Ammonia emission control in China would mitigate haze pollution and nitrogen deposition, but worsen acid rain

Significance

Atmospheric ammonia plays important roles in fine particle pollution, acid rain, and nitrogen deposition. China, known as the world’s top emitter of gaseous ammonia, plans to control ammonia emissions to mitigate the haze pollution that has recently emerged. However, the complex side effects are still unclear. By integrating a chemical transport model, nationwide measurements, and a sophisticated ammonia emission model, we find that ammonia emission control would significantly aggravate acid rain pollution, thereby offsetting the benefit from reduced fine particle pollution and nitrogen deposition. Our work suggests that region-specific ammonia-control strategies provide a more rational and effective way to achieve the dual benefits of protecting human and ecosystem health in China.

Abstract

China has been experiencing fine particle (i.e., aerodynamic diameters ≤ 2.5 µm; PM_2.5) pollution and acid rain in recent decades, which exert adverse impacts on human health and the ecosystem. Recently, ammonia (i.e., NH₃) emission reduction has been proposed as a strategic option to mitigate haze pollution. However, atmospheric NH₃ is also closely bound to nitrogen deposition and acid rain, and comprehensive impacts of NH₃ emission control are still poorly understood in China. In this study, by integrating a chemical transport model with a high-resolution NH₃ emission inventory, we find that NH₃ emission abatement can mitigate PM_2.5 pollution and nitrogen deposition but would worsen acid rain in China. Quantitatively, a 50% reduction in NH₃ emissions achievable by improving agricultural management, along with a targeted emission reduction (15%) for sulfur dioxide and nitrogen oxides, can alleviate PM_2.5 pollution by 11−17% primarily by suppressing ammonium nitrate formation. Meanwhile, nitrogen deposition is estimated to decrease by 34%, with the area exceeding the critical load shrinking from 17% to 9% of China’s terrestrial land. Nevertheless, this NH₃ reduction would significantly aggravate precipitation acidification, with a decrease of as much as 1.0 unit in rainfall pH and a corresponding substantial increase in areas with heavy acid rain. An economic evaluation demonstrates that the worsened acid rain would partly offset the total economic benefit from improved air quality and less nitrogen deposition. After considering the costs of abatement options, we propose a region-specific strategy for multipollutant controls that will benefit human and ecosystem health.

During recent decades, growing fossil fuel consumption resulting from rapid economic development and intense agricultural production has brought about huge emissions of multiple air pollutants and associated environmental issues in China: (i) acid rain, (ii) enhanced nitrogen deposition, and (iii) air quality deterioration caused by fine particulate matter (i.e., aerodynamic diameters ≤ 2.5 µm; PM_2.5). In the 1980s, acid rain was recognized as a top environmental issue in China; it was characterized by precipitation acidification with a pH less than 5.6 as a result of high levels of sulfur dioxide (i.e., SO₂) and nitrogen oxide (i.e., NO_x) emission from fossil fuel combustion (1–3). Moreover, large amounts of NH₃ emission from dense agricultural activities has made China a global hotspot for nitrogen deposition (4, 5). It is estimated that approximately 15% of the land area of China exceeds nitrogen critical load (6).

In recent years, China has been experiencing frequent haze pollution, which is characterized by extremely high PM_2.5 concentration and rapid deterioration of visibility (7, 8). In winter, severe haze pollution with PM_2.5 concentrations of hundreds of micrograms per cubic meter often engulfs tens of thousands of square kilometers and lasts for days or even weeks, drawing increasing attention from the public and the government. Many measurements are indicative of a remarkably high proportion (20−60%) of secondary inorganic aerosols in PM_2.5 mass concentration, including sulfate, nitrate, and ammonium, during haze pollution (9–11).

The Chinese government has devoted great effort to control SO₂ and NO_x emissions to mitigate acid rain and haze pollution (12, 13). In 1990s, China set out to control SO₂ emission for the purpose of mitigating acid rain pollution. Accordingly, SO₂ emissions significantly decreased during the 2000s as a result of widespread implementation of flue gas desulfurization (Fig. 1A). Measurements from ground and space also showed a notable decrease (>50%) of atmospheric SO₂ in the past decade (Fig. 1 B–D). As a result, acid rain pollution was gradually alleviated, even though it remains an important issue in the southern part of China (1, 14). In contrast, NO_x emission control started relatively late and has resulted in a moderate reduction of ∼20% from 2012 to 2015. Although air quality has been mitigated slightly in the past few years as a result of SO₂ and NO_x emission reduction, it is still a challenging problem facing China. According to a report issued by the government, 74.3% of 74 key cities in China exceeded the national air-quality standard of annual mean PM_2.5 concentrations (35 μg⋅m⁻³) in 2017 (15). To further improve the air quality, China will continue to implement stringent emission controls on SO₂ and NO_x with the aim reduce their emissions by 15% during the period of 2016–2020 (16).

