Stagnant pollution mixture Possible
Daily PM2.5 peaked at 80.6 µg/m³ near Riverside across 1 covered metro home to 4.7 million people.
Counting the extra PM2.5 above each metro's typical day, this event handed the average affected resident ≈ 2.9 cigarettes. Across everyone living in the affected metros, that is the collective equivalent of ≈ 14 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
Four western Inland Empire PM2.5 monitors rose together while desert, mountain, and farther-east sites remained much lower. Collocated PM10 and elevated combustion-related gases show a mixed particle episode, while exact-date NWS discussions document dense fog under a shallow inversion, hazy sunshine, and explicitly stagnant conditions. Established South Coast source characterization supports a recurring secondary-plus-combustion aerosol. Together these support stagnant anthropogenic mixture at possible confidence; exact-day speciation is absent, so the record does not claim a unique source or component shares.
Possible This is a transparent hypothesis supported by limited evidence or an explicit inference, not a definitive attribution.
National Weather Service San Diego Area Forecast Discussion via Iowa Environmental Mesonet · Dec 2, 2000 hypothesis
By midafternoon, NWS reported that low clouds had cleared to hazy sunshine but that conditions would remain stagnant through Tuesday. This independently records the persistence of the trapping air mass during the PM2.5 event.
U.S. EPA AirData hypothesis
EPA's certified daily record shows a coherent west-to-east Inland Empire gradient: Rubidoux measured 80.6 micrograms per cubic meter, Riverside-Magnolia 78.1, Ontario 71.6, and Fontana 64.2, while San Bernardino measured 26.9, Victorville 16-17, Indio 12.9, and Palm Springs 6.2. The four elevated western sites make a single-monitor malfunction or hyperlocal plume unlikely, while the sharp gradient argues against treating the entire metro as uniformly exposed.
National Weather Service San Diego Area Forecast Discussion via Iowa Environmental Mesonet · Dec 2, 2000 hypothesis
The exact-date NWS discussion reported that weak-to-moderate offshore pressure gradients lacked upper-level support and compressed the marine layer to about 1,500 feet, allowing dense fog beneath the inversion in coastal and some valley areas. The early-morning discussion also described weak synoptic-scale gradients and increasing subsidence under a strengthening ridge.
Inference, not direct attribution
exact_stagnation_plus_inland_gradient_and_mixed_pollution_fingerprint_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
exact_stagnation_plus_inland_gradient_and_mixed_pollution_fingerprint_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 3.7 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.
Riverside-San Bernardino-Ontario, CA
Monitor comparison window 2000-11-29 to 2000-12-05 · PM2.5 target: Rubidoux
0% above nearby days
11 kt maximum
0 hours at or below 3 miles
About the same as surrounding days
About the same as surrounding days
24% higher than surrounding days
About the same as surrounding days
62% higher than surrounding days
About the same as surrounding days
11 kt maximum · No calm observed hours
0 observed hours at or below 3 miles
53.6°F to 75.2°F
41% 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 |
|---|---|---|---|---|
| Riverside-San Bernardino-Ontario, CA | 80.6 µg/m³ | 3.7 | Dec 2, 2000 | #24 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 San Diego Area Forecast Discussion via Iowa Environmental Mesonet
By midafternoon, NWS reported that low clouds had cleared to hazy sunshine but that conditions would remain stagnant through Tuesday. This independently records the persistence of the trapping air mass during the PM2.5 event.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The forecast discussion uses haze descriptively and does not chemically attribute it.
National Weather Service San Diego Area Forecast Discussion via Iowa Environmental Mesonet
The exact-date NWS discussion reported that weak-to-moderate offshore pressure gradients lacked upper-level support and compressed the marine layer to about 1,500 feet, allowing dense fog beneath the inversion in coastal and some valley areas. The early-morning discussion also described weak synoptic-scale gradients and increasing subsidence under a strengthening ridge.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The discussion describes the regional air mass and does not measure mixing height at each pollution monitor.
U.S. EPA AirData
EPA's certified daily record shows a coherent west-to-east Inland Empire gradient: Rubidoux measured 80.6 micrograms per cubic meter, Riverside-Magnolia 78.1, Ontario 71.6, and Fontana 64.2, while San Bernardino measured 26.9, Victorville 16-17, Indio 12.9, and Palm Springs 6.2. The four elevated western sites make a single-monitor malfunction or hyperlocal plume unlikely, while the sharp gradient argues against treating the entire metro as uniformly exposed.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The daily mass measurements do not identify chemical components or source sectors.
U.S. EPA AirData
Collocated PM10 at Rubidoux was 131 micrograms per cubic meter, compared with PM2.5 of 80.6, so most of the measured particle mass was fine rather than coarse. PM10 was also elevated at Norco, Ontario, and Fontana. Across the event packet, carbon monoxide and nitrogen dioxide were elevated at multiple monitors, supporting an accumulated mixed urban aerosol rather than a pure coarse-dust explanation.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
PM10 and PM2.5 were not measured at every same site, and their network summaries should not be treated as component fractions.
Kim et al., Journal of the Air & Waste Management Association
A South Coast AQMD-led basin characterization found that ammonium, nitrate, sulfate, organic carbon, and elemental carbon dominated PM2.5 mass. Nitrate contributed 28 to 41 percent and organic carbon 18 to 26 percent, establishing the basin's recurring secondary-plus-combustion mixture and its strong inland gradient.
Context for the event; this item does not establish the attributed cause by itself.
This is recurring basin context, not an exact-date December 2 source-apportionment result.
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.