•Urban residential sector in China contributed significantly to health burdens•Chinese urban residential energy switched rapidly to cleaner energy between 1980 and 2014•The energy switching has avoided 2.2 million premature deaths•Indoor exposure to PM2.5 is found to have larger health impacts than outdoor exposure Energy use in China is rapidly changing, and this trend is expected to continue given China's carbon-neutral target. Although the urban residential sector contributes only a small fraction of total energy consumption in China, it can still have a significant impact on human health due to fine particulate matter emissions (PM2.5) associated with the burning of coal. Thanks to the increased adoption of relatively clean energy (e.g., natural gas as a replacement for coal) from 2% in 1980 to 71% in 2014 in urban residential areas, a total number of 2.2 million premature deaths attributable to exposure to PM2.5 have been avoided. However, due to rapid population growth, urbanization, aging, and changes in background mortality rates, premature deaths still increased from 136,000 to 202,000 during the same period. This is a valuable lesson for developing strategic roadmaps to achieve health-climate co-benefits in the residential sector in China and other countries. Exposure to air pollution by burning solid fuels (such as coal) for residential cooking and heating in China has caused significant health impacts in the past. The government has implemented measures to replace coal with relatively clean energy sources (e.g., natural gas). However, the scale and scope of health benefits associated with such energy switching in an urban context, when considering both ambient and indoor air quality, remain unclear. Here we used an atmospheric chemical transport model showing that relatively clean energy use increased from 2% to 71% from 1980 to 2014, and although annual premature deaths attributed to particulate matter with an aerodynamic diameter less than 2.5 μm (PM2.5) from urban residential sources increased from 136,000 (87,000–194,000) to 202,000 (117,000–302,000) during the same period, this was primarily due to population growth, urbanization, aging, and background mortality rate changes. In the absence of energy switching, there would have been 2.2 million additional premature deaths. The results provide solid evidence on health benefits of energy switching, suggesting further switching to cleaner energy for expanded health-climate co-benefits. Exposure to air pollution by burning solid fuels (such as coal) for residential cooking and heating in China has caused significant health impacts in the past. The government has implemented measures to replace coal with relatively clean energy sources (e.g., natural gas). However, the scale and scope of health benefits associated with such energy switching in an urban context, when considering both ambient and indoor air quality, remain unclear. Here we used an atmospheric chemical transport model showing that relatively clean energy use increased from 2% to 71% from 1980 to 2014, and although annual premature deaths attributed to particulate matter with an aerodynamic diameter less than 2.5 μm (PM2.5) from urban residential sources increased from 136,000 (87,000–194,000) to 202,000 (117,000–302,000) during the same period, this was primarily due to population growth, urbanization, aging, and background mortality rate changes. In the absence of energy switching, there would have been 2.2 million additional premature deaths. The results provide solid evidence on health benefits of energy switching, suggesting further switching to cleaner energy for expanded health-climate co-benefits. IntroductionRapid socioeconomic development over the past decades has led to a substantial increase in the coal-dominant energy consumption in China,2International Energy AgencyWorld energy outlook 2016 special report workshop: energy and air pollution.https://www.iea.org/events/world-energy-outlook-2016-special-report-workshop-energy-and-air-qualityDate: 2016Google Scholar causing rapid growth in emissions of both air pollutants and greenhouse gases, resulting in more than 1 million premature deaths per year.3Institute for Health Metrics