Fig. 1.

(A) Interannual trends in SO₂, NO_x, and NH₃ emissions in China during 2005–2015. SO₂ emissions were provided by the Ministry of Ecology and Environment of China (MEE; www.mee.gov.cn/), Lu et al. (35), and Multiresolution Emission Inventory for China; NOx emissions were derived from MEE and Liu et al. (12); and NH₃ emissions were derived from Kang et al. (25). (B) Interannual trends in wet deposition (mean ± SD) of sulfate, nitrate, and ammonium averaged over four Acid Deposition Monitoring Network in East Asia stations (Jinyunshan, Shizhan, Xiaoping, and Xiangzhou) in China. (C) Normalized vertical column densities (VCDs) of SO₂ and NO₂ retrieved from ozone monitoring instrument measurements over China relative to the 2005 levels. (D) Ground-based concentrations of SO₂ and NO₂ averaged over 31 cities in China. These annual averaged values for each city were provided by the China Environmental Statistics Yearbook (www.stats.gov.cn/).

More recently in China, NH₃ control has been advocated as a potential measure by policy makers given that atmospheric NH₃ facilitates secondary PM_2.5 formation, i.e., ammonium sulfate/bisulfate and ammonium nitrate (17). The Chinese Premier recently announced that a new fund will be set up to discover the cause of frequent haze pollution, with special attention devoted to agricultural NH₃ emission control (18). In northern China, a highly polluted area, abatement options for NH₃ agricultural emissions have been integrated in the air pollution control plan for 2016–2020.

However, it should not be neglected that atmospheric NH₃ plays a vital role in neutralizing precipitation acidity and therefore helps to weaken acid rain (19, 20). Decrease in ambient NH₃ level is likely to induce more acidic precipitation and expose terrestrial systems to acid rain pollution, especially in southern and southwestern China, where soil systems are generally less buffered and particularly sensitive to acid input (21). Thus, a problem arises that, under the circumstance of tremendous emissions of acid gas (SO₂ + NO_x), China’s NH₃ emission abatement may significantly worsen acid rain by increasing rainfall acidity. It is still unclear, and we are aware of no study that has quantified this. The impacts of NH₃ emissions are complex in China, with distinct effects on air quality and human and ecosystem health. Previous studies mainly focused on the role of NH₃ emission in particle pollution on global and regional scales, and there is still a lack of studies on these comprehensive impacts in China, particularly in terms of acid rain and nitrogen deposition.

In this study, we performed an integrated analysis of multiple impacts of NH₃ emission control strategy on PM_2.5 pollution, nitrogen deposition, and acid rain by combining the Weather Research and Forecasting model with Chemistry (WRF-Chem) simulation, ground-based and satellite-retrieved observations, as well as economic assessment. We have developed a high-resolution NH₃ emission model and used it to forecast the potential NH₃ emission reduction in the agricultural sector based on Chinese agricultural practice. We then intend to provide optimized and cost-effective strategies for China’s NH₃ emission abatement policy in the near future.

NH₃ Emission and Its Potential Reduction in China

As a large agricultural country, China produces a huge amount of NH₃ emission (Fig. 1A), with agricultural activities accounting for more than 80% (22–24). In this study, we use a comprehensive high-resolution inventory developed by our research group (PKU-NH₃) that covers agricultural activities, biomass burning, industry, transportation, and other sectors (25). It has been demonstrated to accurately describe the magnitude and spatial and seasonal patterns of China’s NH₃ emission and has been applied in chemical transport modeling to reproduce secondary inorganic aerosols (details are shown in the SI Appendix).

