Chengdu Air Quality Index (AQI)
Sichuan Basin Pollution Trap · Western China's Largest City · 成都 · Population 21M
The Sichuan Basin: Why Chengdu Can't Escape Its Own Pollution
Chengdu's pollution problem begins with geography that predates the city itself. The Sichuan Basin (四川盆地) is an ancient geological formation — a sunken plain roughly the size of France, encircled on all sides by dramatic mountain ranges. To the west, the Tibetan Plateau rises to 4,000+ meters. To the north, the Qinling and Daba Mountains form a 2,000 m barrier. Eastern and southern ranges complete the enclosure.
This creates one of the world's most effective natural pollution traps. Air masses enter the basin but rarely exit with enough force to fully flush it. Temperature inversions — where a warm air layer caps cooler air near the ground — form on roughly 200+ days per year in Chengdu, compared to 80–100 days in Beijing. When an inversion forms, every vehicle exhaust, factory emission, and cooking fire is compressed into an increasingly concentrated layer of polluted air with nowhere to go.
Chengdu is also characterized by its overcast climate. The basin averages fewer than 100 sunny days per year — earning Sichuan the saying "the Sichuan dog barks at the sun" (蜀犬吠日). While reduced solar radiation limits ozone formation, the persistent low clouds and fog trap pollution at ground level for extended periods.
Despite these challenges, Chengdu has reduced its PM2.5 by approximately 30–35% since 2013 through vehicle restrictions, industrial relocations, and coal-to-gas conversions. But geography remains the fundamental constraint — achieving the same air quality as coastal cities like Shenzhen or Xiamen would require near-zero emissions from a metro area of 21 million people.
Monthly AQI in Chengdu
Annual average: AQI 112 — PM2.5 44 μg/m³. Source: MEE China, IQAir 2024 World Air Quality Report.
Key Pollutants in Chengdu
| Pollutant | Chengdu Level | WHO Guideline | Primary Source |
|---|---|---|---|
| PM2.5 | 44 μg/m³ | 5 μg/m³ | Coal combustion, vehicles, industry, cooking |
| PM10 | 70 μg/m³ | 15 μg/m³ | Construction, road dust, basin-trapped industry |
| NO₂ | 55 μg/m³ | 10 μg/m³ | 4M+ vehicles, thermal power plants, industry |
| O₃ (ozone) | 72 μg/m³ | 60 μg/m³ | Traffic NOx + summer sunlight in basin |
| SO₂ | 28 μg/m³ | 40 μg/m³ | Coal burning — industrial and residential |
Chengdu vs Major Chinese Cities
| City | AQI 2024 | PM2.5 μg/m³ | Key Factor |
|---|---|---|---|
| Xi'an | 118 | 48 | Wei River Valley bowl, loess dust |
| Chengdu ★ | 112 | 44 | Sichuan Basin, 200+ inversion days/yr |
| Zhengzhou | 125 | 52 | Henan plain, heavy industry |
| Beijing | 92 | 35 | NW winds provide periodic flushing |
| Wuhan | 98 | 38 | Yangtze industrial corridor |
| Chongqing | 88 | 33 | Mountain valley but more winds |
| Shanghai | 78 | 27 | Sea breezes from East China Sea |
| Shenzhen | 60 | 19 | Tropical climate, Pearl River ventilation |
Health Advisory for Chengdu Residents
Limit intense outdoor exercise when AQI exceeds 100. Keep windows closed on winter smog days. Use apps like IQAir or MEE to check daily AQI before outdoor plans.
Chronic PM2.5 exposure impairs lung development. Schools should limit outdoor time above AQI 100. HEPA purifiers in classrooms and bedrooms strongly recommended. N95 masks for outdoor commutes in winter.
PM2.5 particles cross the placenta and affect fetal brain development. Minimize outdoor exposure December–February. Indoor air purification critical — avoid cooking with gas or coal without ventilation.
Asthma and COPD sufferers should have AQI thresholds established with their physician. Keep reliever inhalers accessible. During AQI 150+ events, stay indoors with air purifier running.
Seasonal Air Quality Guide — Chengdu
Coal heating + cold air pooling in basin. Temperature inversions trap everything. AQI regularly 150–200+. Worst weeks can exceed AQI 300 (Hazardous). Keep windows shut, wear N95 outdoors.
Heating season ends in March, bringing rapid improvement. Spring rains wash the basin. April–May conditions variable — occasional inversions still occur when anticyclones stall over the basin.
Southwest monsoon brings regular rainfall to the basin, washing out pollutants. July–August are the cleanest months (AQI 60–72). Outdoor activities safest now. Some ozone risk on hot, sunny days.
Monsoon retreats, dry cool air settles in the basin. Inversions restart in October. Heating season resumes in November. AQI climbs rapidly from ~80 in October to ~155 by November.
Frequently Asked Questions — Chengdu Air Quality
Why does Chengdu have worse air quality than most Chinese cities of its size?
The Sichuan Basin is a natural pollution trap. Chengdu sits in a flat plain completely enclosed by the Tibetan Plateau to the west, Qinling Mountains to the north, and mountain ranges to the east and south. This geography creates frequent temperature inversions — warm air sits above cooler polluted air near the ground, preventing vertical mixing. Combined with a large population (21 million), dense industry, and heavy vehicle use, pollutants accumulate with little dispersal. Chengdu averages fewer than 50 days/year with good ventilation winds.
When is Chengdu's air quality at its worst?
December through February is the worst period, when AQI regularly exceeds 150 (Unhealthy) and peaks above 200 (Very Unhealthy) during severe inversion episodes. Coal-fired heating in winter adds massive PM2.5 and SO₂ loads. The basin's cold air pooling can trap smog for days or weeks without relief. July and August offer the best air quality — summer monsoon rains wash the atmosphere regularly.
Is Chengdu's air quality improving?
Yes, but slowly. China's national clean air action plans have driven improvements across major cities. Chengdu has phased out high-emitting vehicles, expanded metro lines, closed or relocated the most polluting factories, and converted coal-fired boilers to natural gas. PM2.5 has dropped ~30% from its 2013–2015 peaks. However, basin geography means improvement is harder than in cities with better natural ventilation — Chengdu remains significantly more polluted than coastal cities.
How does Chengdu's AQI compare to Beijing?
Chengdu (AQI ~112) is slightly worse than Beijing (AQI ~92) on an annual average basis — partly because both cities have been making improvements, but Chengdu's geography is harder to overcome without addressing every emission source. Beijing has the advantage of strong northwest winds in winter that periodically flush its basin; Chengdu's enclosed bowl receives far less ventilation. On severe pollution events, Chengdu's AQI can exceed 300 (Hazardous).
What should visitors and residents do to protect themselves?
Check AQI daily using apps like IQAir, AirVisual, or the national MEE app. On days with AQI above 100, reduce outdoor exercise duration. Above 150, wear an N95/KN95 mask outdoors and limit outdoor time. Use HEPA air purifiers indoors, especially in bedrooms and children's rooms. Keep windows closed during winter smog events. Best strategy: plan outdoor activities from July–September when air is cleanest.