Regional secondary PM Possible
Daily PM2.5 peaked at 39.7 µg/m³ near New Orleans across 2 covered metros home to 1.8 million people.
Counting the extra PM2.5 above each metro's typical day, this event handed the average affected resident ≈ 1.3 cigarettes. Across everyone living in the affected metros, that is the collective equivalent of ≈ 2.5 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 MODIS review documents a large Midwest-to-Southeast haze episode collecting pollutants over Texas and Louisiana on September 13-14. The catalog event is September 15, so this is a transparent trailing-day hypothesis rather than exact-date attribution.
Possible This is a transparent hypothesis supported by limited evidence or an explicit inference, not a definitive attribution.
U.S. Environmental Protection Agency / Federal Register · Jan 30, 2004 hypothesis
EPA describes a September 8-14, 2002 Midwest-Southeast haze episode and says pollutants collected over Texas and Louisiana on September 13 before cloud cover obscured the region on September 14.
See every retained source and limitation ↓
Sequence: New Orleans (Sep 15) → Baton Rouge (Sep 15)
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
The catalog has a grouping hypothesis, but no retained source directly establishes the proposed cause.
Air-mass movement
NOAA HYSPLIT modeled 24 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 2 metros. 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 “Regional secondary PM” explanation; it does not assign the cause by itself.
Baton Rouge, LA
Monitor comparison window 2002-09-12 to 2002-09-18 · PM2.5 target: Capitol
151% above nearby days
About the same as surrounding days
About the same as surrounding days
151% higher than surrounding days
About the same as surrounding days
About the same as surrounding days
44% lower than surrounding days
78% higher than surrounding days
About the same as surrounding days
About the same as surrounding days
About the same as surrounding days
About the same as surrounding days
New Orleans-Metairie, LA
Monitor comparison window 2002-09-12 to 2002-09-18 · PM2.5 target: Kenner
108% above nearby days
About the same as surrounding days
108% higher than surrounding days
About the same as surrounding days
22% lower than surrounding days
About the same as surrounding days
54% higher than 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 Baton Rouge
Peak-date arrivals · 2002-09-15
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.
Air arriving near New Orleans-Metairie
Peak-date arrivals · 2002-09-15
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 |
|---|---|---|---|---|
| New Orleans-Metairie, LA | 39.7 µg/m³ | 1.8 | Sep 15, 2002 | #18 of its top days |
| Baton Rouge, LA | 39.3 µg/m³ | 1.8 | Sep 15, 2002 | #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. Environmental Protection Agency / Federal Register
EPA describes a September 8-14, 2002 Midwest-Southeast haze episode and says pollutants collected over Texas and Louisiana on September 13 before cloud cover obscured the region on September 14.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The account ends one day before the September 15 catalog day and does not identify a single emitting source.
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.