Temperature inversion Possible
Daily PM2.5 peaked at 66.3 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 ≈ 4.7 cigarettes. Across everyone living in the affected metros, that is the collective equivalent of ≈ 15 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 persisted for two days while the local airport recorded wind at or below three knots during 56 percent of event hours, haze, mist, fog, and visibility at or below three miles during 37 of 48 hours. Every Miramar upper-air profile observed on the two event dates measured a low-level inversion layer, including strong morning caps. The county air district independently identifies lowering inversion heights as a recurring driver of San Diego's highest winter 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 2, 2000 hypothesis
SAN DIEGO/LINDBERG recorded calm conditions during 31% of event hours and wind at or below three knots during 56%. Observations included 10 haze hours, 32 mist hours, minimum visibility 0.12 miles.
NOAA IGRA / University of Wyoming upper-air archive · Dec 2, 2000 hypothesis
All three Miramar upper-air profiles observed on the two event dates measured a low-level inversion layer. On the morning of December 2, temperature rose 7.6 degrees Celsius from the surface through 662 meters. The afternoon profile retained a 3.0-degree elevated inversion, and the December 3 morning profile had a net 6.2-degree rise from the surface through 673 meters with a strong 5.6-degree cap in its upper 215 meters.
U.S. EPA AirData hypothesis
EPA records show median daily PM2.5 of 53.1 micrograms per cubic meter across 5 monitors, 1.866 times the adjacent-window level. No measured co-pollutant met the 10% adjacent-window increase screen. Other measured comparisons were carbon monoxide (1.09x), nitrogen dioxide (0.931x), sulfur dioxide (0.806x). No adjacent-window ratio was available for pm10.
Inference, not direct attribution
local_obscuration_plus_exact_local_persistent_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_obscuration_plus_exact_local_persistent_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 3.0 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 2000-11-29 to 2000-12-06 · PM2.5 target: San Diego-12th Ave
59% above nearby days
15 calm hours observed
Haze reported during 10 hours
59% higher than surrounding days
No adjacent-day comparison
About the same as surrounding days
About the same as surrounding days
17% lower than surrounding days
65% higher than surrounding days
9 kt maximum · 15 of 48 observed hours calm
37 observed hours at or below 3 miles
46.4°F to 62°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.
| Metro | Peak PM2.5 | Cigarettes, whole event | Peak day | All-time standing |
|---|---|---|---|---|
| San Diego-Carlsbad, CA | 66.3 µg/m³ | 5.8 | Dec 2, 2000 | #18 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.
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.
National Weather Service surface observations via Iowa Environmental Mesonet
SAN DIEGO/LINDBERG recorded calm conditions during 31% of event hours and wind at or below three knots during 56%. Observations included 10 haze hours, 32 mist hours, minimum visibility 0.12 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
All three Miramar upper-air profiles observed on the two event dates measured a low-level inversion layer. On the morning of December 2, temperature rose 7.6 degrees Celsius from the surface through 662 meters. The afternoon profile retained a 3.0-degree elevated inversion, and the December 3 morning profile had a net 6.2-degree rise from the surface through 673 meters with a strong 5.6-degree cap in its upper 215 meters.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
Miramar is about 16 kilometers from the highest-PM2.5 monitor used for the event autopsy; the sounding represents the local lower atmosphere, not the monitor itself.
U.S. EPA AirData
EPA records show median daily PM2.5 of 53.1 micrograms per cubic meter across 5 monitors, 1.866 times the adjacent-window level. No measured co-pollutant met the 10% adjacent-window increase screen. Other measured comparisons were carbon monoxide (1.09x), nitrogen dioxide (0.931x), sulfur dioxide (0.806x). No adjacent-window ratio was available for pm10.
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.