Stagnant pollution mixture Possible
Daily PM2.5 peaked at 59.1 µg/m³ near Atlanta across 1 covered metro home to 6.4 million people.
Counting the extra PM2.5 above each metro's typical day, this event handed the average affected resident ≈ 2.1 cigarettes. Across everyone living in the affected metros, that is the collective equivalent of ≈ 14 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
The Atlanta Supersite measured an exact-date PM2.5 peak on August 12 during a documented August 3-23 stagnant regime. The field study attributed that regime to high pressure and weak dispersion, with local carbonaceous accumulation at low wind speeds and transported sulfate during the day. This supports a mixed accumulated-pollution mechanism, but not a single emitting source.
Possible This is a transparent hypothesis supported by limited evidence or an explicit inference, not a definitive attribution.
U.S. EPA Atlanta Supersite / Journal of Geophysical Research · Jan 1, 2003 corroborates
The study identifies August 12 as a local PM2.5 maximum inside a prolonged August 3-23 stagnant regime. It reports weak synoptic dispersion, accumulation of local carbonaceous emissions during low-wind periods, and sulfate-dominated periods consistent with transport from more distant sources.
Inference, not direct attribution
exact_date_field_study_mixed_accumulation_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
The study identifies August 12 as a local PM2.5 maximum inside a prolonged August 3-23 stagnant regime. It reports weak synoptic dispersion, accumulation of local carbonaceous emissions during low-wind periods, and sulfate-dominated periods…
View supporting 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 2.7 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.
Atlanta-Sandy Springs-Roswell, GA
Monitor comparison window 1999-08-09 to 1999-08-15 · PM2.5 target: South DeKalb
24% above nearby days
48% higher than surrounding days
24% higher than surrounding days
48% higher than surrounding days
19% higher than surrounding days
7% lower than surrounding days
22% higher than surrounding days
About the same as 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.
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 |
|---|---|---|---|---|
| Atlanta-Sandy Springs-Roswell, GA | 59.1 µg/m³ | 2.7 | Aug 12, 1999 | #14 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 Atlanta Supersite / Journal of Geophysical Research
The study identifies August 12 as a local PM2.5 maximum inside a prolonged August 3-23 stagnant regime. It reports weak synoptic dispersion, accumulation of local carbonaceous emissions during low-wind periods, and sulfate-dominated periods consistent with transport from more distant sources.
A retained source supports the attributed cause for the stated place and dates.
The paper supports a mixed accumulation mechanism and regional/local source pattern, not a unique source or named incident.
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.