and EvaluationGlobal burden of disease compare and data visualizations.vizhub.healthdata.org/gbd-compareDate: 2017Google Scholar From 1980 to 2019, annual total coal consumption in China increased from 515 to 3,180 Tg.4Department of Energy Statistics National Bureau of Statistics of People’s Republic of China China Energy Statistical Yearbook 1986–2020. China Statistics Press, 1986–2020Google Scholar In 2014 alone, coal consumption resulted in 6,430 and 4.76 Tg emissions of CO2 (carbon dioxide) and primary PM2.5 (particulate matter with an aerodynamic diameter less than 2.5 μm).5Yun X. Meng W. Xu H. Zhang W. Yu X. Shen H. Chen Y. Shen G. Ma J. Li B. et al.Coal is dirty, but where it is burned especially matters.Environ. Sci. Technol. 2021; 55: 7316-7326Google Scholar In response to the challenges associated with achieving carbon neutrality and improving human health, energy use in China is undergoing rapid changes.To battle severe air pollution, the Chinese government launched the Air Pollution Prevention and Control Action Plan in 2013.6China State CouncilAir pollution prevention and control action plan [in Chinese].http://www.gov.cn/zwgk/2013-09/12/content_2486773.htmDate: 2013Google Scholar The implementation of the plan has led to substantial improvement in air quality across China.7Zhang Q. Zheng Y. Tong D. Shao M. Wang S. Zhang Y. Xu X. Wang J. He H. Liu W. et al.Drivers of improved PM2.5 air quality in China from 2013 to 2017.Proc. Natl. Acad. Sci. U S A. 2019; 116: 24463-24469Google Scholar The annual mean PM2.5 concentrations in the monitored cities had declined by 54% from 72 μg/m3 in 2013 to 33 μg/m3 in 2020.8Ministry of Ecology and Environment of the People's Republic of ChinaBulletin on the state of China's ecological environment 2013–2020.https://www.mee.gov.cn/hjzl/sthjzk/zghjzkgb/Date: 2021Google Scholar However, residential emission sources were not documented among the 10 major measures of the plan. Residential energy consumption accounts for a small fraction of total energy use in China.4Department of Energy Statistics National Bureau of Statistics of People’s Republic of China China Energy Statistical Yearbook 1986–2020. China Statistics Press, 1986–2020Google Scholar However, the sector significantly contributes to the emissions of air pollutants because of the generally poor combustion conditions and absence of end-of-pipe control devices.9Huang Y. Shen H. Chen H. Wang R. Zhang Y. Su S. Chen Y. Lin N. Zhuo S. Zhong Q. et al.Quantification of global primary emissions of PM2.5, PM10, and TSP from combustion and industrial process sources.Environ. Sci. Technol. 2014; 48: 13834-13843Google Scholar,10Chen Y. Sheng G. Bi X. Feng Y. Mai B. Fu J. Emission factors for carbonaceous particles and polycyclic aromatic hydrocarbons from residential coal combustion in China.Environ. Sci. Technol. 2005; 39: 1861-1867Google Scholar For example, the average emission factor (EF; the amount of air pollutant emitted per unit of fuel consumption) of primary PM2.5 for residential coal stoves (9.4 g/kg) is nearly one order of magnitude greater than that for coal-fired power plants (1.0 g/kg).9Huang Y. Shen H. Chen H. Wang R. Zhang Y. Su S. Chen Y. Lin N. Zhuo S. Zhong Q. et al.Quantification of global primary emissions of PM2.5, PM10, and TSP from combustion and industrial process sources.Environ. Sci. Technol. 2014; 48: 13834-13843Google Scholar To address such challenges, coal used to fuel cooking and heating in many cities has been replaced with relatively clean energy, defined as energy sources with low on-site pollutant emissions, including pipeline gases (PLGs; including natural gas and coal gas), liquefied petroleum gas (LPG), electricity, or centralized heating.11Mao X. Guo X. Chang Y. Peng Y. Improving air quality in large cities by substituting natural gas for coal in China: changing idea and incentive policy implications.Energy Policy. 