Dust Likely
Daily PM2.5 peaked at 32.4 µg/m³ near Tucson across 1 covered metro home to 1.1 million people. 1 metro placed a top-ten day in their record.
Counting the extra PM2.5 above each metro's typical day, this event handed the average affected resident ≈ 1.2 cigarettes. Across everyone living in the affected metros, that is the collective equivalent of ≈ 1.3 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
Pima County material identifies April 9 as its highest recorded PM10 and PM2.5 day, while peer-reviewed analysis documents a widespread April 8-9 Arizona windblown-dust event with PM2.5 exceedances.
Likely Applicable reporting or a supported inference makes this the best current causal explanation.
Pima County Department of Environmental Quality presentation official
Pima County's air-quality presentation identifies April 9, 2013 as producing the highest PM10 and PM2.5 concentrations in its record and discusses the event within its particulate-matter exceptional-events material.
Atmospheric Environment · Aug 1, 2021 corroborates
The peer-reviewed case study identifies a widespread April 8-9 Arizona windblown-dust event caused by a cold front and high winds over dry soils; both PM10 and PM2.5 standards were exceeded and satellite, visibility, wind, and monitor evidence supported the dust attribution.
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
Pima County's air-quality presentation identifies April 9, 2013 as producing the highest PM10 and PM2.5 concentrations in its record and discusses the event within its particulate-matter exceptional-events material.
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 1.5 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 “Dust” explanation; it does not assign the cause by itself.
Tucson, AZ
Monitor comparison window 2013-04-06 to 2013-04-12 · PM2.5 target: CHILDREN'S PARK NCore
291% above nearby days
40% higher than surrounding days
420% higher than surrounding days
291% higher than surrounding days
420% higher than surrounding days
15% lower than surrounding days
32% lower than surrounding days
94% lower than surrounding days
7% higher than surrounding days
58% higher than surrounding days
14% lower than surrounding days
40% 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.
| Metro | Peak PM2.5 | Cigarettes, whole event | Peak day | All-time standing |
|---|---|---|---|---|
| Tucson, AZ | 32.4 µg/m³ | 1.5 | Apr 9, 2013 | #4 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.
Pima County Department of Environmental Quality presentation
Pima County's air-quality presentation identifies April 9, 2013 as producing the highest PM10 and PM2.5 concentrations in its record and discusses the event within its particulate-matter exceptional-events material.
An official source supports the attributed cause for the stated place and dates.
The accessible copy is a third-party rendering of county presentation material.
Atmospheric Environment
The peer-reviewed case study identifies a widespread April 8-9 Arizona windblown-dust event caused by a cold front and high winds over dry soils; both PM10 and PM2.5 standards were exceeded and satellite, visibility, wind, and monitor evidence supported the dust attribution.
A retained source supports the attributed cause for the stated place and dates.
The modeled core and NWS dust report emphasize Maricopa and Pinal counties, so the Tucson classification also relies on the Pima County record.
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.