Stagnant pollution mixture Possible
Daily PM2.5 peaked at 71.6 µg/m³ near Cleveland across 1 covered metro home to 2.2 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 ≈ 2.7 cigarettes. Across everyone living in the affected metros, that is the collective equivalent of ≈ 5.8 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
Cleveland's eleven-monitor PM2.5 median rose to 63.65 micrograms per cubic meter while PM10, carbon monoxide, nitrogen dioxide, and sulfur dioxide rose together. Nearby surface observations recorded high pressure, light winds, very humid air, fog or mist, and haze. EPA data show a simultaneous December 11-12 PM2.5 rise in the Steubenville area, and SCAMP independently documented one December 2001 episode with the same PM2.5/combustion-gas pattern and inverse ozone relationship. The exact Cleveland evidence supports stagnant mixed-pollution accumulation at possible confidence; the month-only SCAMP table is retained as context rather than treated as exact-date proof.
Possible This is a transparent hypothesis supported by limited evidence or an explicit inference, not a definitive attribution.
National Weather Service surface observations via Iowa Environmental Mesonet · Dec 11, 2001 hypothesis
Cleveland Burke Lakefront reported a median wind of four knots, six hours at or below three knots, three calm hours, median sea-level pressure of 1027.45 millibars, median relative humidity of 95.75%, thirteen fog-or-mist hours, one haze hour, and visibility as low as three miles.
U.S. EPA AirData hypothesis
Across eleven Cleveland-area monitors, median daily PM2.5 reached 63.65 micrograms per cubic meter, about 3.7 times the adjacent-window comparison. PM10 reached 102 micrograms per cubic meter, while carbon monoxide and nitrogen dioxide were about 1.5 and 1.9 times their adjacent comparisons; sulfur dioxide was also elevated before falling sharply after the episode.
U.S. EPA AirData hypothesis
At the daily Steubenville-area monitor, PM2.5 rose from 21.5 micrograms per cubic meter on December 10 to 33.2 on December 11 and 33.9 on December 12, then fell to 11.9 on December 13. This independently shows a same-window regional rise and clearing beyond Cleveland.
Inference, not direct attribution
high_pressure_mixed_pollution_regional_overlap.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
high_pressure_mixed_pollution_regional_overlap.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.
Cleveland-Elyria, OH
Monitor comparison window 2001-12-08 to 2001-12-14 · PM2.5 target: St Theodosius
268% above nearby days
3 calm hours observed
Haze reported during 1 hour
268% higher than surrounding days
325% higher than surrounding days
53% higher than surrounding days
88% higher than surrounding days
10% higher than surrounding days
8 kt maximum · 3 of 24 observed hours calm
2 observed hours at or below 3 miles
32°F to 47°F
100% 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 |
|---|---|---|---|---|
| Cleveland-Elyria, OH | 71.6 µg/m³ | 3.3 | Dec 11, 2001 | #5 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
Cleveland Burke Lakefront reported a median wind of four knots, six hours at or below three knots, three calm hours, median sea-level pressure of 1027.45 millibars, median relative humidity of 95.75%, thirteen fog-or-mist hours, one haze hour, and visibility as low as three miles.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
One airport station cannot represent the complete metropolitan mixing layer.
U.S. EPA AirData
At the daily Steubenville-area monitor, PM2.5 rose from 21.5 micrograms per cubic meter on December 10 to 33.2 on December 11 and 33.9 on December 12, then fell to 11.9 on December 13. This independently shows a same-window regional rise and clearing beyond Cleveland.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The Steubenville-area monitor is roughly 100 miles from Cleveland and corroborates regional timing, not Cleveland's local source mix.
U.S. EPA AirData
Across eleven Cleveland-area monitors, median daily PM2.5 reached 63.65 micrograms per cubic meter, about 3.7 times the adjacent-window comparison. PM10 reached 102 micrograms per cubic meter, while carbon monoxide and nitrogen dioxide were about 1.5 and 1.9 times their adjacent comparisons; sulfur dioxide was also elevated before falling sharply after the episode.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The available evidence identifies an accumulation mechanism and mixed pollution fingerprint, not the shares contributed by traffic, industry, power generation, residential combustion, or secondary aerosol.
Journal of the Air & Waste Management Association
SCAMP identified one 120-hour December 2001 Steubenville episode with maximum hourly PM2.5 of 83.4 micrograms per cubic meter. During that episode PM2.5 tracked sulfur dioxide, carbon monoxide, nitric oxide, and nitrogen dioxide strongly and tracked ozone inversely, a pattern the authors associate with nighttime accumulation and reduced mixing in several cold-season case studies.
Context for the event; this item does not establish the attributed cause by itself.
SCAMP labels its December episode only by month, so it corroborates the pattern but is not used as exact-date proof for Cleveland.
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.