Regional secondary PM Possible
Daily PM2.5 peaked at 47.9 µg/m³ near Louisville/Jefferson County across 1 covered metro home to 1.4 million people.
Counting the extra PM2.5 above each metro's typical day, this event handed the average affected resident ≈ 1.6 cigarettes. Across everyone living in the affected metros, that is the collective equivalent of ≈ 2.2 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
EPA's exact-date CSN sample for Louisville/Jefferson County, KY-IN shows ammonium sulfate plus ammonium nitrate made up 84.2% of reconstructed PM2.5 mass. This supports regional secondary-particle pollution as a possible mechanism for this place only.
Possible This is a transparent hypothesis supported by limited evidence or an explicit inference, not a definitive attribution.
U.S. EPA AirData Chemical Speciation Network · Jul 24, 2001 hypothesis
EPA's exact-date CSN sample reconstructed 27.2 µg/m³ of PM2.5. Ammonium sulfate and ammonium nitrate contributed 22.9 µg/m³, or 84.2% of reconstructed mass; every usable site exceeded 50%.
Inference, not direct attribution
Same-day particle composition: EPA speciation observations on the peak date show a majority secondary-inorganic aerosol signature. This supports a regional secondary-particle mechanism, not a single emitting source.
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
Same-day particle composition. EPA speciation observations on the peak date show a majority secondary-inorganic aerosol signature. This supports a regional secondary-particle mechanism, not a single emitting source.
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 2.2 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.
Louisville/Jefferson County, KY-IN
Monitor comparison window 2001-07-21 to 2001-07-27 · PM2.5 target: Algonquin Parkway
9% above nearby days
5 calm hours observed
Haze reported during 14 hours
9% higher than surrounding days
No adjacent-day comparison
51% higher than surrounding days
21% higher than surrounding days
49% lower than surrounding days
18% higher than surrounding days
7 kt maximum · 5 of 24 observed hours calm
1 observed hours at or below 3 miles
76°F to 92°F
91% 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 |
|---|---|---|---|---|
| Louisville/Jefferson County, KY-IN | 47.9 µg/m³ | 2.2 | Jul 24, 2001 | #22 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 Chemical Speciation Network
EPA's exact-date CSN sample reconstructed 27.2 µg/m³ of PM2.5. Ammonium sulfate and ammonium nitrate contributed 22.9 µg/m³, or 84.2% of reconstructed mass; every usable site exceeded 50%.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
Chemical composition identifies the measured aerosol mechanism, not a particular emitting facility or source region.
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.