Temperature inversion Possible
Daily PM2.5 peaked at 92.5 µg/m³ near Los Angeles across 2 covered metros home to 14 million people.
Counting the extra PM2.5 above each metro's typical day, this event handed the average affected resident ≈ 3.4 cigarettes. Across everyone living in the affected metros, that is the collective equivalent of ≈ 47 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
Manual research connected 3 source events to one temperature-inversion pollution episode using the deterministic grouping rule.
Possible This is a transparent hypothesis supported by limited evidence or an explicit inference, not a definitive attribution.
National Weather Service Los Angeles/Oxnard via Iowa Environmental Mesonet · Oct 20, 2000 hypothesis
NWS Los Angeles/Oxnard measured a 1,500-2,000-foot marine inversion across the Los Angeles Basin with widespread fog in coastal valleys.
National Weather Service Los Angeles/Oxnard via Iowa Environmental Mesonet · Oct 21, 2000 hypothesis
NWS Los Angeles/Oxnard reported that the marine inversion had risen to about 3,000 feet while low clouds and fog became widespread across Los Angeles and Ventura coastal and valley locations.
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.
See every retained source and limitation ↓
Sequence: Los Angeles (Oct 20) → Oxnard (Oct 21)
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 2 metros. The peak daily concentration is roughly 4.2 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.
Los Angeles-Long Beach-Anaheim, CA
Monitor comparison window 2000-10-17 to 2000-10-23 · PM2.5 target: Azusa
134% above nearby days
About the same as surrounding days
134% higher than surrounding days
About the same as surrounding days
About the same as surrounding days
5% higher than surrounding days
20% lower than surrounding days
56% higher than surrounding days
9% higher than surrounding days
About the same as surrounding days
About the same as surrounding days
Oxnard-Thousand Oaks-Ventura, CA
Monitor comparison window 2000-10-18 to 2000-10-24 · PM2.5 target: Simi Valley-Cochran Street
4% above nearby days
About the same as surrounding days
No adjacent-day comparison
About the same as surrounding days
No adjacent-day comparison
About the same as surrounding days
12% lower than surrounding days
54% lower than surrounding days
6% higher than surrounding days
8% higher than surrounding days
About the same as surrounding days
About the same as surrounding days
About the same as 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.
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 |
|---|---|---|---|---|
| Los Angeles-Long Beach-Anaheim, CA | 92.5 µg/m³ | 4.2 | Oct 20, 2000 | #11 of its top days |
| Oxnard-Thousand Oaks-Ventura, CA | 44.2 µg/m³ | 2.0 | Oct 21, 2000 | #14 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 Los Angeles/Oxnard via Iowa Environmental Mesonet
NWS Los Angeles/Oxnard reported that the marine inversion had risen to about 3,000 feet while low clouds and fog became widespread across Los Angeles and Ventura coastal and valley locations.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The source does not directly discuss the Oxnard PM2.5 record.
National Weather Service Los Angeles/Oxnard via Iowa Environmental Mesonet
NWS Los Angeles/Oxnard measured a 1,500-2,000-foot marine inversion across the Los Angeles Basin with widespread fog in coastal valleys.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The source documents the mechanism but not the particle mixture.
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.