Stagnant pollution mixture Possible
Daily PM2.5 peaked at 105.7 µg/m³ near Bakersfield across 1 covered metro home to 922,529 people.
Counting the extra PM2.5 above each metro's typical day, this event handed the average affected resident ≈ 8.3 cigarettes. Across everyone living in the affected metros, that is the collective equivalent of ≈ 7.6 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
California's PM2.5 plan places February 4-5 inside the plotted Bakersfield November 2000-February 2001 episode and describes that winter as persistent high pressure with light-to-calm winds and stable stagnation. CRPAQS source apportionment identifies ammonium nitrate and biomass burning as the largest winter components. The closest intensive composition period ended February 3, so the conclusion is capped at possible.
Possible This is a transparent hypothesis supported by limited evidence or an explicit inference, not a definitive attribution.
California Air Resources Board · Dec 20, 2012 hypothesis
CARB's plan plots Bakersfield PM2.5 through February 2001 and says the 2000-01 winter was characterized by persistent high pressure, light-to-calm winds, low mixing, and stable stagnation. Its CRPAQS summary reports ammonium nitrate and biomass burning as the largest winter mixture components.
Inference, not direct attribution
documented_same_season_particle_episode_mixed_accumulation_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
documented_same_season_particle_episode_mixed_accumulation_possible.v1. This inference is recorded in the catalog methodology.
Air-mass movement
NOAA HYSPLIT modeled 12 backward paths across multiple arrival times and heights. They show where the air traveled, not which source polluted it.
Explore the modeled paths ↓Local response
EPA monitors recorded the event across 1 metro. The peak daily concentration is roughly 4.8 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.
Bakersfield, CA
Monitor comparison window 2001-02-01 to 2001-02-08 · PM2.5 target: Bakersfield-California
70% above nearby days
15% lower than surrounding days
13% higher than surrounding days
70% higher than surrounding days
13% higher than surrounding days
About the same as surrounding days
15% higher than surrounding days
38% higher than surrounding days
About the same as surrounding days
About the same as surrounding days
11% higher than surrounding days
15% 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.
Air-mass reconstruction
NOAA HYSPLIT traces the air arriving at each metro backward for 72 hours. Multiple arrival times and heights show whether the broad path is stable—or sensitive to assumptions about when and where the polluted layer arrived.
Air arriving near Bakersfield
Peak-date arrivals · 2001-02-04
Each line begins 72 hours before arrival and points toward the metro. Color represents arrival height; repeated lines represent 00, 06, 12, and 18 UTC arrival assumptions. A path can show where the air traveled, but it cannot prove which source added the PM2.5.
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 |
|---|---|---|---|---|
| Bakersfield, CA | 105.7 µg/m³ | 9.5 | Feb 4, 2001 | #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.
California Air Resources Board
CARB's plan plots Bakersfield PM2.5 through February 2001 and says the 2000-01 winter was characterized by persistent high pressure, light-to-calm winds, low mixing, and stable stagnation. Its CRPAQS summary reports ammonium nitrate and biomass burning as the largest winter mixture components.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The closest Bakersfield intensive source-apportionment period ended February 3, one day before this event began.
Journal of the Air & Waste Management Association
The CRPAQS comparison documents severe southern San Joaquin Valley winter PM2.5 under high pressure, strong inversions, low mixing heights, and light winds, including the early-January 2001 Bakersfield peak immediately preceding the broader winter period containing this event.
Context for the event; this item does not establish the attributed cause by itself.
The intensive episode ended before February 4-5, so it supports the winter mechanism and mixture but is not an exact-date source apportionment.
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.