Stagnant pollution mixture Possible
Daily PM2.5 peaked at 50.4 µg/m³ near St. Louis across 1 covered metro home to 2.8 million people.
Counting the extra PM2.5 above each metro's typical day, this event handed the average affected resident ≈ 3.3 cigarettes. Across everyone living in the affected metros, that is the collective equivalent of ≈ 9.2 million cigarettes. An awareness illustration of outdoor exposure at 22 µg/m³ per cigarette (Berkeley Earth), counting only the event's ranked days; not a medical dose.
Why we think this happened
Certified observations show a broad multi-pollutant episode across St. Louis monitors while wind speed dropped sharply. NWS independently documented persistent haze under weak flow through the event window. This supports a stagnant mixed-pollution mechanism, but not a unique source. A same-day small facility fire was reviewed and retained as an occurrence only because its scale and transport are insufficient to explain the metro-wide pattern.
Possible This is a transparent hypothesis supported by limited evidence or an explicit inference, not a definitive attribution.
U.S. EPA AirData hypothesis
IAQng's reproducible aggregation of certified EPA records found a broad St. Louis episode across 13 PM2.5 monitors: the event-window median was 44.225 µg/m³, versus 20.0 before and 18.9 after. PM10, CO, NO2, ozone, and SO2 also rose while median wind speed fell to 1.59 knots.
National Weather Service St. Louis via Iowa Environmental Mesonet · Aug 2, 2007 hypothesis
The same-window NWS St. Louis discussion said weak low- and mid-level flow was allowing haze to persist and expected weak flow and haze to continue through early August 3.
Inference, not direct attribution
documented_weak_flow_haze_multimonitor.v1: This inference is recorded in the catalog methodology.
Evidence path
A cause is more convincing when the record connects three things: something happened, the air mass moved toward the region, and local instruments responded. Here is how much of that chain is documented for this event.
Source or mechanism
documented_weak_flow_haze_multimonitor.v1. This inference is recorded in the catalog methodology.
Air-mass movement
The retained record does not yet show how air traveled from the proposed source to these metros. A trajectory replay or dated plume analysis belongs here.
Local response
EPA monitors recorded the event across 1 metro. The peak daily concentration is roughly 2.3 cigarette-equivalents at the outdoor daily rate.
Inspect the pollutant and weather signals ↓Event autopsy
Before, during, and after the episode: nearby pollutant monitors and surface-weather stations show what changed. This context tests the “Stagnant pollution mixture” explanation; it does not assign the cause by itself.
St. Louis, MO-IL
Monitor comparison window 2007-07-30 to 2007-08-06 · PM2.5 target: Blair Street
91% above nearby days
33% lower than surrounding days
107% higher than surrounding days
91% higher than surrounding days
107% higher than surrounding days
27% higher than surrounding days
45% higher than surrounding days
108% higher than surrounding days
24% higher than surrounding days
7% higher than surrounding days
About the same as surrounding days
About the same as surrounding days
33% lower than surrounding days
Source: U.S. EPA AirData daily observations and, where available, nearby ASOS/AWOS surface observations. Values are descriptive medians over the retained monitor set; missing measurements remain missing. Transport and source evidence remain separate layers because this instrument pattern does not assign a cause by itself.
Episode peak
-- cigarettes
The smoke map's scale, replayed over this event: the surface interpolates between covered metros' daily records, framed to the affected region. Hover for a reading, click a metro to pin it to the card. Days a monitor did not report draw nothing.
| Metro | Peak PM2.5 | Cigarettes, whole event | Peak day | All-time standing |
|---|---|---|---|---|
| St. Louis, MO-IL | 50.4 µg/m³ | 4.5 | Aug 3, 2007 | #18 of its top days |
"Cigarettes, whole event" totals a metro's ranked event days at 22 µg/m³ per cigarette: what a person outdoors there breathed across the event, as the illustration. "All-time standing" is where this episode's days sit in each metro's ranked record (worst-area measure, since 1999). Full local context is on each metro's Air Records page.
Public evidence library
Each card states what a source supports—and what it does not. Official records, research, reporting, community observations, and transparent inferences remain visibly different kinds of evidence.
National Weather Service St. Louis via Iowa Environmental Mesonet
The same-window NWS St. Louis discussion said weak low- and mid-level flow was allowing haze to persist and expected weak flow and haze to continue through early August 3.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The discussion identifies the accumulation setting but does not identify the pollutants' emitting sources.
U.S. Coast Guard National Response Center
NRC report 844190 records a small St. Louis facility fire on August 2 involving an unknown quantity of hazardous waste in a roll-off storage container.
Context for the event; this item does not establish the attributed cause by itself.
The report documents a contemporaneous occurrence, not that the small facility fire caused the metro-wide PM2.5 episode.
U.S. EPA AirData
IAQng's reproducible aggregation of certified EPA records found a broad St. Louis episode across 13 PM2.5 monitors: the event-window median was 44.225 µg/m³, versus 20.0 before and 18.9 after. PM10, CO, NO2, ozone, and SO2 also rose while median wind speed fell to 1.59 knots.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The pollution fingerprint demonstrates a broad mixed episode and weak dispersion; it does not identify a unique emitting source.
Possible This is a transparent hypothesis supported by limited evidence or an explicit inference, not a definitive attribution.
This episode was grouped from the monitor record by the catalog pipeline and given its cause from the evidence listed above. It has not been individually reviewed and signed off, so its boundaries, cause, and name may change as evidence accumulates. Grouping policy common_sense_root_cause_grouping.v2. The cause label reflects the assembled evidence above, not an IAQng detection formula. If it is wrong, tell us.