Temperature inversion Possible
Daily PM2.5 peaked at 56.1 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 ≈ 2.2 cigarettes. Across everyone living in the affected metros, that is the collective equivalent of ≈ 1.9 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 City, ID's 4-monitor PM2.5 value was 2.477 times the adjacent-window comparison. Surface observations recorded wind at or below three knots during 96% of event-day hours, and a nearby event-date radiosonde measured a surface-based inversion of 8.3 degrees Celsius through 349 meters above ground. Together, these observations support temperature-inversion accumulation as a possible mechanism. They do not identify the trapped particle sources.
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 54.55 micrograms per cubic meter across 4 monitors, 2.477 times the adjacent-window level. Co-pollutants at least 10% above the adjacent-window comparison were pm10 (2.164x), carbon monoxide (1.616x), nitrogen dioxide (1.225x).
National Weather Service surface observations via Iowa Environmental Mesonet · Jan 6, 1999 hypothesis
CALDWELL recorded calm conditions during 83% of event hours and wind at or below three knots during 96%. Observations included minimum visibility 3.5 miles.
NOAA/NWS radiosonde observations via Iowa Environmental Mesonet · Jan 6, 1999 hypothesis
Boise Arpt ID/US's 12 UTC radiosonde profile measured temperature rising 8.3 degrees Celsius from 871 to 1220 meters, a 349-meter surface-based inversion. The NOAA/IGRA station is 28.4 kilometers from the PM2.5 target monitor.
Inference, not direct attribution
exact_radiosonde_inversion_plus_multimonitor_accumulation_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
exact_radiosonde_inversion_plus_multimonitor_accumulation_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 1999-01-03 to 1999-01-09 · PM2.5 target: NORTHWEST NAZARENE COLLEGE SAMPLER ON ARTS BUILDING IN SUB. SETTING
143% above nearby days
20 calm hours observed
0 hours at or below 3 miles
143% higher than surrounding days
91% higher than surrounding days
60% higher than surrounding days
19% higher than surrounding days
4 kt maximum · 20 of 24 observed hours calm
0 observed hours at or below 3 miles
26.6°F to 39.2°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 | 56.1 µg/m³ | 2.6 | Jan 6, 1999 | #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.
National Weather Service surface observations via Iowa Environmental Mesonet
CALDWELL recorded calm conditions during 83% of event hours and wind at or below three knots during 96%. Observations included minimum visibility 3.5 miles.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
A single surface station documents ventilation and obscuration but not the full vertical profile or particle source mix.
NOAA/NWS radiosonde observations via Iowa Environmental Mesonet
Boise Arpt ID/US's 12 UTC radiosonde profile measured temperature rising 8.3 degrees Celsius from 871 to 1220 meters, a 349-meter surface-based inversion. The NOAA/IGRA station is 28.4 kilometers from the PM2.5 target monitor.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The radiosonde station is 28.4 kilometers from the PM2.5 target monitor and may not represent every part of the metro airshed.
U.S. Environmental Protection Agency
EPA's Boise winter field study found that residential wood combustion and motor vehicles dominated the sampled particle mixture, while shallow mixing depth, inversion height, and low wind controlled winter accumulation across the airshed.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The study characterizes Boise's recurring winter particle regime and does not apportion the January 6, 1999 event itself.
U.S. EPA AirData
EPA records show median daily PM2.5 of 54.55 micrograms per cubic meter across 4 monitors, 2.477 times the adjacent-window level. Co-pollutants at least 10% above the adjacent-window comparison were pm10 (2.164x), carbon monoxide (1.616x), nitrogen dioxide (1.225x).
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The pollution fingerprint does not uniquely apportion traffic, heating, industry, wood combustion, 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.