NH₃ emission in China (∼10.0 million tons annually) exceeds the sum of those in the European Union (3.7 million tons) and the United States (3.9 million tons) (24, 26). Moreover, the NH₃ emission rates per amount of fertilizer applied or animal excretion in farms are almost twice those in developed countries (SI Appendix, Table S2). Much evidence manifests that poor agricultural production management and low efficiency of nitrogen use are the main causes of the large NH₃ emissions over China (details are shown in the SI Appendix). The implementation of feasible abatement measures during agricultural activities could sharply reduce China’s NH₃ emissions.

Here, a series of options for NH₃ emission reduction are compiled (SI Appendix, Table S1) based on existing literature and local agricultural practice. To abate NH₃ emission in croplands, rational fertilization (i.e., avoidance of overfertilization and implementation of deep application of fertilizers) can be implemented during growing seasons (mostly spring and summer months) when emission from nitrogen fertilizers peaks. For livestock NH₃ emissions, an optimization of animal manure management should be promoted. Based on the PKU-NH₃ emission model, it is estimated that a combination of the aforementioned measures can reduce China’s NH₃ emissions by 30−50%. In this work, we choose a 50% NH₃ emission abatement strategy to examine its multiple environmental impacts, which can be taken as an upper bound of NH₃ emission reduction via feasible control options in the near future. In this case, the national annual emissions changed from 3.8 to 1.8 million tons during summer (June, July, and August; hereafter referred to as JJA) and from 1.4 to 0.7 million tons during winter (January, February, and December; referred to as DJF hereafter). The unit emission rates in fertilizer use and animal excretion are comparable with those in developed countries (SI Appendix, Table S2).

To quantitatively understand multiple impacts of NH₃ emission control, we performed WRF-Chem simulations in the summer months of 2015 with frequent rainfall and in the winter months with frequent haze pollution. Three parallel numerical experiments with different emission scenarios are designed: (i) standard emission levels in 2015 (Run_base), (ii) the same as (i) but reducing SO₂ and NO_x emissions by 15% on a national scale (Run_15SN; target for 2020), and (iii) the same as (ii) but with an additional 50% NH₃ emission reduction (Run_15SN50A). We validated the model by comparisons with nationwide measurements of gaseous NH₃ concentrations, PM_2.5 mass concentrations, particle chemical compositions, and rainfall acidity (SI Appendix, Figs. S3–S6 and Tables. S3–S5). After ensuring model performance, multiple environmental effects of NH₃ emission reduction could be disentangled by disparities among the simulations based on different emission scenarios.

Results and Discussion

Multiple Impacts of NH₃ Emission Control.

China’s huge anthropogenic emissions lead to severe particle pollution (Run_base), as shown in Fig. 2. In northern China, south-central China, and the Sichuan Basin (the geographical areas of interest in this study are shown in SI Appendix, Fig. S2), mean PM_2.5 concentrations exceed 75 μg⋅m⁻³, far greater than the Chinese Ambient Air Quality Standard of 35 μg⋅m⁻³ (interim target 1 set by World Health Organization) that would lead to potential public health risks. If China achieves its SO₂ and NO_x emission reduction targets as planned (15% reduction during 2015−2020; Run_15SN), PM_2.5 concentrations will decrease by 1−3 μg⋅m⁻³, or only ∼3% of the baseline level (Fig. 2B). However, an additional 50% NH₃ emissions control (i.e., Run_15SN50A) can accomplish a decrease of 11−17% in PM_2.5 concentrations relative to the 2015 baseline in these three regions (Fig. 2C). Substantial decline in the average PM_2.5 concentrations (∼10 μg⋅m⁻³) primarily occurs in southwestern, south-central, and parts of northern China, where PM_2.5 concentrations already exceed the air quality standard and NH₃ concentrations are relatively high. Clearly, synchronous NH₃ emission control is a more efficient way to mitigate PM_2.5 pollution, rather than merely implementing SO₂ and NO_x emission controls (i.e., Run_15SN).

Fig. 2.

Comparison of simulated PM_2.5 concentrations (A–C), critical load exceedance of nitrogen (D–F), and precipitation-weighted mean pH (G–I) over China in the Run_base, Run_15SN, and Run_15SN50A scenarios. The small islands in the South China Sea were excluded.

