Mixed causes
2020-12-31 2-area mixed-cause episode
Daily PM2.5 peaked at 131.9 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 ≈ 6.6 cigarettes. Across everyone living in the affected metros, that is the collective equivalent of ≈ 41 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.
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.
What happened, in the record
The supplied evidence supports a mixed episode in which holiday fireworks and fireplace smoke accumulated under an inversion.
Sequence: Phoenix (Dec 31) → Tucson (Jan 1)
Affected metros
| Metro | Peak PM2.5 | Cigarettes, whole event | Peak day | All-time standing |
|---|---|---|---|---|
| Phoenix-Mesa-Scottsdale, AZ | 131.9 µg/m³ | 8.9 | Jan 1, 2021 | #5 of its top days |
| Tucson, AZ | 27.7 µg/m³ | 1.3 | Jan 1, 2021 | #13 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.
The evidence
EPA AQS monitor qualifier 1
- AQS PM2.5 qualifier IH — Fireworks. EPA AQS carries an agency-submitted informational qualifier (IH, Fireworks.) on 5 PM2.5 samples in the candidate CBSA on 2020-12-31, 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.
Agency and publisher pages 2
- PM2.5 High Pollution Advisory extended through January 2, 2021 ADEQ extended its PM2.5 High Pollution Advisory through January 2 for Maricopa County because particle-pollutant levels were expected to exceed the federal PM2.5 health standard.
- Valley experiencing poor air quality due to fireworks and weather issues Reporting contemporaneous to the event says ADEQ attributed the Phoenix-area haze and elevated fine particulate matter to an inversion trapping smoke from holiday fireworks and fireplace use near the ground.
What remains uncertain
- An AQS informational qualifier is an official submitting-agency flag, not an EPA-concurred exclusion determination.
- The supplied evidence resolves cause but does not establish identity with another candidate episode.
Status and provenance
Research preview: this record carries research-only status and its boundaries, cause, and name may change as evidence review completes. Grouping policy common_sense_root_cause_grouping.v2. The cause label reflects the assembled evidence above, not an IAQng detection formula.