Stagnant pollution mixture Possible
Daily PM2.5 peaked at 44.8 µg/m³ near Winston across 3 covered metros home to 4.4 million people. 1 metro placed a top-ten day in their record.
Counting the extra PM2.5 above each metro's typical day, this event handed the average affected resident ≈ 1.5 cigarettes. Across everyone living in the affected metros, that is the collective equivalent of ≈ 6.7 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
North Carolina DAQ's exact-date trajectory analysis identifies North Carolina and South Carolina as the primary source regions for high PM2.5 at the Davidson monitor on August 13, 1999, while the program's composite analysis describes short trajectories, regional stagnation, and recirculation. The Atlanta Supersite independently places August 12 inside a prolonged Southeast high-pressure stagnation regime with local carbonaceous accumulation and transported sulfate. Together they support a stagnant mixed-pollution mechanism for the Charlotte and Triad place units at possible confidence.
Possible This is a transparent hypothesis supported by limited evidence or an explicit inference, not a definitive attribution.
North Carolina Division of Air Quality · Feb 26, 2004 hypothesis
For August 13, 1999, North Carolina DAQ meteorologists identified North Carolina and South Carolina as the primary source regions for elevated PM2.5 at the Davidson County monitor. The program's trajectory analysis describes short transport paths, regional stagnation, and recirculation on high-PM days.
Journal of Geophysical Research / U.S. EPA Atlanta Supersite · Apr 16, 2003 hypothesis
The Atlanta Supersite places August 12 inside a prolonged August 3-23 Southeast high-pressure stagnation regime and documents a mixed aerosol pattern involving local carbonaceous accumulation and transported sulfate.
See every retained source and limitation ↓
Sequence: Winston (Aug 13) → Greensboro (Aug 13) → Charlotte (Aug 13)
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
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 3 metros. The peak daily concentration is roughly 2.0 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 “Stagnant pollution mixture” explanation; it does not assign the cause by itself.
Durham-Chapel Hill, NC
Monitor comparison window 1999-08-09 to 1999-08-16
57% above nearby days
7 calm hours observed
Haze reported during 26 hours
57% higher than surrounding days
About the same as surrounding days
28% higher than surrounding days
16 kt maximum · 7 of 48 observed hours calm
6 observed hours at or below 3 miles
68°F to 97°F
94% maximum
Charlotte-Concord-Gastonia, NC-SC
Monitor comparison window 1999-08-09 to 1999-08-16
29% above nearby days
33% higher than surrounding days
29% higher than surrounding days
33% higher than surrounding days
65% higher than surrounding days
42% higher than surrounding days
26% higher than surrounding days
Columbia, SC
Monitor comparison window 1999-08-09 to 1999-08-16 · PM2.5 target: PARKLANE
44% above nearby days
12 calm hours observed
Haze reported during 19 hours
44% higher than surrounding days
26% higher than surrounding days
13% higher than surrounding days
31% higher than surrounding days
About the same as surrounding days
19% higher than surrounding days
11 kt maximum · 12 of 43 observed hours calm
3 observed hours at or below 3 miles
78°F to 101°F
90% maximum
Greensboro-High Point, NC
Monitor comparison window 1999-08-09 to 1999-08-16
25% above nearby days
25% higher than surrounding days
20% higher than surrounding days
Winston-Salem, NC
Monitor comparison window 1999-08-09 to 1999-08-16 · PM2.5 target: Lexington water tower
29% above nearby days
About the same as surrounding days
37% higher than surrounding days
29% higher than surrounding days
37% higher than surrounding days
38% higher than surrounding days
49% higher than surrounding days
16% higher than surrounding days
19% higher than surrounding days
About the same as 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 |
|---|---|---|---|---|
| Winston-Salem, NC | 44.8 µg/m³ | 2.0 | Aug 13, 1999 | #15 of its top days |
| Greensboro-High Point, NC | 44.7 µg/m³ | 2.0 | Aug 13, 1999 | #10 of its top days |
| Charlotte-Concord-Gastonia, NC-SC | 44 µg/m³ | 2.0 | Aug 13, 1999 | #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.
North Carolina Division of Air Quality
For August 13, 1999, North Carolina DAQ meteorologists identified North Carolina and South Carolina as the primary source regions for elevated PM2.5 at the Davidson County monitor. The program's trajectory analysis describes short transport paths, regional stagnation, and recirculation on high-PM days.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The exact trajectory endpoints were Catawba and Davidson Counties. This evidence does not cover Durham or directly measure Columbia, and it identifies source regions and accumulation conditions rather than one emitting source.
Journal of Geophysical Research / U.S. EPA Atlanta Supersite
The Atlanta Supersite places August 12 inside a prolonged August 3-23 Southeast high-pressure stagnation regime and documents a mixed aerosol pattern involving local carbonaceous accumulation and transported sulfate.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The composition measurements were made in Atlanta. For the North Carolina metros this source corroborates the regional meteorological regime, not the local particle mixture.
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.