Stagnant pollution mixture Possible
Daily PM2.5 peaked at 39.1 µg/m³ near Chattanooga across 1 covered metro home to 588,050 people.
Why we think this happened
Four Chattanooga-area regulatory monitors recorded a broad PM2.5 rise. The nearby airport observed calm wind during 21 of 24 hours and high surface pressure, while exact-date National Weather Service discussions documented persistent surface and upper ridging, subsidence warming, and a surface-based inversion. Together these observations support accumulation of an otherwise unidentified anthropogenic particle mixture at possible confidence. They do not support a wildfire-smoke or single-source claim.
Possible This is a transparent hypothesis supported by limited evidence or an explicit inference, not a definitive attribution.
National Weather Service surface observations via Iowa Environmental Mesonet · Nov 25, 2006 hypothesis
Chattanooga airport observations show 21 calm hours and 22 hours at or below three knots. Median sea-level pressure was 1025.45 millibars and the daily maximum was 1028.2 millibars.
U.S. EPA AirData hypothesis
EPA records show median daily PM2.5 of 24.3 micrograms per cubic meter across 4 Chattanooga-area monitors, with a monitor maximum of 39.1. The median was 3.135 times the preceding sampled day.
National Weather Service Morristown via Iowa Environmental Mesonet · Nov 25, 2006 hypothesis
The exact-date forecast discussion says a ridge remained over the region through Monday night and explicitly anticipated another surface-based inversion with warm air aloft. Adjacent discussions described the ridge at the surface and aloft, subsidence warming, generally clear skies, and calm or light winds.
Inference, not direct attribution
broad_pm_rise_with_official_inversion_and_calm_surface_possible.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
broad_pm_rise_with_official_inversion_and_calm_surface_possible.v1. This inference is recorded in the catalog methodology.
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 1.8 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 “Stagnant pollution mixture” explanation; it does not assign the cause by itself.
Chattanooga, TN-GA
Monitor comparison window 2006-11-22 to 2006-11-28 · PM2.5 target: EAST RIDGE CITY HALL
0% above nearby days
21 calm hours observed
0 hours at or below 3 miles
About the same as surrounding days
10% lower than surrounding days
5 kt maximum · 21 of 24 observed hours calm
0 observed hours at or below 3 miles
36°F to 70°F
92% 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
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 Chattanooga
Peak-date arrivals · 2006-11-25
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.
| Metro | Peak PM2.5 | Cigarettes, whole event | Peak day | All-time standing |
|---|---|---|---|---|
| Chattanooga, TN-GA | 39.1 µg/m³ | 1.8 | Nov 25, 2006 | #25 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 Morristown via Iowa Environmental Mesonet
The exact-date forecast discussion says a ridge remained over the region through Monday night and explicitly anticipated another surface-based inversion with warm air aloft. Adjacent discussions described the ridge at the surface and aloft, subsidence warming, generally clear skies, and calm or light winds.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The forecast discussion describes the accumulation environment but does not attribute the measured particles to a particular source sector.
National Weather Service surface observations via Iowa Environmental Mesonet
Chattanooga airport observations show 21 calm hours and 22 hours at or below three knots. Median sea-level pressure was 1025.45 millibars and the daily maximum was 1028.2 millibars.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
One airport station cannot determine mixing height across the full metropolitan area.
U.S. EPA AirData
EPA records show median daily PM2.5 of 24.3 micrograms per cubic meter across 4 Chattanooga-area monitors, with a monitor maximum of 39.1. The median was 3.135 times the preceding sampled day.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The sparse three-day filter schedule shows the broad local rise but not its hourly onset or clearing time.
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.