2005; 33: 307-318Google Scholar, 12Malla S. Timilsina G.R. Household Cooking Fuel Choice and Adoption of Improved Cookstoves in Developing Countries: A Review. World Bank Policy Research Working Paper No. 6903.https://ssrn.com/abstract=2445749Date: 2014Google Scholar, 13China District Heating AssociationReview on the achievements of heat supply industry in China [in Chinese].http://www.china-heating.org.cn/hangyedt/901901200.htmlDate: 2019Google Scholar Per capita consumption of LPG in China increased from 0.4 kg in 1980 to 20.4 kg in 2019, and that of natural gas increased from 0.2 m3 to 35.9 m3.4Department of Energy Statistics National Bureau of Statistics of People’s Republic of China China Energy Statistical Yearbook 1986–2020. China Statistics Press, 1986–2020Google Scholar However, although “cleaner” than coal, it remains unclear to what extent such energy switching could contribute to urban residential (UR) emission reduction, and to what degree the reduced UR emissions benefit outdoor and indoor air quality as well as human health.Residential-sector-associated health consequences attributable to PM2.5 have been assessed previously. Lelieveld et al. estimated that the residential sector in China contributed to 32% of the total premature deaths induced by PM2.5 and O3 (ozone) pollution in 2010.14Lelieveld J. Evans J.S. Fnais M. Giannadaki D. Pozzer A. The contribution of outdoor air pollution sources to premature mortality on a global scale.Nature. 2015; 525: 367-371Google Scholar It was reported that 49,200 (46,600–51,600) premature deaths in China were avoided due to PM2.5 reductions brought by clean residential energy promotion from 2013 to 2017.7Zhang Q. Zheng Y. Tong D. Shao M. Wang S. Zhang Y. Xu X. Wang J. He H. Liu W. et al.Drivers of improved PM2.5 air quality in China from 2013 to 2017.Proc. Natl. Acad. Sci. U S A. 2019; 116: 24463-24469Google Scholar It is known that residential energy mixes differ significantly between urban and rural areas. Several studies on residential energy transition in rural China have been reported in recent years15Tao S. Ru M. Du W. Zhu X. Zhong Q. Li B. Shen G. Pan X. Meng W. Chen Y. et al.Quantifying the rural residential energy transition in China from 1992 to 2012 through a representative national survey.Nat. Energy. 2018; 3: 567-573Google Scholar,16Shen G. Ru M. Du W. Zhu X. Zhong Q. Chen Y. Shen H. Yun X. Meng W. Liu J. et al.Impacts of air pollutants from rural Chinese households under the rapid residential energy transition.Nat. Commun. 2019; 10: 3405Google Scholar; however, such investigations are still lacking for urban China, where rapid energy switching has been happening. Moreover, to achieve an integrated evaluation, assessing population exposure to PM2.5 and associated health impacts is a vital part, linking energy use to human health. So far, most studies targeting the health consequences of PM2.5 were conducted based on ambient exposure.7Zhang Q. Zheng Y. Tong D. Shao M. Wang S. Zhang Y. Xu X. Wang J. He H. Liu W. et al.Drivers of improved PM2.5 air quality in China from 2013 to 2017.Proc. Natl. Acad. Sci. U S A. 2019; 116: 24463-24469Google Scholar,16Shen G. Ru M. Du W. Zhu X. Zhong Q. Chen Y. Shen H. Yun X. Meng W. Liu J. et al.Impacts of air pollutants from rural Chinese households under the rapid residential energy transition.Nat. Commun. 2019; 10: 3405Google Scholar It is important to take indoor exposure into consideration because people spend most of their time indoors17Ministry of Environmental ProtectionExposure Factors Handbook of Chinese Population (Adult Volume). China Environmental Science Press, 2013Google Scholar, 18Ministry of Environmental ProtectionExposure Factors Handbook of Chinese Population (Children 0-5 Years Old Volume). China Environmental Science Press, 2016Google Scholar, 19Ministry of Environmental ProtectionExposure Factors Handbook of Chinese Population (Children 6–17 Years Old Volume). China Environmental Science Press, 2016Google Scholar and the indoor air in households using solid fuels is often severely polluted.20Chen Y. Shen H. Smith K.R. Guan D. Chen Y. Shen G. Liu J. Cheng H. Zeng E. Tao S. Estimating household air pollution exposures and health impacts from space heating in rural China.Environ. Int. 2018; 119: 117-124Google Scholar Thus, to bridge the mentioned gaps, a quantitative evaluation of the impacts of UR emissions and the benefits of UR energy switching, considering both indoor and outdoor paths of exposure, is needed for a multiple-year period in China.Here we set out