The effects of energy paths and emission controls and standards on future trends in China's emissions of primary air pollutants

. To examine the efﬁcacy of China’s actions to control atmospheric pollution, three levels of growth of energy consumption and three levels of implementation of emission controls are estimated, generating a total of nine combined activity-emission control scenarios that are then used to estimate trends of national emissions of primary air pollutants through 2030. The emission control strategies are expected to have more effects than the energy paths on the future emission trends for all the concerned pollutants. As recently promulgated national action plans of air pollution prevention and control (NAPAPPC) are implemented, China’s anthropogenic pollutant emissions should decline. For exam-ple, the emissions of SO 2 , NO x , total suspended particles (TSP), PM 10 , and PM 2 . 5 are estimated to decline 7, 20, 41, 34, and 31 % from 2010 to 2030, respectively, in the “best guess” scenario that includes national commitment of energy saving policy and implementation of NAPAPPC.

and ecosystem acidification (Larssen et al., 2006;Zhao et al., 2009Zhao et al., , 2011a. To alleviate serious air pollution and the resulting environmental impacts, China has conducted a national strategy of energy conservation and emission reduction since 2005. Compulsory control measures are required in major economic sectors to limit fossil energy inputs and high emissions. These measures include replacement of small and inefficient plants or boilers with larger and energy-efficient ones in the power and some other heavy industrial sectors, installation of flue-gas desulfurization (FGD) systems at all newly built and many preexisting thermal power units, and staged implementation of more stringent emission standards on vehicles. The strategy has proven effective: national emissions of SO 2 and PM are estimated to have declined gradually from 2005 to 2010 (Lu et al., 2011;, partly confirmed by satellite and ground observations . Urban air quality, particularly leading up to and during major events (e.g., Beijing Olympics and Shanghai Expo), was improved due to incrementally tightened emission control and closures or relocations of major industrial plants . These measures, however, failed to restrain the growth of NO x emissions that are critical to formation of PM 2.5 and O 3 in the atmosphere. Episodes of extremely severe regional haze have occurred more frequently in recent years and have become a growing concern Ma et al., 2012;Wang and Hao, 2012; L. T. , illustrating the need for expanded efforts in pollution control. Facing these circumstances, the government in succession issued new national ambient air quality standards (NAAQS) and a national action plan of air pollution prevention and control (NAPAPPC), requiring expanded and intensified efforts at emission abatement and air quality improvement. Since 2010, the national emission standards have been updated or proposed across a number of sectors, with much tighter emission limits overall.
The implementation of the above-mentioned measures and standards will significantly affect the source characteristics and emission level of the primary air pollutants that substantially determine regional air quality. The future trends in emissions of individual pollutant species and their interactions are important not only to understanding the physical and chemical cycles of air pollutants but also to evaluating the effectiveness and informing the design of policy strategies on air quality and even climate, using chemical transport models. A number of analysts have predicted future trends of China's air pollutant emissions (Ohara et al., 2007;Klimont et al., 2009;Xing et al., 2011a;B. Zhao et al., 2013;. Most studies except very recent ones (e.g., S. X. , however, have little detailed analyses on the effects of ongoing or upcoming control measures on the trends in emission factors (the emission levels per unit energy input or product output) by source type. In particular, the potential benefits of newly is-sued emission standards across sectors have not been well quantified, raising uncertainty of current estimates.
In this study, different scenarios of emission control and energy consumption in China are designed and the future emissions of SO 2 , NO x and PM in different size classes (total suspended particles (TSP), PM 10 , and PM 2.5 ) are correspondingly estimated, including careful analyses of the changes in emission factors due to implementation of various control measures across sectors. It focuses on the period from 2010 to 2030, during which current plans anticipate most of China's megacities to meet the new NAAQS issued in 2012. Section 2 reviews the bottom-up emission inventory methodology and briefly describes the scenarios applied in this work. Section 3 is a thorough analysis of future trends of emission factors and their driving forces by sector, incorporating the latest information of emission control strategies and standards and data from domestic field measurements and investigations. Section 4 presents the predicted future emissions of primary air pollutants by scenario, and related discussions including the effectiveness of the emission control strategy, the implications of the interaction of different pollutant trends to ecosystems and climate forcing, and comparisons with other studies. Section 5 summarizes the study.

The source categories of the emission inventory
The research domain covers the 31 provinces of mainland China (see Fig. S1 in the Supplement). All main anthropogenic activities are included in the emission inventory framework. At the most aggregated scale, sources fall into four main sector categories: coal-fired power plants (CPP), all other industry (IND), transportation (TRA, including onroad and nonroad subcategories), and the residential and commercial sector (RES, including fossil fuel, biofuel use, and biomass open burning subcategories). IND is further divided into cement production (CEM), iron and steel plants (ISP), brick production (BRI), lime production (LIM), nonferrous metal production (NMP), and other industrial boilers and noncombustion processes (OIN), reflecting the structure of available data. The detailed methods of developing a bottom-up emission inventory for SO 2 , NO x and PM are described in previous studies (Zhao et al., 2011b;. Generally, annual anthropogenic emissions of those pollutants are estimated by sector using Eq. (1): E i,t = k m n AL k,m,n,t × EF i,k,m,t × R k,m,n,t × (1 − η i,n,t ), where i,k, m,n and t stand for species, sector, fuel type, emission control technology and year, respectively; AL is the activity level, either energy consumption or industrial production; EF is the unabated emission factor; R is the penetration rate of a given emission control technology; and η is the removal efficiency of that technology.

