Combined short-term and long-term emission controls improve air quality sustainably in China

China’s rapid economic growth has been accompanied by severe air pollution that impacts ecosystems and human health. Initial controls targeted SO2 and acid rain; since 2010, attention shifted to reactive nitrogen (Nr) and fine particulate matter (PM2.5). In addition to long-term national policies (e.g., 2013 Atmospheric Ten Actions; 2018 Three-Year Blue-Sky Defense) aimed at persistent emission reductions across power, industry, and transport, China has also employed short-term, intensive measures around international events to secure "blue skies" by temporarily restricting industry and vehicles. PM2.5 responds on daily scales, whereas atmospheric nitrogen deposition reflects source-to-sink processes at monthly to annual scales. The key research questions are: How effective are short-term abatement measures relative to long-term policies in reducing PM2.5 and Nr deposition? How do anthropogenic activities and climate change influence PM2.5 and nitrogen deposition dynamics? The study leverages nationwide deposition monitoring and event-based PM2.5 datasets to assess trade-offs between short- and long-term strategies and to identify emerging challenges such as rising NH3, transboundary transport, and climate impacts.

Prior studies and national monitoring have shown significant emission reductions from power, industry, and transport sectors, yielding decreased SO2, NOx, and PM2.5, and mitigating acid rain. Emission inventories, environmental monitoring, and model simulations corroborate these trends. Observations and modeling indicate PM2.5 and nitrogen deposition rose rapidly in the early 2010s and then stabilized or declined with policy implementation. Global warming can modify spatial patterns and trends of Nr emissions and deposition via temperature-sensitive NH3 volatilization, suggesting climate–pollution interactions. Despite success in NOx control, fewer national measures have targeted NH3, a major contributor from agriculture, potentially offsetting gains as acid gases decline. Evidence of pollution transport underscores the need for inter-regional coordination. These lines of work set the context for comparing short-term campaign-style controls with enduring policy measures and for examining evolving Nr species composition under sustained controls.

Data and monitoring: The study integrates (1) event-based PM2.5 and precursor observations in Beijing around four international events (2008 Olympic Games, 2014 APEC, 2015 Military Parade, 2017 Belt and Road Summit), using literature-sourced PM2.5 and ionic components for some events, and group observations for BRS; gaseous SO2 and NO2 from CNEMC; monthly NH3 from ALPHA passive samplers as an approximation for event-period NH3; and (2) the Nationwide Nitrogen Deposition Monitoring Network (NNDMN) with ~59 sites and >40,000 samples (2011–2020), measuring monthly gaseous NH3 and HNO3 and particulate pNH4 and pNO3 with active DELTA samplers; NO2 with Gradko tubes; precipitation chemistry via event-based rain sampling. Deposition estimation: Dry deposition (Fd) was computed as concentration (C) times deposition velocity (Vd); hourly Vd for five Nr species was simulated by GEOS-Chem. Bulk deposition (Fb) used precipitation amount times precipitation concentration. Total deposition F = Fd + Fb. Spatial summaries used both arithmetic site averages and geographic (area-weighted) averages across six regions (NC, NE, NW, SE, SW, TP) to address site heterogeneity. Event analyses: Pre-, during-, and post-event windows were defined per event. Meteorological confounders (wind speed, precipitation, RH, pressure, temperature) were assessed to contextualize concentration changes. Statistical analyses: One-way ANOVA and nonparametric tests compared groups across land-use types and regions. Mann–Kendall tests and Sen’s slope quantified trends in deposition and emissions. Linear/nonlinear regressions linked bulk deposition to precipitation and aqueous Nr concentrations. Machine learning: Random forest models related monthly oxidized, reduced, and total deposition to emissions (SO2, NOx, NH3), meteorology (precipitation, temperature, wind speed, pressure, boundary layer height), soil parameters (temperature, moisture, evapotranspiration), and NDVI; performance validated via 10-fold cross-validation. Emission data: Monthly emissions (2011–2020) from the MEIC inventory; additional MIX inventory utilized for modeling. Modeling future scenarios: The WRF (v3.9.1)–EMEP (rv4.17) system (27 km grid) simulated nonlinear responses of PM2.5-SNA and N deposition under Base-2017 conditions and two policy-climate scenarios: Case 1 (2030, Carbon Emission Peaking) and Case 2 (2060, Carbon Neutrality), with meteorology from FNL and RCP4.5 (2030/2060). Anthropogenic emissions reflected stringent policy assumptions: acid gases ~−40% (2030), ~−90% (2060); NH3 ~−10% (2030), ~−50% (2060). Model performance was evaluated against TAP PM2.5 observations (spatial R2≈0.77 in 2017) and NNDMN-calibrated deposition fields.