To shed more light on such a high efficiency of NH₃ emission control on PM_2.5 mitigation, we further investigate responses of different compositions of PM_2.5 in the Run_15SN50A case. Our results indicate that particulate nitrate dominates the reductions in PM_2.5 concentrations for the areas of interest, including northern China, southern China, and the Sichuan Basin. Particulate ammonium nitrate concentrations decrease by 30−60% relative to the baseline levels, which contributes approximately 70% of PM_2.5 mass reduction in these regions during JJA and DJF (other reductions are from particulate ammonium sulfate). According to the aerosol thermodynamic equilibrium, less available ambient NH₃ favors more HNO₃ staying in gas phase instead of partitioning into aerosols and then inhibits the formation of ammonium nitrate (17). The simulations demonstrate that particulate nitrate reductions resulting from NH₃ emission control are more pronounced in winter (mostly 3–9 μg⋅m⁻³) than in summer (<3 μg⋅m⁻³) in the regions on which we focused (SI Appendix, Fig. S7). This disparity can be ascribed to the seasonality of air temperature (i.e., highest in JJA and lowest in DJF) that determines the NH₄NO₃ phase equilibrium and NH₃ emission rate. First, higher temperature in summertime increases the NH₄NO₃ equilibrium constant and lowers the concentrations of ammonium nitrate. Moreover, NH₃ emissions are significantly enhanced in summer, leading to NH₃-rich regimes. These factors jointly make particle nitrate concentrations less sensitive to NH₃ reductions in summer. As a result, PM_2.5 mitigation, which is dominated by ammonium nitrate, is more substantial in DJF than in JJA (SI Appendix, Fig. S7).

Regionally, the mass reductions of nitrate in summer are greater in northern China than in southern China (SI Appendix, Fig. S7) because higher temperatures in the southern area of the country favor the thermodynamic phase partitioning of HNO₃ and NH₃ into gas phase. In contrast, the reductions of nitrate appear more pronounced in southern China in winter, which is likely attributable to relatively lower NH₃ emissions in this region that make the formation of NH₄NO₃ more sensitive to NH₃ reductions. In general, the NH₄NO₃ thermodynamic equilibriums and heterogeneous NH₃ emissions together determine the seasonal and regional responses of NH₄NO₃ to NH₃. It is also noteworthy that NH₃ emission control could be much more effective in reducing particulate nitrate during days with haze pollution than during days without it (SI Appendix, Fig. S5). It could be attributable to rapidly increased PM_2.5 nitrate concentrations during haze conditions, making the formation of ammonium nitrate NH₃-limited. The results highlight the importance of NH₃ abatement in PM_2.5 mitigation during winter haze events in China.

Given that NH_x-N (gaseous NH₃ and particulate NH₄⁺) contributes to more than half of the inorganic nitrogen deposition in China (27), this NH₃ emission control will inevitably alter nitrogen deposition flux through dry and wet processes. By averaging simulation results in summer and winter, the annual nitrogen deposition over China is estimated to be 13.6 Tg N⋅y⁻¹, in which 8.6 Tg N⋅y⁻¹ (63%) is NH_x-N and 3.8 Tg N⋅y⁻¹ (28%) is via dry deposition processes, consistent with previous studies (6, 27). To quantitatively investigate the effects of nitrogen deposition on terrestrial ecosystems, exceedance of critical load is introduced in this study. It is derived from the simulation of nitrogen deposition and a critical load map over China based on the steady-state mass balance method (28). Here, the critical loads of nitrogen deposition refer to quantitative estimates of the highest deposition amount that will not cause harmful effects on sensitive ecological systems, which range between 10–150 kg N⋅ha⁻¹⋅y⁻¹ over China (SI Appendix, Fig. S8).

As illustrated in Fig. 2 D–F, exceedance of critical load can reach more than 20 kg N⋅ha⁻¹⋅y⁻¹ in the northeastern, central, and southern regions of China in the baseline case of 2015. The area exceeding the critical load is estimated to account for approximately 17% of China’s terrestrial land. If SO₂ and NO_x emission is cut by 15% (Run_15SN), the total nitrogen deposition decreases by only 1% and the area with critical load exceedance is almost the same. When NH₃ emission is halved (i.e., Run_15SN50A), nitrogen deposition could decrease by 34% (from 13.6 to 8.9 Tg N⋅y⁻1), thereby reducing the area with critical load exceedance to 9% of the country’s land. Spatially, decrease in critical load exceedance is quite pronounced in central and southern China, which have high nitrogen deposition and vulnerable ecosystems. The reduction in NH_x-N deposition will help weaken soil and aquatic acidification and eutrophication over the terrestrial ecosystems with nitrogen saturation.

