Satellite data reveal a common combustion emission pathway for major cities in China

Extensive fossil fuel combustion in rapidly developing cities severely affects air quality and public health.We report observational evidence of decadal changes in the efficiency and cleanness of bulk combustion over large cities in mainland China. In order to estimate the trends in enhancement ratios of CO and SO₂ to NO₂ (ΔCO/ΔNO₂ and ΔSO₂/ΔNO₂) and infer emergent bulk combustion properties over these cities, we combine air quality retrievals from widely used satellite instruments across 2005-2014. We present results for four Chinese cities (Shenyang, Beijing, Shanghai, and Shenzhen) representing four levels of urban development. Our results show a robust coherent progression of declining to growing ΔCO/ΔNO₂ relative to 2005 (-5.4±0.7 to +8.3±3.1%yr^-1) and slowly declining ΔSO₂/ΔNO₂ (-6.0±1.0 to-3.4±1.0%yr^-1) across the four cities. The coherent progression we find is not evident in the trends of emission ratios reported in Representative Concentration Pathway (RCP8.5) inventory. This progression is likely due to a shift towards cleaner combustion from industrial and residential sectors in Shanghai and Shenzhen that is not yet seen in Shenyang and Beijing. This overall trend is presently obfuscated by China's still relatively higher dependence on coal. Such progression is well-correlated with economic development and traces a common emission pathway that resembles evolution of air pollution in more developed cities. Our results highlight the utility of augmenting observing and modeling capabilities by exploiting enhancement ratios in constraining the time variation in emission ratios in current inventories. As cities and/or countries continue to socioeconomically develop, the ability to monitor combustion efficiency and effectiveness of pollution control becomes increasingly important in assessing sustainable control strategies.