Scenarios and activity levels
The year 2010, for which the authors estimated the emissions previously (Y. , is set as the base year, and annual emissions of SO 2 , NO x , and PM are predicted for 2015, 2020, and 2030 based on different scenarios. To better understand the effects of activity levels (e.g., fuel consumption and industrial production) and emission control strategies on future emission trends, three levels of growth of energy and industrial production (explained later in this section) and three levels of implementation of emission controls (explained in detail in Sect. 3) are considered. This generates a total of nine combined activity-emission control scenarios evaluated for each of the target years. Table 1 summarizes the general assumptions about the energy trends and emission control polices and the derived scenarios. Regarding activity levels, the principles behind scenarios of the International Energy Agency (IEA, 2012) are followed in this work, resulting in a Current Policy Scenario (CPS), a New Policy Scenario (NPS), and a 450 Scenario (450S). The assumptions of the three energy scenarios are explained in detail by IEA (2012). Briefly, as indicated in Table 1, NPS is identified as the "best guess" of future energy trends, taking account of the national energy policy commitments that have been announced (e.g., the plans to reduce fossil energy use and to reduce greenhouse gas emissions). In comparison, CPS is a conservative estimate that assumes those commitments will not be implemented and the national energy policy will not change in practice after 2010. The 450S is an aggressive scenario, which sets a Chinese energy path (as part of a global strategy) to limit the concentration of greenhouse gases in the atmosphere to approximately 450 ppm (parts per million) of CO 2 equivalent.
The current study applies most of the activity levels projected by IEA (2012) for all the scenarios, with some revisions for given emission sources based on more recent domestic information, as described below. Electric power generation is China's biggest coal-consuming sector, and since 2005 the government has implemented a series of measures to improve energy efficiency and to conserve coal use in the sector. Small and inefficient units have been scheduled for staged shutdown, first those less than 100 MW electric (MWe) and then up to 200 MWe. In this work, two steps are used to predict future coal consumption by power plants. First, the coal use of "old" units (i.e., those built before the end of 2010) are estimated based on (1) the detailed power unit database compiled by the authors, which includes the parameter of coal use per unit generation of electricity (ex-pressed as gce kWh −1 ) plant by plant (Zhao et al., 2008), and (2) the government measures to phase out small units. These include assuming the retirement of all pulverized-coal units less than 100 MWe and grate units for electricity generation only (i.e., not including combined heat and power units, CHP) by 2015, all units less than 200 MWe for electricity generation only by 2020, and all CHP units less than 200 MWe by 2030. These actions lead to total capacities of old units decreasing from 588 in 2015 to 555 GW in 2030, and coal consumption for these units from 1265 to 1179 Mt. (Note the average heat value of China's coal is 21 MJ kg −1 ; Zhao et al., 2010.) Second, the growth of "new" units (i.e., those built after 2010) is predicted based on the power plant construction plans in the State Grid. In the reference plans made in 2013, the average growth of national capacity is estimated at 50 GW yr −1 from 2010 to 2020 and 30 GW yr −1 from 2020 to 2030 (internal data, unpublished), and this assumption is applied in the CPS scenario in this work. Regarding NPS and 450S, the total thermal capacity is estimated to further decline for two reasons. First, the growth of national electricity demand would slow down, since China's electricity-consuming industry is expected to be approaching the peak, and thus the electricity consumption per unit Gross Domestic Production (GDP) would decrease in the future under the national policy of energy conservation (Wu, 2013). The second, more penetration of renewable power would be expected under the development plan of renewable energy by the National Energy Administration of China. The coal consumption paths are thus projected based on the capacity, average coal use per unit generation of electricity for advanced big units (320 gce kWh −1 ), and the average annual operation hours (4700 h, personal communication with L. Pan from China Electricity Council, 2013). Table 2 lists the total capacities and coal consumption of power plants projected in this work, compared with those by IEA (2012). In most cases (except 450S for 2020 and 2030), the IEA has less capacity but higher coal consumption, likely due to less consideration of improved coal combustion efficiency and reduced running hours for China's power units in the future.
The energy use (coal, oil, natural gas, and biofuel) of industry and residential and commercial sectors is taken from the IEA projection. It should be noted, however, that the total industry sector in this work is further divided into several subcategories to get a better understanding of the emission trends of those sources. Therefore, the coal consumption of CEM, ISP, LIM, and BRI, which depends on the output of relevant industrial products, must be estimated and then subtracted from the total industrial consumption to derive the coal use from other industrial boilers (OIN), for which sufficient independent data are lacking. The cement and steel production is projected by the IEA. The differences between scenarios are very small, e.g., the cement production in 2030 is estimated at 1972, 1954, and 1947 Mt for CPS, NPS, and 450S, respectively. The IEA does not project lime and brick production, for which activity levels are also less developed Table 1. Basic principles and assumptions of energy path and emission control strategies for scenarios in this work.

Scenarios Descriptions
Energy and industrial production path CPS Current Policy Scenario (following IEA): conservative estimates with the assumption that national energy policy will not change since 2010; modification on power and transportation sectors based on most recent domestic plans or research. NPS New Policy Scenario (following IEA): "best guess" of future energy trend including the national energy policy commitments that have been announced; modification on power and transportation sectors based on most recent domestic plans or research. 450S 450 Scenario (following IEA): the most aggressive scenario in terms of energy conservation and greenhouse emission reduction; modification on power and transportation sectors based on most recent domestic plans or research.  in China's emission inventory literature (Zhao et al., 2011b). In this work, therefore, a rough estimate is made that the relative changes in lime and brick production in all scenarios follow those of cement in NPS. The coal consumption of those industrial processes is then calculated following the methods described in previous studies (Zhao et al., 2011b;. In particular, it should be noted that coal consumption will be influenced by technology changes of emission sources, and that fact is considered in the activity level estimation by sector. For example, precalciner kilns consume 30 % less coal than rotary ones for the same amount of cement production , and the production of solid clay bricks requires twice the coal consumption of hollow ones . The details of technology changes by sector will be explained in Sect. 3. Figure 1a summarizes the coal con-sumption by source and scenario for 2015, 2020, 2025 and 2030 estimated in this work. The future oil consumption by the transportation sector is a big concern in China's energy research and has been addressed by a series of studies Huo et al., 2012a;Ou et al., 2010;IEA, 2012). Compared to IEA (2012), Huo et al. (2012a) incorporated the latest data from investigations into the fuel economy and intensity of use of China's vehicles. Thus the oil consumption of on-road and rural vehicles (the vehicles that have much smaller engine power, lower designed maximum speed, and lower cost than lightduty diesel trucks, used for transportation of goods and passengers in countryside) in the high-and low-vehicle-growth scenarios by Huo et al. (2012a) are applied in this work, replacing the data of the original CPS and NPS by IEA (2012).  For 450S, we keep the values of IEA (2012) as the most aggressive case for energy conservation. Regarding nonroad sources, the diesel consumption of railways and inland ships is adopted from the IEA; for other off-road equipment, the growth rates after 2010 are taken from an assumption in a recent domestic study (Li, 2011). Figure 1b summarizes the future oil consumption from China's transportation sector by source and fuel type for different scenarios. With revisions based on the various aforementioned sources, the difference between CPS and NPS in this work tends to be larger than that in IEA (2012).