As mentioned earlier, another environmental concern that is closely linked to atmospheric NH₃ abundance is acid rain. We evaluated the model performance on reproducing precipitation acidity by comparing it with nationwide measurements (SI Appendix, Fig. S6). The measurements and simulations indicate that acid rain pollution (pH < 5.6) is mainly distributed in southern China, the Sichuan Basin, and parts of northern China in 2015 (Fig. 2G). On the contrary, almost no acid rain area is found in western parts of China because abundant windblown dust in the atmosphere is capable of neutralizing precipitation acidity in this region. Under the assumption of 15% SO₂ and NO_x emission reduction, we find that acid rain can be effectively mitigated. Specifically, precipitation pH values are elevated by 0.1−0.5 units in northern and southern China, and the area with pH values less than 5.6 simultaneously decreases from 1.7 (Run_base) to 1.0 million km² (Run_15SN).

However, unlike the impact on PM_2.5 pollution and nitrogen deposition, the NH₃ emission reduction aggravates precipitation acidification and expands the acid rain area over China (Fig. 2I). Our simulations show that 50% NH₃ emission reduction will significantly decrease the concentrations of ammonium ions in precipitation, with an average of 20−80 μeq/L over eastern China. This would enhance precipitation acidity via two pathways (SI Appendix, Fig. S9). One is the decreased wet removal of gas-phase NH₃ by rainwater. According to the gas–liquid equilibrium of the NH₃(g)-NH₃(aq)-NH₄⁺(aq) system, decreases in ambient gas-phase NH₃ concentrations make this equilibrium shift toward the gas phase, consequently releasing hydrogen ions (H₃O⁺) in the aqueous phase. In addition, less ambient NH₃ favors the formation of ammonium bisulfate, which is more acidic than ammonium sulfate and thereby increases the aerosol acidity (29). When dissolved into cloud/rain water drops, ammonium bisulfate could produce more hydrogen ions than ammonium sulfate with the same wet scavenging amounts of sulfate ions, and the precipitation acidity therefore tends to be elevated. Our simulations demonstrate that the aforementioned two pathways account for almost all of the increase in H₃O⁺ concentration in precipitation over China.

The simulations show that the volume-mean precipitation pH values decrease by as much as 1.0 unit in northern China and 0.1−0.5 units in southern areas. Given that precipitation acidity is much greater in southern China and the Sichuan Basin, even a small decrease in the pH would undoubtedly worsen the already grave situation. According to our simulations, acid rain area with pH < 5.6 over China increases from 1.7 (Run_base) to 2.0 million km² if synchronous control is applied on SO₂, NO_x, and NH₃ emission (Run_15SN50A). However, the heavy acid rain area (pH < 5.0) will expand by approximately 65% (from 0.23 to 0.38 million km²), which is three times larger than that under the Run_15SN scenario (0.7 million km²). In spatial terms, southern China and the Sichuan Basin undergo fast expansion of acid rain areas (Fig. 2I). Overall, NH₃ emission control inevitably results in a general increase in precipitation acidification over China. In addition, precipitation acidity under the Run_15SN50A scenario is almost the same or even higher than that under the Run_base scenario. In other words, the alleviation of acid rain over China caused by the control of SO₂ and NO_x emissions is very likely to be completely forfeited by NH₃ emission reductions.

Economic Valuation.

As demonstrated, 50% NH₃ emission reduction can help to mitigate fine particle pollution and reduce nitrogen deposition to sensitive terrestrial land, but, in the meantime, worsen the acid rain problem, a significant side effect that should not be neglected. To qualitatively compare these three impacts, we estimate the associated economic benefit or loss induced by the NH₃ emission reduction in terms of human health, crop production, and the ecosystem. Note that the differences between the scenario Run_15SN and Run_15SN50A represent the effects of the NH₃ emission reduction.

The assessment on the human health effects of decreased PM_2.5 concentrations is performed based on exposure–response functions (SI Appendix, Materials and Methods). The estimated total premature mortality in the baseline case is 1.15 million persons (0.54–1.75 million, within the 95% confidence level), which is similar to the findings of recent studies (30, 31). The NH₃ emission reduction in China would reduce the premature mortalities attributable to PM_2.5 exposure by approximately 80,000 people in 2015, and the resultant economic benefit is estimated to be 10.1 billion US$. Second, the reduction in nitrogen deposition decreases the area of critical load exceedance and yields an economic benefit of approximately 1.9 billion US$. However, the enhanced precipitation acidification caused by NH₃ emission control would cause an economic loss of approximately 5.0 billion US$ over China, with a dominant contribution from reduced crop yields (90%). This will partly offset the positive effects of the mitigation of PM_2.5 pollution and nitrogen critical load exceedance, and the overall benefit from these three effects is therefore 7.0 billion US$.