to assess the impacts of UR emissions and UR energy switching on outdoor and indoor air quality, as well as human health, from 1980 to 2014 in China. During this period, China has experienced rapid changes in socioeconomic transition and UR energy switching. From 1980 to 2014, the total urban population increased from around 190 million to 750 million,21National Bureau of Statistics of the People’s Republic of ChinaChina Statistics Yearbook 1981–2017. China Statistics Press, 1981–2017Google Scholar but the urban residents who use coal for cooking (heating) declined from 78% to 4% (80% to 30%),5Yun X. Meng W. Xu H. Zhang W. Yu X. Shen H. Chen Y. Shen G. Ma J. Li B. et al.Coal is dirty, but where it is burned especially matters.Environ. Sci. Technol. 2021; 55: 7316-7326Google Scholar driven by better living conditions and easier access to natural gas and centralized heating facilities.22Zhou Q. Shi W. Socio-economic transition and inequality of energy consumption among urban and rural residents in China.Energy Build. 2019; 190: 15-24Google Scholar These changes thereby provide valuable data and evidence that allow us to discern more clear signals in the influences of UR energy switching on human health. In light of this, we quantified UR energy consumption and switching, and compiled emissions based on energy use and EF data for various air pollutants, including primary PM2.5 and secondary PM2.5 precursors.1Peking UniversityPeking University emission inventory.http://inventory.pku.edu.cnGoogle Scholar PM2.5 concentrations in both ambient and indoor air were modelled using an atmospheric chemical transport model23Grell G.A. Peckham S.E. Schmitz R. McKeen S.A. Frost G. Skamarock W.C. Eder B. Fully coupled “online” chemistry within the WRF model.Atmos. Environ. 2005; 39: 6957-6975Google Scholar and a statistical model,20Chen Y. Shen H. Smith K.R. Guan D. Chen Y. Shen G. Liu J. Cheng H. Zeng E. Tao S. Estimating household air pollution exposures and health impacts from space heating in rural China.Environ. Int. 2018; 119: 117-124Google Scholar,24Yun X. Shen G. Shen H. Meng W. Chen Y. Xu H. Ren Y. Zhong Q. Du W. Ma J. et al.Residential solid fuel emissions contribute significantly to air pollution and associated health impacts in China.Sci. Adv. 2020; 6: eaba7621Google Scholar respectively. Population exposure and health impacts were quantified by using dose-response functions with ambient and indoor exposure paths.25Cohen A.J. Brauer M. Burnett R. Anderson H.R. Frostad J. Estep K. Balakrishnan K. Brunekreef B. Dandona L. Dandona R. et al.Estimates and 25-year trends of the global burden of disease attributable to ambient air pollution: an analysis of data from the Global Burden of Diseases Study 2015.Lancet. 2017; 389: 1907-1918Google Scholar We performed model simulations and the normalized marginal method for UR, non-UR, and total emissions, to quantify UR contributions.26Trudinger C. Enting I. Comparison of formalisms for attributing responsibility for climate change: non-linearities in the Brazilian Proposal approach.Clim. Change. 2005; 68: 67-99Google Scholar,27United Nations Framework Convention on Climate ChangeMethodological issues: scientific and methodological assessment of contributions to climate change, report of the expert meeting, note by the secretariat. New Delhi.http://unfccc.int/resource/docs/2002/sbsta/inf14.pdfDate: 2002Google Scholar The net health benefit of UR energy switching was derived based on the difference between realistic cases and a hypothesized scenario with fixed per capita UR energy throughout the study period from 1980 to 2014. The driving forces that affect energy switching and health benefits were also addressed quantitatively. Detailed methodology can be found in the section “experimental procedures.” We find that, although relative contributions of coal use to UR energy decreased from 98% in 1980 to 29% in 2014, the increased emissions combined with changes in population, aging, and background mortality rate led to growing annual premature deaths attributable to PM2.5 from UR emissions from 136,000 in 1980 to 202,000 in 2014. However, an additional 2.2 million premature deaths would have occurred in the absence of UR energy