1999-08-08 Atlanta-Sandy Springs-Roswell, GA stagnant mixed-pollution episode
August 8, 1999
Daily PM2.5 peaked at 53.3 µg/m³ near Atlanta across
1 covered metro home to 6.4 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
≈ 12 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 “Stagnant pollution mixture”
An Atlanta Supersite sample spanning the evening of August 7 into the morning of August 8 directly measured a mixed fine-particle aerosol, with sulfate and ammonium alongside substantial organic carbon. The campaign analysis distinguishes transported sulfate-rich periods from low-wind accumulation of local carbonaceous emissions. This supports a possible mixed accumulation mechanism for the catalog day, not a unique source.
PossibleThis is a transparent hypothesis supported by limited evidence or an explicit inference, not a definitive attribution.
U.S. EPA Atlanta Supersite / Journal of Geophysical Research · Apr 16, 2003corroborates
The Atlanta Supersite sample from the evening of August 7 through early August 8 measured 47.8 µg/m³ of fine-particle mass. Its reported composition included substantial sulfate and ammonium together with organic carbon, and the campaign analysis associated sulfate-rich periods with regional transport while low-wind carbon peaks reflected accumulated local emissions.
Inference, not direct attribution
exact_window_field_study_mixed_accumulation_possible.v1: This inference is recorded in the catalog methodology.
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.
The Atlanta Supersite sample from the evening of August 7 through early August 8 measured 47.8 µg/m³ of fine-particle mass. Its reported composition included substantial sulfate and ammonium together with organic carbon, and the campaign analysis…
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.
Research gap
3
Local response
53.3 µg/m³ near Atlanta
EPA monitors recorded the event across 1 metro.
The peak daily concentration is roughly 2.4 cigarette-equivalents at the outdoor daily rate.
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.
Observed context · not attribution
Atlanta-Sandy Springs-Roswell, GA
Monitor comparison window 1999-08-05 to 1999-08-11 · PM2.5 target: Doraville
5 pollutant signals
Fine-particle pulse46.8 µg/m³
27% above nearby days
Carbon monoxide0.924 ppm
22% higher than surrounding days
Pollution fingerprint
Metro-wide median of reporting monitors
PM2.546.8 µg/m³
27% higher than surrounding days
Before37.4 µg/m³ Event46.8 µg/m³ After36.3 µg/m³
Carbon monoxide0.924 ppm
22% higher than surrounding days
Before0.769 ppm Event0.924 ppm After0.713 ppm
Nitrogen dioxide18.1 ppb
23% lower than surrounding days
Before26.7 ppb Event18.1 ppb After15.2 ppb
Sulfur dioxide2.46 ppb
60% lower than surrounding days
Before6.38 ppb Event2.46 ppb After6.17 ppb
Ozone0.046 ppm
15% lower than surrounding days
Before0.066 ppm Event0.046 ppm After0.053 ppm
Atmospheric conditions
Nearby EPA weather observations
Humidity79.7 %
10% higher than surrounding days
Pressure982 Millibars
About the same as surrounding days
Temperature78.3 °F
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.
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.
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.
"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.
1 retained source
Scientific research
Discrete measurements of reactive gases and fine particle mass and composition during the 1999 Atlanta Supersite Experiment
U.S. EPA Atlanta Supersite / Journal of Geophysical Research
The Atlanta Supersite sample from the evening of August 7 through early August 8 measured 47.8 µg/m³ of fine-particle mass. Its reported composition included substantial sulfate and ammonium together with organic carbon, and the campaign analysis associated sulfate-rich periods with regional transport while low-wind carbon peaks reflected accumulated local emissions.
What it supports
A retained source supports the attributed cause for the stated place and dates.
Limit
The sample covers the evening and overnight portion leading into August 8 rather than the entire local calendar day.
What remains uncertain
The field study supports a broad mixed-pollution mechanism at possible confidence, not a definitive source attribution.
Status and provenance
PossibleThis 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.