U.S. EPA AirData / Chemical Speciation Network hypothesis
EPA AirData daily_SPEC 1999
Exact-peak-date CSN composition estimated a 38.2% secondary-inorganic share of reconstructed PM2.5 mass at the covered knoxville monitor set.
Stagnant pollution mixture Possible
1999-01-27 Knoxville, TN stagnant mixed-pollution episode
Daily PM2.5 peaked at 72.7 µg/m³ near Knoxville across 1 covered metro home to 957,608 people. 1 metro 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 ≈ 2.8 cigarettes. Across everyone living in the affected metros, that is the collective equivalent of ≈ 2.7 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
The basis for “Stagnant pollution mixture”
The exact-date NWS discussion says surface winds decoupled overnight around Knoxville, with fog and calm valleys, while EPA's same-day speciation shows a substantial but non-majority secondary-inorganic share. Together they support a possible stagnant mixed-pollution mechanism rather than a unique source or purely secondary episode.
Possible This is a transparent hypothesis supported by limited evidence or an explicit inference, not a definitive attribution.
National Weather Service Morristown via Iowa Environmental Mesonet · Jan 27, 1999 hypothesis
Area Forecast Discussion: overnight decoupling and fog
On January 27, NWS Morristown reported that surface winds had decoupled overnight, allowing fog and relatively cool temperatures, while nearby valleys had experienced calm wind earlier in the night.
Inference, not direct attribution
exact_date_decoupling_plus_mixed_composition_possible.v1: This inference is recorded in the catalog methodology.
Evidence path
From source to city
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.
- 1Inferred
Source or mechanism
Pattern-based explanation
exact_date_decoupling_plus_mixed_composition_possible.v1. This inference is recorded in the catalog methodology.
- 2Research gap
Air-mass movement
The transport path has not been reconstructed
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.
- 3Documented
Local response
72.7 µg/m³ near Knoxville
EPA monitors recorded the event across 1 metro. The peak daily concentration is roughly 3.3 cigarette-equivalents at the outdoor daily rate.
Inspect the pollutant and weather signals ↓
Event autopsy
What the instruments saw
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.
Knoxville, TN
Monitor comparison window 1999-01-24 to 1999-01-30 · PM2.5 target: Rule High School (relo)
135% above nearby days
5 calm hours observed
0 hours at or below 3 miles
Pollution fingerprint
Metro-wide median of reporting monitors PM2.5 48.8 µg/m³
135% higher than surrounding days
Before14.3 µg/m³ Event48.8 µg/m³ After20.8 µg/m³
PM10 91 µg/m³
168% higher than surrounding days
Before34 µg/m³ Event91 µg/m³ After27 µg/m³
Carbon monoxide 1.27 ppm
12% higher than surrounding days
Before1.14 ppm Event1.27 ppm After0.954 ppm
Nitrogen dioxide 2.26 ppb
About the same as surrounding days
Before2.39 ppb Event2.26 ppb After2.09 ppb
Sulfur dioxide 2.70 ppb
16% lower than surrounding days
Before3.25 ppb Event2.70 ppb After1.83 ppb
Ozone 0.056 ppm
49% higher than surrounding days
Before0.037 ppm Event0.056 ppm After0.037 ppm
Atmospheric conditions
KNOXVILLE MUNICIPAL, 18.1 km from the PM2.5 target22 kt maximum · 5 of 24 observed hours calm
0 observed hours at or below 3 miles
36°F to 68°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.
Measured daily PM2.5 at EPA monitors, as cigarette-equivalents
EPA monitors sit in metro areas, so the colour clusters there. Land between and beyond
them is unmeasured, not clean: this map shows where IAQng has readings, not the extent
of the smoke.
—
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
Camera views near
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.
Affected metros
| Metro | Peak PM2.5 | Cigarettes, whole event | Peak day | All-time standing |
|---|---|---|---|---|
| Knoxville, TN | 72.7 µg/m³ | 3.3 | Jan 27, 1999 | #6 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
What supports this account
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.
Area Forecast Discussion: overnight decoupling and fog
National Weather Service Morristown via Iowa Environmental Mesonet
On January 27, NWS Morristown reported that surface winds had decoupled overnight, allowing fog and relatively cool temperatures, while nearby valleys had experienced calm wind earlier in the night.
What it supports
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
Limit
The forecast discussion documents poor overnight dispersion but does not identify the particle sources or prove a persistent inversion throughout the full day.
EPA AirData daily_SPEC 1999
U.S. EPA AirData / Chemical Speciation Network
Exact-peak-date CSN composition estimated a 38.2% secondary-inorganic share of reconstructed PM2.5 mass at the covered knoxville monitor set.
What it supports
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
Limit
Composition supports an aerosol mechanism, not an emitting source or named episode.
What remains uncertain
- Confidence is capped at possible because no source apportions the full local daily PM2.5 mass.
- The forecast discussion documents poor overnight dispersion but does not identify the particle sources or prove a persistent inversion throughout the full day.
Status and provenance
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.