Regional secondary PM Possible
Daily PM2.5 peaked at 41 µg/m³ near Chattanooga across 1 covered metro home to 588,050 people.
Why we think this happened
Chattanooga, TN-GA's 3-monitor PM2.5 episode reached 40.1 µg/m³, 2.089 times the adjacent-window level, with 7 coded haze hours. 4 exact-date regional chemistry sites agreed that 64.1% to 83.3% of reconstructed mass was ammonium sulfate plus ammonium nitrate, and the largest reconstructed mass was 73.9% 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.
Possible This is a transparent hypothesis supported by limited evidence or an explicit inference, not a definitive attribution.
U.S. EPA AirData hypothesis
EPA records show median daily PM2.5 of 40.1 micrograms per cubic meter across 3 monitors, 2.089 times the adjacent-window level. No measured co-pollutant met the 10% adjacent-window increase screen. No adjacent-window ratio was available for pm10.
National Weather Service surface observations via Iowa Environmental Mesonet · Jul 18, 2001 hypothesis
CHATTANOOGA/LOVELL recorded calm conditions during 33% of event hours and wind at or below three knots during 58%. Observations included 7 haze hours, 4 mist hours, minimum visibility 2 miles.
U.S. EPA AirData Chemical Speciation · Jul 18, 2001 hypothesis
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: Fort Mountain (60.9 km; 75.9% secondary inorganic share; 29.6 µg/m³ reconstructed mass); Great Smoky Mountains NP (137.8 km; 76.5% secondary inorganic share; 26.9 µg/m³ reconstructed mass); Sipsey Wilderness (205 km; 64.1% secondary inorganic share; 20.7 µg/m³ reconstructed mass); Frying Pan Mountain (226.8 km; 83.3% secondary inorganic share; 24.1 µg/m³ reconstructed mass).
Inference, not direct attribution
coherent_nearby_exact_date_secondary_inorganic_context.v1: This inference is recorded in the catalog methodology.
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
coherent_nearby_exact_date_secondary_inorganic_context.v1. This inference is recorded in the catalog methodology.
Air-mass movement
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.
Local response
EPA monitors recorded the event across 1 metro. The peak daily concentration is roughly 1.9 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.
Chattanooga, TN-GA
Monitor comparison window 2001-07-15 to 2001-07-21 · PM2.5 target: EAST RIDGE CITY HALL
69% above nearby days
8 calm hours observed
Haze reported during 7 hours
69% higher than surrounding days
No adjacent-day comparison
12% higher than surrounding days
6 kt maximum · 8 of 24 observed hours calm
7 observed hours at or below 3 miles
68°F to 89°F
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.
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.
| Metro | Peak PM2.5 | Cigarettes, whole event | Peak day | All-time standing |
|---|---|---|---|---|
| Chattanooga, TN-GA | 41 µg/m³ | 1.9 | Jul 18, 2001 | #21 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.
National Weather Service surface observations via Iowa Environmental Mesonet
CHATTANOOGA/LOVELL recorded calm conditions during 33% of event hours and wind at or below three knots during 58%. Observations included 7 haze hours, 4 mist hours, minimum visibility 2 miles.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
A single surface station documents ventilation and obscuration but not the full vertical profile or particle source mix.
U.S. EPA AirData Chemical Speciation
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: Fort Mountain (60.9 km; 75.9% secondary inorganic share; 29.6 µg/m³ reconstructed mass); Great Smoky Mountains NP (137.8 km; 76.5% secondary inorganic share; 26.9 µg/m³ reconstructed mass); Sipsey Wilderness (205 km; 64.1% secondary inorganic share; 20.7 µg/m³ reconstructed mass); Frying Pan Mountain (226.8 km; 83.3% secondary inorganic share; 24.1 µg/m³ reconstructed mass).
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The chemistry sites were 60.9 km and 137.8 km and 205 km and 226.8 km from the affected-place PM2.5 target and did not directly measure the city air.
U.S. EPA AirData
EPA records show median daily PM2.5 of 40.1 micrograms per cubic meter across 3 monitors, 2.089 times the adjacent-window level. No measured co-pollutant met the 10% adjacent-window increase screen. No adjacent-window ratio was available for pm10.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The pollution fingerprint does not uniquely apportion traffic, heating, industry, wood combustion, or secondary aerosol.
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.