Stagnant pollution mixture Officially verified
Daily PM2.5 peaked at 67 µg/m³ near Washington across 1 covered metro home to 6.4 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 ≈ 2.5 cigarettes. Across everyone living in the affected metros, that is the collective equivalent of ≈ 16 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
MARAMA's official Washington table reports 67.0 µg/m³ on September 27, 1999 and assigns the day to CART Bin 19. The assessment explicitly characterizes Bin 19 as regional-scale PM buildup during transitional seasons with relatively low winds.
Officially verified A directly applicable government determination or qualified scientific attribution product identifies this cause.
Maryland Department of the Environment / MARAMA · Sep 30, 2004 official
The official Washington table reports 67.0 µg/m³ on September 27, 1999 and assigns the day to CART Bin 19. The assessment defines Bin 19 as a transitional-season regional-scale PM buildup regime with the second-lowest average wind speeds.
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 official Washington table reports 67.0 µg/m³ on September 27, 1999 and assigns the day to CART Bin 19. The assessment defines Bin 19 as a transitional-season regional-scale PM buildup regime with the second-lowest average wind speeds.
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 3.0 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.
Washington-Arlington-Alexandria, DC-VA-MD-WV
Monitor comparison window 1999-09-24 to 1999-09-30 · PM2.5 target: MCMILLAN NCore-PAMS
25% above nearby days
No adjacent-day comparison
25% higher than surrounding days
No adjacent-day comparison
About the same as surrounding days
About the same as surrounding days
5% 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 |
|---|---|---|---|---|
| Washington-Arlington-Alexandria, DC-VA-MD-WV | 67 µg/m³ | 3.0 | Sep 27, 1999 | #7 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.
Maryland Department of the Environment / MARAMA
The official Washington table reports 67.0 µg/m³ on September 27, 1999 and assigns the day to CART Bin 19. The assessment defines Bin 19 as a transitional-season regional-scale PM buildup regime with the second-lowest average wind speeds.
An official source supports the attributed cause for the stated place and dates.
The regime classification identifies the accumulation mechanism but not the emitting-source mix behind the daily PM2.5 mass.
Officially verified A directly applicable government determination or qualified scientific attribution product identifies this cause.
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.