Stagnant pollution mixture Possible
Daily PM2.5 peaked at 86 µg/m³ near Stockton across 1 covered metro home to 816,108 people.
Counting the extra PM2.5 above each metro's typical day, this event handed the average affected resident ≈ 3.5 cigarettes. Across everyone living in the affected metros, that is the collective equivalent of ≈ 2.8 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
Exact Stockton observations document persistent fog and haze with repeated calm or light winds, while a Stockton-inclusive winter source study identifies the recurring Valley mixture. This is a local possible estimate and is not merged with Fresno solely because the dates coincide.
Possible This is a transparent hypothesis supported by limited evidence or an explicit inference, not a definitive attribution.
NOAA/NWS Stockton surface observations via Iowa Environmental Mesonet · Jan 15, 1999 hypothesis
Stockton airport observations show dense fog and visibility below one mile on January 14, followed by repeated calm or light winds and persistent haze with roughly one to three miles of visibility through much of January 15.
EPA HERO / Atmospheric Environment · Jan 1, 1992 hypothesis
A yearlong source-apportionment study that sampled urban Stockton found winter secondary ammonium nitrate contributions were the Valley's largest seasonal fine-particle contribution, within a broader mixture of urban and regional sources.
Inference, not direct attribution
local_seasonal_profile_with_exact_trapping_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_seasonal_profile_with_exact_trapping_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 3.9 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 “Stagnant pollution mixture” explanation; it does not assign the cause by itself.
Stockton-Lodi, CA
Monitor comparison window 1999-01-12 to 1999-01-18 · PM2.5 target: Stockton-Hazelton
100% above nearby days
18% lower than surrounding days
48% higher than surrounding days
100% higher than surrounding days
48% higher than surrounding days
5% higher than surrounding days
36% higher than surrounding days
About the same as surrounding days
About the same as surrounding days
About the same as surrounding days
18% 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.
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 |
|---|---|---|---|---|
| Stockton-Lodi, CA | 86 µg/m³ | 3.9 | Jan 15, 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.
NOAA/NWS Stockton surface observations via Iowa Environmental Mesonet
Stockton airport observations show dense fog and visibility below one mile on January 14, followed by repeated calm or light winds and persistent haze with roughly one to three miles of visibility through much of January 15.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
Surface observations describe dispersion and visibility but do not measure the chemical source of the PM2.5.
EPA HERO / Atmospheric Environment
A yearlong source-apportionment study that sampled urban Stockton found winter secondary ammonium nitrate contributions were the Valley's largest seasonal fine-particle contribution, within a broader mixture of urban and regional sources.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The study covers 1988-1989 rather than the January 1999 monitor-day.
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.