Regional secondary PM Likely
Daily PM2.5 peaked at 119 µg/m³ near Fresno across 2 covered metros home to 2.0 million people.
Counting the extra PM2.5 above each metro's typical day, this event handed the average affected resident ≈ 4.1 cigarettes. Across everyone living in the affected metros, that is the collective equivalent of ≈ 8.3 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
A peer-reviewed CRPAQS analysis identifies the supplied dates as part of a San Joaquin Valley episode dominated by ammonium nitrate under persistent stability.
Likely Applicable reporting or a supported inference makes this the best current causal explanation.
Journal of the Air & Waste Management Association · Apr 1, 2006 corroborates
The study defines a December 14, 1999-January 1, 2000 episode, reports a 129 microgram-per-cubic-meter peak, and finds ammonium nitrate dominated the northern Valley under unusually strong stability.
See every retained source and limitation ↓
Sequence: Stockton (Dec 17) → Fresno (Dec 18)
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 study defines a December 14, 1999-January 1, 2000 episode, reports a 129 microgram-per-cubic-meter peak, and finds ammonium nitrate dominated the northern Valley under unusually strong stability.
View supporting source ↗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 5.4 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 “Regional secondary PM” explanation; it does not assign the cause by itself.
Stockton-Lodi, CA
Monitor comparison window 1999-12-14 to 1999-12-21 · PM2.5 target: Stockton-Hazelton
0% above nearby days
11% higher than surrounding days
5% higher than surrounding days
About the same as surrounding days
5% higher than surrounding days
About the same as surrounding days
About the same as surrounding days
39% lower than surrounding days
About the same as surrounding days
About the same as surrounding days
11% higher than surrounding days
Fresno, CA
Monitor comparison window 1999-12-14 to 1999-12-21 · PM2.5 target: 3425 N FIRST ST, FRESNO
30% above nearby days
29% higher than surrounding days
14% lower than surrounding days
30% higher than surrounding days
14% lower than surrounding days
About the same as surrounding days
28% higher than surrounding days
About the same as surrounding days
About the same as surrounding days
About the same as surrounding days
29% higher 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 |
|---|---|---|---|---|
| Fresno, CA | 119 µg/m³ | 5.4 | Dec 18, 1999 | #19 of its top days |
| Stockton-Lodi, CA | 78 µg/m³ | 3.5 | Dec 17, 1999 | #25 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.
Journal of the Air & Waste Management Association
The study defines a December 14, 1999-January 1, 2000 episode, reports a 129 microgram-per-cubic-meter peak, and finds ammonium nitrate dominated the northern Valley under unusually strong stability.
A retained source supports the attributed cause for the stated place and dates.
The abstract supports the regional episode and dominant component but not a single named emitting source.
Likely Applicable reporting or a supported inference makes this the best current causal explanation.
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.