Regional secondary PM Possible
Daily PM2.5 peaked at 78.5 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 ≈ 5.5 cigarettes. Across everyone living in the affected metros, that is the collective equivalent of ≈ 13 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
EPA identifies February 9, 2000 as Pittsburgh's 79 microgram-per-cubic-meter PM2.5 day. An exact-date NWS discussion documents a strong low-level inversion, while airport observations show persistent southerly flow rather than prolonged calm. A same-day state HYSPLIT assessment places the air arriving downstream at Martinsburg over Kentucky, Ohio, and Indiana, and a Pittsburgh NETL source-apportionment study spanning October 1999 through September 2001 found transported ammonium sulfate and local secondary material were the two largest contributors. Their convergence supports regional secondary PM at possible confidence for Pittsburgh.
Possible This is a transparent hypothesis supported by limited evidence or an explicit inference, not a definitive attribution.
U.S. Environmental Protection Agency hypothesis
EPA's AQS-based Attachment 4 identifies February 9, 2000 in its Pittsburgh figure and reports peak 24-hour PM2.5 of 79 micrograms per cubic meter and peak PM10 of 150 micrograms per cubic meter.
National Weather Service Pittsburgh via Iowa Environmental Mesonet · Feb 9, 2000 hypothesis
The exact-date Pittsburgh forecast discussion says a strong low-level inversion would persist until breaking around midday over the Appalachians. Discussions the preceding evening described tremendous low-level moisture and inversion conditions.
National Weather Service surface observations via Iowa Environmental Mesonet · Feb 10, 2000 hypothesis
Across 44 observed event hours, Allegheny County Airport reported median wind of 9 knots, only four calm hours, a circular mean wind-from direction of 194 degrees, median sea-level pressure of 1016.75 millibars, and two haze-coded hours. The persistent southerly flow is more consistent with transport plus morning trapping than with a two-day calm-air episode.
Inference, not direct attribution
exact_pm_inversion_transport_plus_same_place_source_regime_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
exact_pm_inversion_transport_plus_same_place_source_regime_possible.v1. This inference is recorded in the catalog methodology.
Air-mass movement
For the same elevated-PM2.5 date, Maryland's designation attachment reports 24-hour HYSPLIT back trajectories into Martinsburg, West Virginia from Kentucky, Ohio, and Indiana at 12 UTC and from Kentucky at 20 UTC. The document reports 47.9…
View transport source ↗Local response
EPA monitors recorded the event across 1 metro. The peak daily concentration is roughly 3.6 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.
Pittsburgh, PA
Monitor comparison window 2000-02-06 to 2000-02-13 · PM2.5 target: Liberty
38% above nearby days
333% higher than surrounding days
38% higher than surrounding days
333% higher than surrounding days
118% higher than surrounding days
60% higher than surrounding days
29% higher than surrounding days
14% 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.
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 |
|---|---|---|---|---|
| Pittsburgh, PA | 78.5 µg/m³ | 6.8 | Feb 9, 2000 | #12 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
Across 44 observed event hours, Allegheny County Airport reported median wind of 9 knots, only four calm hours, a circular mean wind-from direction of 194 degrees, median sea-level pressure of 1016.75 millibars, and two haze-coded hours. The persistent southerly flow is more consistent with transport plus morning trapping than with a two-day calm-air episode.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
One airport does not measure the full metropolitan mixing layer or establish where the transported particles formed.
National Weather Service Pittsburgh via Iowa Environmental Mesonet
The exact-date Pittsburgh forecast discussion says a strong low-level inversion would persist until breaking around midday over the Appalachians. Discussions the preceding evening described tremendous low-level moisture and inversion conditions.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The forecast discussion establishes restricted vertical mixing during part of the event day but does not identify the particle source.
Maryland Department of the Environment
For the same elevated-PM2.5 date, Maryland's designation attachment reports 24-hour HYSPLIT back trajectories into Martinsburg, West Virginia from Kentucky, Ohio, and Indiana at 12 UTC and from Kentucky at 20 UTC. The document reports 47.9 micrograms per cubic meter at Martinsburg, showing a same-day Ohio River Valley transport regime extending downstream of Pittsburgh.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The trajectory destination is Martinsburg, not Pittsburgh, so this is regional transport context rather than a Pittsburgh source path.
U.S. Environmental Protection Agency
EPA's AQS-based Attachment 4 identifies February 9, 2000 in its Pittsburgh figure and reports peak 24-hour PM2.5 of 79 micrograms per cubic meter and peak PM10 of 150 micrograms per cubic meter.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The attachment confirms the exact Pittsburgh particle episode but does not attribute its source.
U.S. Department of Energy National Energy Technology Laboratory and research partners
The Pittsburgh NETL study used 202 daily samples collected from October 1999 through September 2001 with PMF and HYSPLIT. Transported secondary material dominated by ammonium sulfate contributed 47 percent on average, and local secondary material contributed 19 percent. The authors traced the transported sulfate-rich factor mainly to distant sources west and southwest in the Ohio River Valley.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The published percentages summarize the study period and are not an exact February 9 factor allocation.
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.