switching over the period 1980 to 2014. These findings imply that, in addition to reducing the emissions from energy generation, industry, and transportation, residential emissions should be continuously addressed in the future to achieve significant health-climate co-benefits in the context of carbon-neutral targets in China, and very likely other countries that encounter similar challenges.ResultsUR energy switchingFigure 1 shows the temporal trends of the total and per capita consumption (Ecap) for the major UR energy types from 1980 to 2014. Although the urban population grew by 148%28Shen H. Tao S. Chen Y. Ciais P. Güneralp B. Ru M. Zhong Q. Yun X. Zhu X. Huang T. et al.Urbanization-induced population migration has reduced ambient PM2.5 concentrations in China.Sci. Adv. 2017; 3: e1700300Google Scholar from 1980 to 2000, the annual energy consumption increased only slightly, from 2.08 × 109 GJ to 2.26 × 109 GJ, which was partly due to the increased use of more efficient and relatively clean energy sources, including LPG, heat, and electricity.4Department of Energy Statistics National Bureau of Statistics of People’s Republic of China China Energy Statistical Yearbook 1986–2020. China Statistics Press, 1986–2020Google Scholar For example, millions of coal stoves were replaced with LPG ranges, which was driven by the rapid growth of the petrochemical industry and LPG supply.29China Petrochemical Industry ConferenceGas supply mode in the future [in Chinese].http://finance.sina.com.cn/money/future/roll/2020-06-18/doc-iirczymk7710756.shtmlGoogle Scholar Meanwhile, centralized heating was promoted extensively in northern cities.21National Bureau of Statistics of the People’s Republic of ChinaChina Statistics Yearbook 1981–2017. China Statistics Press, 1981–2017Google Scholar,30Ministry of Housing and Urban-Rural Development of People’s Republic of ChinaChina Urban Construction Statistical Yearbook 2006–2017. China Planning Press, 2006–2017Google Scholar In fact, from 1980 to 2000, the relatively clean energy fraction (Fc) increased from 2% to 45%, whereas Ecap decreased from 11.1 GJ/cap to 4.87 GJ/cap.The year 2000 was a turning point, after which both the total and per capita UR energy consumption increased, although the population growth remained nearly constant. This is primarily due to the fact that relatively clean energy consumption increased from 45% in 2000 to 71% in 2014.4Department of Energy Statistics National Bureau of Statistics of People’s Republic of China China Energy Statistical Yearbook 1986–2020. China Statistics Press, 1986–2020Google Scholar In addition to the continuous increases in LPG use and the expansion of centralized heating,30Ministry of Housing and Urban-Rural Development of People’s Republic of ChinaChina Urban Construction Statistical Yearbook 2006–2017. China Planning Press, 2006–2017Google Scholar the use of PLG was promoted by west-to-east natural gas transmission capabilities.31Xinhua News AgencyThe People’s Republic of China Yearbook 1998. The People’s Republic of China Yearbook Press, 1998Google Scholar The rapid increase in electricity use was due to the improved living conditions and the increased home appliance ownership.32McNeil M.A. Letschert V.E. Forecasting Electricity Demand in Developing Countries: A Study of Household Income and Appliance Ownership. European Council for an Energy Efficient Economy-2005 Summer Study, 2005Google Scholar For example, the number of air conditioners owned per household increased from 0.30 in 2000 to 1.1 in 2014.21National Bureau of Statistics of the People’s Republic of ChinaChina Statistics Yearbook 1981–2017. China Statistics Press, 1981–2017Google Scholar The slight increase in coal consumption was likely caused by a massive rural-to-urban migration,28Shen H. Tao S. Chen Y. Ciais P. Güneralp B. Ru M. Zhong Q. Yun X. Zhu X. Huang T. et al.Urbanization-induced population migration has reduced ambient PM2.5 concentrations in China.Sci. Adv. 2017; 3: e1700300Google Scholar while the majority of the 480 million migrants had no access to PLG or centralized heating.33Ru M. Tao S. Smith K. Shen G. Shen H. Huang Y. Chen H. Chen Y. Chen X. Liu J. et al.Direct energy consumption associated emissions by rural-to-urban migrants in Beijing.Environ. Sci. Technol. 