Stagnant pollution mixture Possible
Daily PM2.5 peaked at 128 µg/m³ near Fresno across 1 covered metro home to 1.2 million people.
Counting the extra PM2.5 above each metro's typical day, this event handed the average affected resident ≈ 5.3 cigarettes. Across everyone living in the affected metros, that is the collective equivalent of ≈ 6.2 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 local NWS meteorology documents dense Valley fog after a prolonged ridge pattern, while Fresno-specific winter source apportionment identifies the recurring mixed aerosol expected under those conditions. The sources do not apportion the 1999 daily value, so the estimate remains possible.
Possible This is a transparent hypothesis supported by limited evidence or an explicit inference, not a definitive attribution.
National Weather Service San Joaquin Valley/Hanford via Iowa Environmental Mesonet · Jan 14, 1999 hypothesis
On the evening before Fresno's January 15 PM2.5 sample, NWS Hanford said fog would redevelop in the San Joaquin Valley, with dense fog likely to become more extensive in the southern Valley, and described an upper-level ridge that had dominated the West Coast pattern for several weeks.
San Joaquin Valley Air Pollution Control District · Dec 20, 2012 hypothesis
The District's Fresno winter source-apportionment profile identifies a mixed fine-particle regime led by secondary ammonium nitrate, with substantial biomass-burning and motor-vehicle contributions.
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 5.8 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.
Fresno, CA
Monitor comparison window 1999-01-12 to 1999-01-18 · PM2.5 target: 3425 N FIRST ST, FRESNO
68% above nearby days
About the same as surrounding days
About the same as surrounding days
68% higher than surrounding days
About the same as surrounding days
About the same as surrounding days
65% higher than surrounding days
8% lower 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 |
|---|---|---|---|---|
| Fresno, CA | 128 µg/m³ | 5.8 | Jan 15, 1999 | #15 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 Joaquin Valley Air Pollution Control District
The District's Fresno winter source-apportionment profile identifies a mixed fine-particle regime led by secondary ammonium nitrate, with substantial biomass-burning and motor-vehicle contributions.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The profile summarizes selected 1995-1996 winter samples, not the January 15, 1999 filter.
National Weather Service San Joaquin Valley/Hanford via Iowa Environmental Mesonet
On the evening before Fresno's January 15 PM2.5 sample, NWS Hanford said fog would redevelop in the San Joaquin Valley, with dense fog likely to become more extensive in the southern Valley, and described an upper-level ridge that had dominated the West Coast pattern for several weeks.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The discussion documents a local trapping setting but does not attribute the Fresno PM2.5 measurement.
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.