Emission factor projection
With such an expansive and complex emission category structure, the emission characteristics vary largely across both individual sources and sectors in China. Current and future emission control measures under the NAPAPPC and possible implementation of new emission standards for given sectors are expected to further change the penetration levels of different energy efficiency technologies and emission control devices, and thus to significantly affect the emission fac-tors of primary pollutants. Therefore, quantification of the potential trends of emission factors by source is crucial for projecting national emissions and is thereby the primary undertaking of this study. A base case (BAS), a reference case (REF), and a case of fully implemented emission standards (STD) are applied, as summarized in Table 1. In BAS, the emission control levels for each given technology are conservatively assumed unchanged in the future. This does not imply, however, that the penetration levels of advanced technologies and emission control devices for a whole subsector or sector will necessarily be the same as in the base year 2010. For example, the policy of replacing small power units with bigger and cleaner ones will undoubtedly raise the application of FGD systems for the power sector overall, even if the FGD application rates for small and large units remain at 2010 levels. REF is set as the "best guess" in terms of emission control strategies for the future. The proposed polices and possible new improvements in emission control according to the NAPAPPC are included in the scenario and their benefits are analyzed with currently available domestic information. As a plan issued by the central government, however, NAPAPPC itself does not specify the emission limits of different types of emission sources, and the actual emission levels do not necessarily satisfy the emission standards (issued or proposed), particularly for small and energy-inefficient industrial sources. The benefits of emission control measures may not be as sufficient as expected, attributed mainly to possible poor management and operation of air pollutant control devices for saving costs Wang, 2013). STD, therefore, is a case assuming that the series of emission standards for sources across power and industrial sectors that China has issued or updated since 2010 will be strictly enforced (these standards are listed in Table S1 in the Supplement). Accordingly, STD is an aggressive control strategy and provides an ideal case exploring the potential of China's emission standards on emission abatement. In summary, the trends of emission factors for NPS by source and scenario are illustrated in Fig. 2 (similar trends are projected for CPS and 450S, and they are not shown here for paper length but in Figs. S4 and S5 in the Supplement, respectively). In each panel of Fig. 2, the penetrations of various technologies are indicated on the left-hand vertical axis and the emission factors for BAS, REF, and STD on the righthand vertical axis (except for nonferrous metal production). The emission factor trends by sector are analyzed in detail below.

Coal-fired power plants
Since 2005, coal-fired power plants have been targeted for the most stringent emission controls, and the capacity shares of big power units (≥300 MWe) and FGD systems reached 78 and 86 % at the end of 2010, respectively, leading to great SO 2 and PM emission reductions (Y. . For NO x control, the capacity share of selective catalytic  reduction (SCR) technology was still small at 13 % in 2010, and another 100 GW of SCR systems are under construction (Wang, 2013). The future emission trend depends largely on the penetration and removal efficiency of those technologies. In BAS, the emission control strategies for the power sector in 2010 are assumed to continue; i.e., FGD systems will be required at all new power units and the national average of SO 2 removal efficiency will remain 70 %, consistent with the results of a national survey of emission sources conducted in 2007 (Y. ; no additional penetration of SCR is expected. REF assumes a higher (80 %) SO 2 removal efficiency of FGD systems, due to improved operations, as indicated by recent investigation of power plants by the authors (Qiu et al., in preparation; see Table S2 in the Supplement). It also assumes SCR technologies are required for all newly built units and "old" ones (i.e., those built before the end of 2010) equal to or bigger than 300 MWe in "key" regions (i.e., those with relatively concentrated industry and heavy pollution: eastern, north-central, and south-central China in this work, shown in Fig. S1 in the Supplement). For old units located in other areas, selective noncatalytic reduction (SNCR) technology is assumed, which has lower cost but also lower benefits of NO x control. The penetrations of different technologies by scenario and year are indicated in Fig. S2a in the Supplement. The average NO x removal efficiencies for SCR and SNCR are cautiously set at 60 and 50 %, respectively, less than those of full operation (Wang, 2013; personal communication with L. Pan from China Electricity Council, 2013). The lower removal efficiencies found in China than in developed countries result mainly from relatively poor operation and maintenance of installed technologies in order to save costs Wang, 2013). For example, less ammonia used in SCR systems leads to lower removal efficiency of NO x . For PM control, increased penetration of fabric filters (FF) or electrostatic-bag dust collectors is assumed, accounting for one-third of newly built unit capacity (see Fig. S3a for the trends of dust collector shares). All of the unabated emission factors and PM removal efficiencies for various types of dust collectors are obtained from the power plant emission factor database compiled by the authors ; see also Table S2 in the Supplement).
In STD, the new national emission standard for power plants issued in 2011 (GB 13223-2011) is applied to determine the emission factors of power units. Since the standard requires all of the units with different ages to satisfy the same emission limits at the beginning of 2014, the emission levels of SO 2 , NO x , and PM are simply set to the standard limits of 200, 200, and 30 mg m −3 , respectively, for 2015 and after. The emission factors (kg t −1 ) can then be derived following the method provided by Zhao et al. (2010). There are no specific limits for PM differentiated by size class; thus the mass fractions of PM 10 and PM 2.5 to PM are respectively assumed to be 0.81 and 0.45, based on the field measurement results of power units with FF that can achieve the emission standard for TSP . This method will also be applied for other source types in similar cases to determine the PM 10 and PM 2.5 mass fractions in STD. As shown in Fig. 2a, NO x emission factors are expected to decrease faster than the other two pollutants in REF and STD, reflecting the intended benefits of the country's NO x control measures in the future.

Cement production
At the end of 2010, China's cement production was dominated by precalciner kilns, the most energy-efficient technology, with its share exceeding 80 % (Y. . Although the PM emission control performance of precalciner kilns is strong due to high penetration of FF (as shown in Fig. S3b in the Supplement), they are liable to generate more NO x emissions because of higher combustion temperatures than other types of kilns . In BAS, all of the cement plants built after 2010 are assumed to use precalciner kilns but without the requirement of additional NO x controls, such as SCR/SNCR systems. In REF, all nonprecalciner kilns are assumed to be entirely shutdown by 2020 and the application of SNCR, with the NO x removal efficiency tentatively set at 50 %, is assumed for precalciner kilns in the aforementioned key regions after 2010. For kilns in other areas, low-NO x burners (LNB) are assumed to be used after 2015, with a lower NO x control effect (see the penetrations and removal efficiencies of those systems in Fig. S2b and Table S2 in the Supplement, respectively). Although the new emission standard for cement production has not yet been issued, a reduction in the NO x standard from 800 to 400 mg m −3 has been proposed (unpublished), leading to a reduced average emission factor of 1.2 kg NO x t −1 clinker. Based on the assumption that 1 t of cement is produced from 0.72 t of clinker, and that production of 1 t of cement requires 125 kg of coal in precalciner kilns , the NO x emission factor of the proposed limit is calculated at 6.8 kg t −1 coal, and applied in STD of this work. Given the time needed for standard implementation, however, the old plants (defined as those built before the end of 2010) are assumed to satisfy the limit from 2020 on. All of the other unabated emission factors of SO 2 , NO x , and PM and the PM removal efficiencies of dust collectors are taken from the country-specific database compiled by Lei et al. (2011b) and Zhao et al. (2011b). As seen in Fig. 2b, PM emission factors in REF and STD will keep declining due to additional penetration of FF, and the NO x limit of the proposed standard could be approached by use of SCR systems, even without full operation of the technology.