2015; 49: 13708-13715Google Scholar A similar switch toward relatively clean energy also occurred in rural China at a relatively slow pace.34Zhu X. Yun X. Meng W. Xu H. Du W. Shen G. Cheng H. Ma J. Tao S. Stacked use and transition trends of rural household energy in mainland China.Environ. Sci. Technol. 2019; 53: 521-529Google ScholarBecause of the physical and socioeconomic variations, UR energy consumption and mixes varied extensively across the country, which are mapped in Figure S1 as the Fc and Ecap of the total residential energy and the six individual residential energy types in 2014. The general increasing trends of Ecap of the total energy as well as lump coal, coal briquettes, and heat from south to north can be partially explained by the heating needs at high latitudes. In fact, the annual total heating degree days (HDD; the number of degrees that the daily average temperature is below the base temperature of 5°C)35Zhu D. Tao S. Wang R. Shen H. Huang Y. Shen G. Wang B. Li W. Zhang Y. Chen H. et al.Temporal and spatial trends of residential energy consumption and air pollutant emissions in China.Appl. Energy. 2013; 106: 17-24Google Scholar is 955 ± 690 degree⋅days in the north and 92 ± 71 degree⋅days in the south.35Zhu D. Tao S. Wang R. Shen H. Huang Y. Shen G. Wang B. Li W. Zhang Y. Chen H. et al.Temporal and spatial trends of residential energy consumption and air pollutant emissions in China.Appl. Energy. 2013; 106: 17-24Google Scholar Extensive use of LPG in the south eastern areas was associated with better living conditions,36Shen G. Lin W. Chen Y. Yue D. Liu Z. Yang C. Factors influencing the adoption and sustainable use of clean fuels and cookstoves in China-A Chinese literature review.Renew. Sust. Energ. Rev. 2015; 51: 741-750Google Scholar whereas a higher PLG consumption in the west was related to high local production levels.4Department of Energy Statistics National Bureau of Statistics of People’s Republic of China China Energy Statistical Yearbook 1986–2020. China Statistics Press, 1986–2020Google Scholar,37Lin W. Chen B. Luo S. Liang L. Factor analysis of residential energy consumption at the provincial level in China.Sustainability. 2014; 6: 7710-7724Google Scholar The trends were confirmed by the significant dependence (p < 0.05) of Ecap (MJ) on HDD (Figure S2A) and Fc (Figure S2B). The combined effects of HDD and Fc on Ecap are quantified by Equation 1, and the model-calculated results are plotted against the recorded values, as shown in Figure S2C.log(Ecap) = − 0.003 Fc + 0.229 log(HDD) + 3.486, R2 = 0.56.(Equation 1) The choice of UR energy type depends on affordability and accessibility.38Li J. Chen C. Liu H. Transition from non-commercial to commercial energy in rural China: insights from the accessibility and affordability.Energy Policy. 2019; 127: 392-403Google Scholar It was reported that the Fc increase in rural China was primarily driven by improvements in living conditions, which can be quantified by the per capita gross domestic product (GDPc) as a proxy.15Tao S. Ru M. Du W. Zhu X. Zhong Q. Li B. Shen G. Pan X. Meng W. Chen Y. et al.Quantifying the rural residential energy transition in China from 1992 to 2012 through a representative national survey.Nat. Energy. 2018; 3: 567-573Google Scholar A similar relationship is seen for urban areas, as indicated by the nonlinear correlation between Fc and GDPc at the provincial level in Figure S3. No differences among years (different colors) can be observed. It implies that the same effects of GDPc on Fc exist both spatially (provinces) and temporally (years), which is in line with a similar relationship revealed earlier.35Zhu D. Tao S. Wang R. Shen H. Huang Y. Shen G. Wang B. Li W. Zhang Y. Chen H. et al.Temporal and spatial trends of residential energy consumption and air pollutant emissions in China.Appl. Energy. 2013; 106: 17-24Google Scholar According to the S-shaped trend, Fc began to increase at a GDPc of approximately US$500 and was roughly when the transition from low-income to median-income countries occurred.39Ozturk I. Aslan A. Kalyoncu H. Energy consumption and economic growth relationship: evidence from panel data for low and middle income countries.Energy Policy. 