Regional secondary PM Possible
Daily PM2.5 peaked at 58.4 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 ≈ 2.1 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
EPA's exact-date CSN sample for Knoxville, TN shows sulfate plus nitrate made up a literal majority of reconstructed PM2.5 mass. This supports regional secondary-particle pollution as a possible mechanism without naming a specific emitter.
Possible This is a transparent hypothesis supported by limited evidence or an explicit inference, not a definitive attribution.
U.S. EPA AirData Chemical Speciation Network hypothesis
EPA's exact-date CSN sample reconstructed a median 10.3 µg/m³ of PM2.5. Ammonium sulfate and ammonium nitrate contributed a median 5.3 µg/m³, or 51.1% of reconstructed mass; every usable site exceeded 50%.
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
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
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.
Air-mass movement
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 ↓Local response
EPA monitors recorded the event across 1 metro. The peak daily concentration is roughly 2.7 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 “Regional secondary PM” explanation; it does not assign the cause by itself.
Knoxville, TN
Monitor comparison window 1999-05-02 to 1999-05-08 · PM2.5 target: Rule High School (relo)
229% above nearby days
69% higher than surrounding days
46% higher than surrounding days
229% higher than surrounding days
46% higher than surrounding days
6% higher than surrounding days
About the same as surrounding days
21% lower than surrounding days
10% lower than surrounding days
26% higher than surrounding days
About the same as surrounding days
69% 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
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 · 1999-05-05
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.
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 |
|---|---|---|---|---|
| Knoxville, TN | 58.4 µg/m³ | 2.7 | May 5, 1999 | #11 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.
U.S. EPA AirData Chemical Speciation Network
EPA's exact-date CSN sample reconstructed a median 10.3 µg/m³ of PM2.5. Ammonium sulfate and ammonium nitrate contributed a median 5.3 µg/m³, or 51.1% of reconstructed mass; every usable site exceeded 50%.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
Chemical composition identifies the measured aerosol mechanism, not a particular emitting facility or upwind source region.
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 51% (minimum 51% across 1 site).
Measured composition consistent with secondary-inorganic-dominated PM2.5 at sampled monitors on the local peak date.
A component majority describes aerosol composition at sampled monitors; it does not identify an emitting source.
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.
A possible regional secondary PM hypothesis; this is a reproducible inference, not direct source attribution.
A component majority describes aerosol composition at sampled monitors; it does not identify an emitting source.
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.