Temperature inversion Possible
Daily PM2.5 peaked at 52.7 µg/m³ near Minneapolis across 1 covered metro home to 3.8 million people.
Counting the extra PM2.5 above each metro's typical day, this event handed the average affected resident ≈ 2.0 cigarettes. Across everyone living in the affected metros, that is the collective equivalent of ≈ 7.4 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
Minneapolis-St. Paul's 13-monitor PM2.5 spike and compatible carbon-monoxide and nitrogen-dioxide increases occurred with six hours of haze, 18 hours of mist, restricted visibility, and mostly calm or light wind. An exact-window NWS sounding documented a formidable inversion. This supports temperature-inversion trapping at possible confidence while leaving the trapped emissions unidentified.
Possible This is a transparent hypothesis supported by limited evidence or an explicit inference, not a definitive attribution.
U.S. EPA AirData hypothesis
EPA records show median daily PM2.5 of 52.7 micrograms per cubic meter across 13 monitors, 2.158 times the adjacent-window level. Co-pollutants at least 10% above the adjacent-window comparison were carbon monoxide (1.328x), nitrogen dioxide (1.183x). Other measured comparisons were sulfur dioxide (1.077x).
National Weather Service Twin Cities via Iowa Environmental Mesonet · Nov 25, 2000 hypothesis
At the start of the event day, NWS Twin Cities reported a very formidable inversion in the evening sounding, light southerly flow, and a setup favorable for fog that could become difficult to clear.
National Weather Service surface observations via Iowa Environmental Mesonet · Nov 25, 2000 hypothesis
ST. PAUL recorded calm conditions during 46% of observed event hours and wind at or below three knots during 58%. The station recorded 6 haze hours, 18 mist hours, minimum visibility of 1.25 miles.
Inference, not direct attribution
Documented inversion overlap: The event overlaps a documented inversion period, but the retained material does not directly attribute the measured PM2.5 episode.
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 inversion overlap. The event overlaps a documented inversion period, but the retained material does not directly attribute the measured PM2.5 episode.
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 2.4 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 “Temperature inversion” explanation; it does not assign the cause by itself.
Minneapolis-St. Paul-Bloomington, MN-WI
Monitor comparison window 2000-11-22 to 2000-11-28 · PM2.5 target: Harding High School
88% above nearby days
11 calm hours observed
Haze reported during 6 hours
88% higher than surrounding days
30% higher than surrounding days
21% higher than surrounding days
About the same as surrounding days
9 kt maximum · 11 of 24 observed hours calm
19 observed hours at or below 3 miles
22°F to 37°F
100% 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.
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 |
|---|---|---|---|---|
| Minneapolis-St. Paul-Bloomington, MN-WI | 52.7 µg/m³ | 2.4 | Nov 25, 2000 | #22 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.
Minnesota Legislature
Minnesota's official environment report explains that temperature inversions trap pollution on calm winter days and can create unhealthy regional air quality.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
This describes Minnesota's recurring winter mechanism, not the November 25, 2000 episode specifically.
National Weather Service Twin Cities via Iowa Environmental Mesonet
At the start of the event day, NWS Twin Cities reported a very formidable inversion in the evening sounding, light southerly flow, and a setup favorable for fog that could become difficult to clear.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The NWS record establishes the vertical trapping mechanism near the start of the event day but does not identify the particle sources below it.
National Weather Service surface observations via Iowa Environmental Mesonet
ST. PAUL recorded calm conditions during 46% of observed event hours and wind at or below three knots during 58%. The station recorded 6 haze hours, 18 mist hours, minimum visibility of 1.25 miles.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
A single surface station documents poor dispersion and obscuration but does not directly measure the vertical temperature profile or identify particle sources.
U.S. EPA AirData
EPA records show median daily PM2.5 of 52.7 micrograms per cubic meter across 13 monitors, 2.158 times the adjacent-window level. Co-pollutants at least 10% above the adjacent-window comparison were carbon monoxide (1.328x), nitrogen dioxide (1.183x). Other measured comparisons were sulfur dioxide (1.077x).
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The multi-pollutant fingerprint demonstrates a broad accumulation episode but cannot apportion local combustion, industry, traffic, or secondary aerosol.
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.