Iron and steel production
The emissions of the iron and steel industry include mainly gaseous and particle pollutant emissions from coking, sintering, and pig-iron production (in blast furnaces), and particle emissions from steel production (nearly 90 % of which occurs in basic oxygen furnaces) and casting processes. In this work, we assume that the shares of various types of dust collectors would remain the same as those in 2010 for all processes in BAS, while new emission sources (reflected by the annual net growth of production) would apply the most advanced dust collectors that have already been deployed in analogous processes in REF; thus reflecting the effect of national policies to foster energy conservation and emission reduction. This assumption leads to a strong reduction of PM emission factors of iron and steel production in REF compared to those in BAS (Fig. 2c), and it will still hold for other industrial sources described in Sect. 3.4 (see Fig. S3ck in the Supplement for detailed information on penetration levels of various dust collectors in iron and steel production and other industrial processes). The unabated PM emission factors and removal efficiencies of dust collectors used in BAS and REF are taken from a previous study by the authors (Zhao et al., 2011b).
For coke production, the emission factors are updated to 1.0 (machinery coking ovens)-4.3 kg t −1 coke (indigenous ovens) for SO 2 and 1.7 kg t −1 coke for NO x , based on recent domestic measurements (He, 2006;Huo et al., 2012b). The emission factors used for STD are determined based on the emission standard of pollutants for the coking chemical industry issued in 2012 (GB 16171-2012), which sets the same emission limits for 2015 and beyond for both newly built and existing ovens. For example, the SO 2 concentration in flue gas is limited at 50 mg m −3 for machinery coking ovens, equal to 0.24 kg t −1 coke based on an average of flue-gas volume at 5.0 N m 3 kg −1 coke (He, 2006). The big reduction of emission factors from REF to STD would force the adoption of coke gas desulfurization in coke production to achieve sufficient removal efficiency. Similarly, the NO x and PM emission factors for machinery coking are calculated at 2.4 and 0.3 kg t −1 coke, respectively.
Based on domestic measurements (AISGC, 2007;MEP, 2010), the SO 2 and NO x emission factors for sintering processes were respectively estimated at 2.9 and 1.3 kg t −1 product, as used in previous work (Y.  and in the BAS of this study. Under REF, the adoption of FGD systems is assumed for sintering, with the mean removal efficiency of SO 2 conservatively set at 50 % according to a limited field investigation by the authors in 2012 (unpublished). As a result, the SO 2 emission factor for the whole iron and steel sector will be significantly reduced, as sintering is the main source of SO 2 emissions in the sector (Fig. 2c). However, no extra measure for NO x control is assumed to be implemented. The STD emission factors are determined based on emission standards of air pollutants for sintering and pelletizing issued in 2012 (GB 28662-2012); i.e., 0.7, 0.8, and 0.25 kg t −1 product for SO 2 , NO x , and PM, respectively.
For blast-furnace iron production, a national survey (MEP, 2010) determined the current averages of emission factors for SO 2 and NO x as 0.15 and 0.20 kg t −1 iron, respectively. These factors were applied in previous work (Y.  and used in the BAS and REF cases of this study. Regarding STD, domestic plants with the highest energy efficiencies and most advanced emission control technologies were investigated (SSC, 2007). The report determined the emission factors at 0.05, 0.10, and 0.12 kg t −1 iron for furnace SO 2 , NO x , and PM. These values were used as the emission standards for air pollutants from the iron smelting industry (GB 28663-2012). The mass fraction of PM 10 and PM 2.5 in STD are estimated based on the removal efficiencies of FF by particle size. The fugitive PM emissions, however, are difficult to quantify for STD and are assumed to be the same as REF. In steel production, very little SO 2 or NO x is emitted from basic oxygen/electric furnaces. According to a national investigation (BGC, 2007), the PM emission levels of big plants with improved emission control are approximately 60 % lower than the national average and set as the limit of the emission standard for the steel smelting industry (GB 28664-2012) and thereby assumed as the emission factor of STD in this work.
The casting process also generates particles. However, there is no clear emission standard or other information about emission control for the process yet. Therefore we assume the same PM emission factors in STD as those in REF of this work.

Other industrial sources
Emission factors of brick production, lime production, nonferrous metal smelting, and other boilers are described in this section.
Coal combustion in kilns is the main source of SO 2 and NO x emissions in brick production. In the base year 2010, solid bricks accounted for approximately 40 % of total production and the national average of coal consumption per 10 000 bricks was estimated at 0.88 t equivalent (tce), based on the coal consumption rates of 1.3 tce/10 000 solid clay bricks and 0.6 tce/10 000 hollow ones (Xu and Wang, 2007;Zhao et al., 2012). According to a national survey of brick kilns (CRAES, 2009), the average SO 2 and NO x emission levels are 345 and 160 mg m −3 in flue gas, respectively. The emission factors can then be calculated at 17 and 8.0 kg/10 000 bricks based on the average flue-gas amount of ∼ 50 000 nm 3 /10 000 bricks, i.e., 14.0 and 6.5 kg t −1 coal, respectively. We assume the fraction of solid bricks is unchanged in BAS, and thus the SO 2 and NO x emission factors remain the same as in 2010. In REF, however, the use of solid bricks is assumed to be strictly limited at the 2010 level and the coal consumption per 10 000 bricks decreases accordingly, leading to a lower level of SO 2 and NO x emission levels, as shown in Fig. 2d. Regarding PM, as mentioned in Sect. 3.3, the penetration of various types of dust collectors and thereby the emission factors for BAS are assumed the same as that in 2010 (Y. . In REF, more application of wet scrubbers with higher PM removal efficiency than cyclones is assumed, resulting in a considerable reduction of PM emission factors. For STD, as proposed in the new standard (CRAES, 2009), the PM limit is set roughly 80 % lower than the current level, while no significant reduction is required for SO 2 or NO x . The mass fractions of PM 10 and PM 2.5 are determined based on the size-dependent removal efficiencies of wet scrubbers.
Until now there is little information about nor any proposed standard for emission control in lime production. The emission factors for SO 2 and NO x in base year 2010 are applied for all scenarios in this work. PM emission factors for BAS remain the same as the base year, while they are expected to decline in REF and STD, attributed to increased penetration of FF with higher PM removal efficiencies.
Nonferrous metals include production of copper (Cu), lead (Pb), zinc (Zn), electrolytic aluminum (Al) and alumina that generate SO 2 and PM emissions. For SO 2 , a national survey was conducted (MEP, 2010); based on which the current level, the emission limit for existing sources, and the limit for newly built sources, are determined and applied for emission factors of BAS, REF, and STD in this work, respectively. For Cu smelting, as an example, the current average level of fluegas SO 2 concentration, the limit for existing sources, and the limit for the new sources were estimated at 2116, 960, and 400 mg m −3 . The emission factors of BAS, REF, and STD can then be accordingly calculated at 49, 22, and 9 kg t −1 Cu, respectively, based on the average flue-gas amount of 23 000 N m 3 t −1 Cu (CRAES, 2007;MEP, 2010). The PM emission factors of BAS are assumed the same as in the base year (Y.  and those of REF are expected to decrease due to more application of advance dust collectors. For STD, the newly issued emission standards for nonferrous metals (GB 25465-2010;GB 25466-2010;and GB 25467-2010) require a significantly enhanced control, and the extremely low limits imply the need to apply the most effective dust collectors like FF. Compared to BAS, roughly 80 % of emission abatement is required by the standards for both SO 2 and PM (Fig. 2e).
For other industrial coal-combustion boilers, the emission factors in BAS are assumed to be the same as in 2010. In REF and STD, FGD and LNB are expected to be used for newly built sources following the instructions of the NAPAPPC with the average SO 2 and NO x removal efficiencies of 70 and 30 %, respectively. Figure S2c in the Supplement shows the penetrations of those technologies in detail. For PM control, similar to other industrial sources, the new sources are expected to be installed with wet scrubbers, leading to reduced PM emission factors. The emission factors of oil and gas boilers are assumed to remain the same as in 2010 for all the scenarios.

