Abstract
Abstract. 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.
Highlights
Urban agglomeration, megacities, are expected to continue growing over the coming decades (Jalkanen, 2012; World Bank, 2015)
Naturally produced carbon monoxide (CO) and NO2 like biogenic CO and lightning nitrogen oxides (NOx) introduce a strong seasonality in these ratios even within the megacity, we find that when we average the monthly ratios using only the months corresponding to a particular season, we still find a similar temporal pattern in derived
Enhancement ratios of CO to NO2 and SO2 to NO2 over megacities in mainland China that are derived from Measurement of Pollution In The Troposphere (MOPITT) and Ozone Monitoring Instrument (OMI) satellite instruments show a coherent longterm progression in recent years of decreasing to increasing ratios relative to 2005
Summary
Megacities (i.e., cities with > 10 million inhabitants), are expected to continue growing (in size and number) over the coming decades (Jalkanen, 2012; World Bank, 2015). Estimates of city- to national-scale emissions from fossil fuel combustion remain uncertain, especially in rapidly developing regions where combustion is still poorly characterized due to the lack of detailed information on energy use, combustion practices, and pollution control strategies (Streets et al, 2013; Creutzig et al, 2015). This is confounded by larger uncertainties on other sources of pollution that may be associated with urbanization (e.g., deforestation, agriculture, and fires). These alone prevent us from accurately assessing the changes in atmospheric composition due to anthropogenic activities at scales that are relevant to AQ, energy, and environmental policy (National Academies of Sciences, Engineering, and Medicine, 2016)
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