MDA8) ozone, and to identify the driving factor of its interannual variability from 2015 to 2019.

The five-year-mean CNB ozone is estimated as 37.8 ppbv, showing a general west-to-east downward gradient. The national-mean CNB ozone levels are the largest in summer (42.5 ppbv), but distinct seasonality can be seen at different regions. Using the tagged ozone technique, we show that the high background levels in western China are due to abundant transport from the free troposphere and adjacent foreign regions, while in eastern China, domestic ozone formation near surface from natural precursors is also important and exhibits intensive seasonal variation. CNB ozone enhancements from lightning NOx, soil NOx, and biogenic volatile organic compound (BVOC) emissions are estimated as 8.1, 6.4 and 3.9 ppbv, respectively, in summertime. We found the greater importance of BVOC over soil NOx to ozone as reported in previous studies is reversed when domestic anthropogenic emissions are turned off, reflecting a more NOx-sensitive ozone chemical regime in a “clean” atmosphere.

The interannual variability (IAV) of CNB ozone shows the peak in summer, with standard deviation values during five years of ~5 ppbv over Qinghai-Tibet Plateau (QTP) and >3.5 ppbv over vast eastern China. CNB levels in QTP are found to be well correlated with horizontal circulation anomalies at 500 hPa, while in the east, year-to-year changes in soil NOx emissions dominate the IAV of CNB ozone. We further explore the role of El Nino-Southern Oscillation (ENSO) in modulating the IAV of CNB ozone over southern China in spring. Compared to the La Nina event, the enhanced precipitation and decreased temperature during El Nino inhibit CNB ozone formation in southeast China. However, heat and drought events, as well as enhanced biomass burning emissions during El Nino in Mainland Southeast Asia can largely worsen ozone there, and further rise CNB levels in southwest China under the southerly wind.