Worst air pollution Gateway Arch - St. Louis, today and tomorrow
Today (20 September): Air quality in Gateway Arch is fair. AQI, PM2.5, and NO2 are fair; PM10, O3, SO2, CO, HCHO, and PM1 are good. Pollution peaks around 4:00 PM (NO2: 30.3/100).
Tomorrow (21 September 2026): Air quality in Gateway Arch is moderate. AQI worsens to moderate; NO2 increases to moderate; all other pollutants stay as today. Pollution peaks around 6:00 PM (NO2: 41.9/100). Airmine updates local air quality forecasts four times daily.
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Developed with support from the European Space Agency (ESA)’s Business Applications and Space Solutions (BASS) team
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St. Louis' Gateway Arch air quality benefits from the Mississippi River waterfront position, with the river's wide waterway providing limited natural ventilation around the extraordinary 192-metre stainless steel monument. Vehicle traffic on the surrounding I-44 and I-70 corridors contributes local sources, while the riverfront's open geometry provides reasonable dispersal and the Gateway Arch generally maintains moderate St. Louis metropolitan readings throughout most of the year.
Today - 20 September | Pollution peaks at 16:00
AQI:
Fair
30.3/100
PM2.5 (fine particles):
Fair
28.9/100
PM10 (coarse particles):
Good
14.7/100
NO2 (nitrogen dioxide):
Fair
30.3/100
O3 (ozone):
Good
17.0/100
SO2 (sulfur dioxide):
Good
1.3/100
CO (carbon monoxide):
Good
0.6/100
HCHO (formaldehyde):
Good
3.2/100
PM1 (ultrafine particles):
Good
5.2/100
Levels show the poorest local air quality during the day
Tomorrow - 21 September | Pollution peaks at 18:00
AQI:
Moderate
41.9/100
PM2.5 (fine particles):
Fair
29.0/100
PM10 (coarse particles):
Good
14.7/100
NO2 (nitrogen dioxide):
Moderate
41.9/100
O3 (ozone):
Good
18.5/100
SO2 (sulfur dioxide):
Good
1.2/100
CO (carbon monoxide):
Good
0.6/100
HCHO (formaldehyde):
Good
3.6/100
PM1 (ultrafine particles):
Good
5.7/100
Other Landmarks in St. Louis
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Frequently Asked Questions
Is air quality bad in Gateway Arch right now?
On 20 September 2026, overall air quality in Gateway Arch is fair. AQI is the main air quality indicator at 30.3/100. Pollution peaks around 4:00 PM (NO2: 30.3/100).
Will air quality be worse tomorrow in Gateway Arch?
On 21 September 2026, air quality conditions in Gateway Arch will be worse than today, with AQI forecast at moderate at 41.9/100. Pollution peaks around 6:00 PM (NO2: 41.9/100).
When is air pollution at its worst in Gateway Arch?
Gateway Arch follows St. Louis's typical seasonal air quality pattern, as winter generally brings the cleanest air, aside from brief inversion episodes during calm cold spells; by contrast, summer heat waves and the dust of planting and harvest seasons are the main annual concerns. Farm-heavy surrounding land is the main source of the seasonal dust spikes.
Are air quality levels in Gateway Arch based on measurements or forecasts?
It is forecasts derived by downscaling forecasts provided by EU’s Copernicus Atmosphere Monitoring Service (CAMS) by taking into account local conditions such as traffic patterns. CAMS bases its forecast on satellite measurements of particles and chemical compounds in the atmosphere. Airmine’s services were developed with support from the European Space Agency (ESA)’s Business Applications and Space Solutions (BASS) team.
Why doesn’t the forecast always reflect wildfire impacts?
Airmine’s forecast uses CAMS (Copernicus Atmosphere Monitoring Service) as its background atmospheric model. While CAMS includes wildfire emissions, these are derived from satellite observations and are not available in real time. During rapidly evolving wildfire events, there may therefore be a delay before increased emissions are incorporated into the model. As a result, the forecast may temporarily underestimate PM₂.₅ and other pollutants associated with wildfire smoke.
During rapidly evolving wildfire events, CAMS may lag by approximately 1–2 days before increased wildfire emissions are fully represented, which can lead to temporary underestimation of PM₂.₅ concentrations in Airmine’s forecast.
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Data provided by Airmine — Last update: 20 September 2026 at 14:33 CEST (Data is updated four times daily.)
This forecast is produced independently using Airmine's own atmospheric data and models.
Unless explicitly made clear, the content on this site has not been paid for by external organisations and is Airmine's sole responsibility.
Disclaimer: Forecasts are provided for informational purposes only and should not be relied upon as medical advice. Actual pollen and air quality conditions may vary from the forecast. If you have allergies, asthma, or another medical condition, consult a healthcare professional. Please see our Terms of Service for important information about limitations of liability.
Some text on this page may be generated or summarised using artificial intelligence (AI). Wherever possible, AI-generated content is enriched with Airmine's proprietary data to improve its accuracy and relevance. However, AI-generated content may still contain errors, omissions, or inaccuracies.
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