U.S. EPA AirData / Chemical Speciation Network hypothesis
EPA AirData daily_SPEC 2005
Exact-peak-date CSN composition estimated a 58.3% secondary-inorganic share of reconstructed PM2.5 mass at the covered pittsburgh monitor set.
Regional secondary PM Possible
2005-04-19 Pittsburgh, PA regional secondary-particle pollution episode
Daily PM2.5 peaked at 69.6 µg/m³ near Pittsburgh across 1 covered metro home to 2.4 million people.
Counting the extra PM2.5 above each metro's typical day, this event handed the average affected resident ≈ 2.5 cigarettes. Across everyone living in the affected metros, that is the collective equivalent of ≈ 6.1 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 basis for “Regional secondary PM”
Secondary inorganic aerosol formed a majority of reconstructed PM2.5 on the exact peak date.
Possible This is a transparent hypothesis supported by limited evidence or an explicit inference, not a definitive attribution.
Inference, not direct attribution
Same-day particle composition: EPA speciation observations on the peak date show a majority secondary-inorganic aerosol signature. This supports a regional secondary-particle mechanism, not a single emitting source.
Evidence path
From source to city
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.
- 1Inferred
Source or mechanism
Pattern-based explanation
Same-day particle composition. EPA speciation observations on the peak date show a majority secondary-inorganic aerosol signature. This supports a regional secondary-particle mechanism, not a single emitting source.
- 2Research gap
Air-mass movement
The transport path has not been reconstructed
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.
- 3Documented
Local response
69.6 µg/m³ near Pittsburgh
EPA monitors recorded the event across 1 metro. The peak daily concentration is roughly 3.2 cigarette-equivalents at the outdoor daily rate.
Inspect the pollutant and weather signals ↓
Event autopsy
What the instruments saw
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.
Pittsburgh, PA
Monitor comparison window 2005-04-16 to 2005-04-22 · PM2.5 target: Liberty
112% above nearby days
82% higher than surrounding days
Pollution fingerprint
Metro-wide median of reporting monitors PM2.5 29.5 µg/m³
112% higher than surrounding days
Before13.5 µg/m³ Event29.5 µg/m³ After13.9 µg/m³
PM10 60 µg/m³
82% higher than surrounding days
Before33 µg/m³ Event60 µg/m³ After20 µg/m³
Carbon monoxide 0.454 ppm
49% higher than surrounding days
Before0.298 ppm Event0.454 ppm After0.306 ppm
Nitrogen dioxide 29.5 ppb
70% higher than surrounding days
Before17.9 ppb Event29.5 ppb After16.8 ppb
Sulfur dioxide 6.92 ppb
About the same as surrounding days
Before6.83 ppb Event6.92 ppb After6.52 ppb
Ozone 0.036 ppm
About the same as surrounding days
Before0.041 ppm Event0.036 ppm After0.023 ppm
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.
Measured daily PM2.5 at EPA monitors, as cigarette-equivalents
EPA monitors sit in metro areas, so the colour clusters there. Land between and beyond
them is unmeasured, not clean: this map shows where IAQng has readings, not the extent
of the smoke.
—
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
Camera views near
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.
Affected metros
| Metro | Peak PM2.5 | Cigarettes, whole event | Peak day | All-time standing |
|---|---|---|---|---|
| Pittsburgh, PA | 69.6 µg/m³ | 3.2 | Apr 19, 2005 | #25 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
What supports this account
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.
EPA AirData daily_SPEC 2005
U.S. EPA AirData / Chemical Speciation Network
Exact-peak-date CSN composition estimated a 58.3% secondary-inorganic share of reconstructed PM2.5 mass at the covered pittsburgh monitor set.
What it supports
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
Limit
Composition supports an aerosol mechanism, not an emitting source or named episode.
Peak-date reconstructed aerosol composition
U.S. EPA Chemical Speciation Network
On the affected place peak date, reconstructed ammonium sulfate plus ammonium nitrate represented a majority of EPA reconstructed mass at every usable sampled site. pittsburgh: median 58% (minimum 58% across 1 site).
What it supports
Measured composition consistent with secondary-inorganic-dominated PM2.5 at sampled monitors on the local peak date.
Limit
A component majority describes aerosol composition at sampled monitors; it does not identify an emitting source.
Secondary-inorganic component majority
IAQng deterministic inference policy
Under csn_secondary_inorganic_majority_on_peak_date.v1, complete affected-place peak-date coverage with a secondary-inorganic majority creates a possible regional-secondary-PM hypothesis.
What it supports
A possible regional secondary PM hypothesis; this is a reproducible inference, not direct source attribution.
Limit
A component majority describes aerosol composition at sampled monitors; it does not identify an emitting source.
What remains uncertain
- Mechanism only; capped at possible.
Status and provenance
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.