Temperature inversion Possible
Daily PM2.5 peaked at 55.3 µg/m³ near New York across 1 covered metro home to 20 million people.
Counting the extra PM2.5 above each metro's typical day, this event handed the average affected resident ≈ 2.0 cigarettes. Across everyone living in the affected metros, that is the collective equivalent of ≈ 39 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
A 32-monitor New York PM2.5 increase coincided with broad increases in combustion and particle co-pollutants, weak surface flow, observed haze, and an exact-date NWS report of clouds trapped beneath an inversion. The evidence supports a temperature-inversion trapping mechanism at possible confidence. It does not establish which anthropogenic sources supplied the particle mixture.
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 46.2 micrograms per cubic meter across 32 monitors, about 2.1 times the adjacent-window comparison. Carbon monoxide, nitrogen dioxide, PM10, and sulfur dioxide were each roughly 1.9 to 2.2 times their adjacent comparisons, while ozone was lower.
National Weather Service New York Area Forecast Discussion via Iowa Environmental Mesonet · Jan 23, 2001 hypothesis
The exact-date NWS discussion reported low clouds trapped beneath an inversion over the forecast area and light coastal winds. A later discussion that day retained weak high pressure and described clouds still trapped beneath the inversion.
National Weather Service surface observations via Iowa Environmental Mesonet · Jan 23, 2001 hypothesis
LaGuardia recorded a median wind speed of 4.5 knots, nine of 24 hours at or below three knots, two calm hours, and two hours coded as haze. EPA's metro-level daily wind series was also less than half its adjacent-window comparison.
Inference, not direct attribution
Documented inversion overlap: The event overlaps a documented inversion period, but the retained material does not directly attribute the measured PM2.5 episode.
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
Documented inversion overlap. The event overlaps a documented inversion period, but the retained material does not directly attribute the measured PM2.5 episode.
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 2.5 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.
New York-Newark-Jersey City, NY-NJ-PA
Monitor comparison window 2001-01-20 to 2001-01-26 · PM2.5 target: IS 52
76% above nearby days
2 calm hours observed
Haze reported during 2 hours
76% higher than surrounding days
63% higher than surrounding days
98% higher than surrounding days
73% higher than surrounding days
55% higher than surrounding days
47% lower than surrounding days
7 kt maximum · 2 of 24 observed hours calm
0 observed hours at or below 3 miles
27°F to 37°F
81% 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 |
|---|---|---|---|---|
| New York-Newark-Jersey City, NY-NJ-PA | 55.3 µg/m³ | 2.5 | Jan 23, 2001 | #17 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 New York Area Forecast Discussion via Iowa Environmental Mesonet
The exact-date NWS discussion reported low clouds trapped beneath an inversion over the forecast area and light coastal winds. A later discussion that day retained weak high pressure and described clouds still trapped beneath the inversion.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The forecast discussion documents the trapping mechanism, not the sources or chemical composition of the accumulated particles.
National Weather Service surface observations via Iowa Environmental Mesonet
LaGuardia recorded a median wind speed of 4.5 knots, nine of 24 hours at or below three knots, two calm hours, and two hours coded as haze. EPA's metro-level daily wind series was also less than half its adjacent-window comparison.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
One airport station cannot describe every part of the New York metro or the complete vertical temperature profile.
U.S. EPA AirData
EPA records show median daily PM2.5 of 46.2 micrograms per cubic meter across 32 monitors, about 2.1 times the adjacent-window comparison. Carbon monoxide, nitrogen dioxide, PM10, and sulfur dioxide were each roughly 1.9 to 2.2 times their adjacent comparisons, while ozone was lower.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The multi-pollutant response supports accumulated anthropogenic pollution but does not identify the individual emitting sectors or particle composition.
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.