Stagnant pollution mixture Possible
Daily PM2.5 peaked at 52.8 µg/m³ near Indianapolis across 1 covered metro home to 2.2 million people.
Counting the extra PM2.5 above each metro's typical day, this event handed the average affected resident ≈ 1.8 cigarettes. Across everyone living in the affected metros, that is the collective equivalent of ≈ 3.9 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
Exact-date NWS discussions document strong ridging, surface high pressure, deep subsidence, clear skies, and calm-to-light wind. Indiana and EPA observations independently show a metro-wide particle episode with elevated combustion tracers. The evidence supports accumulation of an unidentified anthropogenic mixture rather than a named exceptional source.
Possible This is a transparent hypothesis supported by limited evidence or an explicit inference, not a definitive attribution.
National Weather Service Indianapolis via Iowa Environmental Mesonet · Nov 7, 2020 corroborates
NWS Indianapolis documented strong ridging aloft and high pressure at the surface, with calm to six-knot overnight winds followed by light flow during the event day.
U.S. EPA AirData hypothesis
EPA observations show elevated PM2.5 across five monitoring series, with higher PM10, carbon monoxide, and nitrogen dioxide than the following comparison window and lower event-window wind than the preceding period.
Indiana Department of Environmental Management hypothesis
Indiana's official annual table lists elevated November 7 PM2.5 across Indianapolis-area monitors, including 52.8 micrograms per cubic meter at West 18th Street and 36.4 at I-70.
Inference, not direct attribution
documented_ridge_weak_flow_multimonitor_accumulation.v1: This inference is recorded in the catalog methodology.
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
NWS Indianapolis documented strong ridging aloft and high pressure at the surface, with calm to six-knot overnight winds followed by light flow during the event day.
View supporting source ↗Air-mass movement
NOAA HYSPLIT modeled 12 backward paths across multiple arrival times and heights. They show where the air traveled, not which source polluted it.
Explore the modeled paths ↓Local response
EPA monitors recorded the event across 1 metro. The peak daily concentration is roughly 2.4 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.
Indianapolis-Carmel-Anderson, IN
Monitor comparison window 2020-11-04 to 2020-11-10 · PM2.5 target: Indpls- W. 18th St./ Ernie Pyle School 90
109% above nearby days
20% lower than surrounding days
69% higher than surrounding days
109% higher than surrounding days
69% higher than surrounding days
60% higher than surrounding days
82% higher than surrounding days
10% lower than surrounding days
About the same as surrounding days
About the same as surrounding days
About the same as surrounding days
About the same as surrounding days
20% 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.
Air-mass reconstruction
NOAA HYSPLIT traces the air arriving at each metro backward for 72 hours. Multiple arrival times and heights show whether the broad path is stable—or sensitive to assumptions about when and where the polluted layer arrived.
Air arriving near Indianapolis-Carmel-Anderson
Peak-date arrivals · 2020-11-07
Each line begins 72 hours before arrival and points toward the metro. Color represents arrival height; repeated lines represent 00, 06, 12, and 18 UTC arrival assumptions. A path can show where the air traveled, but it cannot prove which source added the PM2.5.
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 |
|---|---|---|---|---|
| Indianapolis-Carmel-Anderson, IN | 52.8 µg/m³ | 2.4 | Nov 7, 2020 | #16 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.
National Weather Service Indianapolis via Iowa Environmental Mesonet
NWS Indianapolis documented strong ridging aloft and high pressure at the surface, with calm to six-knot overnight winds followed by light flow during the event day.
A retained source supports the attributed cause for the stated place and dates.
The discussion documents poor-dispersion ingredients but does not identify a dominant source or explicitly attribute the PM2.5 measurement.
U.S. EPA AirData
EPA observations show elevated PM2.5 across five monitoring series, with higher PM10, carbon monoxide, and nitrogen dioxide than the following comparison window and lower event-window wind than the preceding period.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The fingerprint supports broad accumulation but cannot distinguish local emissions, regional secondary aerosol, smoke, or another mixture.
Indiana Department of Environmental Management
Indiana's official annual table lists elevated November 7 PM2.5 across Indianapolis-area monitors, including 52.8 micrograms per cubic meter at West 18th Street and 36.4 at I-70.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The table confirms a broad exact-date episode but does not assign a cause.
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.