While great efforts have been made to China’s clean air actions since 2013 and effectively mitigated PM2.5 pollution, the emission-concentration relationships and cost-effectiveness may have changed substantially and are poorly constrained. Large emission reductions during the COVID-19 lockdown period in early 2020 did not similarly alleviate PM2.5 pollution in North China, reflecting a distinct nonlinear chemical response of PM2.5 formation to emission changes. At the same time, strengthened emissions standards and elimination of outdated industrial capacities have replaced the existing technologies and increased the cost of pollution control. Here we apply emission-concentration relationships for PM2.5 diagnosed using the adjoint approach to quantitatively assess how chemical nonlinearity affects PM2.5 over Beijing in February 2020 in response to two emission reduction scenarios: the COVID-19 lockdown and 2013-2017 emission controls, and further evaluate the marginal cost and benefit of possible technological alternatives. We find that, in the absence of chemical nonlinearity, the COVID-19 lockdown would decrease PM2.5 in Beijing by 10.6 μg m-3, and the 2013-2017 emission controls resulted in a larger decrease of 54.2 μg m-3 because of greater reductions of SO2 and primary aerosol emissions. Chemical nonlinearity offset the decrease for Beijing PM2.5 by 4.7 μg m-3 in lockdown, which was mainly attributed to enhanced sensitivity of aerosol nitrate to NOx emissions, but enhanced the efficiency of 2013-2017 emission controls by 12.5 μg m-3 due to the weakened heterogeneous reaction of sulfate. For further PM2.5 mitigation, emission reductions in ammonia by urea substitution and primary PM2.5 with electrostatic precipitator have high PM2.5 reduction potential and cost effectiveness. Such chemical nonlinearity and cost optimization are important to estimate and consider when designing or assessing air pollution control strategies.

Ammonia emissions in China mainly came from agricultural activities. Excess emissions could lead to degraded air quality and excess nitrogen deposition. Therefore, it is essential to improve air quality and nitrogen deposition through agricultural ammonia reduction measures. On the basis of the existing research, this study established an Agricultural Management Technology-Ammonia emission assessment platform with 51 measures of fertilizer application and 53 measures of livestock farming derived from a literature review and adopted the Monte Carlo method to apply this platform to Beijing-Tianjing-Heibei (BTH) region where active agricultural activities occur. An updated agricultural ammonia emission inventory at 3-km resolution in BTH region was used in this study. We find that ammonia emissions from livestock farming could be reduced by 79-151Gg (30%-57%) and from fertilizer application by 58-163Gg (18%-51%) in BTH region in 2019. We applied two reduction scenarios that could achieve average and maximum ammonia emission reduction based on the Monte Carlo results, and evaluated the resulting improvements of air quality and deposition using the GCHP model with a resolution of 10km × 10km in BTH region.The results show that the baseline of PM2.5 concentration, NHX and NOy deposition in BTH region in 2019 is 27-61 µg/m3, 8-57 Gg N/month and 3-51 Gg N/month. Under two ammonia emission reduction scenarios, PM2.5 concentration and NHx deposition would, respectively, reduce 1.38-3.89 µg/m3, 3-14 Gg N/month while NOy deposition would increase 0.5-2 Gg N/month. Our research shows that agricultural ammonia has great emission reduction potential that would benefit to the reduction of nitrogen pollution.

2 Dry deposition is an important ozone sink in the planetary boundary layer (PBL), but how different PBL parameterization schemes affect the ozone dry deposition impacts has not been well quantified. Here we investigate the influences of PBL mixing parameterizations on surface ozone and dry deposition in eastern China using a regional air quality model (WRF-Chem) and further quantify the contributions of dry deposition to ozone tendencies both near the surface and in the PBL with integrated process rates (IPR) analysis. We analyze three PBL schemes coupled with their corresponding surface layer (SFL) schemes, including Yonsei University (YSU), Mellor–Yamada–Janjić (MYJ), and Asymmetric Convective Model v2 (ACM2). We find that using different PBL-SFL schemes in general produces similar monthly mean meteorological fields, leading to relatively small uncertainties, ranging from 6.5% to 18.5% in monthly mean surface ozone concentrations and 3.6% to 15.3% in total ozone dry deposition in eastern China. However, these uncertainties can frequently exceed 30% in summer at the daily scale. IPR analyses with YSU_MM5 show that ozone dry deposition accounts for 80-89% of surface ozone losses and 45-58% of PBL ozone losses. Model sensitivity simulations by suppressing ozone dry deposition would enhance surface ozone in eastern China by 24-30% (20.6-35.7 μg m-3) during the daytime and 61-82% (26.0-46.1 μg m-3) at night. Ozone dry deposition significantly promotes the downward vertical mixing of ozone to the surface layer, which contributes to smoothing the near-surface ozone gradients as caused by ozone dry deposition and titration, thus explaining the smaller contribution estimated by the sensitivity approach than IPR analyses. We further show that different meteorological conditions simulated by different PBL-SFL schemes affect daily ozone simulations not only through the chemistry process as previously recognized but also considerably by the dry deposition process.

