Mixed causes Possible
Daily PM2.5 peaked at 50.5 µg/m³ near Washington across 2 covered metros home to 8.2 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 ≈ 14 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
NASA documented a large exact-period aerosol shroud and poor-air advisories over the eastern U.S., while explicitly describing the aerosol as a possible mixture of pollution and smoke.
Possible This is a transparent hypothesis supported by limited evidence or an explicit inference, not a definitive attribution.
NASA Earth Observatory · Jun 30, 2001 corroborates
NASA documented an aerosol shroud over parts of Virginia and Maryland on June 28 and reported poor-air-quality advisories in Washington and Baltimore from June 26-29. It supports regional event grouping for the Virginia Beach-Washington observations, not a source attribution.
NASA Earth Observatory · Jun 28, 2001 hypothesis
NASA imaged an aerosol shroud over much of the eastern United States during June 26-29 poor-air advisories and described it as potentially containing pollution and smoke.
See every retained source and limitation ↓
Sequence: Washington (Jun 29) → Virginia Beach (Jun 29)
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
The catalog has a grouping hypothesis, but no retained source directly establishes the proposed cause.
Air-mass movement
NASA imaged an aerosol shroud over much of the eastern United States during June 26-29 poor-air advisories and described it as potentially containing pollution and smoke.
View transport source ↗Local response
EPA monitors recorded the event across 2 metros. The peak daily concentration is roughly 2.3 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 “Mixed causes” explanation; it does not assign the cause by itself.
Virginia Beach-Norfolk-Newport News, VA-NC
Monitor comparison window 2001-06-26 to 2001-07-02 · PM2.5 target: MONITORS ON GROUND IN FENCED COMPOUND AT O.F. SMITH STADIUM
166% above nearby days
45% higher than surrounding days
166% higher than surrounding days
45% higher than surrounding days
19% higher than surrounding days
65% higher than surrounding days
66% higher than surrounding days
Washington-Arlington-Alexandria, DC-VA-MD-WV
Monitor comparison window 2001-06-26 to 2001-07-02 · PM2.5 target: RIVER TERRACE
93% above nearby days
About the same as surrounding days
93% higher than surrounding days
About the same as surrounding days
47% higher than surrounding days
54% higher than surrounding days
17% higher than surrounding days
About the same as surrounding days
About the same as 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 |
|---|---|---|---|---|
| Washington-Arlington-Alexandria, DC-VA-MD-WV | 50.5 µg/m³ | 2.3 | Jun 29, 2001 | #24 of its top days |
| Virginia Beach-Norfolk-Newport News, VA-NC | 43.4 µg/m³ | 2.0 | Jun 29, 2001 | #20 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.
NASA Earth Observatory
NASA imaged an aerosol shroud over much of the eastern United States during June 26-29 poor-air advisories and described it as potentially containing pollution and smoke.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The source does not separate smoke from anthropogenic aerosol or prove surface attribution at every affected metro.
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.