2010; 38: 4422-4428Google ScholarRegarding energy accessibility, the consumption of lump coal, coal briquettes, LPG, and PLG depends positively (p < 0.05) on coal, LPG, and PLG production, respectively (Figure S4). This can be explained by the different prices of fuels that are locally produced and imported. For example, coal briquettes were sold at 0.37 RMB/kg in Zhejiang and 0.18 RMB/kg in the coal-rich province of Shanxi.40National Development and Reform CommissionPrice Yearbook of China 2007. People’s Publishing House Press, 2007Google Scholar Similarly, the natural gas prices are 2.61 RMB/m3 and 1.25 RMB/m3 in Beijing and Xining, respectively, as the natural gas used in Beijing is piped from the west.41Beijing Municipal Commission of Development and ReformGas prices [in Chinese].http://fgw.beijing.gov.cn/bmcx/djcx/jzldj/202003/t20200331_1752797.htmDate: 2019Google Scholar,42People's Government of Qilian CountyGas prices in Qinghai [in Chinese].http://www.qilian.gov.cn/html/5124/260493.htmlDate: 2018Google Scholar The effects of affordability and accessibility on Fc can be quantified as Equation 2:Fc = 1/1+325e−2.194logGDPc−0.087logHDD− 0.014logQcoal + 0.030logQLPG + 0.002logQPLG, R2 = 0.58,(Equation 2) where Qcoal (104 t), QLPG (104 t), and QPLG (108 m3) are the production amounts of coal, LPG, and PLG, respectively. The regression coefficients show the positive effects of the LPG and PLG production and the negative effects of cold weather and coal production on relatively clean energy use. The model-calculated Fc values are plotted against the observed values in Figure 2 and generally show good agreement. Among the independent variables, logGDPc contributed dominantly (R2 = 0.43) to the total variation, which indicates a primary contribution by affordability (experimental procedures).Figure 2Relationship between the observed and calculated FcShow full captionThe calculated values are based on Equation 2. The normalized mean bias (NMB), normalized mean error (NME), and the 1:1 line are shown.View Large Image Figure ViewerDownload Hi-res image Download (PPT)Effects of UR energy switching on emissions and PM2.5 in airThe temporal trends of the absolute and relative contributions of UR energy to emissions of major air pollutants in urban areas from 1980 to 2014 are shown in Figure S5. The relative contributions of UR energy varied extensively from 0.49% (NH3 in 2010) to 16.2% (black carbon [BC] in 1980). Among the major UR energy types, coal predominantly contributed to the total UR emissions of incomplete combustion products, including organic carbon (OC; 99%, in 2014), BC (99%), NH3 (92%), CO (86%), PM2.5 (77%), and particulate matter with an aerodynamic diameter less than 10 μm (PM10; 68%), due to high EFs.9Huang Y. Shen H. Chen H. Wang R. Zhang Y. Su S. Chen Y. Lin N. Zhuo S. Zhong Q. et al.Quantification of global primary emissions of PM2.5, PM10, and TSP from combustion and industrial process sources.Environ. Sci. Technol. 2014; 48: 13834-13843Google Scholar,43Huang Y. Shen H. Chen Y. Zhong Q. Chen H. Wang R. Shen G. Liu J. Li B. Tao S. Global organic carbon emissions from primary sources from 1960 to 2009.Atmos. Environ. 2015; 122: 505-512Google Scholar, 44Huang Y. Shen H. Chen H. Wang R. Zhang Y. Su S. Chen Y. Lin N. Zhuo S. Zhong Q. et al.Trend in global black carbon emissions from 1960 to 2007.Environ. Sci. Technol. 2014; 48: 6780-6787Google Scholar, 45Meng W. Zhong Q. Yun X. Zhu X. Huang T. Shen H. Chen Y. Chen H. Zhou F. Liu J. et al.Improvement of a global high-resolution ammonia emission inventory for combustion and industrial sources with new data from the residential and transportation sectors.Environ. Sci. Technol. 2017; 51: 2821-2829Google Scholar, 46Zhong Q. Huang Y. Shen H. Chen Y. Chen H. Huang T. Zeng E.Y. Tao S. Global estimates of carbon monoxide emissions from 1960 to 2013.Environ. Sci. Pollut. Res. 2017; 24: 864-873Google Scholar The temporal trends exhibited a shallow V-shaped pattern, which was similar to that of coal consumption. On the other hand, electricity and heating use predominantly and increasingly contributed to SO2 and nitrogen oxides (NOx) emissions and moderately to