Mixed causes Possible
Daily PM2.5 peaked at 52.2 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 ≈ 1.8 cigarettes. Across everyone living in the affected metros, that is the collective equivalent of ≈ 5.0 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
EPA's PMF source-apportionment appendix contains an actual St. Louis sample on January 22, 2001. Ammonium nitrate and coal combustion were the two largest modeled factors, with mobile and smaller industrial and crustal factors also present. That exact-date mixture supports mixed-source pollution at possible confidence without assigning one emitter or one atmospheric mechanism.
Possible This is a transparent hypothesis supported by limited evidence or an explicit inference, not a definitive attribution.
U.S. Environmental Protection Agency · Sep 24, 2003 hypothesis
EPA's Positive Matrix Factorization time series includes an actual sample at site 29-510-0085 on 2001-01-22; ammonium nitrate and coal combustion were the two largest modeled factors, with mobile sources and smaller industrial and crustal factors also present.
Inference, not direct attribution
epa_pmf_exact_sample_factor_pattern.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
epa_pmf_exact_sample_factor_pattern.v1. This inference is recorded in the catalog methodology.
Air-mass movement
NOAA HYSPLIT modeled 12 backward paths across multiple arrival times and heights. They show where the air traveled, not which source polluted it.
Explore the modeled paths ↓Local response
EPA monitors recorded the event across 1 metro. The peak daily concentration is roughly 2.4 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 “Mixed causes” explanation; it does not assign the cause by itself.
St. Louis, MO-IL
Monitor comparison window 2001-01-19 to 2001-01-25 · PM2.5 target: IEPA-RAPS TRAILER
66% above nearby days
14 calm hours observed
Haze reported during 3 hours
66% higher than surrounding days
32% lower than surrounding days
109% higher than surrounding days
28% higher than surrounding days
17% higher than surrounding days
61% lower than surrounding days
8 kt maximum · 14 of 24 observed hours calm
0 observed hours at or below 3 miles
19.9°F to 44°F
96% maximum
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.
Air-mass reconstruction
NOAA HYSPLIT traces the air arriving at each metro backward for 72 hours. Multiple arrival times and heights show whether the broad path is stable—or sensitive to assumptions about when and where the polluted layer arrived.
Air arriving near St. Louis
Peak-date arrivals · 2001-01-22
Each line begins 72 hours before arrival and points toward the metro. Color represents arrival height; repeated lines represent 00, 06, 12, and 18 UTC arrival assumptions. A path can show where the air traveled, but it cannot prove which source added the PM2.5.
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 | 52.2 µg/m³ | 2.4 | Jan 22, 2001 | #11 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.
U.S. Environmental Protection Agency
EPA's St. Louis site summary identifies coal combustion, ammonium nitrate, mobile sources, steel production, smelting, a zinc-refinery factor, and crustal material. The report documents the site's modeled date range, factor interpretation, average apportioned mass, local source setting, and back-trajectory context.
Context for the event; this item does not establish the attributed cause by itself.
The site summary defines and contextualizes the factors across the modeled period; the separate appendix supplies the event-date source strengths.
U.S. Environmental Protection Agency
EPA's Positive Matrix Factorization time series includes an actual sample at site 29-510-0085 on 2001-01-22; ammonium nitrate and coal combustion were the two largest modeled factors, with mobile sources and smaller industrial and crustal factors also present.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The appendix reports modeled factor contributions at one chemical-speciation monitor, not the complete metro-wide emissions inventory.
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.