Mixed causes EPA monitor flag
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.
Why we think this happened
The supplied evidence supports a mixed episode in which holiday fireworks and fireplace smoke accumulated under an inversion.
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 · Dec 31, 2020 official
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.
Maricopa County Air Quality Department / Arizona Department of Environmental Quality · Jan 1, 2021 official
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.
FOX 10 Phoenix · Jan 1, 2021 corroborates
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.
See every retained source and limitation ↓
Sequence: Phoenix (Dec 31) → 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 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,…
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
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.
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 2020-12-28 to 2021-01-04 · PM2.5 target: WEST PHOENIX
154% above nearby days
About the same as surrounding days
10% higher than surrounding days
154% higher than surrounding days
10% higher than surrounding days
30% higher than surrounding days
8% higher than surrounding days
31% higher than surrounding days
About the same as surrounding days
12% lower 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 2020-12-28 to 2021-01-04 · PM2.5 target: GERONIMO
112% above nearby days
About the same as surrounding days
18% higher than surrounding days
112% higher than surrounding days
18% higher than surrounding days
8% higher than surrounding days
9% lower than surrounding days
45% higher than surrounding days
14% higher than surrounding days
8% 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 | 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.
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 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.
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.
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.