Temperature inversion Possible
Daily PM2.5 peaked at 81.3 µg/m³ near Los Angeles across 1 covered metro home to 13 million people.
Counting the extra PM2.5 above each metro's typical day, this event handed the average affected resident ≈ 3.0 cigarettes. Across everyone living in the affected metros, that is the collective equivalent of ≈ 39 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 NWS discussion preceding the January 18 measurement documented a roughly 1,500-foot inversion over the Los Angeles area. The retained scope and limitations support possible confidence for this local episode.
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 · Jan 17, 1999 hypothesis
The NWS discussion preceding the January 18 measurement documented a roughly 1,500-foot inversion over the Los Angeles area.
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
The catalog has a grouping hypothesis, but no retained source directly establishes the proposed cause.
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 “Temperature inversion” explanation; it does not assign the cause by itself.
Los Angeles-Long Beach-Anaheim, CA
Monitor comparison window 1999-01-15 to 1999-01-21 · PM2.5 target: Azusa
64% above nearby days
23% lower than surrounding days
7% lower than surrounding days
64% higher than surrounding days
7% lower than surrounding days
29% higher than surrounding days
9% higher than surrounding days
21% higher than surrounding days
11% lower than surrounding days
About the same as surrounding days
About the same as surrounding days
About the same as surrounding days
23% 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.
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 |
|---|---|---|---|---|
| Los Angeles-Long Beach-Anaheim, CA | 81.3 µg/m³ | 3.7 | Jan 18, 1999 | #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 Los Angeles/Oxnard via Iowa Environmental Mesonet
The NWS discussion preceding the January 18 measurement documented a roughly 1,500-foot inversion over the Los Angeles area.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The discussion establishes a trapping layer but does not apportion the particles or independently quantify its effect on the next day's PM2.5.
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.