Daily PM2.5 peaked at 42.4 µg/m³ near Nashville across
1 covered metro home to 2.2 million people.
Counting the extra PM2.5 above each metro's typical day, this event handed the
average affected resident ≈ 1.4 cigarettes.
Across everyone living in the affected metros, that is the collective equivalent of
≈ 3.1 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 39 µg/m³, 1.814 times the adjacent-window level, with 12 coded haze hours. 4 exact-date regional chemistry sites agreed that 68.6% to 81.4% of reconstructed mass was ammonium sulfate plus ammonium nitrate, and the largest reconstructed mass was 71.0% 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 · Jul 20, 2003hypothesis
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.
coherent_nearby_exact_date_secondary_inorganic_context.v1. This inference is recorded in the catalog methodology.
Inferred
2
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.
Research gap
3
Local response
42.4 µg/m³ near Nashville
EPA monitors recorded the event across 1 metro.
The peak daily concentration is roughly 1.9 cigarette-equivalents at the outdoor daily rate.
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 2003-07-17 to 2003-07-23 · PM2.5 target: Lockeland
6 pollutant signals
Fine-particle pulse39 µg/m³
61% above nearby days
Air movement4 kt median wind
8 calm hours observed
What observers saw1.5 mi minimum visibility
Haze reported during 12 hours
Pollution fingerprint
Metro-wide median of reporting monitors
PM2.539 µg/m³
61% higher than surrounding days
Before27.8 µg/m³ Event39 µg/m³ After15.2 µg/m³
PM1058 µg/m³
No adjacent-day comparison
Before— Event58 µg/m³ After—
Carbon monoxide0.985 ppm
9% higher than surrounding days
Before0.944 ppm Event0.985 ppm After0.810 ppm
Nitrogen dioxide1.54 ppb
About the same as surrounding days
Before1.71 ppb Event1.54 ppb After1.08 ppb
Sulfur dioxide9.13 ppb
50% higher than surrounding days
Before6.08 ppb Event9.13 ppb After5.67 ppb
Ozone0.040 ppm
20% higher than surrounding days
Before0.036 ppm Event0.040 ppm After0.029 ppm
Atmospheric conditions
NASHVILLE METRO, 7.8 km from the PM2.5 target
Wind4 kt median
8 kt maximum · 8 of 24 observed hours calm
Visibility1.5 mi minimum
4 observed hours at or below 3 miles
Temperature80.5°F median
68°F to 91°F
Humidity71% median
96% 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.
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, 4 EPA speciation sites within 175 km agreed that ammonium sulfate plus ammonium nitrate comprised a majority of reconstructed PM2.5: Mammoth Cave NP (118.8 km; 81.4% secondary inorganic share; 27.7 µg/m³ reconstructed mass); OLD DOVER HIGHWAY CADIZ,KY (120.3 km; 79.4% secondary inorganic share; 26.8 µg/m³ reconstructed mass); Sipsey Wilderness (211 km; 68.6% secondary inorganic share; 13.3 µg/m³ reconstructed mass); Fort Mountain (246 km; 73.5% secondary inorganic share; 24.5 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 118.8 km and 120.3 km and 211 km and 246 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 118.8 km and 120.3 km and 211 km and 246 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.