Publications·August 03, 2025
This region-wide assessment uses satellite remote sensing (notably GEMS), combined with targeted ground measurements, to map where, when, and why air pollution spikes across Asia and the Pacific. It reframes the evidence through two operational lenses—hotspots and airsheds—to help governments move from city-by-city responses to coordinated, cross-border action. The report also connects trends to health, economic loss, agriculture, and equity, and closes with priority recommendations to strengthen regional cooperation under RAPAP.
Health, economic and equity stakes
Air pollution remains one of the region’s largest environmental risks: Asia-Pacific accounts for roughly two-thirds of global premature deaths from air pollution. A time-series of Years Lived with Disability (YLD) shows a persistent rise since 1990 in most countries. Economic impacts include trillions in welfare losses and labour-income losses, while crop damage—especially from O₃— threatens food security. The burden is uneven: lower-middle-income countries, and poor or vulnerable populations, bear disproportionate exposure.
Present status from space: AOD, NO₂, SO₂, O₃
Aerosol Optical Depth (AOD) pinpoints major PM loadings. In 2023, AOD was persistently elevated over eastern China and seasonally high across South and mainland Southeast Asia. Two strong seasonal signals dominate: (i) the Indo-Gangetic Plain (IGP) peaking around November (post-harvest burning) and (ii) northern Lao PDR–northern Viet Nam–northern Thailand during February–April (dry-season burning). Monsoons lower AOD mid-year via wet deposition. Dust from the Gobi and Thar deserts adds episodic surges.
NO₂ (2023) shows persistent, year-round urban/industrial hotspots in Beijing-Tianjin-Hebei (BTH), the Yangtze River Delta, northern India, and large metros across Southeast Asia, Japan, and the Republic of Korea—with winter (Oct–Mar) peaks at higher latitudes. Elevated NO₂ also traces major shipping lanes (Malacca/Singapore Straits, Indian Ocean approaches).
SO₂ patterns differ: industrial power belts (e.g., Inner Mongolia in China; Singrauli in India) stand out, with winter maxima from heating and stable boundary layers; volcanoes (e.g., Marapi, Sumatra) contribute episodically. The report cautions that SO₂ retrievals near the western edge of the GEMS domain (high solar-zenith angles) can overestimate values; those are excluded from interpretation.
O₃ exposure has risen globally and in South Asia. Because O₃ is secondary and retrieval at the surface from satellites remains challenging, the paper emphasises careful integration of satellite columns with in-situ data and models. Seasonally, O₃ rises with warmth/sunlight (Mar–Sep) and stagnation in megacity airsheds, then eases with monsoon cleansing.
Can we trust GEMS for operations? (Validation signal)
Cross-validation of GEMS AOD with AERONET (≈25k co-observations over 69 stations, 2022–2023) yields R² ≈ 0.54 overall, improving for certain scans/hours; correlations are strongest in Korea, northern China, and mainland Southeast Asia. Conclusion: GEMS is operationally valuable if used with well-calibrated ground networks and rigorous QA/QC.
Interannual and decadal trends—divergent stories
East Asia: Since 2013, large-scale policy (China’s Air Pollution Prevention and Control Action Plan) drove marked declines in NO₂/SO₂/AOD, with spillover benefits to neighbours. Information disclosure and stricter controls reduced concentrations and health burdens. Japan and ROK show the familiar arc: earlier peaks followed by long-run reductions.
South & Southeast Asia: Hotspots persist in the same broad locations, with SO₂ increases in parts of eastern India and AOD rises over mainland Southeast Asia (notably 2023). Maritime Southeast Asia experienced extreme AOD during El Niño (2015–16). The analysis excludes 2020–22 to avoid COVID-19 distortions. Overall, seasonal biomass burning, coal/industry growth corridors, and stagnant synoptic patterns dominate variability.
Ground perspectives: days above WHO thresholds & inequality
Using AirNow city stations for 2023, many South Asian cities exceeded WHO PM₂.₅ interim targets on large fractions of days—some >40% of the year over IT-1, with never falling below the 15 μg/m³ 24-h guideline. In Southeast Asia, exceedances are generally fewer in major cities, but this understates rural haze: city monitors miss agricultural-fire plumes, underscoring the need to fuse satellite + model + ground for a regional picture.
Linking to inequality, lower-middle-income countries show more days above WHO targets; simple comparisons of Gini indices and PM₂.₅ metrics hint at relationships that require careful treatment (different spatial/statistical bases). The practical takeaway: satellites can fill data gaps in countries with sparse monitoring, improving equity-aware policymaking.
Hotspots & airsheds—how to target action
The report formalises regional hotspots as extended areas with persistent or extreme pollution that often cross administrative borders; airsheds describe the meteorology/topography-bounded catchments that trap or transport pollution. Three emblematic hotspots are profiled:
Indo-Gangetic Plain & Himalayan Foothills: multi-source emissions (domestic energy, open burning, brick kilns, traffic); frequent winter smog; major health and food-security risks (O₃ impacts).
Beijing–Tianjin–Hebei / North China Plain Industrial Belt: dense population, industry, coal/power; strong winter inversions; significant post-2013 improvements but continued vigilance needed.
Mainland Southeast Asia: Feb–Apr burning across Lao PDR, Thailand, Myanmar, Viet Nam, Yunnan (China) produces large, transboundary haze footprints.
These examples argue for airshed-scale governance, not just city limits.
What the authors recommend (for RAPAP implementation)
Tighten & align standards with WHO AQGs/Interim Targets; review regularly.
Build a hybrid monitoring fabric: reference-grade stations as backbone; EO (GEMS + LEO) for coverage and transport; calibrated low-cost sensors to densify; fusion/ML products for decision-grade fields.
Make data fully open: units (not only AQI), station geolocation, timeliness, and APIs; harmonise QA/QC and clearly flag sensor vs reference.
Focus on airsheds & transboundary flows: shared dashboards, episode rooms, and joint forensics; prioritise seasonal burn management and industrial corridors.
Invest in O&M, training, SOPs, and institutional capacity so pilots become operational services.
Use this evidence base to deepen multilateral cooperation through RAPAP working groups and best-practice exchange.
Key words
Airsheds; Hotspots; Transboundary air pollution; AOD/PM₂.₅; NO₂/SO₂/O₃; GEMS geostationary satellite; Validation (AERONET); Biomass burning & El Niño; Dust (Gobi/Thar); WHO AQGs & Interim Targets; Open data & APIs; Data fusion / ML; Equity & vulnerability.