Winter inversion Documented
Daily PM2.5 peaked at 70.5 µg/m³ near Allentown across 4 covered metros home to 4.9 million people.
Counting the extra PM2.5 above each metro's typical day, this event handed the average affected resident ≈ 2.3 cigarettes. Across everyone living in the affected metros, that is the collective equivalent of ≈ 12 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
Maryland’s retrospective analysis identifies the 12–15 January episode, including Baltimore, with stagnation and a strong morning inversion, plus a nitrate-rich local component.
Documented An agency or publisher page assesses this cause, without an agency record naming the event or an EPA qualifier on the data.
Maryland Department of the Environment official
Maryland’s retrospective episode analysis reports regionwide PM2.5 enhancement on 13 January 2001 and a 12–15 January episode with westerly transport, recirculation, high humidity, stagnation, and a strong morning inversion. It identifies a nitrate-rich local-scale component associated with motor-vehicle and home-heating emissions for the Maryland portion, including Baltimore.
See every retained source and limitation ↓
Sequence: Baltimore (Jan 13) → Allentown (Jan 14) → Harrisburg (Jan 14) → Scranton (Jan 14)
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
Maryland’s retrospective episode analysis reports regionwide PM2.5 enhancement on 13 January 2001 and a 12–15 January episode with westerly transport, recirculation, high humidity, stagnation, and a strong morning inversion. It identifies a…
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 4 metros. 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 “Winter inversion” explanation; it does not assign the cause by itself.
Baltimore-Columbia-Towson, MD
Monitor comparison window 2001-01-10 to 2001-01-18
38% above nearby days
No adjacent-day comparison
38% higher than surrounding days
No adjacent-day comparison
93% higher than surrounding days
About the same as surrounding days
7% lower than surrounding days
About the same as surrounding days
8% higher than surrounding days
About the same as surrounding days
About the same as surrounding days
Allentown-Bethlehem-Easton, PA-NJ
Monitor comparison window 2001-01-10 to 2001-01-18 · PM2.5 target: Allentown
71% above nearby days
70% higher than surrounding days
71% higher than surrounding days
70% higher than surrounding days
104% higher than surrounding days
39% higher than surrounding days
73% higher than surrounding days
Harrisburg-Carlisle, PA
Monitor comparison window 2001-01-10 to 2001-01-18 · PM2.5 target: LAT/LON POINT=LOCATION OF PM-2.5 MONITOR INLET
57% above nearby days
65% higher than surrounding days
57% higher than surrounding days
65% higher than surrounding days
21% higher than surrounding days
12% higher than surrounding days
28% lower than surrounding days
Scranton--Wilkes-Barre--Hazleton, PA
Monitor comparison window 2001-01-10 to 2001-01-18 · PM2.5 target: Wilkes-Barre
99% above nearby days
67% higher than surrounding days
99% higher than surrounding days
67% higher than surrounding days
81% higher than surrounding days
57% higher than surrounding days
60% 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.
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 |
|---|---|---|---|---|
| Allentown-Bethlehem-Easton, PA-NJ | 70.5 µg/m³ | 3.2 | Jan 14, 2001 | #12 of its top days |
| Harrisburg-Carlisle, PA | 59.4 µg/m³ | 2.7 | Jan 14, 2001 | #14 of its top days |
| Baltimore-Columbia-Towson, MD | 53.6 µg/m³ | 2.4 | Jan 13, 2001 | #14 of its top days |
| Scranton--Wilkes-Barre--Hazleton, PA | 52.6 µg/m³ | 4.5 | Jan 14, 2001 | #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.
Maryland’s retrospective episode analysis reports regionwide PM2.5 enhancement on 13 January 2001 and a 12–15 January episode with westerly transport, recirculation, high humidity, stagnation, and a strong morning inversion. It identifies a nitrate-rich local-scale component associated with motor-vehicle and home-heating emissions for the Maryland portion, including Baltimore.
Documented An agency or publisher page assesses this cause, without an agency record naming the event or an EPA qualifier on the data.
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.