Dust EPA monitor flag
Daily PM2.5 peaked at 102.3 µg/m³ near Phoenix across 1 covered metro home to 5.2 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 ≈ 4.1 cigarettes. Across everyone living in the affected metros, that is the collective equivalent of ≈ 21 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
National Weather Service, ADEQ, and monitor-qualifier evidence consistently support windblown dust as the broad cause of the July 5 Phoenix PM2.5 episode.
EPA monitor flag EPA attached an exceptional-event qualifier to the affected samples. The agency flagged the measurement itself; no separate agency record describes the event.
U.S. EPA Air Quality System and the submitting air monitoring agency · Jul 5, 2011 official
EPA AQS carries an agency-submitted request-exclusion qualifier (RJ, High Winds.) on 101 PM2.5 samples in the candidate CBSA on 2011-07-05, identifying windblown or transported dust as affecting the samples. A request-exclusion qualifier is an official submitting-agency causal claim, but this endpoint does not establish EPA concurrence.
U.S. EPA Air Quality System and the submitting air monitoring agency · Jul 5, 2011 official
EPA AQS carries an agency-submitted informational qualifier (IJ, High Winds.) on 1 PM2.5 sample in the candidate CBSA on 2011-07-05, identifying windblown or transported dust as affecting the sample. An informational qualifier is an official submitting-agency flag, not a request for exclusion or an EPA-concurred determination.
National Weather Service Phoenix · Jul 5, 2011 official
The National Weather Service documented a large dust storm driven by thunderstorm outflow winds that entered and passed through the Phoenix metropolitan area during the evening of July 5.
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
EPA AQS carries an agency-submitted request-exclusion qualifier (RJ, High Winds.) on 101 PM2.5 samples in the candidate CBSA on 2011-07-05, identifying windblown or transported dust as affecting the samples. A request-exclusion qualifier is an…
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
Arizona DEQ's July 2011 report documents the major July 5 dust storm and PM2.5 exceedances in the Phoenix metropolitan area.
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.
Phoenix-Mesa-Scottsdale, AZ
Monitor comparison window 2011-07-02 to 2011-07-08 · PM2.5 target: MESA
241% above nearby days
About the same as surrounding days
280% higher than surrounding days
241% higher than surrounding days
280% higher than surrounding days
About the same as surrounding days
17% lower than surrounding days
About the same as surrounding days
6% lower than surrounding days
15% higher than 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.
| Metro | Peak PM2.5 | Cigarettes, whole event | Peak day | All-time standing |
|---|---|---|---|---|
| Phoenix-Mesa-Scottsdale, AZ | 102.3 µg/m³ | 4.6 | Jul 5, 2011 | #8 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.
AQS PM2.5 qualifier RJ — High Winds.
EPA AQS carries an agency-submitted request-exclusion qualifier (RJ, High Winds.) on 101 PM2.5 samples in the candidate CBSA on 2011-07-05, identifying windblown or transported dust as affecting the samples. A request-exclusion qualifier is an official submitting-agency causal claim, but this endpoint does not establish EPA concurrence.
AQS PM2.5 qualifier IJ — High Winds.
EPA AQS carries an agency-submitted informational qualifier (IJ, High Winds.) on 1 PM2.5 sample in the candidate CBSA on 2011-07-05, identifying windblown or transported dust as affecting the sample. An informational qualifier is an official submitting-agency flag, not a request for exclusion or an EPA-concurred determination.
NWS Dust Storm record, GREATER PHOENIX AREA, ARIZONA (2011-07-05)
Strong thunderstorms developed to the southeast of the greater Phoenix area during the afternoon hours on July 5.
July 5, 2011 - Historic Dust Storm across south-central Arizona
The National Weather Service documented a large dust storm driven by thunderstorm outflow winds that entered and passed through the Phoenix metropolitan area during the evening of July 5.
Monthly Air Quality Report for July 2011
Arizona DEQ's July 2011 report documents the major July 5 dust storm and PM2.5 exceedances in the Phoenix metropolitan area.
EPA monitor flag EPA attached an exceptional-event qualifier to the affected samples. The agency flagged the measurement itself; no separate agency record describes the event.
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.