2001-07-17 Atlanta-Sandy Springs-Roswell, GA regional secondary-particle pollution episode
July 17, 2001
Daily PM2.5 peaked at 77.4 µg/m³ near Atlanta across
1 covered metro home to 6.4 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 ≈ 2.9 cigarettes.
Across everyone living in the affected metros, that is the collective equivalent of
≈ 19 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”
A peer-reviewed daily source-apportionment table for Atlanta's Jefferson Street monitor reports secondary sulfate as the largest identified contributor on July 17, 2001. Secondary sulfate, nitrate, and ammonium together account for 18.72 micrograms per cubic meter, about 52 percent of measured PM2.5, and the authors conclude that secondary aerosol formation dominates Atlanta's summer samples.
PossibleThis is a transparent hypothesis supported by limited evidence or an explicit inference, not a definitive attribution.
Journal of the Air & Waste Management Association · Feb 1, 2007corroborates
The study's exact-date Atlanta table apportions 35.96 micrograms per cubic meter on July 17, 2001. Secondary sulfate is the largest identified contributor at 13.69, with secondary nitrate and ammonium bringing the secondary inorganic contribution to 18.72 micrograms per cubic meter, about 52 percent of measured PM2.5. The authors conclude that secondary aerosol formation dominates Atlanta summertime PM2.5.
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 study's exact-date Atlanta table apportions 35.96 micrograms per cubic meter on July 17, 2001. Secondary sulfate is the largest identified contributor at 13.69, with secondary nitrate and ammonium bringing the secondary inorganic contribution to…
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
77.4 µg/m³ near Atlanta
EPA monitors recorded the event across 1 metro.
The peak daily concentration is roughly 3.5 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 “Regional secondary PM” explanation; it does
not assign the cause by itself.
Observed context · not attribution
Atlanta-Sandy Springs-Roswell, GA
Monitor comparison window 2001-07-14 to 2001-07-20 · PM2.5 target: E. Rivers School
6 pollutant signals
Fine-particle pulse34.2 µg/m³
0% above nearby days
PM1047 µg/m³
12% higher than surrounding days
Pollution fingerprint
Metro-wide median of reporting monitors
PM2.534.2 µg/m³
About the same as surrounding days
Before30 µg/m³ Event34.2 µg/m³ After34.9 µg/m³
PM1047 µg/m³
12% higher than surrounding days
Before34 µg/m³ Event47 µg/m³ After50 µg/m³
Carbon monoxide0.810 ppm
About the same as surrounding days
Before0.602 ppm Event0.810 ppm After0.795 ppm
Nitrogen dioxide8.96 ppb
About the same as surrounding days
Before6.13 ppb Event8.96 ppb After16.6 ppb
Sulfur dioxide1.32 ppb
19% lower than surrounding days
Before1.63 ppb Event1.32 ppb After1.79 ppb
Ozone0.044 ppm
About the same as surrounding days
Before0.041 ppm Event0.044 ppm After0.044 ppm
Atmospheric conditions
Nearby EPA weather observations
Humidity75.6 %
About the same as surrounding days
Pressure988 Millibars
About the same as surrounding days
Temperature76.2 °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
Source Apportionment of Daily Fine Particulate Matter at Jefferson Street, Atlanta, GA, during Summer and Winter
Journal of the Air & Waste Management Association
The study's exact-date Atlanta table apportions 35.96 micrograms per cubic meter on July 17, 2001. Secondary sulfate is the largest identified contributor at 13.69, with secondary nitrate and ammonium bringing the secondary inorganic contribution to 18.72 micrograms per cubic meter, about 52 percent of measured PM2.5. The authors conclude that secondary aerosol formation dominates Atlanta summertime PM2.5.
What it supports
A retained source supports the attributed cause for the stated place and dates.
Limit
The source-apportionment monitor is at Jefferson Street; the catalog event uses a metro-level observed peak that may come from a different monitor.
What remains uncertain
The classification names a regional secondary-particle mechanism, not a unique emitting facility or precursor source.
The source-apportionment monitor is at Jefferson Street; the catalog event uses a metro-level observed peak that may come from a different monitor.
This evidence applies only to Atlanta; Knoxville, Chattanooga, and Greenville remain unresolved within the original observation cluster.
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.