Regional secondary PM Likely
Daily PM2.5 peaked at 47.3 µg/m³ near Memphis across 2 covered metros home to 2.1 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 ≈ 1.6 cigarettes. Across everyone living in the affected metros, that is the collective equivalent of ≈ 3.3 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 scientific case study places Little Rock and Memphis inside an exact-date continental fine-particle haze event that accumulated in the Midwest and moved south and southwest.
Likely Applicable reporting or a supported inference makes this the best current causal explanation.
NASA Technical Reports Server · Nov 1, 2004 corroborates
The case study identifies an intense September 8-14 fine-particle haze event that accumulated in the Midwest and was transported south and southwest as a tropical system approached.
See every retained source and limitation ↓
Sequence: Memphis (Sep 10) → Little Rock (Sep 10)
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 case study identifies an intense September 8-14 fine-particle haze event that accumulated in the Midwest and was transported south and southwest as a tropical system approached.
View supporting source ↗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 2.1 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.
Little Rock-North Little Rock-Conway, AR
Monitor comparison window 2002-09-07 to 2002-09-13 · PM2.5 target: DOYLE SPRINGS ROAD
54% above nearby days
69% higher than surrounding days
54% higher than surrounding days
69% higher than surrounding days
27% lower than surrounding days
About the same as surrounding days
24% higher than surrounding days
Memphis, TN-MS-AR
Monitor comparison window 2002-09-07 to 2002-09-13 · PM2.5 target: GUTHRIE CLINIC PM 2.5
71% above nearby days
13% higher than surrounding days
71% higher than surrounding days
13% higher than surrounding days
About the same as surrounding days
About the same as surrounding days
30% 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 Little Rock-North Little Rock-Conway
Peak-date arrivals · 2002-09-10
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 Memphis
Peak-date arrivals · 2002-09-10
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.
Views from the selected metro
Select a metro on the map to see nearby cameras. Where an operator archive reaches the event, drag the divider to compare two moments, adjust either date and time, or play the available event-day frames. Camera imagery is visual context, not an air-quality measurement or proof of smoke.
No useful public camera views are currently available within 80 km of this metro.
| Metro | Peak PM2.5 | Cigarettes, whole event | Peak day | All-time standing |
|---|---|---|---|---|
| Memphis, TN-MS-AR | 47.3 µg/m³ | 2.1 | Sep 10, 2002 | #6 of its top days |
| Little Rock-North Little Rock-Conway, AR | 41.2 µg/m³ | 1.9 | Sep 10, 2002 | #13 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.
NASA Technical Reports Server
The case study identifies an intense September 8-14 fine-particle haze event that accumulated in the Midwest and was transported south and southwest as a tropical system approached.
A retained source supports the attributed cause for the stated place and dates.
The study describes the regional episode and transport rather than a city-specific chemical mass balance.
Likely Applicable reporting or a supported inference makes this the best current causal explanation.
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.