Advantages of a city-scale emission inventory for urban air quality research and policy: the case of Nanjing, a typical industrial city in the Yangtze River Delta, China

摘要

With most eastern Chinese cities facing major air quality challenges, thereis a strong need for city-scale emission inventories for use in both chemicaltransport modeling and the development of pollution control policies. In thispaper, a high-resolution emission inventory (with a horizontal resolution of3 × 3 km) of air pollutants and CO₂ for Nanjing, a typicallarge city in the Yangtze River Delta, is developed, incorporating the bestavailable information on local sources. Emission factors and activity data atthe unit or facility level are collected and compiled using a thorough on-sitesurvey of major sources. Over 900 individual plants, which account for97 % of the city's total coal consumption, are identified as pointsources, and all of the emission-related parameters including combustiontechnology, fuel quality, and removal efficiency of air pollution controldevices (APCD) are analyzed. New data-collection approaches includingcontinuous emission monitoring systems and real-time monitoring of trafficflows are employed to improve spatiotemporal distribution of emissions.Despite fast growth of energy consumption between 2010 and 2012, relativelysmall interannual changes in emissions are found for most air pollutantsduring this period, attributed mainly to benefits of growing APCD deploymentand the comparatively strong and improving regulatory oversight of the largepoint sources that dominate the levels and spatial distributions of Nanjingemissions overall. The improvement of this city-level emission inventory isindicated by comparisons with observations and other inventories at largerspatial scale. Relatively good spatial correlations are found for SO₂,NO_x, and CO between the city-scale emission estimates and concentrationsat nine state-operated monitoring sites (R = 0.58, 0.46, and 0.61,respectively). The emission ratios of specific pollutants including BC to CO,OC to EC, and CO₂ to CO compare well to top-down constraints from groundobservations. The interannual variability and spatial distribution ofNO_x emissions are consistent with NO₂ vertical column densitymeasured by the Ozone Monitoring Instrument (OMI). In particular, the Nanjingcity-scale emission inventory correlates better with satellite observationsthan the downscaled Multi-resolution Emission Inventory for China (MEIC) doeswhen emissions from power plants are excluded. This indicates improvement inemission estimation for sectors other than power generation, notably industryand transportation. A high-resolution emission inventory may also provide abasis to consider the quality of instrumental observations. To furtherimprove emission estimation and evaluation, more measurements of bothemission factors and ambient levels of given pollutants are suggested; theuncertainties of emission inventories at city scale should also be fullyquantified and compared with those at national scale.