Stagnant pollution mixture Possible
Daily PM2.5 peaked at 65.8 µg/m³ near Detroit across 1 covered metro home to 4.4 million people.
Counting the extra PM2.5 above each metro's typical day, this event handed the average affected resident ≈ 2.4 cigarettes. Across everyone living in the affected metros, that is the collective equivalent of ≈ 11 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 observations show a broad, multi-pollutant Detroit episode across 10 PM2.5 monitors while wind speed fell sharply. Exact-date NWS discussions document a surface ridge, light winds, fog, and broad high pressure. Together these support stagnant accumulation of an anthropogenic mixture, but do not identify source-sector shares. A same-day natural-gas line fire was reviewed and retained only as context because it does not explain the regional particulate response.
Possible This is a transparent hypothesis supported by limited evidence or an explicit inference, not a definitive attribution.
National Weather Service Detroit/Pontiac via Iowa Environmental Mesonet · Dec 11, 2002 hypothesis
The exact-date NWS Detroit/Pontiac discussion described light winds and patchy fog that night, followed by weak ridging aloft and broad surface high pressure. An earlier discussion that day placed a 1020 mb surface ridge across southern Lower Michigan.
U.S. EPA AirData hypothesis
IAQng's reproducible aggregation of EPA records found a broad Detroit episode across 10 PM2.5 monitors: the event-window median was 62.35 ug/m3, versus 23.75 before. CO, NO2, and SO2 also rose while median wind speed fell to 0.84 knots, from 3.73 knots before the event.
Inference, not direct attribution
documented_ridge_low_wind_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_ridge_low_wind_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 3.0 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.
Detroit-Warren-Dearborn, MI
Monitor comparison window 2002-12-08 to 2002-12-14 · PM2.5 target: Allen Park
124% above nearby days
75% lower than surrounding days
26% higher than surrounding days
124% higher than surrounding days
26% higher than surrounding days
150% higher than surrounding days
41% higher than surrounding days
9% higher than surrounding days
About the same as surrounding days
About the same as surrounding days
About the same as surrounding days
75% 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.
Views from the selected metro
Select a metro on the map to see nearby cameras. Where an operator archive reaches the event, drag the divider to compare two moments, adjust either date and time, or play the available event-day frames. Camera imagery is visual context, not an air-quality measurement or proof of smoke.
No useful public camera views are currently available within 80 km of this metro.
| Metro | Peak PM2.5 | Cigarettes, whole event | Peak day | All-time standing |
|---|---|---|---|---|
| Detroit-Warren-Dearborn, MI | 65.8 µg/m³ | 3.0 | Dec 11, 2002 | #14 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 Detroit/Pontiac via Iowa Environmental Mesonet
The exact-date NWS Detroit/Pontiac discussion described light winds and patchy fog that night, followed by weak ridging aloft and broad surface high pressure. An earlier discussion that day placed a 1020 mb surface ridge across southern Lower Michigan.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The discussion documents poor dispersion conditions but does not identify the emitting sources responsible for the measured pollution.
U.S. Coast Guard National Response Center
NRC report 631509 records a natural-gas service-line fire in Romulus, Wayne County, on December 11. The report lists an unknown quantity of natural gas released to the atmosphere.
Context for the event; this item does not establish the attributed cause by itself.
The report documents a contemporaneous local occurrence, not the cause of the metro-wide PM2.5 episode.
U.S. EPA AirData
IAQng's reproducible aggregation of EPA records found a broad Detroit episode across 10 PM2.5 monitors: the event-window median was 62.35 ug/m3, versus 23.75 before. CO, NO2, and SO2 also rose while median wind speed fell to 0.84 knots, from 3.73 knots before the event.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The pollution fingerprint supports a metro-wide accumulation episode; it does not apportion the pollution among source sectors.
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.