Mixed causes Documented
Daily PM2.5 peaked at 149.1 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 ≈ 6.2 cigarettes. Across everyone living in the affected metros, that is the collective equivalent of ≈ 32 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 local and ADEQ evidence supports a mixed New Year's episode involving residential fireplace and fire-pit smoke together with fireworks.
Documented An agency or publisher page assesses this cause, without an agency record naming the event or an EPA qualifier on the data.
Arizona Department of Environmental Quality · Jan 1, 2024 official
ADEQ's retrospective assessment identifies the West Phoenix 149.1 µg/m3 PM2.5 value on January 1, 2020 and describes New Year's Eve/Day conditions, including a midnight fireworks peak and persistent high concentrations into the morning.
FOX 10 Phoenix · Jan 1, 2020 corroborates
Contemporaneous local reporting attributed poor Valley air to smoke from fireplaces and fire pits mixed with New Year's fireworks.
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
ADEQ's retrospective assessment identifies the West Phoenix 149.1 µg/m3 PM2.5 value on January 1, 2020 and describes New Year's Eve/Day conditions, including a midnight fireworks peak and persistent high concentrations into the morning.
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 1 metro. The peak daily concentration is roughly 6.8 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 2019-12-29 to 2020-01-04 · PM2.5 target: WEST PHOENIX
319% above nearby days
12% lower than surrounding days
39% higher than surrounding days
319% higher than surrounding days
39% higher than surrounding days
12% higher than surrounding days
16% lower than surrounding days
6% higher than surrounding days
About the same as surrounding days
7% higher than surrounding days
About the same as surrounding days
12% lower than 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 | 149.1 µg/m³ | 6.8 | Jan 1, 2020 | #3 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.
ADEQ's retrospective assessment identifies the West Phoenix 149.1 µg/m3 PM2.5 value on January 1, 2020 and describes New Year's Eve/Day conditions, including a midnight fireworks peak and persistent high concentrations into the morning.
Hikers unfazed by poor air quality on New Year's Day
Contemporaneous local reporting attributed poor Valley air to smoke from fireplaces and fire pits mixed with New Year's fireworks.
Documented An agency or publisher page assesses this cause, without an agency record naming the event or an EPA qualifier on the data.
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.