Daily PM2.5 peaked at 52.4 µg/m³ near Nashville across
1 covered metro home to 2.2 million 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 ≈ 1.9 cigarettes.
Across everyone living in the affected metros, that is the collective equivalent of
≈ 4.1 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”
Nashville-Davidson--Murfreesboro--Franklin, TN's 5-monitor PM2.5 episode reached 38.6 µg/m³, 1.503 times the adjacent-window level, with 16 coded haze hours. 3 exact-date regional chemistry sites agreed that 65.0% to 74.8% of reconstructed mass was ammonium sulfate plus ammonium nitrate, and the largest reconstructed mass was 80.4% of the affected-place PM2.5 value. This supports regional secondary PM at possible confidence; the chemistry sites did not directly sample the city and no particular source region is assigned.
PossibleThis is a transparent hypothesis supported by limited evidence or an explicit inference, not a definitive attribution.
U.S. EPA AirData Chemical Speciation · Oct 21, 2000hypothesis
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.
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.
Observed context · not attribution
Nashville-Davidson--Murfreesboro--Franklin, TN
Monitor comparison window 2000-10-18 to 2000-10-24
6 pollutant signals
Fine-particle pulse38.6 µg/m³
38% above nearby days
Air movement5 kt median wind
3 calm hours observed
What observers saw3 mi minimum visibility
Haze reported during 16 hours
Pollution fingerprint
Metro-wide median of reporting monitors
PM2.538.6 µg/m³
38% higher than surrounding days
Before18.6 µg/m³ Event38.6 µg/m³ After32.8 µg/m³
PM1049.5 µg/m³
No adjacent-day comparison
Before— Event49.5 µg/m³ After—
Carbon monoxide0.829 ppm
45% lower than surrounding days
Before1.56 ppm Event0.829 ppm After1.50 ppm
Nitrogen dioxide20.2 ppb
28% lower than surrounding days
Before28 ppb Event20.2 ppb After35.5 ppb
Sulfur dioxide3.21 ppb
23% lower than surrounding days
Before5.01 ppb Event3.21 ppb After4.04 ppb
Ozone0.019 ppm
43% lower than surrounding days
Before0.032 ppm Event0.019 ppm After0.038 ppm
Atmospheric conditions
NASHVILLE METRO, 4.6 km from the PM2.5 target
Wind5 kt median
9 kt maximum · 3 of 24 observed hours calm
Visibility3 mi minimum
2 observed hours at or below 3 miles
Temperature66°F median
64°F to 72°F
Humidity81% median
93% 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.
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.
Transport context · not source attribution
Air arriving near Nashville-Davidson--Murfreesboro--Franklin
Peak-date arrivals · 2000-10-21
Broad historyMostly southeast of the metro
72-hour reach200–450 km
Sensitivity runs12 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.
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.
"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.
On the exact event date, 3 EPA speciation sites within 175 km agreed that ammonium sulfate plus ammonium nitrate comprised a majority of reconstructed PM2.5: Mammoth Cave NP (126.3 km; 65.0% secondary inorganic share; 26.7 µg/m³ reconstructed mass); Sipsey Wilderness (203 km; 68.1% secondary inorganic share; 27.9 µg/m³ reconstructed mass); Fort Mountain (240.5 km; 74.8% secondary inorganic share; 31.0 µg/m³ reconstructed mass).
What it supports
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
Limit
The chemistry sites were 126.3 km and 203 km and 240.5 km from the affected-place PM2.5 target and did not directly measure the city air.
What remains uncertain
Composition supports an aerosol mechanism but cannot identify a particular emitting facility or source region.
No exact or same-window source with enough causal and geographic specificity was recovered from the accessible web record.
Same-date regional composition and distance do not prove that every site sampled one shared air mass.
The 50% chemistry share, 175 km distance, and regional-mass comparability gates are transparent IAQng catalog conventions, not EPA attribution thresholds.
The chemistry sites were 126.3 km and 203 km and 240.5 km from the affected-place PM2.5 target and did not directly measure the city air.
Status and provenance
PossibleThis 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.