Mixed causes EPA monitor flag
Daily PM2.5 peaked at 102.8 µg/m³ near Phoenix across 2 covered metros home to 6.3 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 ≈ 3.6 cigarettes. Across everyone living in the affected metros, that is the collective equivalent of ≈ 22 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
Official evidence identifies fireworks across the Arizona metros and separately identifies an inversion trapping emissions at the West Phoenix monitor. Both are retained as material components of the 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 · Jan 1, 2024 official
EPA AQS carries an agency-submitted request-exclusion qualifier (RH, Fireworks.) on 144 PM2.5 samples in the candidate CBSA on 2024-01-01, identifying fireworks 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 · Jan 1, 2024 official
EPA AQS carries an agency-submitted informational qualifier (IH, Fireworks.) on 10 PM2.5 samples in the candidate CBSA on 2024-01-01, identifying fireworks as affecting the samples. An informational qualifier is an official submitting-agency flag, not a request for exclusion or an EPA-concurred determination.
Maricopa Association of Governments official
The regional government air-quality update identifies the West Phoenix monitor's January 1, 2024 PM2.5 episode and describes an inversion trapping emissions as a contributing condition.
See every retained source and limitation ↓
Sequence: Phoenix (Jan 1) → Tucson (Jan 1)
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 (RH, Fireworks.) on 144 PM2.5 samples in the candidate CBSA on 2024-01-01, identifying fireworks as affecting the samples. A request-exclusion qualifier is an official submitting-agency…
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 2 metros. The peak daily concentration is roughly 4.7 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 “Mixed causes” explanation; it does not assign the cause by itself.
Phoenix-Mesa-Scottsdale, AZ
Monitor comparison window 2023-12-29 to 2024-01-04 · PM2.5 target: WEST PHOENIX
290% above nearby days
About the same as surrounding days
57% higher than surrounding days
290% higher than surrounding days
57% higher than surrounding days
45% higher than surrounding days
16% higher than surrounding days
34% higher than surrounding days
32% lower than surrounding days
10% higher than surrounding days
About the same as surrounding days
About the same as surrounding days
About the same as surrounding days
Tucson, AZ
Monitor comparison window 2023-12-29 to 2024-01-04 · PM2.5 target: ROSE ELEMENTARY
197% above nearby days
About the same as surrounding days
75% higher than surrounding days
197% higher than surrounding days
75% higher than surrounding days
About the same as surrounding days
7% higher than surrounding days
20% higher than surrounding days
About the same as surrounding days
7% lower than surrounding days
About the same as 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 | 102.8 µg/m³ | 4.7 | Jan 1, 2024 | #7 of its top days |
| Tucson, AZ | 25.8 µg/m³ | 1.2 | Jan 1, 2024 | #19 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 RH — Fireworks.
EPA AQS carries an agency-submitted request-exclusion qualifier (RH, Fireworks.) on 144 PM2.5 samples in the candidate CBSA on 2024-01-01, identifying fireworks 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 IH — Fireworks.
EPA AQS carries an agency-submitted informational qualifier (IH, Fireworks.) on 10 PM2.5 samples in the candidate CBSA on 2024-01-01, identifying fireworks as affecting the samples. An informational qualifier is an official submitting-agency flag, not a request for exclusion or an EPA-concurred determination.
Air Quality Update meeting minutes
The regional government air-quality update identifies the West Phoenix monitor's January 1, 2024 PM2.5 episode and describes an inversion trapping emissions as a contributing condition.
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.