15% below nearby days
Regional secondary PM Possible
2004-09-27 Knoxville, TN regional secondary-particle pollution episode
Daily PM2.5 peaked at 56.1 µg/m³ near Knoxville across 1 covered metro home to 957,608 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 ≈ 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
The basis for “Regional secondary PM”
Exact-peak-date CSN observations show secondary inorganic aerosol above the frozen majority threshold.
Possible This is a transparent hypothesis supported by limited evidence or an explicit inference, not a definitive attribution.
Inference, not direct attribution
Same-day particle composition: EPA speciation observations on the peak date show a majority secondary-inorganic aerosol signature. This supports a regional secondary-particle mechanism, not a single emitting source.
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
Same-day particle composition. EPA speciation observations on the peak date show a majority secondary-inorganic aerosol signature. This supports a regional secondary-particle mechanism, not a single emitting source.
- 2Modeled
Air-mass movement
The 72-hour air-mass history is reconstructed
NOAA HYSPLIT modeled 12 backward paths across multiple arrival times and heights. They show where the air traveled, not which source polluted it.
Explore the modeled paths ↓ - 3Documented
Local response
56.1 µg/m³ near Knoxville
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
What the instruments saw
Before, during, and after the episode: nearby pollutant monitors and surface-weather stations show what changed. This context tests the “Regional secondary PM” explanation; it does not assign the cause by itself.
Knoxville, TN
Monitor comparison window 2004-09-24 to 2004-09-30 · PM2.5 target: Loudon Pope site
56% higher than surrounding days
47% lower than surrounding days
Pollution fingerprint
Metro-wide median of reporting monitors PM2.5 19.1 µg/m³
15% lower than surrounding days
Before22.8 µg/m³ Event19.1 µg/m³ After22.6 µg/m³
PM10 17 µg/m³
47% lower than surrounding days
Before35 µg/m³ Event17 µg/m³ After31 µg/m³
Sulfur dioxide 13.3 ppb
250% higher than surrounding days
Before4.79 ppb Event13.3 ppb After2.99 ppb
Ozone 0.028 ppm
21% lower than surrounding days
Before0.042 ppm Event0.028 ppm After0.030 ppm
Atmospheric conditions
Nearby EPA weather observationsAbout the same as surrounding days
About the same as surrounding days
56% higher than 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.
Air-mass reconstruction
Where the air had been
NOAA HYSPLIT traces the air arriving at each metro backward for 72 hours. Multiple arrival times and heights show whether the broad path is stable—or sensitive to assumptions about when and where the polluted layer arrived.
Air arriving near Knoxville
Peak-date arrivals · 2004-09-27
Broad history Mostly east of the metro
72-hour reach 300–2,300 km
Sensitivity runs 12 modeled paths
Near surface · 100 mLower atmosphere · 500 mHigher layer · 1,000 m
Each line begins 72 hours before arrival and points toward the metro. Color represents arrival height; repeated lines represent 00, 06, 12, and 18 UTC arrival assumptions. A path can show where the air traveled, but it cannot prove which source added the PM2.5.
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 | 56.1 µg/m³ | 2.6 | Sep 27, 2004 | #15 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.
Peak-date reconstructed aerosol composition
U.S. EPA Chemical Speciation Network
On the affected place peak date, reconstructed ammonium sulfate plus ammonium nitrate represented a majority of EPA reconstructed mass at every usable sampled site. knoxville: median 70% (minimum 70% across 1 site).
What it supports
Measured composition consistent with secondary-inorganic-dominated PM2.5 at sampled monitors on the local peak date.
Limit
A component majority describes aerosol composition at sampled monitors; it does not identify an emitting source.
Secondary-inorganic component majority
IAQng deterministic inference policy
Under csn_secondary_inorganic_majority_on_peak_date.v1, complete affected-place peak-date coverage with a secondary-inorganic majority creates a possible regional-secondary-PM hypothesis.
What it supports
A possible regional secondary PM hypothesis; this is a reproducible inference, not direct source attribution.
Limit
A component majority describes aerosol composition at sampled monitors; it does not identify an emitting source.
No news search has been run for this episode. Its evidence is agency records only, and an absent article is not a finding that none exists.
What remains uncertain
- The classification is bounded to the retained evidence and does not apportion every source contributing to the daily PM2.5 value.
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.