While China’s clean air actions implemented since 2013 have been effective in mitigating PM _2.5 air pollution, the large emission reductions during the COVID-19 lockdown period in early 2020 did not similarly alleviate PM _2.5 pollution in North China, reflecting a distinct nonlinear chemical response of PM _2.5 formation to emission changes. Here we apply emission-concentration relationships for PM _2.5 diagnosed using the adjoint approach to quantitatively assess how chemical nonlinearity affects PM _2.5 over Beijing in February 2020 in response to two emission reduction scenarios: the COVID-19 lockdown and 2013–2017 emission controls. We find that, in the absence of chemical nonlinearity, the COVID-19 lockdown would decrease PM _2.5 in Beijing by 17.9 μ g m ^–3 , and the 2013–2017 emission controls resulted in a larger decrease of 54.2 μ g m ^–3 because of greater reductions of SO _2 and primary aerosol emissions. Chemical nonlinearity offset the decrease for Beijing PM _2.5 by 3.4 μ g m ^–3 during the lockdown due to enhanced sensitivity of aerosol nitrate to NO _x emissions, but enhanced the efficiency of 2013–2017 emission controls by 11.9 μ g m ^–3 due to the weakened heterogeneous reaction of sulfate. Such nonlinear chemical effects are important to estimate and consider when designing or assessing air pollution control strategies.

Nitrogen oxides (NOx) and ammonia (NH3) contribute substantially to current global fine particulate matter (PM2.5) pollution. Their future role remains unclear and is complicated by interactions with background emissions. Here, we show that under climate mitigation scenarios, by 2050, a hypothetical phaseout of anthropogenic NH3 emissions would reduce PM2.5 by 20%-60% locally and be more effective than phasing out NOx. Reducing NH3 by 25%, instead, would be less effective than 25% NOx reduction for many regions. Future reductions of NOx and sulfuric dioxides from clean energy transitions would shift the nonlinear chemical regime of secondary inorganic aerosol formation toward NH3 saturation. The later NH3 controls are installed, the deeper the required reductions will be to be effective, although for many regions such levels are still within technical feasibility, while NOx controls will always remain effective. Nitrogen reductions remain useful for achieving the World Health Organization guideline target for PM2.5, and NH3 controls need to happen sooner rather than later.

Ammonia (NH3) is critical to the nitrogen cycle and PM2.5 formation, yet a great deal of uncertainty exists in its urban emission quantifications. Model-underestimated NH3 concentrations have been reported for cities, yet few studies have provided an explanation. Here, we explore reasons for severe WRF-Chem model underestimations of NH3 concentrations in Beijing in August 2018, including simulated gas-particle partitioning, meteorology, regional transport, and emissions, using spatially refined (3 km resolution) NH3 emission estimates in the agricultural sector for Beijing-Tianjin-Hebei and in the traffic sector for Beijing. We find that simulated NH3 concentrations are significantly lower than ground-based and satellite observations during August in Beijing, while wintertime underestimations are much more moderate. Further analyses and sensitivity experiments show that such discrepancies cannot be attributed to factors other than biases in NH3 emissions. Using site measurements as constraints, we estimate that both agricultural and non-agricultural NH3 emission totals in Beijing shall increase by ∼5 times to match the observations. Future research should be performed to allocate underestimations to urban fertilizer, power, traffic, or residential sources. Dense and regular urban NH3 observations are necessary to constrain and validate bottom-up inventories and NHx simulation.

Abstract Atmospheric ammonia (NH3) has multiple impacts on the environment, climate change and human health. China is the largest emitter of NH3 globally, with the dynamic inventory of NH3 emission remaining uncertain. Here, we use the high-resolution secondary national pollution survey, integrated satellite data, 15N isotope source apportionment and multiple models to better understand those key features of NH3 emissions and its impacts in China. Our results show that the total NH3 emissions were estimated to be 12.3 Tg yr-1 in 2017 with three emission peaks in April, June and October. NH3 emissions have contributed approximately 23% to secondary PM2.5 formation, a 19.7% increase in nitrogen loading of surface waters, while ammonium deposition led to a decrease in soil pH by 0.0035 units and an increase in the terrestrial carbon sink by 83.4 Tg C yr-1. Reducing NH3 emissions in China would contribute to the mitigation of air and water pollution challenges, saving damage costs estimated at around 22 billion US dollars due to avoided human and ecosystem health impacts.

Surface ozone air pollution is unequally distributed in space and varies over urban and surrounding nonurban areas. Traditionally, urban ozone levels tend to be lower than their nonurban counterparts, resulting from discrepancies in emissions and nonlinearity in photochemistry. However, how the differences in urban vs nonurban ozone evolve over the past decades is uncertain. Here, we construct 6361 pairs of urban and nonurban ozone measurement sites based on available surface monitoring networks to analyze the long-term changes of their ozone differences. We show that urban vs nonurban ozone differences have narrowed substantially in North America, Europe, South Korea, and Japan over the summers of 1990-2020. The hemispheric mean urban vs nonurban ozone differences have decreased by 90% from -5.0 ppbv in the 1990s to -0.5 ppbv in the 2010s. We estimate that the anthropogenic emission reduction of nitrogen oxides is the dominant driver of the narrowing trends. It has suppressed the urban ozone titration and led to closer ozone formation regimes over urban and nonurban areas.