Short-term controls (event periods): - During Olympic, APEC, and Parade events, SO2 and NO2 fell by 30–56%, PM2.5 by ~60%, and secondary inorganic ions (SNA: SO4²−, NO3−, NH4+) by >70%, indicating marked air quality improvement; NH3 did not decrease substantially and slightly increased during APEC. - After controls ended, pollutant levels rebounded: PM2.5 increased by 157% (post-Olympic), 156% (post-APEC), 139% (post-Parade), and showed ~−1% change post-BRS (no significant effect during BRS under resident-friendly controls). SNA rose from event to post-event (up to 5-fold after the Parade). - Meteorology modulated concentrations (negative links with wind, precipitation, RH; positive with pressure), but emission reductions were the dominant driver of event-period declines. Long-term trends (Beijing and national): - Beijing (Aug 2005–Aug 2020): PM2.5 −68%, SNA −66%, gaseous precursors −52%; SO2 −90%, NO2 −38%, NH3 −26% (summer perspective). Winter trends (Jan 2005–Jan 2020): PM2.5 −65%, SNA −51%, SO2 −94%, NO2 −50%; winter NH3 increased ~50% and stabilized near 7 µg m−3 after 2013. - From 2013 to 2020, sustained policies yielded ~54% PM2.5 decrease in Beijing. Short-term events did not change the long-term decline slope (p>0.05); short-term net effects remained large (e.g., 67% at APEC, 61% at Parade). Nitrogen deposition (China, 2011–2020): - Site-average dry, bulk, total deposition: 20.2±1.60, 19.0±0.82, 39.2±2.42 kg N ha−1 yr−1; geographic means: 14.8±1.71 (dry), 12.8±0.82 (bulk), 27.7±2.42 (total). - Declines from 2012 to 2020: dry −19%, bulk −26%, total −23% (p<0.01). Strong coupling with emissions: R2=0.90 between Nr emissions and total deposition. NOx emissions decreased ~33% (2012–2020), NH3 emissions declined modestly (10.6→9.07 Tg). - Composition shifts: Dry deposition of NO2, HNO3, pNH4+, pNO3− decreased (−36%, −64%, −33%, −47%), but NH3 dry deposition increased. Gaseous fraction of NH3 in reduced N deposition rose (0.39→0.52). NHx deposition decreased ~13%, consistent with ~14% NH3 emission decrease; oxidized N deposition fell ~31%, tracking NOx. Spatial and land-use patterns: - Mean total deposition at urban, rural, background sites: 47.8±7.32, 39.3±2.32, 24.6±6.49 kg N ha−1 yr−1. Urban NHx and NOy deposition exceeded rural by ~10% and ~37%, respectively. Urban NHx and NOy deposition declined 3.5% and 3.9% per year (p<0.01). In rural areas, decreases in NH4+ bulk deposition were offset by slight increases in NH3 dry deposition; during COVID-19 (2019→2020), reduced N dry deposition decreased in urban but increased in rural areas, implying persistent agricultural NH3 sources. - Regional differences: North China (NC) exhibited the strongest Nr emission abatement and N deposition decline (−3.9% yr−1 since 2011) and decoupling from GDP growth. In SW China, NH3+pNH4+ rose after 2017 despite decreases in acid gases, indicating more gaseous NH3 under reduced neutralization. - Transboundary effects: At provincial scale in NC, about 50% of NOx is deposited locally with substantial export to adjacent regions (northwest/southeast), highlighting inter-regional pollution transport and external mortality burdens. Future scenarios (WRF–EMEP): - PM2.5-SNA national mean: 22.9 µg m−3 (2017) → 12.8 (2030, −44%) → 5.4 (2060, −76%); area >15 µg m−3: 51% (2017), 20% (2030), 0.3% (2060). - N deposition mean: 14.6 kg N ha−1 yr−1 (2017) → 9.8 (2030, −33%) → 6.3 (2060, −57%). Responses are weaker for deposition than for PM2.5-SNA due to complex source–sink and transformation processes.

