Status and quality evaluation of precursor emission inventories for PM2.5 and ozone in China

Developing precursor emission inventories is fundamental to deepen our understanding of air pollution and formulate refined co-control strategies for PM2.5 and ozone. Thanks to numerous research projects and the strong support by local governments, precursor emissions for PM2.5 and ozone have been rapidly advanced in China over the past two decades, achieving the following key progresses: (1) Feasible methodologies and guidelines or manuals for precursor emission developments have been established. These include nine technical guidelines for the development of emission inventories of air pollutant sources for PM10, PM2.5, volatile organic compounds (VOCs), and NH3 emissions as well as on-road mobile sources, non-road mobile sources, biomass combustion sources, particulate matter (PM) from dust fugitive sources, and residential coal combustion sources published in 2014-2016; the technical manual for Urban Air Pollutant Emission Inventory Development and workbook for the Study on Air Pollution Prevention and Control in "2+26" Cities released in 2017. (2) Comprehensive datasets of local emission factors and speciated profiles of PM and VOCs have been established to cover major anthropogenic emission sources and reduce the uncertainty in emission estimation. In recent years, emission factors for isoprene, nitrous acid (HONO), chlorine (Cl), and other key precursor species of PM2.5 and ozone formation have also been developed. The most recent VOC profile datasets contain 400 to 500 VOC species from nearly 100 different sources. The less-studied oxygenated VOC (OVOC) species were also included. (3) With the progress in local emission factors, speciated profiles, and activity data, emission sources and species involved in emission inventories are more comprehensive and inclusive. For instance, the number of VOC species has increased by about 140 in China and by about 280 in the Pearl River Delta (PRD) region. The inclusion of more VOC species boosts the ozone formation potential in PRD by 30%. Emissions of semi-volatile and intermediate VOCs (S/IVOCs), Cl and HONO have also been developed. (4) A large number of precursor emissions at national, regional/provincial and city scales have been established to meet the requirements of PM2.5 and ozone control in different levels. These regional or provincial emissions mainly focused in East China (30%), South China (24%), and North China (13%), whereas city-level emissions were concentrated in North China (32%), East China (30%), and Southwest China (10%). (5) To improve the spatial and temporal accuracy and timeliness, big data have been applied to improve the anthropogenic emission inventories from power plants, industrial sources, motor vehicles, dust sources, biomass open burning, ship emissions, etc. in China. (6) Novel techniques, including source apportionments based on isotope analysis and emission inversion, have been applied to calibrate emissions. Overall, the temporal-spatial resolution, timeliness, and reliability of precursor emissions for PM2.5 and ozone in China have been significantly improved in recent years. Nonetheless, uncertainty in precursor emission inventories remain significant, particularly for emissions of speciated VOCs, organic carbon (OC), and black carbon (BC), as well as dust fugitive sources, biomass burning, industrial combustion sources, and solvent-use sources, whose upper limits of uncertainty range typically exceed 100%. Meanwhile, official emission factors and speciated profiles are unavailable, and the capacity of speciated emission validation is not well developed. Methods for assessing inventory quality remain largely vacuum. Future research should focus on expanding datasets for standardized emission factors and speciated profiles, investigating methods and compiling guidebooks for emission validation and quality assessment, developing and commercializing near real-time and short-term prediction inventories, and establishing and evaluating multi-type precursor emission inventories. This study proposes a conceptual framework to assess the quality of precursor emission inventories, which could serve as a reference to assess the quality of precursor emission inventories for inventory users and help standardize the development of precursor emission inventories.