Regional secondary PM Possible
Daily PM2.5 peaked at 56.7 µg/m³ near Harrisburg across 1 covered metro home to 615,361 people.
Counting the extra PM2.5 above each metro's typical day, this event handed the average affected resident ≈ 2.1 cigarettes. Across everyone living in the affected metros, that is the collective equivalent of ≈ 1.3 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 exact event is a coherent, fine-particle-dominant multi-monitor spike under exceptionally stagnant and hazy conditions. Pennsylvania's same-place, same-year composition profile is dominated by sulfate, nitrate, and ammonium. Together these support a regional secondary-aerosol episode with accumulated urban combustion tracers, but no exact-day composition sample was available.
Possible This is a transparent hypothesis supported by limited evidence or an explicit inference, not a definitive attribution.
U.S. EPA Air Quality System; IAQng event-autopsy calculation v1 hypothesis
EPA observations show median PM2.5 of 51.85 µg/m³ across three monitors, versus 9.7 before and 18.95 after. PM10 reached 66 µg/m³, making the event strongly fine-particle dominant, while carbon monoxide and nitrogen dioxide each rose to nearly three times their preceding values and sulfur dioxide remained roughly flat.
NOAA/NWS surface observations via Iowa Environmental Mesonet · Mar 12, 2003 hypothesis
The airport 3.3 km from the target PM2.5 monitor recorded calm wind during 17 hours, wind at or below 3 knots during 20 hours, haze during 7 hours, and visibility at or below 3 miles during 15 hours.
Inference, not direct attribution
multi_monitor_fingerprint_with_local_seasonal_profile_possible.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
multi_monitor_fingerprint_with_local_seasonal_profile_possible.v1. This inference is recorded in the catalog methodology.
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 2.6 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.
Harrisburg-Carlisle, PA
Monitor comparison window 2003-03-09 to 2003-03-15 · PM2.5 target: Harrisburg
174% above nearby days
230% higher than surrounding days
174% higher than surrounding days
230% higher than surrounding days
56% higher than surrounding days
85% higher than surrounding days
10% 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 Harrisburg-Carlisle
Peak-date arrivals · 2003-03-12
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 |
|---|---|---|---|---|
| Harrisburg-Carlisle, PA | 56.7 µg/m³ | 2.6 | Mar 12, 2003 | #17 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.
Pennsylvania Department of Environmental Protection
Pennsylvania DEP's 2003 Harrisburg PM2.5 composition chart assigns 30% of annual mass to sulfate, 18% to nitrate, and 15% to ammonium, with another 23% organic carbon and 4% elemental carbon. The same-place, same-year profile is dominated by secondary inorganic aerosol.
Context for the event; this item does not establish the attributed cause by itself.
The chart is an annual Harrisburg profile, not an exact-date source-apportionment sample.
NOAA/NWS surface observations via Iowa Environmental Mesonet
The airport 3.3 km from the target PM2.5 monitor recorded calm wind during 17 hours, wind at or below 3 knots during 20 hours, haze during 7 hours, and visibility at or below 3 miles during 15 hours.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The airport and pollution monitors are nearby but not co-located.
U.S. EPA Air Quality System; IAQng event-autopsy calculation v1
EPA observations show median PM2.5 of 51.85 µg/m³ across three monitors, versus 9.7 before and 18.95 after. PM10 reached 66 µg/m³, making the event strongly fine-particle dominant, while carbon monoxide and nitrogen dioxide each rose to nearly three times their preceding values and sulfur dioxide remained roughly flat.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
Pollutants were not necessarily measured at the same sites, and the fingerprint is descriptive rather than formal source apportionment.
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.