Spatial and temporal variability of ozone sensitivity over China observed from the Ozone Monitoring Instrument

Abstract Surface ozone (O 3 ) air pollution in populated regions has been attributed to emissions of nitrogen oxides (NO + NO 2 = NO x ) and reactive volatile organic compounds (VOCs). These constituents react with hydrogen oxide radicals (OH + HO 2 = HO x ) in the presence of sunlight and heat to produce O 3 . The question of whether to reduce NO x emissions, VOC emissions, or both is complicated by spatially and temporally heterogeneous ozone‐NO x ‐VOC sensitivity. This study characterizes spatial and temporal variations in O 3 sensitivity by analyzing the ratio of formaldehyde (HCHO, a marker of VOCs) to nitrogen dioxide (NO 2 ), a metric known as the formaldehyde nitrogen ratio (FNR). Level 3 gridded retrievals from the Ozone Monitoring Instrument (OMI) aboard the NASA Aura satellite were used to calculate FNR, with our analysis focusing on China. Based on previous studies, we take FNR 2.0 as indicating NO x ‐limited regime, and FNR between 1.0 and 2.0 as indicating transitional regime (where either NO x reductions or VOC reductions would be expected to reduce O 3 ). We find that the transitional regime is widespread over the North China Plain (NCP), the Yangtze River Delta, and the Pearl River Delta during the ozone season (defined as having near‐surface air temperatures >20°C at the early afternoon OMI overpass time). Outside of these regions, the NO x ‐limited regime is dominant. Because HCHO and NO 2 have distinct seasonal patterns, FNR also has a pronounced seasonality, consistent with the seasonal cycle of surface O 3 . Examining trends from 2005 to 2013 indicates rapid growth in NO 2 , especially over less‐developed areas where O 3 photochemistry is NO x limited. Over this time period, HCHO decreased in southern China, where VOC emissions are dominated by biogenic sources, but increased slightly over the NCP, where VOC emissions are dominated by anthropogenic sources. A linear regression approach suggests that most of China (70% of grid cells) will be characterized by a transitional regime during the O 3 season by 2030. However, in megacities such as Guangzhou, Shanghai, and Beijing, NO 2 has decreased such that the chemical regime has shifted from VOC limited in 2005 to transitional in 2013.