Transportation
China's new on-road vehicles have been required to meet staged emission standards (stages I-IV, equivalent to Euro I-IV) since 1999. Based on the vehicle population growth and assumed vehicle lifetimes by type, the fleet compositions by control stage can be determined and thus the emission factors by type can be calculated as described in previous work . Therefore the times of implementation of stricter emission standards (e.g., stage V or VI) in the future are crucial for projecting emission levels in transportation.
Although stages I-III were implemented roughly at the same time across the country (except for certain megacities like Beijing), the standards under latter stages may come into operation asynchronously by province/region, making the projection quite difficult and uncertain. In BAS, a conservative assumption is made in that stages IV and V for on-road vehicles would take effect as of 2013 and 2016 in the entire country, respectively, while those implementation years are expected to move forward to 2011 and 2014 in REF and STD for selected provinces with large and dense vehicle populations, including Tianjin in north-central China, Shanghai, Jiangsu and Zhejiang in eastern China, and Guangdong in south-central China (see Fig. S1). It should be also noted that certain nonroad sources are required as well to gradually meet standards but no differences are assumed between the scenarios of this work. Table S3 in the Supplement summarizes the time schedule of implementation of emission standards for China's transportation sector.
In previous work, the on-road measurements of NO x and PM 2.5 emission factors for China's vehicles by type and control stage were thoroughly investigated and the emission factors were obtained in real-life operating conditions and applied in emission inventory development (Y. . In this work, most recent domestic studies Huo et al., 2012c) are included to update the emission factor database, and those updated emission factors are applied in the BAS and REF scenarios. For STD, an aggressive assumption is made in that standard limits would be strictly satisfied, with little effect of vehicle deterioration from poor inspection and maintenance considered. This is of course an ideal case providing minimum emission levels for a given vehicle population with a fixed fleet composition. Figure 2f-h show the fleet composition and changes in emission factors for typical vehicle types. Faster reduction of NO x emission factors is expected for light-duty vehicles (as they already have relatively low PM emissions factors), while a faster reduction of PM is expected for heavy-duty diesel and rural vehicles.

Residential and commercial combustion
For the residential and commercial sector, very few measures or standards of emission control have been announced or are expected to be implemented in the near future. In most cases, therefore, the emission factors for all of the scenarios are assumed unchanged from those in 2010, including for coal, oil, gas, and biofuel combustion. One exception is the coal combustion in REF and STD, in which the share of small-coal stoves is assumed to decrease due to penetration of more district heating with advanced grate boilers, resulting in reduced PM emission factors. Most recent domestic field studies (H. Zhang et al., 2012;Shen et al., 2013) are included to update the emission factors used in previous work (Y. .

Emission trends by scenario
The national emissions in 2015, 2020, 2025 and 2030 are summarized by the scenario in Table 3, and the emissions in 2010 (Y.  are provided as well for comparison. The sector distributions of emissions are indicated in Fig. 3 for scenarios with different emission control levels BAS, REF, and STD under a common energy and industrial production trend, the best-guess NPS. Compared to energy and industrial production, the emission control strategies tend to have more effect on the estimated future trends of emissions for all of the concerned pollutants. Under the BAS scenarios representing current emission control strategies, national SO 2 emissions are estimated to increase compared to 2010, except for the case of 450S in 2030. Since SO 2 emissions are mainly from fossil fuel In STD, the shares of the power sector decrease by approximately 20 %, spotlighting the importance of other industrial boilers as significant emission sources that should be of greater concern. A modest reduction of emissions is found for residential and commercial activities, due to the gradually reduced coal consumption for the sector. The NO x emission trends in BAS cases would be dominated by fossil fuel use, and the emissions in 2030 are estimated at 38.6, 32.8 and 24.9 Tg (i.e., 134, 114, and 86 % of emissions in 2010) for CPS, NPS, and 450S cases, respectively. In the REF scenarios, significant benefits would be achieved from the penetration of SCR and LNB technologies in power and industrial sectors. Even with a conservative estimate of the national average NO x -removal efficiencies, the power and cement sectors would see their emissions reduced by 54 and 48 % respectively from the BAS to the REF case in 2030 under a common NPS energy path, leading to a 20 % reduction of the national total emissions from 2010 to 2030, i.e., from 28.8 to 22.9 Tg. In STD, the very aggressive emission standard for power plants would limit emissions of the sector to 1591 (for 450S) and to 3079 Gg (for CPS) in 2030, approximately 80 % lower than the levels in BAS cases. In contrast to power plants, the share of transportation would not significantly decrease for any of the scenarios until 2030, as shown in Fig. 3b. For heavy duty vehicles, the biggest sources of transportation NO x emissions, current on-road tests failed to find a statistically significant improvement of NO x emission factors as emission standards became more stringent . Similar results were also found for rural vehicles (Yao et al., 2011). Stage III and IV vehicles are thus believed to have emission levels close to those of stages I and II, most likely attributable to similar driving patterns and diesel fuel quality. Additionally, the NO x emissions from vehicles will be relatively difficult to reduce in the near future.
Coming largely from industrial processes, PM emissions are projected to be less affected by the energy path than SO 2 or NO x , since the output levels of the main industrial products are estimated to be similar among different energy scenarios. A considerable reduction in emissions is expected from the further penetration of advanced dust collectors at industrial sources under the national action plan of air pollution control. In the NPS-REF case, as an example, the PM emissions from cement, iron and steel, brick, and nonferrous metal production in 2030 are estimated to decrease by 74, 22, 24, and 56 % compared to 2010, respectively, leading to a decline of TSP emissions from 28.7 to 16.9 Tg during the period. Under the implementation of issued or proposed standards (the STD case), PM emissions from brick and nonferrous metal production in 2030 would be further reduced by 73 and 52 %, respectively, compared to the REF case. The benefit of emission standards for iron and steel production, however, is limited, since the fugitive dust requirement by the standards, expressed as the ambient dust concentration near the plant, cannot be directly accounted as an emission abatement benefit. Regarding PM by size, the shares of finer particles would grow as more stringent control reduces PM overall (chiefly through reduction of coarser particles), raising the difficulty of further reductions because smaller primary particles are more difficult to abate. The mass fractions of PM 2.5 to TSP, as an example, are estimated to increase from 40 % in BAS cases to 55 % in STD cases for 2030. Shown in Fig. 3ce, certain industrial sources (e.g., brick and lime production) play important roles in TSP emissions but contribute much less to finer particles. Transportation and residential combustion of fossil fuels and biofuel are thus identified as more important sources of fine particles, particularly as control of industrial emission continues to progress in the future.