To figure out the overall cost effectiveness of the 50% NH₃ emission reduction in China, we further assess the cost of NH₃ abatement options in agricultural activities. The annual cost is estimated to be 6.6 billion US$, which is dominated by control measures for the improvement of livestock manure management (58%), followed by rational fertilizer application. Thus, the net benefit (subtracting the cost from the benefit) of NH₃ emission reduction is 0.4 US$, only approximately 6% of the expenditure. In other words, the loss from worsened acid rain and the abatement costs almost offset the benefits from mitigation of PM_2.5 pollution and nitrogen deposition. Therefore, under a target of a 15% reduction of SO₂ and NO_x emission in 2020, a reduction of 50% or more in NH₃ emissions may be not applicable because the benefits could not outweigh the costs.

To explore optimum control strategy for NH₃ emissions, we further vary the NH₃ emission reductions from 10% to 50% and compare the economic benefits with the corresponding abatement costs under each individual reduction scenario. Fig. 3 presents the comparisons in the three regions of concern. For northern China, NH₃ reductions always bring positive net benefits (i.e., revenue greater than cost), but the cost effectiveness (i.e., marginal revenue vs. marginal cost) varies with the NH₃ emission reductions. In response to the 0−20% options, the growth rate of economic benefit (i.e., marginal revenue) is greater than the abatement cost (i.e., marginal cost), which can be inferred from the slopes of the curves. A 20% reduction in NH₃ emission in northern China yields a relatively high benefit of 1.3 billion US$ relative to a cost of 0.3 billion US$, with a net benefit of 1.0 billion US$. Meanwhile, when NH₃ emissions are not as rigorously controlled (e.g., 20% reduction), the increase in acid rain area is relatively small (<4 ×10⁴ km²), and famers would select cheaper and more feasible options to attain such a moderate NH₃ emission reduction, such as low-protein feeding and deep application of manure. However, further reductions (≥ 30%) in NH₃ emissions result in an accelerated growth in acid rain pollution and rapidly increased expenditure for NH₃ emission abatement, making the unit reduction in NH₃ emissions less beneficial and more expensive. As a consequence, the marginal cost outweighs the marginal revenue when NH₃ emission reduction exceeds 30% (i.e., the slopes of the curves are <1 compared with the 1:1 line). Therefore, the most cost-effective strategy for NH₃ emission control is defined at a critical point at which marginal cost equals marginal revenue. In our study, that is estimated to be 20−30% for northern China.

Fig. 3.

Comparison of total economic benefit (in millions of US dollars; i.e., benefit from reduced PM_2.5 and nitrogen deposition minus loss from acid rain) vs. the costs of NH₃ abatement options under each NH₃ emission reduction scenario in northern China, southern China, and the Sichuan Basin. The cases shown in the figure are NH₃ emission reductions of 0 (■), 10% (∆), 20% (★), 30% (□), 40% (▼), and 50% (●).

In southern China, the economic loss from worsened acid rain pollution is much more noteworthy. Under a 20% NH₃ reduction, the area exposed to acid rain in southern China increases by 2.1 × 10⁵ km² (from 3.3 to 5.4 × 10⁵ km²), leading to a negative economic effect of as much as 0.6 billion US$. The resultant net benefit is only 0.4 billion US$ in the case that PM_2.5 pollution and nitrogen deposition is alleviated. Moreover, the economic loss from enhanced acid rain in southern China is 40−260% greater than that in northern China under the same emission reduction percentages. This further indicates that acid rain pollution remains a noteworthy problem in southern China and is quite sensitive to changes in NH₃ emissions.

Sichuan Basin is one of the most intensive emitters of NH₃ in China as a result of a large amount of livestock production. Heavy acid rain pollution also emerged in the region in the 1980s and has been a major environmental problem for decades (3). As shown in Fig. 3, the benefits of NH₃ emission reductions are almost the same as the corresponding costs (both ≤ 0.5 billion US$). Under a 20% NH₃ reduction, even though the marginal revenues are higher than the costs, acid rain pollution is obviously worsened. The simulation results show that the area exposed to heavy acid rain (pH < 5.0) in the Sichuan Basin increases by approximately 50% under the 20% case.

