U.S. EPA AirData hypothesis
Charlotte multi-monitor PM2.5 fingerprint, August 7, 1999
EPA records show median daily PM2.5 of 38.5 micrograms per cubic meter across six Charlotte-area monitors, compared with 33.8 before and 23.95 after the event day.
Regional secondary PM Possible
1999-08-07 Charlotte-Concord-Gastonia, NC-SC regional secondary-particle pollution episode
Daily PM2.5 peaked at 41.1 µg/m³ near Charlotte across 1 covered metro home to 2.9 million people.
Counting the extra PM2.5 above each metro's typical day, this event handed the average affected resident ≈ 1.4 cigarettes. Across everyone living in the affected metros, that is the collective equivalent of ≈ 3.9 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”
NWS documented an extensive exact-date haze plume across the Piedmont and Midlands, official surface observations recorded nine hours of haze and weak wind, and EPA measured elevated PM2.5 across six Charlotte-area monitors. North Carolina's official recurring profile identifies sulfate-dominated regional haze as the leading summertime mechanism under these conditions. Regional secondary PM is therefore possible, but the visible plume was described as mainly aloft and no exact-day composition sample was recovered.
Possible This is a transparent hypothesis supported by limited evidence or an explicit inference, not a definitive attribution.
National Weather Service surface observations via Iowa Environmental Mesonet hypothesis
Charlotte surface conditions, August 7, 1999
Charlotte's official surface station reported haze during nine hours, with six calm hours and 11 hours at or below three knots.
National Weather Service Greenville-Spartanburg via Iowa Environmental Mesonet hypothesis
Area Forecast Discussion: extensive Piedmont haze plume
The exact-date NWS discussion described an extensive haze plume across the Piedmont and Midlands and called it a hot and dusty day, while noting that the visible plume appeared mainly aloft.
Inference, not direct attribution
multi_monitor_fingerprint_with_local_seasonal_profile_possible.v1: This inference is recorded in the catalog methodology.
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
multi_monitor_fingerprint_with_local_seasonal_profile_possible.v1. This inference is recorded in the catalog methodology.
- 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
41.1 µg/m³ near Charlotte
EPA monitors recorded the event across 1 metro. The peak daily concentration is roughly 1.9 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.
Charlotte-Concord-Gastonia, NC-SC
Monitor comparison window 1999-08-04 to 1999-08-10
27% above nearby days
About the same as surrounding days
Pollution fingerprint
Metro-wide median of reporting monitors PM2.5 38.5 µg/m³
27% higher than surrounding days
Before33.8 µg/m³ Event38.5 µg/m³ After23.9 µg/m³
Carbon monoxide 0.539 ppm
About the same as surrounding days
Before0.705 ppm Event0.539 ppm After0.422 ppm
Nitrogen dioxide 24.3 ppb
About the same as surrounding days
Before32.2 ppb Event24.3 ppb After11.5 ppb
Sulfur dioxide 3.25 ppb
8% lower than surrounding days
Before3.83 ppb Event3.25 ppb After2.68 ppb
Ozone 0.062 ppm
About the same as surrounding days
Before0.065 ppm Event0.062 ppm After0.053 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 |
|---|---|---|---|---|
| Charlotte-Concord-Gastonia, NC-SC | 41.1 µg/m³ | 1.9 | Aug 7, 1999 | #21 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.
Charlotte multi-monitor PM2.5 fingerprint, August 7, 1999
U.S. EPA AirData
EPA records show median daily PM2.5 of 38.5 micrograms per cubic meter across six Charlotte-area monitors, compared with 33.8 before and 23.95 after the event day.
What it supports
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
Limit
Co-pollutant measurements did not show a matching combustion-tracer increase, and PM10 coverage was unavailable.
North Carolina's Air Quality Multi-Pollutant Process
U.S. Environmental Protection Agency and North Carolina Division of Air Quality
The official North Carolina assessment says sulfate is the largest contributor to regional haze and that peak southeastern hazy days occur in summer under stagnant weather, high relative humidity, high temperatures, and low wind speeds.
What it supports
Context for the event; this item does not establish the attributed cause by itself.
Limit
The assessment is a recurring regional profile, not exact-day source apportionment.
Charlotte surface conditions, August 7, 1999
National Weather Service surface observations via Iowa Environmental Mesonet
Charlotte's official surface station reported haze during nine hours, with six calm hours and 11 hours at or below three knots.
What it supports
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
Limit
Surface haze does not distinguish sulfate, smoke, dust, or another fine-particle mixture.
Area Forecast Discussion: extensive Piedmont haze plume
National Weather Service Greenville-Spartanburg via Iowa Environmental Mesonet
The exact-date NWS discussion described an extensive haze plume across the Piedmont and Midlands and called it a hot and dusty day, while noting that the visible plume appeared mainly aloft.
What it supports
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
Limit
The discussion does not identify the haze composition, and its statement that the plume appeared mainly aloft weakens any direct surface attribution.
What remains uncertain
- Co-pollutant measurements did not show a matching combustion-tracer increase, and PM10 coverage was unavailable.
- Surface haze does not distinguish sulfate, smoke, dust, or another fine-particle mixture.
- The assessment is a recurring regional profile, not exact-day source apportionment.
- The discussion does not identify the haze composition, and its statement that the plume appeared mainly aloft weakens any direct surface attribution.
- The fingerprint supports a broad fine-particle haze episode but not a unique source or chemical mechanism.
- The word dusty is retained as a competing clue, not treated as proof of a dust-driven PM2.5 event.
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.