Regional secondary PM Likely
Daily PM2.5 peaked at 51.1 µg/m³ near Milwaukee across 1 covered metro home to 1.6 million people.
Counting the extra PM2.5 above each metro's typical day, this event handed the average affected resident ≈ 1.9 cigarettes. Across everyone living in the affected metros, that is the collective equivalent of ≈ 3.0 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
Contemporaneous reporting explicitly places Milwaukee in a persistent statewide combustion-particle alert, while the LADCO winter nitrate study identifies January 21-23 as a stagnant, nitrate-rich regional PM2.5 episode in Milwaukee.
Likely Applicable reporting or a supported inference makes this the best current causal explanation.
LADCO winter nitrate study · Oct 15, 2012 corroborates
The Milwaukee-area field study identifies January 21-23, 2009 as a winter PM2.5 episode, describes warm moist stagnant conditions and inversion-related accumulation, and finds that nitrate was predominantly aerosol-phase and largely regional during the campaign.
The Badger Herald · Jan 23, 2009 corroborates
A contemporaneous report says Wisconsin DNR issued a statewide alert for persistent fine-particle pollution, explicitly lists Milwaukee at the orange level, and attributes the particle mixture to combustion sources including power plants, factories, vehicles, and wood fires.
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 Milwaukee-area field study identifies January 21-23, 2009 as a winter PM2.5 episode, describes warm moist stagnant conditions and inversion-related accumulation, and finds that nitrate was predominantly aerosol-phase and largely regional during…
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 2.3 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 “Regional secondary PM” explanation; it does not assign the cause by itself.
Milwaukee-Waukesha-West Allis, WI
Monitor comparison window 2009-01-19 to 2009-01-25 · PM2.5 target: HARRINGTON BEACH PARK
144% above nearby days
93% higher than surrounding days
144% higher than surrounding days
93% higher than surrounding days
71% higher than surrounding days
72% lower than surrounding days
About the same as surrounding days
27% higher 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.
Views from the selected metro
Select a metro on the map to see nearby cameras. Where an operator archive reaches the event, drag the divider to compare two moments, adjust either date and time, or play the available event-day frames. Camera imagery is visual context, not an air-quality measurement or proof of smoke.
No useful public camera views are currently available within 80 km of this metro.
| Metro | Peak PM2.5 | Cigarettes, whole event | Peak day | All-time standing |
|---|---|---|---|---|
| Milwaukee-Waukesha-West Allis, WI | 51.1 µg/m³ | 2.3 | Jan 22, 2009 | #15 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.
LADCO winter nitrate study
The Milwaukee-area field study identifies January 21-23, 2009 as a winter PM2.5 episode, describes warm moist stagnant conditions and inversion-related accumulation, and finds that nitrate was predominantly aerosol-phase and largely regional during the campaign.
A retained source supports the attributed cause for the stated place and dates.
The study characterizes the episode mechanism and regional nitrate behavior but does not assign the entire daily PM2.5 mass to one source.
The Badger Herald
A contemporaneous report says Wisconsin DNR issued a statewide alert for persistent fine-particle pollution, explicitly lists Milwaukee at the orange level, and attributes the particle mixture to combustion sources including power plants, factories, vehicles, and wood fires.
A retained source supports the attributed cause for the stated place and dates.
The news report describes a mixed source category rather than performing source apportionment.
Likely Applicable reporting or a supported inference makes this the best current causal explanation.
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.