Stagnant pollution mixture Possible
Daily PM2.5 peaked at 80.2 µg/m³ near Louisville/Jefferson County across 1 covered metro home to 1.4 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 ≈ 7.2 cigarettes. Across everyone living in the affected metros, that is the collective equivalent of ≈ 10 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
A coherent seven-monitor PM2.5 rise occurred with compatible carbon monoxide, nitrogen dioxide, and sulfur dioxide increases. Exact-window NWS discussions documented sinking beneath high pressure, very weak boundary-layer flow, and persistent haze, while the official surface station recorded extensive calm and light wind. This supports stagnant mixed-pollution accumulation as a possible broad mechanism, but does not identify source-sector shares.
Possible This is a transparent hypothesis supported by limited evidence or an explicit inference, not a definitive attribution.
U.S. EPA AirData · Jul 15, 2002 hypothesis
EPA records show median daily PM2.5 of 53.55 micrograms per cubic meter across seven monitors, about twice the adjacent-window level, with compatible increases in carbon monoxide, nitrogen dioxide, and sulfur dioxide.
National Weather Service Louisville via Iowa Environmental Mesonet · Jul 15, 2002 hypothesis
The exact-window NWS discussion placed Louisville beneath high pressure and a deformation-zone sinking region, forecast zero-to-eight-knot boundary-layer wind, and expected haze through most of July 16.
National Weather Service surface observations via Iowa Environmental Mesonet · Jul 15, 2002 hypothesis
Louisville's official surface station recorded haze during 21 event hours; 54.2% of observed hours had wind at or below three knots and 40.3% were calm.
Inference, not direct attribution
official_surface_weak_dispersion_multimonitor_accumulation.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
official_surface_weak_dispersion_multimonitor_accumulation.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 3.6 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.
Louisville/Jefferson County, KY-IN
Monitor comparison window 2002-07-12 to 2002-07-20 · PM2.5 target: Southwick
79% above nearby days
51% higher than surrounding days
79% higher than surrounding days
51% higher than surrounding days
29% higher than surrounding days
About the same as surrounding days
39% higher than surrounding days
10% lower than surrounding days
About the same as surrounding days
About the same as 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 Louisville/Jefferson County
Peak-date arrivals · 2002-07-17
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 |
|---|---|---|---|---|
| Louisville/Jefferson County, KY-IN | 80.2 µg/m³ | 8.9 | Jul 17, 2002 | #3 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
EPA records show median daily PM2.5 of 53.55 micrograms per cubic meter across seven monitors, about twice the adjacent-window level, with compatible increases in carbon monoxide, nitrogen dioxide, and sulfur dioxide.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The co-pollutant pattern supports mixed accumulation but cannot identify individual source sectors or exclude an unmeasured transported contribution.
National Weather Service Louisville via Iowa Environmental Mesonet
The exact-window NWS discussion placed Louisville beneath high pressure and a deformation-zone sinking region, forecast zero-to-eight-knot boundary-layer wind, and expected haze through most of July 16.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The discussion documents poor dispersion and haze, not the chemical composition or source shares of the particles.
National Weather Service surface observations via Iowa Environmental Mesonet
Louisville's official surface station recorded haze during 21 event hours; 54.2% of observed hours had wind at or below three knots and 40.3% were calm.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
One surface station cannot describe the complete metropolitan boundary layer, and haze is not source-specific.
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.