The 2005–16 Trends of Ozone Pollution and Formaldehyde Columns over China Observed by Satellites

We use satellite observations to study the 2005-2016 trends of ozone pollution and formaldehyde columns in China.

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 tropospheric ozone retrievals from the OMI satellite instrument and to detect boundary layer ozone pollution in China. We focus on summer when ozone pollution in China is most severe and when OMI has the strongest sensitivity. After subtracting the Pacific background, we find that the 2013-2017 mean OMI ozone enhancements over eastern China have strong spatial correlation with the corresponding multiyear means in the surface afternoon observations (R = 0.73), and that OMI can estimate these multiyear means in summer afternoon surface ozone with a precision of 8 ppb. The OMI data show significantly higher values on observed surface ozone episode days (>82 ppb) than on non-episode days. Day-to-day correlations with surface ozone are much weaker due to OMI instrument noise, and are stronger for sites in southern China (<34^oN; 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. Ozonesonde data show that much of the variability of OMI ozone over southern China in summer is 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±3.0 ppb over the 8-year period and that the number of episode days per summer has increased by 2.2±0.4 (as diagnosed by an extreme value model), generally consistent with the few long-term surface records.

We use 2005-2016 observations of formaldehyde (HCHO) columns over China from the OMI, GOME-2, and SCIAMACHY satellite instruments to evaluate long-term trends in emission inventories of volatile organic compounds (VOCs) that affect air quality. The observations show large increases over 2005-2016 in the North China Plain (NCP, +1.1±0.5% a^-1 relative to 2005) and the Yangtze River Delta region (YRD, +1.5±0.4 % a^-1 relative to 2005), consistent with the trend of anthropogenic VOC emissions in the Multi-resolution Emission Inventory for China (MEIC). Unlike other pollutants, VOC emissions have not been decreasing in recent years. An exception is the Huai River Basin (HRB) in rural eastern China where the satellite data show rapidly decreasing VOC emissions since the early 2010s that appear to reflect bans on agricultural fires.