Temperature inversion Possible
Daily PM2.5 peaked at 83 µg/m³ near Provo across 3 covered metros home to 2.7 million people. 3 metros placed a top-ten day in their record.
Counting the extra PM2.5 above each metro's typical day, this event handed the average affected resident ≈ 9.1 cigarettes. Across everyone living in the affected metros, that is the collective equivalent of ≈ 25 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
Manual research connected 2 source events to one temperature-inversion pollution episode using the deterministic grouping rule.
Possible This is a transparent hypothesis supported by limited evidence or an explicit inference, not a definitive attribution.
National Weather Service via Iowa Environmental Mesonet · Dec 29, 2001 hypothesis
On the final day of the Wasatch Front event, the NWS discussion said temperature inversions would continue in the northern valleys and described pollution or haze trapped in populated valleys.
National Weather Service Salt Lake City via Iowa Environmental Mesonet · Dec 29, 2001 hypothesis
The NWS Salt Lake City discussion said temperature inversions would continue in northern Utah valleys as a weak system crossed the area immediately before the December 30-31 episode.
National Weather Service Salt Lake City via Iowa Environmental Mesonet · Dec 31, 2001 hypothesis
On December 31, the NWS said the disturbance crossing Utah would not be strong enough to break the inversion over most valleys, directly overlapping the final event day.
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.
: This inference is recorded in the catalog methodology.
See every retained source and limitation ↓
Sequence: Provo (Dec 27) → Salt Lake City (Dec 27) → Ogden (Dec 27)
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 3 metros. The peak daily concentration is roughly 3.8 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.
Ogden-Clearfield, UT
Monitor comparison window 2001-12-27 to 2002-01-03 · PM2.5 target: Ogden
30% above nearby days
About the same as surrounding days
30% higher than surrounding days
About the same as surrounding days
15% lower than surrounding days
About the same as surrounding days
24% lower than surrounding days
About the same as surrounding days
Provo-Orem, UT
Monitor comparison window 2001-12-27 to 2002-01-03 · PM2.5 target: North Provo
17% above nearby days
10% higher than surrounding days
17% higher than surrounding days
10% higher than surrounding days
About the same as surrounding days
10% higher than surrounding days
Salt Lake City, UT
Monitor comparison window 2001-12-27 to 2002-01-03 · PM2.5 target: Hawthorne
6% above nearby days
6% lower than surrounding days
6% higher than surrounding days
6% lower than surrounding days
21% lower than surrounding days
About the same as surrounding days
13% lower than surrounding days
About the same as surrounding days
About the same as 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 |
|---|---|---|---|---|
| Provo-Orem, UT | 83 µg/m³ | 7.3 | Dec 30, 2001 | #9 of its top days |
| Salt Lake City, UT | 80.8 µg/m³ | 14.0 | Dec 29, 2001 | #7 of its top days |
| Ogden-Clearfield, UT | 66.6 µg/m³ | 5.9 | Dec 30, 2001 | #9 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 Salt Lake City via Iowa Environmental Mesonet
On December 31, the NWS said the disturbance crossing Utah would not be strong enough to break the inversion over most valleys, directly overlapping the final event day.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
This is meteorological evidence for an accumulation mechanism, not a source-specific PM2.5 attribution.
National Weather Service via Iowa Environmental Mesonet
On the final day of the Wasatch Front event, the NWS discussion said temperature inversions would continue in the northern valleys and described pollution or haze trapped in populated valleys.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The forecast discussion documents the accumulation meteorology, not the emitting sources or chemical composition.
National Weather Service Salt Lake City via Iowa Environmental Mesonet
The NWS Salt Lake City discussion said temperature inversions would continue in northern Utah valleys as a weak system crossed the area immediately before the December 30-31 episode.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
This is meteorological evidence for an accumulation mechanism, not a source-specific PM2.5 attribution.
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.