Event-focused short-term measures can rapidly lower PM2.5 and secondary inorganic aerosols, creating visible "blue skies" that build public confidence and political momentum. However, these improvements are temporary, with pollutants rebounding once controls cease, and such measures impose economic and social costs if prolonged. Long-term national policies, by contrast, drive sustained declines in SO2, NOx, PM2.5, and total nitrogen deposition, without altering the economy’s overall trajectory, but they require years to deliver observable benefits. The study demonstrates that combining short-term campaigns (for immediate, demonstrable gains and public buy-in) with durable long-term reforms (for stability and permanence) yields a robust pathway to sustained air quality improvement. Emerging challenges include a transition to NH3-rich atmospheres as acid gases fall, slower declines in N deposition after early gains, persistent rural NH3 from agriculture, and substantial inter-regional transport that externalizes pollution burdens. Climate change, through warming and altered precipitation patterns, can modulate monthly and seasonal deposition and influence PM2.5 distributions, potentially offsetting some policy gains. Effective progress thus requires synergistic multi-pollutant control (especially adding NH3 mitigation), regionally coordinated policies addressing transboundary flows, and technological innovation, particularly in agriculture, to continue advancing toward 2030/2060 goals.

The study shows that short-term, high-intensity emissions controls during major events substantially reduce PM2.5 and SNA, but effects are transient with post-event rebounds. Long-term policies since 2013 have delivered durable and substantial improvements: PM2.5 reductions in Beijing (~54% since 2013) and national total nitrogen deposition declines (~23% since 2012), with strong coupling between emissions and deposition. The composition of nitrogen deposition is shifting toward higher gaseous NH3 fractions as acid gases decline, revealing the importance of NH3 control. Modeling indicates that under stringent decarbonization and air pollution policies, PM2.5-SNA and N deposition will continue to fall markedly by 2030 and 2060, though deposition responds less strongly than PM2.5. The main contributions are a quantitative comparison of short-term versus long-term control effectiveness, nationwide observational evidence of N deposition trends and component shifts, documentation of regional and land-use differences and transboundary transport, and scenario-based projections under climate change. Future work should prioritize: (1) scalable NH3 mitigation via precision agriculture, manure management, and technology diffusion; (2) integrated, multi-pollutant strategies balancing oxidized and reduced nitrogen; (3) cross-regional coordination with fair compensation mechanisms; (4) innovation to sustain gains as easy reductions are exhausted; and (5) continued long-term monitoring to evaluate policy efficacy and climate influences.

- NH3 monitoring during events relied on monthly ALPHA passive sampling as an approximation of ambient NH3, limiting temporal resolution relative to daily PM2.5 and NO2/SO2 data. - For inter-regional transport quantification, the analysis assumed national emission–deposition balance within China and no cross-border exchanges, simplifying complex transboundary fluxes. - Emission inventories (MEIC/MIX) and deposition velocities (GEOS-Chem) carry uncertainties that propagate into deposition estimates and modeled scenarios. - Climate impact assessments used RCP4.5 pathways and assumed “most stringent” policy implementations; alternative pathways or policy stringency could alter projections. - Site distribution and land-use representation may introduce spatial sampling biases despite area-weighting; winter air quality and rural NH3 dynamics remain less responsive, indicating gaps in control coverage. - Declines in N deposition slowed after 2015, suggesting that further improvements depend on technological innovations not fully captured by historical trend analyses.