Temperature inversion Likely
Daily PM2.5 peaked at 59.6 µg/m³ near Philadelphia across 3 covered metros home to 16 million people.
Counting the extra PM2.5 above each metro's typical day, this event handed the average affected resident ≈ 2.9 cigarettes. Across everyone living in the affected metros, that is the collective equivalent of ≈ 45 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 forecasting assessment lists the January 23-24, 2001 high-PM episode in Baltimore, Philadelphia, and Washington and assigns the January 23 day in each city to a winter CART regime characterized by very low winds and deep, strongly stable layers.
Likely Applicable reporting or a supported inference makes this the best current causal explanation.
MARAMA / Maryland Department of the Environment · Sep 30, 2004 official
MARAMA lists January 23-24, 2001 as simultaneous high-PM days in Baltimore, Philadelphia, and Washington. The January 23 city classifications fall in winter CART regimes that the report characterizes by very low surface winds and deep or strongly stable layers.
See every retained source and limitation ↓
Sequence: Philadelphia (Jan 23) → Baltimore (Jan 23) → Washington (Jan 23)
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
MARAMA lists January 23-24, 2001 as simultaneous high-PM days in Baltimore, Philadelphia, and Washington. The January 23 city classifications fall in winter CART regimes that the report characterizes by very low surface winds and deep or strongly…
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 3 metros. 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 “Temperature inversion” explanation; it does not assign the cause by itself.
Baltimore-Columbia-Towson, MD
Monitor comparison window 2001-01-20 to 2001-01-27 · PM2.5 target: Oldtown
247% above nearby days
165% higher than surrounding days
247% higher than surrounding days
165% higher than surrounding days
60% higher than surrounding days
34% higher than surrounding days
24% lower than surrounding days
About the same as surrounding days
About the same as surrounding days
About the same as surrounding days
Philadelphia-Camden-Wilmington, PA-NJ-DE-MD
Monitor comparison window 2001-01-20 to 2001-01-27 · PM2.5 target: MLK CORNER OF MLK BLVD AND JUSTISON ST
126% above nearby days
About the same as surrounding days
135% higher than surrounding days
126% higher than surrounding days
135% higher than surrounding days
126% higher than surrounding days
72% higher than surrounding days
133% higher than surrounding days
46% lower 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
Washington-Arlington-Alexandria, DC-VA-MD-WV
Monitor comparison window 2001-01-20 to 2001-01-27 · PM2.5 target: RIVER TERRACE
197% above nearby days
99% higher than surrounding days
197% higher than surrounding days
99% higher than surrounding days
76% higher than surrounding days
28% higher than surrounding days
22% lower than 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 |
|---|---|---|---|---|
| Philadelphia-Camden-Wilmington, PA-NJ-DE-MD | 59.6 µg/m³ | 5.4 | Jan 23, 2001 | #15 of its top days |
| Baltimore-Columbia-Towson, MD | 56.7 µg/m³ | 2.6 | Jan 23, 2001 | #12 of its top days |
| Washington-Arlington-Alexandria, DC-VA-MD-WV | 54.9 µg/m³ | 2.5 | Jan 23, 2001 | #19 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.
MARAMA / Maryland Department of the Environment
MARAMA lists January 23-24, 2001 as simultaneous high-PM days in Baltimore, Philadelphia, and Washington. The January 23 city classifications fall in winter CART regimes that the report characterizes by very low surface winds and deep or strongly stable layers.
An official source supports the attributed cause for the stated place and dates.
The assessment identifies the meteorological accumulation mechanism but does not allocate measured PM2.5 among individual emitting sources.
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.