Excess reactive nitrogen (Nr), including nitrogen oxides (NO x ) and ammonia (NH 3 ), contributes strongly to fine particulate matter (PM 2.5 ) air pollution in Europe, posing challenges to public health. Designing cost-effective Nr control roadmaps for PM 2.5 mitigation requires considering both mitigation efficiencies and implementation costs. Here we identify optimal Nr control pathways for Europe by integrating emission estimations, air quality modeling, exposure-mortality modeling, Nr control experiments and cost data. We find that phasing out Nr emissions would reduce PM 2.5 by 2.3 ± 1.2 μg·m −3 in Europe, helping many locations achieve the World Health Organization (WHO) guidelines and reducing PM 2.5 -related premature deaths by almost 100 thousand in 2015. Low-ambition NH 3 controls have similar PM 2.5 mitigation efficiencies as NO x in Eastern Europe, but are less effective in Western Europe until reductions exceed 40%. The efficiency for NH 3 controls increases at high-ambition reductions while NO x slightly decreases. When costs are considered, strategies for both regions uniformly shift in favor of NH 3 controls, as NH 3 controls up to 50% remain 5-11 times more cost-effective than NO x per unit PM 2.5 reduction, emphasizing the priority of NH 3 control policies for Europe.

Nitrogen is an essential nutrient and a major limiting element for the ocean ecosystem. Since the preindustrial era, substantial amounts of nitrogen from terrestrial sources have entered the ocean via rivers, groundwater, and atmospheric deposition. China serves as a key hub in the global nitrogen cycle, but the pathways, sources, and potential mitigation strategies for land-ocean nitrogen transport are unclear. By combining the CHANS, WRF-Chem, and WNF models, we estimated that 8 million tonnes (Tg) of nitrogen was transferred into the ocean in 2017 in China, with atmospheric deposition contributing 1/3. About half variation of the offshore chlorophyll concentration was explained by atmospheric deposition. The Bohai Sea was the hot spot of nitrogen input, estimated at 214 kg N ha(-1), while other areas were around 25-51 kg N ha(-1). The largest contributors are agricultural systems (4 Tg, 55%), followed by domestic sewage (2 Tg, 21%). Abatement measures could reduce nitrogen export to the ocean by 43%, and mitigating ammonia and nitrogen oxide emissions accounts for 33% of this reduction, highlighting the importance of addressing air pollution in resolving ocean pollution. The cost-benefit analysis suggests the priority of nitrogen reduction in cropland and transport systems for the ocean environment.

Large amounts of energy consumption in recent years have not only increased air pollution and greenhouse gas emissions, but have also released more anthropogenic heat into the atmosphere. However, the latter was overlooked in previous air quality and pollution-related health impacts studies. Here we use the atmospheric chemistry model coupled the exposure mortality model to investigate the effects of increased anthropogenic heat flux on PM2.5 pollution and related health burden in China. We find the ignoring anthropogenic heat leads nighttime PM2.5 concentrations to be overestimated, especially in metropolitan areas. The rising anthropogenic heat flux between 2000-2016 decreases surface PM2.5 by 4 ug·m-3 in Chinese urban region through altering microphysical processes and enhancing vertical mixing. Furthermore, the anthropogenic heat changes could avoid additional 15% (47 thousand) premature deaths, compare to anthropogenic emission reductions. Our findings indicate that anthropogenic heat should be included in air quality modeling and reveal the health benefit of energy use from a microphysical standpoint.

Recent Chinese air pollution actions have significantly lowered the levels of fine particulate matter (PM2.5) in North China via controlling emissions of sulfur dioxide (SO2) and nitrogen oxides (NOx) together with primary aerosols, while the emissions of another precursor, ammonia (NH3), have not yet been regulated. This raises a question that how effective the NH3 emission controls can be on the mitigation of PM2.5 pollution along with the reduction of SO2 and NOx emissions. Here we use a regional air quality model to investigate this issue focusing on the PM2.5 pollution in North China for January and July 2015. We find that the efficiency of the PM2.5 reduction is highly sensitive to the NH3 emission and its reduction intensity. Reductions in the population-weighted PM2.5 concentration (PWC) in the Beijing–Tianjin–Hebei region are only 1.4–3.8 μg m−3 (1.1%–2.9% of PM2.5) with 20%–40% NH3 emission reductions, but could reach 8.1–26.7 μg m−3 (6.2%–21%) with 60%–100% NH3 emission reductions in January 2015. Besides, the 2015–2017 emission changes (mainly reduction in SO2 emissions) could lower the PM2.5 control efficiency driven by the NH3 reduction by up to 30% for high NH3 emission conditions, while lead to no change or increase in the efficiency when NH3 emissions become low. NOx emission reductions may enhance the wintertime PM2.5 pollution due to the weakened titration effect and can be offset by simultaneously controlling NH3 emissions. Our results emphasize the need to jointly consider NH3 with SO2 and NOx emission controls when designing PM2.5 pollution mitigation strategies.