Mixed causes Officially verified
Daily PM2.5 peaked at 107.5 µg/m³ near San Francisco across 1 covered metro home to 4.6 million people.
Counting the extra PM2.5 above each metro's typical day, this event handed the average affected resident ≈ 4.4 cigarettes. Across everyone living in the affected metros, that is the collective equivalent of ≈ 20 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 Bay Area Air Quality Management District staff report explicitly labels January 7, 2001 as a Top 20 Bay Area PM2.5 day caused by transport and local emissions. Exact-date EPA speciation independently shows that secondary inorganic aerosol dominated the reconstructed mass at the covered San Francisco monitor.
Officially verified A directly applicable government determination or qualified scientific attribution product identifies this cause.
Bay Area Air Quality Management District via California Air Resources Board · Nov 1, 2019 official
The Air District's official Top 20 chart identifies the Bay Area PM2.5 peak on January 7, 2001 as caused by transport and local emissions.
U.S. EPA AirData / Chemical Speciation Network hypothesis
Exact-peak-date CSN composition produced secondary-inorganic shares of reconstructed mass as follows: san_francisco 74.0%.
Inference, not direct attribution
Same-day particle composition: EPA speciation observations on the peak date show a majority secondary-inorganic aerosol signature. This supports a regional secondary-particle mechanism, not a single emitting source.
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 Air District's official Top 20 chart identifies the Bay Area PM2.5 peak on January 7, 2001 as caused by transport and local emissions.
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 1 metro. The peak daily concentration is roughly 4.9 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 “Mixed causes” explanation; it does not assign the cause by itself.
San Francisco-Oakland-Hayward, CA
Monitor comparison window 2001-01-04 to 2001-01-10 · PM2.5 target: Livermore
41% above nearby days
No adjacent-day comparison
41% higher than surrounding days
No adjacent-day comparison
About the same as surrounding days
About the same as surrounding days
About the same as 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 |
|---|---|---|---|---|
| San Francisco-Oakland-Hayward, CA | 107.5 µg/m³ | 4.9 | Jan 7, 2001 | #11 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.
Bay Area Air Quality Management District via California Air Resources Board
The Air District's official Top 20 chart identifies the Bay Area PM2.5 peak on January 7, 2001 as caused by transport and local emissions.
An official source supports the attributed cause for the stated place and dates.
The regional label establishes a mixed transported-and-local cause but does not identify individual sources or quantify their shares at the San Francisco monitor.
U.S. EPA AirData / Chemical Speciation Network
Exact-peak-date CSN composition produced secondary-inorganic shares of reconstructed mass as follows: san_francisco 74.0%.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
Composition supports an aerosol mechanism, not an emitting source or named regional episode.
U.S. EPA Chemical Speciation Network
On the affected place peak date, reconstructed ammonium sulfate plus ammonium nitrate represented a majority of EPA reconstructed mass at every usable sampled site. san_francisco: median 74% (minimum 74% across 1 site).
Measured composition consistent with secondary-inorganic-dominated PM2.5 at sampled monitors on the local peak date.
A component majority describes aerosol composition at sampled monitors; it does not identify an emitting source.
IAQng deterministic inference policy
Under csn_secondary_inorganic_majority_on_peak_date.v1, complete affected-place peak-date coverage with a secondary-inorganic majority creates a possible regional-secondary-PM hypothesis.
A possible regional secondary PM hypothesis; this is a reproducible inference, not direct source attribution.
A component majority describes aerosol composition at sampled monitors; it does not identify an emitting source.
Officially verified A directly applicable government determination or qualified scientific attribution product identifies this cause.
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.