Stagnant pollution mixture Possible
Daily PM2.5 peaked at 82.7 µg/m³ near Riverside across 1 covered metro home to 4.7 million people.
Counting the extra PM2.5 above each metro's typical day, this event handed the average affected resident ≈ 3.0 cigarettes. Across everyone living in the affected metros, that is the collective equivalent of ≈ 14 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
Riverside's April 30 PM2.5 peak coincided with increases in carbon monoxide, nitrogen dioxide, and sulfur dioxide while block-average PM10 stayed near its surrounding-day level. The exact NWS discussion documents a strong, shallow inversion under high pressure and subsidence. A peer-reviewed Riverside study from February-May 2001 identifies the area's recurring mixture of aged basin aerosol, traffic emissions, and secondary ammonium nitrate. Together these support stagnant anthropogenic mixture at possible confidence, without assigning exact source shares or excluding every undocumented episodic source.
Possible This is a transparent hypothesis supported by limited evidence or an explicit inference, not a definitive attribution.
National Weather Service San Diego Area Forecast Discussion via Iowa Environmental Mesonet · Apr 30, 2001 hypothesis
The exact-date NWS discussion reported high pressure aloft, warming and subsidence, a marine inversion lowered to about 1,500 feet, and an inversion strength near 10 degrees Celsius. A later discussion said the strong inversion remained intact and prevented significant mixing.
U.S. EPA AirData hypothesis
EPA's certified daily record shows 82.7 micrograms per cubic meter of PM2.5 at Rubidoux on April 30 after 64.15 on the preceding sampled day and before 33.85 on the following sampled day. Across the metro's reporting monitors, median carbon monoxide rose to 0.6022 ppm, nitrogen dioxide to 26.0652 ppb, and sulfur dioxide to 1.2391 ppb, while same-day block-average PM10 remained 27 micrograms per cubic meter. The joint pattern is more compatible with accumulated fine combustion and secondary pollution than a coarse-dust event.
Inference, not direct attribution
exact_inversion_plus_fine_combustion_fingerprint_and_same_period_source_context_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
exact_inversion_plus_fine_combustion_fingerprint_and_same_period_source_context_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 3.8 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.
Riverside-San Bernardino-Ontario, CA
Monitor comparison window 2001-04-27 to 2001-05-03 · PM2.5 target: Rubidoux
64% above nearby days
11 kt maximum
0 hours at or below 3 miles
64% higher than surrounding days
About the same as surrounding days
26% higher than surrounding days
55% higher than surrounding days
97% higher than surrounding days
About the same as surrounding days
11 kt maximum · No calm observed hours
0 observed hours at or below 3 miles
46.4°F to 91.4°F
76% maximum
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 |
|---|---|---|---|---|
| Riverside-San Bernardino-Ontario, CA | 82.7 µg/m³ | 3.8 | Apr 30, 2001 | #21 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.
National Weather Service San Diego Area Forecast Discussion via Iowa Environmental Mesonet
The exact-date NWS discussion reported high pressure aloft, warming and subsidence, a marine inversion lowered to about 1,500 feet, and an inversion strength near 10 degrees Celsius. A later discussion said the strong inversion remained intact and prevented significant mixing.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The NWS discussion documents the trapping conditions but does not attribute the measured PM2.5 to a source sector.
U.S. EPA AirData
EPA's certified daily record shows 82.7 micrograms per cubic meter of PM2.5 at Rubidoux on April 30 after 64.15 on the preceding sampled day and before 33.85 on the following sampled day. Across the metro's reporting monitors, median carbon monoxide rose to 0.6022 ppm, nitrogen dioxide to 26.0652 ppb, and sulfur dioxide to 1.2391 ppb, while same-day block-average PM10 remained 27 micrograms per cubic meter. The joint pattern is more compatible with accumulated fine combustion and secondary pollution than a coarse-dust event.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
Only one FRM PM2.5 monitor sampled on the event date, and the co-pollutant comparison is descriptive rather than source apportionment.
Fine et al., Aerosol Science and Technology
A peer-reviewed mobile supersite study sampled Riverside from February through May 2001. It describes Riverside and Rubidoux as downwind receptors for aged Los Angeles Basin aerosol, identifies morning traffic emissions and low mixing heights, and notes secondary ammonium-nitrate formation associated with downwind ammonia sources. This establishes the plausible recurring anthropogenic mixture during the exact season, not the composition of April 30 itself.
Context for the event; this item does not establish the attributed cause by itself.
The study covers the same place and measurement period but does not publish an exact April 30 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.