152% above nearby days
Temperature inversion Possible
2009-12-10 Portland-Vancouver-Hillsboro, OR-WA temperature-inversion pollution episode
Daily PM2.5 peaked at 56.2 µg/m³ near Portland across 1 covered metro home to 2.5 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 ≈ 5.7 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
The basis for “Temperature inversion”
An official regional forecast documented a cold low-level air dome, decreasing winds, and poor ventilation through the exact event date. Those conditions make trapped local and regional pollution a possible explanation, but no source apportioned Portland's PM2.5 that day.
Possible This is a transparent hypothesis supported by limited evidence or an explicit inference, not a definitive attribution.
Inference, not direct attribution
exact_window_stagnation_possible.v1: This inference is recorded in the catalog methodology.
Evidence path
From source to city
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.
- 1Inferred
Source or mechanism
Pattern-based explanation
exact_window_stagnation_possible.v1. This inference is recorded in the catalog methodology.
- 2Research gap
Air-mass movement
The transport path has not been reconstructed
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.
- 3Documented
Local response
56.2 µg/m³ near Portland
EPA monitors recorded the event across 1 metro. The peak daily concentration is roughly 2.6 cigarette-equivalents at the outdoor daily rate.
Inspect the pollutant and weather signals ↓
Event autopsy
What the instruments saw
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.
Portland-Vancouver-Hillsboro, OR-WA
Monitor comparison window 2009-12-07 to 2009-12-13 · PM2.5 target: VANCOUVER - 4TH PL BLVD E
39% lower than surrounding days
About the same as surrounding days
Pollution fingerprint
Metro-wide median of reporting monitors PM2.5 56.2 µg/m³
152% higher than surrounding days
Before13.9 µg/m³ Event56.2 µg/m³ After32.5 µg/m³
Carbon monoxide 0.548 ppm
About the same as surrounding days
Before0.430 ppm Event0.548 ppm After0.642 ppm
Nitrogen dioxide 20.5 ppb
9% higher than surrounding days
Before9.70 ppb Event20.5 ppb After19.6 ppb
Sulfur dioxide 2.77 ppb
9% higher than surrounding days
Before2.54 ppb Event2.77 ppb After2.48 ppb
Ozone 0.011 ppm
24% lower than surrounding days
Before0.025 ppm Event0.011 ppm After0.007 ppm
Atmospheric conditions
Nearby EPA weather observationsAbout the same as surrounding days
About the same as surrounding days
About the same as surrounding days
39% lower 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.
Measured daily PM2.5 at EPA monitors, as cigarette-equivalents
EPA monitors sit in metro areas, so the colour clusters there. Land between and beyond
them is unmeasured, not clean: this map shows where IAQng has readings, not the extent
of the smoke.
—
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
Camera views near
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.
Affected metros
| Metro | Peak PM2.5 | Cigarettes, whole event | Peak day | All-time standing |
|---|---|---|---|---|
| Portland-Vancouver-Hillsboro, OR-WA | 56.2 µg/m³ | 2.6 | Dec 10, 2009 | #22 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
What supports this account
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.
Willamette Valley forecast issued December 8, 2009
Oregon Department of Agriculture Smoke Management Program
Oregon's smoke-management forecast described a cold low-level air dome, a low ventilation index, and decreasing surface winds expected through December 10 in the Willamette Valley.
What it supports
Context for the event; this item does not establish the attributed cause by itself.
Limit
The forecast does not say an inversion caused Portland's December 10 PM2.5 value, and the linked December 10 update could not be retrieved.
Recent changes in winter PM2.5 contributions from wood smoke, motor vehicles, and other sources in the Northwest U.S.
Journal of the Air & Waste Management Association
The receptor-model study includes Portland observations from 2007-2009 and identifies winter contributions from fresh and aged wood smoke, gasoline engines, nitrate-dominant aerosol, and other combustion sources. It supplies composition context for pollution trapped during the December 2009 cold-air episode.
What it supports
Context for the event; this item does not establish the attributed cause by itself.
Limit
The published table is seasonal and does not apportion Portland's December 10 event specifically.
What remains uncertain
- The classification describes the accumulation mechanism, not a unique emissions source.
Status and provenance
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.