Regional secondary PM Possible
Daily PM2.5 peaked at 57.8 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 ≈ 2.0 cigarettes. Across everyone living in the affected metros, that is the collective equivalent of ≈ 4.4 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
Indianapolis-Carmel-Anderson, IN's 8-monitor PM2.5 episode reached 54.3 µg/m³, 2.007 times the adjacent-window level, with 70.8% light-wind hours. 2 exact-date regional chemistry sites agreed that 73.1% to 80.0% of reconstructed mass was ammonium sulfate plus ammonium nitrate, and the largest reconstructed mass was 53.7% of the affected-place PM2.5 value. This supports regional secondary PM at possible confidence; the chemistry sites did not directly sample the city and no particular source region is assigned.
Possible This is a transparent hypothesis supported by limited evidence or an explicit inference, not a definitive attribution.
National Weather Service surface observations via Iowa Environmental Mesonet · Mar 1, 2003 hypothesis
INDIANAPOLIS recorded calm conditions during 46% of event hours and wind at or below three knots during 71%. Observations included 2 haze hours, 22 mist hours, minimum visibility 0.75 miles.
U.S. EPA AirData hypothesis
EPA records show median daily PM2.5 of 54.3 micrograms per cubic meter across 8 monitors, 2.007 times the adjacent-window level. Co-pollutants at least 10% above the adjacent-window comparison were carbon monoxide (1.517x), nitrogen dioxide (1.212x), sulfur dioxide (1.736x).
U.S. EPA AirData Chemical Speciation · Mar 1, 2003 hypothesis
On the exact event date, 2 EPA speciation sites within 175 km agreed that ammonium sulfate plus ammonium nitrate comprised a majority of reconstructed PM2.5: 18-175-9000 (142.5 km; 73.1% secondary inorganic share; 29.2 µg/m³ reconstructed mass); ISWS CLIMATE STATION (194.4 km; 80.0% secondary inorganic share; 28.0 µg/m³ reconstructed mass).
Inference, not direct attribution
coherent_nearby_exact_date_secondary_inorganic_context.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
coherent_nearby_exact_date_secondary_inorganic_context.v1. This inference is recorded in the catalog methodology.
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 2.6 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.
Indianapolis-Carmel-Anderson, IN
Monitor comparison window 2003-02-26 to 2003-03-04 · PM2.5 target: Indpls- Washington Park/ in parking lot next to police station
137% above nearby days
11 calm hours observed
Haze reported during 2 hours
137% higher than surrounding days
27% higher than surrounding days
7% higher than surrounding days
60% higher than surrounding days
6 kt maximum · 11 of 24 observed hours calm
21 observed hours at or below 3 miles
32°F to 36°F
100% maximum
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 |
|---|---|---|---|---|
| Indianapolis-Carmel-Anderson, IN | 57.8 µg/m³ | 2.6 | Mar 1, 2003 | #11 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 surface observations via Iowa Environmental Mesonet
INDIANAPOLIS recorded calm conditions during 46% of event hours and wind at or below three knots during 71%. Observations included 2 haze hours, 22 mist hours, minimum visibility 0.75 miles.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
A single surface station documents weak ventilation and obscuration but not the full vertical profile or particle source mix.
U.S. EPA AirData Chemical Speciation
On the exact event date, 2 EPA speciation sites within 175 km agreed that ammonium sulfate plus ammonium nitrate comprised a majority of reconstructed PM2.5: 18-175-9000 (142.5 km; 73.1% secondary inorganic share; 29.2 µg/m³ reconstructed mass); ISWS CLIMATE STATION (194.4 km; 80.0% secondary inorganic share; 28.0 µg/m³ reconstructed mass).
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The chemistry sites were 142.5 km and 194.4 km from the affected-place PM2.5 target and did not directly measure the city air.
U.S. EPA AirData
EPA records show median daily PM2.5 of 54.3 micrograms per cubic meter across 8 monitors, 2.007 times the adjacent-window level. Co-pollutants at least 10% above the adjacent-window comparison were carbon monoxide (1.517x), nitrogen dioxide (1.212x), sulfur dioxide (1.736x).
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The broad multi-pollutant response does not apportion traffic, heating, industry, wood combustion, or secondary aerosol.
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.