Stagnant pollution mixture Likely
Daily PM2.5 peaked at 47 µg/m³ near Omaha across 2 covered metros home to 1.8 million people.
Counting the extra PM2.5 above each metro's typical day, this event handed the average affected resident ≈ 1.6 cigarettes. Across everyone living in the affected metros, that is the collective equivalent of ≈ 2.9 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 official regional assessment directly describes the December 18, 2009 northern-plains PM2.5 episode as transport and buildup under light winds and local inversions.
Likely Applicable reporting or a supported inference makes this the best current causal explanation.
South Dakota Department of Environment and Natural Resources / U.S. EPA archive · Jan 1, 2010 official
The official assessment reports exceedances across Iowa and Omaha on December 18, attributing the regional episode to fine-particle transport and buildup encouraged by light winds and local temperature inversions; it explicitly distinguishes the event from exceptional wildfire smoke.
South Dakota Department of Environment and Natural Resources official
The state assessment identifies a December 17–22, 2009 northern-Plains PM2.5 transport event, with air stagnation contributing. It records December 18 exceedances in Iowa and at all three Omaha monitors, supporting regional event grouping and transport context; it does not identify a definitive emitting source or specifically assess Kansas City.
See every retained source and limitation ↓
Sequence: Omaha (Dec 18) → Des Moines (Dec 18)
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 official assessment reports exceedances across Iowa and Omaha on December 18, attributing the regional episode to fine-particle transport and buildup encouraged by light winds and local temperature inversions; it explicitly distinguishes the…
View supporting source ↗Air-mass movement
The state assessment identifies a December 17–22, 2009 northern-Plains PM2.5 transport event, with air stagnation contributing. It records December 18 exceedances in Iowa and at all three Omaha monitors, supporting regional event grouping and…
View transport source ↗Local response
EPA monitors recorded the event across 2 metros. The peak daily concentration is roughly 2.1 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 “Stagnant pollution mixture” explanation; it does not assign the cause by itself.
Des Moines-West Des Moines, IA
Monitor comparison window 2009-12-15 to 2009-12-21 · PM2.5 target: INDIAN HILLS JR HIGH, CLIVE IA, FRM R&P SEQ. SAMPLER (1),ROOF
66% above nearby days
163% higher than surrounding days
66% higher than surrounding days
163% higher than surrounding days
25% higher than surrounding days
37% higher than surrounding days
77% higher than surrounding days
About the same as surrounding days
About the same as surrounding days
Kansas City, MO-KS
Monitor comparison window 2009-12-15 to 2009-12-21 · PM2.5 target: Troost
112% above nearby days
9 kt maximum
Haze reported during 21 hours
112% higher than surrounding days
About the same as surrounding days
7% higher than surrounding days
49% higher than surrounding days
About the same as surrounding days
9 kt maximum · No calm observed hours
3 observed hours at or below 3 miles
28°F to 39°F
82% maximum
Omaha-Council Bluffs, NE-IA
Monitor comparison window 2009-12-15 to 2009-12-21 · PM2.5 target: COUNCIL BLUFFS, FRANKLIN SCH.
148% above nearby days
126% higher than surrounding days
148% higher than surrounding days
126% higher than surrounding days
14% higher than 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.
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 |
|---|---|---|---|---|
| Omaha-Council Bluffs, NE-IA | 47 µg/m³ | 2.1 | Dec 18, 2009 | #13 of its top days |
| Des Moines-West Des Moines, IA | 41.5 µg/m³ | 1.9 | Dec 18, 2009 | #19 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.
South Dakota Department of Environment and Natural Resources / U.S. EPA archive
The official assessment reports exceedances across Iowa and Omaha on December 18, attributing the regional episode to fine-particle transport and buildup encouraged by light winds and local temperature inversions; it explicitly distinguishes the event from exceptional wildfire smoke.
An official source supports the attributed cause for the stated place and dates.
Kansas City is not specifically documented in the cited passage.
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.