Limitations
In this work, the benefits of national emission control strategies and the most recent emission standards are quantified, although there are still considerable uncertainties. The NO x emission levels from coal combustion, for example, are influenced by many factors including coal quality, burner types, and combustion operation . The actual removal efficiencies of SCR and SNCR systems can vary significantly between individual plants, particularly when those plants are required to reach or even approach a specific emission standard. It is thus difficult to determine accurate removal efficiencies for the country, even by the power companies themselves (personal communication with L. Zhu from China Guodian Corporation, one of the largest power companies in China, 2013), and the national averages derived from limited tests and expert judgment had to be applied in this work. Similarly, due to a lack of information regarding individual small boilers, the NAPAPPC could not be followed precisely in this work, and local plans for air pollution control are being issued, or will be issued, subsequent to the national one, particularly in provinces with heavy pollution. This leads to divergent emission controls by region in practice. Finally, uniform emission standards have been set in STD for a given type of emission sources due to a lack of detailed individual plant data, resulting in possible overestimates of emissions from plants that actually have lower emission levels than the standards. All of these facts imply a need for further investigation of individual plants in key regions and sectors, to better understand the future trends of emission factors for China's air pollutants. Besides emission control levels, the activity levels (i.e., energy consumption and industrial production) also play significant roles on future emission trends. In this work, the energy projection by IEA (2012) is mainly applied with revisions for specific sources combining the latest information. The future energy trends, however, would be significantly independent of national and local energy policies, which could not be fully followed or predicted at the current stage. Thus the estimated emissions based on current energy projections should be interpreted cautiously and necessary revisions are strongly suggested when new energy policies become available.
Because of the above-mentioned limitations, there are discrepancies between the projected emissions in this work and the national emission targets that are intended to be achieved through full implementation of the emission control strategies, particularly in the near future. For example, the country has announced that the annual emissions of SO 2 and NO x in 2015 will be reduced respectively by 8 and 10 % compared to those in 2010, while much smaller abatement is found following the NPS-REF path (the "best-guess" case) in this work. To further understand the recent trends of emissions, the vertical column density (VCD) of tropospheric NO 2 from the Ozone Monitoring Instrument (OMI) are used as indirect evidence. The annual NO 2 VCD over mainland China from 2010 to 2013 is calculated and mapped in Fig. S4 in the Supplement based on the monthly data with resolution of 0.125 • ×0.125 • retrieved by the Royal Netherlands Meteorological Institute (Boersma et al., 2007; http://www.temis.nl/ airpollution/no2col/no2regioomimonth_v2.php). Since the data in November and December of 2013 were unavailable at the time of writing, 10-month (from January to October) averages instead of annual means are used for interannual comparisons. Although the 10-month mean NO 2 VCD reached a peak in 2011 and started to decline afterwards, the mean VCDs in 2012 and 2013 are still 13 and 8 % higher than those in 2010, implying incomplete implementation of the emission controls and illustrating the difficulty in national emission reduction. Moreover, as shown in Fig. S4, clear regional differences are found in VCD trends. Compared to megacity areas (e.g., the Yangtze River delta region), bigger growth in VCDs from 2010 to 2013 are found in less-developed areas, such as north-central and south-central China, consistent with Zhang et al. (2012b). These limited satellite-retrieved VCD data thus reconfirm (1) the importance of careful investigation of emission control implementation to improve the accuracy of emission projections and to ensure the success of national polices, and (2) the necessity of emission trend analysis by region.
Besides the species concerned in this work, some other pollutants including volatile organic compounds (VOCs), NH 3 and CO are confirmed to play important roles in atmospheric chemistry and pollution formation in China (Xing et al., 2011b;Wang et al., 2011;Chen et al., 2009). Those pollutants come largely from industrial and agricultural processes with big diversities of manufacturing technologies and pollution control levels. Much larger uncertainties of emissions were found for them than for SO 2 and NO x that come more from big energy-related sources . Till now they are relatively less stressed by the current emission control strategies or standards compared to SO 2 , NO x and PM, resulting in even higher uncertainty of emission projection for them. Therefore, the study on future trends of emissions for those species is strongly suggested once the improved and more specific emission control policies and standards become available.

