Stagnant pollution mixture Possible
Daily PM2.5 peaked at 69.9 µg/m³ near Pittsburgh across 1 covered metro home to 2.4 million people.
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 ≈ 6.1 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
An exact-date NWS discussion documents a slow-moving ridge of high pressure and subsidence. EPA observations show PM2.5 and PM10 increasing across many monitors along with carbon monoxide and nitrogen dioxide, while ozone remained near zero. The combined evidence supports a trapped combustion-associated mixture, not a uniquely identified source.
Possible This is a transparent hypothesis supported by limited evidence or an explicit inference, not a definitive attribution.
National Weather Service Pittsburgh via Iowa Environmental Mesonet · Nov 15, 2004 corroborates
NWS Pittsburgh documented a slow-moving ridge of surface high pressure and an area of subsidence over the forecast region during the event.
U.S. EPA AirData hypothesis
EPA observations show event-window PM2.5 elevated across 17 daily monitoring series, PM10 across multiple monitors, and higher carbon monoxide and nitrogen dioxide than the preceding period while ozone was suppressed.
Inference, not direct attribution
documented_ridge_weak_flow_multimonitor_accumulation.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
NWS Pittsburgh documented a slow-moving ridge of surface high pressure and an area of subsidence over the forecast region during the event.
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 3.2 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 “Stagnant pollution mixture” explanation; it does not assign the cause by itself.
Pittsburgh, PA
Monitor comparison window 2004-11-12 to 2004-11-18 · PM2.5 target: Liberty
84% above nearby days
131% higher than surrounding days
84% higher than surrounding days
131% higher than surrounding days
133% higher than surrounding days
59% higher than surrounding days
19% higher than surrounding days
43% lower 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.
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 |
|---|---|---|---|---|
| Pittsburgh, PA | 69.9 µg/m³ | 3.2 | Nov 15, 2004 | #23 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.
National Weather Service Pittsburgh via Iowa Environmental Mesonet
NWS Pittsburgh documented a slow-moving ridge of surface high pressure and an area of subsidence over the forecast region during the event.
A retained source supports the attributed cause for the stated place and dates.
The discussion establishes atmospheric trapping conditions but does not attribute the measured particles to a particular source.
U.S. EPA AirData
EPA observations show event-window PM2.5 elevated across 17 daily monitoring series, PM10 across multiple monitors, and higher carbon monoxide and nitrogen dioxide than the preceding period while ozone was suppressed.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The multi-pollutant fingerprint is compatible with trapped combustion emissions but cannot distinguish source sectors or local from transported contributions.
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.