Overall, the results suggest a region-specific NH₃ emission abatement strategy. Over existing areas of acid rain like southern China and the Sichuan Basin (in southwestern China), it would be prudent to prioritize SO₂ and NO_x emission controls over NH₃ control to effectively alleviate the potential risks of precipitation acidification. In contrast, for areas with scarce acid rain but heavy fine particle pollution like northern China, implementation of 20−30% NH₃ emissions control would be cost-effective, particularly for reducing PM_2.5 pollution in winter haze conditions during 2016−2020.

Conclusion

NH₃ emission reduction has been proposed recently as a strategic option to mitigate severe haze pollution in China. However, atmospheric NH₃ is closely bound to other environmental concerns like nitrogen deposition and acid rain. In this study, we comprehensively investigate these multiple impacts of NH₃ emission reduction for further policy consideration in China. On the basis of a sophisticated NH₃ emission model, we indicate that improving agricultural production management can readily reduce the total NH₃ emission by as much as 50% across China. This reduction, together with a 15% reduction in SO₂ and NO_x emission, could accomplish a decrease of 11−17% in PM_2.5 concentrations, far more than when applying only SO₂ and NO_x control (3%). The dominant contributor is particulate nitrate, the concentration of which can be reduced by 30−60%, accounting for ∼70% of PM_2.5 mitigation. As a result of NH₃ emission reduction, the nitrogen deposition is estimated to decrease by 33%, with the area of critical load exceedance decreasing from 17% to 9% of Chinese terrestrial land. However, as an important side effect, there arises an overall enhancement in precipitation acidity in China, with a decrease in pH values of as much as 1.0 unit and a 65% increase in heavy acid rain areas (pH < 5.0).

Our results emphasize the complexity embedded in NH₃ emission control in China: improved air quality, reduced nitrogen deposition, but worsened acid rain. Further quantitative assessments of the economic effects of NH₃ emission reduction demonstrate that the economic loss resulting from enhanced acid rain would partly offset the benefit from PM_2.5 mitigation and alleviated nitrogen deposition. After considering the costs of our abatement options, we propose regional-specific emission control strategies in the near future: implementing NH₃ emission reduction by 20−30% in areas with scarce acid rain but heavy fine particle pollution (e.g., northern China) while prioritizing SO₂ and NO_x emission controls in areas of existing acid rain like southern China. Such emission-control strategies will benefit air quality improvement and ecosystem health across China.

Materials and Methods

To investigate the multiple impacts of potential reductions in NH₃ emissions in China on PM_2.5 pollution, nitrogen deposition, and acid rain, we integrated numerical model simulations with a comprehensive NH₃ emission inventory (i.e., PKU-NH₃) as well as nationwide observations of precipitation acidity and aerosol chemical constituents. We performed simulations by using WRF-Chem (version 3.6.1) during summer (i.e., JJA) and winter (i.e., DJF) of 2015 to represent the pollution of acid rain, PM_2.5, and nitrogen critical load exceedance. The model domain and the geographic areas of interest are shown in SI Appendix, Fig. S2. Based on the PKU-NH₃ model and a combination of feasible emission abatement options, we designed a potential NH₃ emission reduction scenario and examined its influences on simulated PM_2.5 concentrations, nitrogen deposition, and precipitation acidity. We then estimated the economic effects of PM_2.5 pollution, nitrogen deposition, and acid rain caused by the NH₃ emission reduction. The premature mortality attributable to long-term exposure to PM_2.5 was calculated following the methods of Liu et al. (30). We quantitatively assessed the economic loss of reduced crop yields and forestry caused by enhanced precipitation acidification based on the experiments of Feng et al. (32). The potential benefit from reduced nitrogen deposition in those areas with critical load exceedance was estimated based on a unit damage cost for acidification and eutrophication of ecosystems (33). Furthermore, to obtain a cost-effective NH₃ control strategy, we estimated the cost of NH₃ abatement options applied in this study following the Greenhouse Gas—Air Pollution Interactions and Synergies model (34). Details of the observation datasets, evaluation of model performance, and economic assessment are provided in the SI Appendix.