Ability of the OMI satellite instrument to observe surface ozone pollution in China : application to 2005-2017 ozone trends

Nadir-viewing satellite observations of tropospheric ozone in the UV have been shown to detect surface ozone pollution episodes but no quantitative evaluation of this ability has been done so far. Here we use 2013-2017 surface ozone data from the new China Ministry of Ecology and Environment (MEE) network of ~1000 sites, together with vertical profiles from ozonesondes and aircraft, to quantify the ability of OMI tropospheric ozone retrievals to characterize surface ozone pollution in China. After subtracting the Pacific background, the 2013-2017 mean OMI ozone enhancements over 15 eastern China can quantify the spatial distribution of mean summer afternoon surface ozone with a precision of 8.4 ppb and a spatial correlation coefficient R=0.73. The OMI data show significantly higher values on observed surface ozone episode days (>82 ppb) than on non-episode days. OMI is much more successful at capturing the day-to-day variability of surface ozone at sites in southern China <34N (R = 0.3-0.6) than in northern China (R = 0.1-0.3) because of weaker retrieval sensitivity and larger upper tropospheric variability in the north. Much of the variability of OMI ozone over southern China 20 in summer appears to be driven by the boundary layer. Comparison of 2005-2009 and 2013-2017 OMI data indicates that mean summer afternoon surface ozone in southern China (including urban and rural regions) has increased by 3.5 ppb over the 8-year period and the number of episode days per summer has increased by 2.2 (as diagnosed by an extreme value model), fairly consistent with the few long-term surface records. Ozone increases have been particularly large in the Yangtze River Delta and in the Hubei, Guangxi, and Hainan provinces. 25