Temperature inversion Possible
Daily PM2.5 peaked at 63.3 µg/m³ near San Diego across 1 covered metro home to 3.3 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 ≈ 7.6 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
San Diego's five-monitor PM2.5 rise coincided with elevated nitrogen dioxide. Lindbergh Field was calm during 42 percent of event hours and at or below three knots during 75 percent. The local Miramar morning sounding measured an 8.2-degree Celsius surface inversion through only 265 meters, while the evening profile retained a weaker elevated inversion. The county air district independently identifies lowering winter inversion heights as a recurring driver of San Diego's highest PM2.5 days. Temperature-inversion accumulation is therefore a possible mechanism, while the evidence does not establish exact source shares.
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 5, 2006 hypothesis
SAN DIEGO/LINDBERG recorded calm conditions during 42% of event hours and wind at or below three knots during 75%. Observations included minimum visibility 7 miles.
NOAA IGRA / University of Wyoming upper-air archive · Dec 5, 2006 hypothesis
Both Miramar upper-air profiles corresponding to the local December 5 event date measured low-level inversion structure. The morning profile warmed 8.2 degrees Celsius from the surface through 265 meters. The evening profile had a much weaker 1.0-degree elevated inversion between 351 and 824 meters, so the strongest retention layer was concentrated in the morning.
U.S. EPA AirData hypothesis
EPA records show median daily PM2.5 of 38.75 micrograms per cubic meter across 5 monitors, 2.788 times the adjacent-window level. Co-pollutants at least 10% above the adjacent-window comparison were nitrogen dioxide (1.151x). Other measured comparisons were carbon monoxide (1.024x), sulfur dioxide (0.937x).
Inference, not direct attribution
local_light_wind_plus_exact_local_inversion_possible.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
local_light_wind_plus_exact_local_inversion_possible.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 2.9 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.
San Diego-Carlsbad, CA
Monitor comparison window 2006-12-02 to 2006-12-08 · PM2.5 target: San Diego - Beardsley Street
129% above nearby days
10 calm hours observed
0 hours at or below 3 miles
129% higher than surrounding days
About the same as surrounding days
11% higher than surrounding days
5% lower than surrounding days
16% lower than surrounding days
6 kt maximum · 10 of 24 observed hours calm
0 observed hours at or below 3 miles
45°F to 67°F
80% 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.
| Metro | Peak PM2.5 | Cigarettes, whole event | Peak day | All-time standing |
|---|---|---|---|---|
| San Diego-Carlsbad, CA | 63.3 µg/m³ | 2.9 | Dec 5, 2006 | #20 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
SAN DIEGO/LINDBERG recorded calm conditions during 42% of event hours and wind at or below three knots during 75%. Observations included minimum visibility 7 miles.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
A single surface station documents ventilation and obscuration but not the full vertical profile or particle source mix.
NOAA IGRA / University of Wyoming upper-air archive
Both Miramar upper-air profiles corresponding to the local December 5 event date measured low-level inversion structure. The morning profile warmed 8.2 degrees Celsius from the surface through 265 meters. The evening profile had a much weaker 1.0-degree elevated inversion between 351 and 824 meters, so the strongest retention layer was concentrated in the morning.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
Miramar is about 17 kilometers from the highest-PM2.5 monitor used for the event autopsy; the sounding represents the local lower atmosphere, not the monitor itself.
San Diego County Air Pollution Control District
The district's analysis of San Diego's highest-PM2.5 days found evening peaks that generally reflect lowering inversion heights trapping pollution near the surface, together with commute traffic and winter residential wood combustion.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The district analysis describes a recurring high-PM pattern and does not establish an inversion or source shares for this exact event.
U.S. EPA AirData
EPA records show median daily PM2.5 of 38.75 micrograms per cubic meter across 5 monitors, 2.788 times the adjacent-window level. Co-pollutants at least 10% above the adjacent-window comparison were nitrogen dioxide (1.151x). Other measured comparisons were carbon monoxide (1.024x), sulfur dioxide (0.937x).
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The pollution fingerprint does not uniquely apportion traffic, heating, industry, wood combustion, or secondary aerosol.
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.