Comparisons with other studies
A series of studies focuses on projections and future trends of China's primary air pollutants. Earlier studies (e.g., Waldhoff, 2000, andKlimont et al., 2001) were based on relatively conservative projections of energy growth and emission control strategies, and thus are not included in the comparisons of this work. Since 2005, three main groups or research programs have conducted thorough analysis of Chinese future emission trends using different scenarios: the Regional Emission inventory in ASia (REAS; Ohara et al. 2007), the International Institute for Applied System Analysis (IIASA; Amann et al. 2008, andCofala et al. 2012),   and Tsinghua University , B. Zhao et al., 2013. Ohara et al. (2007) set three cases to evaluate China's SO 2 and NO x emissions through 2020: PSC (policy success case), REF (reference case), and PFC (policy failure case). Amann et al. (2008) set CLE (current legislation), ACT (advanced control technology) and OPT (a least-cost optimization scenario that would achieve the same health benefit as ACT) cases to analyze the effects of control strategies on the emissions. Cofala et al. (2012) based their study on CPS, NPS, HE (high energy efficiency scenario), and 450S from the IEA to analyze the effects of energy path, with few revisions from recent domestic information that is included in this work. Xing et al. (2011a), B. Zhao et al. (2013 and S. X.  devised energy (REF/BAU/PC)-emission control (0/1/2) combination cases to evaluate the possible trends of air pollutants in the future. Since those three studies follow the same methods, we merge the results and refer to them as "Tsinghua" in Fig. 4. The results of the above-mentioned studies are compared with this work for SO 2 , NO x , and PM 2.5 emissions, as shown in Fig. 4a, b and c, respectively.
Given the different energy and emission control assumptions, clear differences exist among these studies for projections of China's future emissions of primary air pollutants. However, this work and other studies share some common judgments: (1) the growth of China's primary air pollutant emissions could be constrained through implementation of pollution control strategies in the country, and (2) improved control strategies will play a more important role in emission abatement than variations in possible energy paths, as indicated by the larger differences between scenarios in Amann et al. (2008) than those in Cofala et al. (2012). In most cases, the emissions estimated in this work are higher than similar cases from other studies. As shown in Fig. 4b, for example, the NO x emissions in BAU1 and PC1 by Tsinghua in 2020 are 28 and 39 % lower than those in NPS-REF and 450S-REF of this work, respectively, and the analogous values in 2030 would be 31 and 38 %. The most probable reason for this is that more conservative NO x removal efficiencies of LNB, SCR, and SNCR are applied in this work, attributed to the unclear overall operational conditions of those devices at the national level, as described in Sect. 4.2. There are relatively few studies that include PM 2.5 because of the higher methodological complexity of its projection than that of other pollutants. Lower emissions are found for this work than those by Cofala et al. (2012), possibly due to the use of unabated emission factors and removal efficiencies for several dust collector technologies based on recent domestic measurements by the authors (e.g., Zhao et al., 2010).

Emission control: the imperative to broaden the disproportionate focus on coal-fired power, cement, and iron and steel sectors
For a long time, China's serious air pollution has been strongly associated with its heavy dependence on coal use for industrial production and electricity generation. Almost half of China's coal is consumed by the coal-fired power sector (CPP), and the coal-fired fraction of total electricity has remained relatively stable at approximately 80 %. Compiling information at the level of generating units to estimate emissions, the SO 2 , NO x , and PM 10 emissions from CPP increased by 49, 64, and 7 % from 200049, 64, and 7 % from to 200549, 64, and 7 % from (Zhao et al., 2008, and CPP accounted for 52, 36, and 11 % of national emissions of those pollutants in 2005, respectively (Y. . Given these large shares of air pollutant emissions, CPP has been considered the most important target for emission control across the country since 2005 and great efforts have been made to reduce the emissions from electricity generation. Compulsory requirements in energy conservation and emission control have also been implemented in certain major industrial sources other than CPP, including cement (CEM) and iron and steel production (ISP). To evaluate the effectiveness of the policies targeting large sources, the historical emissions of those sectors and shares of national totals from 2000 to 2012 are analyzed and compared with future trends projected in this work. The historical data come from various sources that follow the same bottom-up emission inventory principles: emissions from CPP are estimated with unit-based information from Zhao et al. (2008); SO 2 , NO x , and PM 10 emissions from other sources in 2000-2004 are taken, respectively, from Lu et al. (2011 and Lei et al. (2011a); emissions in 2005-2010 are from Y. ; and emissions for 2011 and 2012 are updated following the methodology of Y. , with the most recent commitment of emission control polices included. For future trends, NPS-REF (the best guess scenario) and NPS-STD (with the best guess of the energy path combined with the most aggressive emission control strategies) are selected for comparison. Figure 5 shows the annual emissions of SO 2 , NO x and PM 10 for CPP (Fig. 5a) and CPP + CEM + ISP (Fig. 5b) and as well as their shares in total national emissions. It is found that the emissions and fractions of SO 2 and PM 10 for those concerned sources started decreasing around 2005, reflecting the benefits of national pollution control policies set mainly under the 11th Five Year Plan. Even with tightened controls in CPP, CEM, and ISP, however, no further significant reduction of SO 2 and PM 10 emissions are expected in The only exception is the emissions of PM 10 from CPP + CEM + ISP after 2020. This spotlights the very limited abatement potential remaining in those sectors from a national emission perspective, due to the near saturation of emission control technologies in these industries. For instance, penetration of FGD in the power sector and FF in precalciner cement kilns had reached an estimated 86 and 83 % of total capacity by 2010, respectively, as discussed earlier. NO x emissions from these three sectors increased continuously from 2000 to 2012, but the sharply expanded deployment of SCR and SNCR is expected to lead to considerable abatement in these sectors from 2010 to 2015. Similar to SO 2 , however, it will become more difficult to reduce NO x emissions and national fractions from CPP, CEM, and ISP after 2015 because of saturation of these control technologies. It appears clear that a continued disproportionate focus on the major sources of CPP, CEM, and ISP to achieve the national goals regarding air pollution will be inadequate, under currently projected trends of energy growth, to get anywhere near to the scale of reductions in SO 2 emissions achieved from 2005 to 2010. If emission standards can be strictly implemented (NPS-STD in Fig. 5), more abatement of emissions could be achieved for these three sectors, but their emissions and fractions would still increase over time despite the aggressive controls. Given the high costs from the improved operation of emission control systems to meet the standards, the STD case is less likely to occur than REF case.
To alleviate China's air pollution effectively in the near future, therefore, it is imperative to broaden the control targets from big sources to other swiftly growing sectors; including building material (brick and lime) production, chemical production, small industrial boilers, and residential stoves. The implementation of energy saving and emission control measures in those sectors will be challenging because of the geographic dispersion of sources and much greater diversities of production technologies. Thus their emissions will often be much harder to monitor and supervise than those of big sources. In order to restrain national emissions and to prevent deteriorating the air quality, however, China has almost no choice but to urgently extend the kind of control measures enacted in CPP, CEM, and ISP to more sectors than ever before.

