Mixed causes Possible
Daily PM2.5 peaked at 63 µg/m³ near Phoenix across 1 covered metro home to 5.2 million people.
Counting the extra PM2.5 above each metro's typical day, this event handed the average affected resident ≈ 2.3 cigarettes. Across everyone living in the affected metros, that is the collective equivalent of ≈ 12 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
An exact-date high-pollution advisory and no-burn restrictions support holiday combustion accumulated under winter conditions, but do not isolate the source mixture.
Possible This is a transparent hypothesis supported by limited evidence or an explicit inference, not a definitive attribution.
Maricopa County Air Quality Department · Dec 23, 2020 official
Maricopa County published ADEQ's PM2.5 High Pollution Advisory for December 24 and 25 because particle levels in the Phoenix area were expected to exceed the federal health standard.
Maricopa County · Dec 23, 2020 hypothesis
Maricopa County issued a PM2.5 High Pollution Advisory and prohibited wood burning for Christmas Day 2020.
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
The catalog has a grouping hypothesis, but no retained source directly establishes the proposed cause.
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
Maricopa County published ADEQ's PM2.5 High Pollution Advisory for December 24 and 25 because particle levels in the Phoenix area were expected to exceed the federal 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 2020-12-22 to 2020-12-28 · PM2.5 target: Thirty-Third
91% above nearby days
About the same as surrounding days
25% lower than surrounding days
91% higher than surrounding days
25% lower than surrounding days
31% higher than surrounding days
8% higher than surrounding days
7% higher than surrounding days
About the same as surrounding days
21% 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 | 63 µg/m³ | 2.9 | Dec 25, 2020 | #23 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.
Maricopa County
Maricopa County issued a PM2.5 High Pollution Advisory and prohibited wood burning for Christmas Day 2020.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The advisory does not quantify the relative roles of wood smoke, other local combustion, and inversion accumulation.
Possible This is a transparent hypothesis supported by limited evidence or an explicit inference, not a definitive attribution.
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.