Reddit r/Tucson · Oct 12, 2021 corroborates
Dust Question
A Tucson resident reported an AQI reading around 325 on October 12 and identified dust after the prior night's strong winds; replies said the dust was already settling later that day.
Dust Possible
2021-10-12 Tucson, AZ dust episode
Daily PM2.5 peaked at 27.5 µg/m³ near Tucson across 1 covered metro home to 1.1 million people.
Counting the extra PM2.5 above each metro's typical day, this event handed the average affected resident ≈ 1.0 cigarettes. Across everyone living in the affected metros, that is the collective equivalent of ≈ 1.1 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
The basis for “Dust”
A contemporaneous Tucson community report explicitly describes AQI around 325 as dust after an overnight wind event. An official Arizona atypical-event assessment independently documents the same regional high-wind and blowing-dust system continuing from October 11 into October 12, but its regulatory monitor analysis is centered north of Tucson, so the Tucson attribution remains possible rather than likely.
Possible This is a transparent hypothesis supported by limited evidence or an explicit inference, not a definitive attribution.
Arizona Department of Transportation · Jun 30, 2022 hypothesis
Arizona Dust Mitigation Plan Review and Evaluation: Atypical Event October 11, 2021
The official assessment says a low-pressure system caused widespread blowing dust across Arizona beginning late October 11 and continuing into October 12, with forecasts warning of elevated dust on both days.
Evidence path
From source to city
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.
- 1Documented
Source or mechanism
Dust
A Tucson resident reported an AQI reading around 325 on October 12 and identified dust after the prior night's strong winds; replies said the dust was already settling later that day.
View supporting source ↗ - 2Research gap
Air-mass movement
The transport path has not been reconstructed
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.
- 3Documented
Local response
27.5 µg/m³ near Tucson
EPA monitors recorded the event across 1 metro. The peak daily concentration is roughly 1.3 cigarette-equivalents at the outdoor daily rate.
Inspect the pollutant and weather signals ↓
Event autopsy
What the instruments saw
Before, during, and after the episode: nearby pollutant monitors and surface-weather stations show what changed. This context tests the “Dust” explanation; it does not assign the cause by itself.
Tucson, AZ
Monitor comparison window 2021-10-09 to 2021-10-15 · PM2.5 target: ORANGE GROVE
186% above nearby days
118% higher than surrounding days
394% higher than surrounding days
Pollution fingerprint
Metro-wide median of reporting monitors PM2.5 26.1 µg/m³
186% higher than surrounding days
Before6.03 µg/m³ Event26.1 µg/m³ After10.2 µg/m³
PM10 178 µg/m³
394% higher than surrounding days
Before20.5 µg/m³ Event178 µg/m³ After37.5 µg/m³
Carbon monoxide 0.118 ppm
36% lower than surrounding days
Before0.176 ppm Event0.118 ppm After0.217 ppm
Nitrogen dioxide 2.89 ppb
55% lower than surrounding days
Before5.56 ppb Event2.89 ppb After9.43 ppb
Sulfur dioxide 0.054 ppb
41% lower than surrounding days
Before0.071 ppb Event0.054 ppb After0.113 ppb
Ozone 0.034 ppm
About the same as surrounding days
Before0.035 ppm Event0.034 ppm After0.038 ppm
Atmospheric conditions
Nearby EPA weather observationsAbout the same as surrounding days
About the same as surrounding days
7% lower than surrounding days
118% 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.
Measured daily PM2.5 at EPA monitors, as cigarette-equivalents
EPA monitors sit in metro areas, so the colour clusters there. Land between and beyond
them is unmeasured, not clean: this map shows where IAQng has readings, not the extent
of the smoke.
—
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.
Affected metros
| Metro | Peak PM2.5 | Cigarettes, whole event | Peak day | All-time standing |
|---|---|---|---|---|
| Tucson, AZ | 27.5 µg/m³ | 1.3 | Oct 12, 2021 | #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
What supports this account
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.
Arizona Dust Mitigation Plan Review and Evaluation: Atypical Event October 11, 2021
Arizona Department of Transportation
The official assessment says a low-pressure system caused widespread blowing dust across Arizona beginning late October 11 and continuing into October 12, with forecasts warning of elevated dust on both days.
What it supports
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
Limit
The report's regulatory exceedance analysis focuses on Maricopa and Pinal counties rather than a Tucson monitor.
Dust Question
Reddit r/Tucson
A Tucson resident reported an AQI reading around 325 on October 12 and identified dust after the prior night's strong winds; replies said the dust was already settling later that day.
What it supports
A retained source supports the attributed cause for the stated place and dates.
Limit
This is community reporting rather than an official causal determination.
What remains uncertain
- The exact Tucson attribution comes from community reporting; the official dust assessment establishes the regional episode but not the Tucson PM2.5 monitor-day.
Status and provenance
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.