Stagnant pollution mixture Possible
Daily PM2.5 peaked at 73.5 µg/m³ near Allentown across 1 covered metro home to 886,418 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 ≈ 4.9 cigarettes. Across everyone living in the affected metros, that is the collective equivalent of ≈ 4.3 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
Allentown's three-monitor PM2.5 episode rose with PM10, carbon monoxide, nitrogen dioxide, and sulfur dioxide. NWS forecast very light wind beneath crossing high pressure with strong radiational conditions over snowpack, and the airport recorded extensive mist and weak flow. This supports trapped mixed-pollution accumulation as a possible mechanism, not a uniquely identified source.
Possible This is a transparent hypothesis supported by limited evidence or an explicit inference, not a definitive attribution.
U.S. EPA AirData hypothesis
EPA records show median daily PM2.5 of 58.375 micrograms per cubic meter across three monitors, about 4.1 times the prior adjacent-window level. PM10 rose to 52 micrograms per cubic meter from nine, while carbon monoxide rose to roughly three times its prior level.
National Weather Service Philadelphia/Mount Holly via Iowa Environmental Mesonet · Jan 22, 2001 hypothesis
The discussion covering the start of the event forecast clear skies as high pressure crossed the region, very light winds, excellent radiational conditions, and enhanced cooling over widespread snowpack; later updates mentioned patchy fog.
National Weather Service surface observations via Iowa Environmental Mesonet · Jan 23, 2001 hypothesis
Allentown-Bethlehem Airport was calm during 16 of 47 observed event hours and at or below three knots during 25 hours. It recorded mist during 21 hours and haze during five hours, with visibility falling to 1.25 miles.
Inference, not direct attribution
official_surface_weak_dispersion_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
official_surface_weak_dispersion_multimonitor_accumulation.v1. This inference is recorded in the catalog methodology.
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 3.3 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.
Allentown-Bethlehem-Easton, PA-NJ
Monitor comparison window 2001-01-20 to 2001-01-27 · PM2.5 target: Freemansburg
161% above nearby days
16 calm hours observed
Haze reported during 5 hours
161% higher than surrounding days
167% higher than surrounding days
117% higher than surrounding days
67% higher than surrounding days
59% higher than surrounding days
8 kt maximum · 16 of 47 observed hours calm
9 observed hours at or below 3 miles
6°F to 37°F
92% maximum
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 | 73.5 µg/m³ | 5.8 | Jan 24, 2001 | #9 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 surface observations via Iowa Environmental Mesonet
Allentown-Bethlehem Airport was calm during 16 of 47 observed event hours and at or below three knots during 25 hours. It recorded mist during 21 hours and haze during five hours, with visibility falling to 1.25 miles.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
Surface observations demonstrate weak ventilation and obscuration but do not prove a temperature inversion or identify particle sources.
National Weather Service Philadelphia/Mount Holly via Iowa Environmental Mesonet
The discussion covering the start of the event forecast clear skies as high pressure crossed the region, very light winds, excellent radiational conditions, and enhanced cooling over widespread snowpack; later updates mentioned patchy fog.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The discussion does not explicitly call the condition an inversion or identify the particles' source mixture.
U.S. EPA AirData
EPA records show median daily PM2.5 of 58.375 micrograms per cubic meter across three monitors, about 4.1 times the prior adjacent-window level. PM10 rose to 52 micrograms per cubic meter from nine, while carbon monoxide rose to roughly three times its prior level.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The co-pollutant fingerprint supports mixed accumulation but does not identify individual source sectors or the local-versus-regional split.
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.