Stagnant pollution mixture Possible
Daily PM2.5 peaked at 51.1 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 ≈ 1.7 cigarettes. Across everyone living in the affected metros, that is the collective equivalent of ≈ 4.9 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
A Midwest PM2.5 case study identifies September 9-15, 2003 as a widespread episode affected by surface stagnation, a stalled frontal boundary, and blocked synoptic flow. St. Louis's catalog event begins on the exact first day. The study supports accumulated regional and local pollution under stagnant meteorology, not a unique source.
Possible This is a transparent hypothesis supported by limited evidence or an explicit inference, not a definitive attribution.
American Meteorological Society / Sonoma Technology · Jan 1, 2004 corroborates
The case study identifies September 9-15, 2003 as a Midwest-wide PM2.5 episode and documents a blocking ridge, surface-wind stagnation, a stalled frontal boundary, and other meteorological conditions that allowed fine-particle concentrations to remain elevated.
Inference, not direct attribution
exact_window_regional_field_study_stagnation_possible.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
The case study identifies September 9-15, 2003 as a Midwest-wide PM2.5 episode and documents a blocking ridge, surface-wind stagnation, a stalled frontal boundary, and other meteorological conditions that allowed fine-particle concentrations to…
View supporting source ↗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 2003-09-06 to 2003-09-12 · PM2.5 target: Blair Street
52% above nearby days
6% lower than surrounding days
29% higher than surrounding days
52% higher than surrounding days
29% higher than surrounding days
About the same as surrounding days
7% higher than surrounding days
143% higher than surrounding days
19% higher than surrounding days
About the same as surrounding days
About the same as surrounding days
About the same as surrounding days
6% 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 | 51.1 µg/m³ | 2.3 | Sep 9, 2003 | #15 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.
American Meteorological Society / Sonoma Technology
The case study identifies September 9-15, 2003 as a Midwest-wide PM2.5 episode and documents a blocking ridge, surface-wind stagnation, a stalled frontal boundary, and other meteorological conditions that allowed fine-particle concentrations to remain elevated.
A retained source supports the attributed cause for the stated place and dates.
The report treats the Midwest region and does not provide a St. Louis-specific source split.
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.