The trends of PM chemical species and their environmental implications
The future emission trends of PM chemical species, including alkaline dust (calcium (Ca) and magnesium (Mg)) and carbonaceous aerosols (black carbon (BC) and organic carbon (OC)), are projected using the methods provided in our previous work (Zhao et al., 2011;. The emission factor database of PM chemical species is updated particularly for the transportation and residential sectors, combining the most recent field tests (Song et al., 2012;Shen et al., 2013;Wei et al., 2014). It should be noted that the mass fractions of those species for the industrial and transportation sectors have to be assumed constant due to a lack of time-series data from measurements. Uncertainty thus may exist in the projection, and long-term field tests are suggested to support the time-series analysis of emission factor evolution for those species. As shown in Table 4, significant abatement of anthropogenic alkaline dust emissions is expected in the REF and STD scenarios. As an example, two-thirds of Ca emissions are expected to be cut from 2010 to 2030 in the NSP-REF case. Since the alkaline dust is produced mostly by industrial sources, particularly building material production (cement, lime, and brick), the emissions can be efficiently reduced through the expanded use of advanced dust collectors in those sectors. Fewer benefits are found for carbonaceous aerosols, as the emissions are largely from coal and biofuel combustion in the residential and commercial sectors and biomass open burning. In this work, little progress of emission control is assumed for those sources, and the declines of carbonaceous aerosols result mainly from the decreased use of fossil fuel and biomass burning. The reduced alkaline dust may increase the risks of ecosystem acidification, as the acid-neutralizing capacity would likely decrease as a result. Illustrated in Fig. 6 are the relative changes of China's Ca, PM 10 , and SO 2 emissions from 2010 to 2030 for two selected scenarios in this work, NPS-REF and NPS-STD. For comparison, the analogous data on PM 10 and SO 2 emissions for the US (USEPA, 2011) and for the European Union (CEIP, 2011) for the period 1990-2010 are shown. In contrast to the greater reductions of SO 2 than PM historically in the US and EU, a faster decrease in PM (and thus also Ca) emissions than SO 2 is projected for China in the future, implying that recovery of acidification in the country will be a considerable challenge under the current national emission control commitment. Long-term observations at different sites across China have partly confirmed the increased acidity of precipitation and decreased alkaline species, particularly in rural areas (Tang et al., 2010;. As the serious haze and PM pollution is now becoming the biggest focus of air quality improvement, further abatement of primary PM (and thereby alkaline base cations) would gradually lead to clearer skies in urban areas but may exacerbate regional acid deposition, if further reduction of SO 2 emissions is constrained.
Besides local and regional conventional air pollution impacts, the future trends of aerosols can affect regional climate as well. In this work the changes of radiative forcing (RF) from trends in China's short-lived species are evaluated as the sum of emission-based constituent global forcing (CGF) of SO 2 , NO x , BC, and OC, weighted by the changes in China's emissions as percentages of total global emissions, following the methods of Carmichael et al. (2002). The emission-based CGF represents that emissions of a single primary precursor can affect several related forcing agents, and the values are taken from Carmichael et al. (2002) and IPCC (2007). It should be acknowledged that the forcing efficacy from those short-lived species is assumed constant for a rough estimate. Figure 7 shows the effects of changes in Chinese emissions between 2005 and 2030 on radiative www.atmos-chem-phys.net/14/8849/2014/   Cofala et al. (2012), while those of carbonaceous aerosols are from Klimont et al. (2014). The latter applies the same methodology as Klimont et al. (2009) and Bond et al. (2013). Emissions from global biomass open burning are taken from Bond et al. (2004).
As shown in Fig. 7, the reduced primary carbonaceous emissions play similar roles in RF change from CPS-REF to 450S-REF, as the fossil fuel energy paths (driven by industrial and power generation demand) have limited effects on BC and OC emissions. The warming effects of SO 2 reduction are expected to increase, as the emissions would be reduced resulting from reduced coal combustion. Under a common energy path (NPS), both the cooling effects from BC reduction and warming effects from SO 2 reduction are expected to grow with the improved implementation of emission controls. The SO 2 reduction would dominate the RF changes because current emission control commitments and standards focus little on the residential coal and biofuel combustion that generates most of the carbonaceous aerosols. The tightened controls in China are thus expected to result in enhanced RF in the future. Efforts to reduce emissions from the dispersed residential sources such as small coal and biofuel stoves would not only lead to improved local and regional air quality, but also help limit regional warming from short-lived climate forcing.

Conclusions
Given the high frequency of serious haze and heavy urban and regional air pollution events, China has announced a national action plan of air pollution control, and a series of measures aimed at energy conservation and emission reduction will be implemented in the coming years. The effectiveness and benefits of those measures on future national emission abatement are evaluated in this work using scenarios coupling different levels of energy consumption with several emission control strategies. Should current policy commitments be implemented, China's primary pollutant emissions are expected to be restrained in the future, including emissions of NO x that have grown almost continuously in recent years. However, compromised operational conditions of swiftly disseminating air pollutant control devices (e.g., SCR systems) lead to lower removal efficiencies of air pollutants than expected, and thus partly undermine the benefits of emission abatement in the near future, as suggested by satellite-retrieved NO 2 vertical column densities observed across the country. Compared to the energy path, the levels of emission control implementation will play more important roles in future trends of emissions for all of the concerned pollutants, particularly for primary aerosols that depend largely on noncombustion industrial processes. Should the emission standards across sectors that have been either issued or proposed be fully met, they could prove highly effective for emission abatement.
Limitations exist for current policy commitments and emission standards. Because of high penetration of emission control devices in key sectors in recent years, the potential for further emission reductions from those sources may be limited despite aggressive control efforts. Therefore, greater efforts should be made to target the small coal and biofuel combustion stoves that generate significantly more fine particles and carbonaceous aerosols than large, energy-efficient boilers, resulting in benefits both to air quality and regional climate. As it is becoming more and more difficult to further reduce SO 2 , the swift declines in alkaline dust emissions in the future would likely increase acidification risks to ecosystems, as indicated by long-term observations at a number of sites. Thus not only the total amounts of emissions but also the linkages between emissions and various environmental impacts need to be comprehensively considered in policy making. Finally, the highly imbalanced economic development and urbanization by region in China are currently leading to major differences in atmospheric pollution and control strategies across the country. Limited data preclude a better understanding of the regional differentiation with future technology innovation, emission control plans, and thereby emission trends. Further investigations of emission source changes and their spatial distributions are thus urgently needed to reduce uncertainties in the analysis of future emission trends and related impacts, and to better support air pollution control planning across the country.
The Supplement related to this article is available online at doi:10.5194/acp-14-8849-2014-supplement.