Temperature inversion Possible
Daily PM2.5 peaked at 58.3 µg/m³ near Boise City across 1 covered metro home to 845,877 people.
Counting the extra PM2.5 above each metro's typical day, this event handed the average affected resident ≈ 2.3 cigarettes. Across everyone living in the affected metros, that is the collective equivalent of ≈ 2.0 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
Boise's four-monitor particle episode occurred during cold, humid, high-pressure conditions with calm or light wind, freezing fog, mist, and haze. NWS Boise explains that this exact local winter pattern commonly reflects cold air trapped beneath an inversion, which concentrates fireplace smoke and vehicle exhaust. The evidence supports a temperature-inversion accumulation mechanism at possible confidence, not a unique emissions source.
Possible This is a transparent hypothesis supported by limited evidence or an explicit inference, not a definitive attribution.
National Weather Service surface observations via Iowa Environmental Mesonet · Jan 6, 2001 hypothesis
Boise Airport recorded a median temperature of 25 degrees Fahrenheit, median pressure of 1026.2 millibars, calm conditions during 11 of 24 hours, and freezing fog, mist, or haze through much of the day.
U.S. EPA AirData hypothesis
EPA records show daily PM2.5 of 58.3 micrograms per cubic meter across four monitors, with elevated PM10 and carbon monoxide and substantially lower particle levels after the event.
National Weather Service Boise hypothesis
NWS Boise explains that winter cold air commonly becomes trapped at the surface beneath an inversion in the Treasure Valley, where fireplace smoke and automobile exhaust are concentrated with fog and degraded air quality.
Inference, not direct attribution
surface_cold_pool_with_local_inversion_climatology_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
surface_cold_pool_with_local_inversion_climatology_possible.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 2.6 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.
Boise City, ID
Monitor comparison window 2001-01-03 to 2001-01-09
76% above nearby days
11 calm hours observed
Haze reported during 6 hours
76% higher than surrounding days
76% higher than surrounding days
22% higher than surrounding days
About the same as surrounding days
11 kt maximum · 11 of 24 observed hours calm
12 observed hours at or below 3 miles
18°F to 33°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 |
|---|---|---|---|---|
| Boise City, ID | 58.3 µg/m³ | 2.6 | Jan 6, 2001 | #19 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 surface observations via Iowa Environmental Mesonet
Boise Airport recorded a median temperature of 25 degrees Fahrenheit, median pressure of 1026.2 millibars, calm conditions during 11 of 24 hours, and freezing fog, mist, or haze through much of the day.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
One surface station does not directly measure the vertical temperature profile.
National Weather Service Boise
NWS Boise explains that winter cold air commonly becomes trapped at the surface beneath an inversion in the Treasure Valley, where fireplace smoke and automobile exhaust are concentrated with fog and degraded air quality.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
This is a local climatological mechanism description, not an exact-date inversion diagnosis.
U.S. EPA AirData
EPA records show daily PM2.5 of 58.3 micrograms per cubic meter across four monitors, with elevated PM10 and carbon monoxide and substantially lower particle levels after the event.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The fingerprint demonstrates particle accumulation but cannot by itself identify an inversion or apportion emissions.
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.