Regional secondary PM Likely
Daily PM2.5 peaked at 61.7 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 ≈ 2.2 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
A peer-reviewed daily source-apportionment table for Atlanta's Jefferson Street monitor identifies secondary sulfate as the largest contributor on July 10, 2001. Secondary sulfate, nitrate, and ammonium together account for 13.79 micrograms per cubic meter, about 52 percent of the measured PM2.5.
Likely Applicable reporting or a supported inference makes this the best current causal explanation.
Journal of the Air & Waste Management Association · Feb 1, 2007 corroborates
The exact-date Atlanta table reports 26.50 micrograms per cubic meter of measured PM2.5 on July 10, 2001. Secondary sulfate is the largest identified contributor at 10.86; secondary sulfate, nitrate, and ammonium total 13.79 micrograms per cubic meter, about 52 percent of the measured mass.
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 exact-date Atlanta table reports 26.50 micrograms per cubic meter of measured PM2.5 on July 10, 2001. Secondary sulfate is the largest identified contributor at 10.86; secondary sulfate, nitrate, and ammonium total 13.79 micrograms per cubic…
View supporting source ↗Air-mass movement
NOAA HYSPLIT modeled 12 backward paths across multiple arrival times and heights. They show where the air traveled, not which source polluted it.
Explore the modeled paths ↓Local response
EPA monitors recorded the event across 1 metro. The peak daily concentration is roughly 2.8 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 “Regional secondary PM” explanation; it does not assign the cause by itself.
Atlanta-Sandy Springs-Roswell, GA
Monitor comparison window 2001-07-07 to 2001-07-13 · PM2.5 target: Doraville
5% below nearby days
10% lower than surrounding days
About the same as surrounding days
10% lower than surrounding days
6% lower than surrounding days
13% higher than surrounding days
105% higher than surrounding days
About the same as 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.
Air-mass reconstruction
NOAA HYSPLIT traces the air arriving at each metro backward for 72 hours. Multiple arrival times and heights show whether the broad path is stable—or sensitive to assumptions about when and where the polluted layer arrived.
Air arriving near Atlanta-Sandy Springs-Roswell
Peak-date arrivals · 2001-07-10
Each line begins 72 hours before arrival and points toward the metro. Color represents arrival height; repeated lines represent 00, 06, 12, and 18 UTC arrival assumptions. A path can show where the air traveled, but it cannot prove which source added the PM2.5.
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 |
|---|---|---|---|---|
| Atlanta-Sandy Springs-Roswell, GA | 61.7 µg/m³ | 2.8 | Jul 10, 2001 | #11 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.
Journal of the Air & Waste Management Association
The exact-date Atlanta table reports 26.50 micrograms per cubic meter of measured PM2.5 on July 10, 2001. Secondary sulfate is the largest identified contributor at 10.86; secondary sulfate, nitrate, and ammonium total 13.79 micrograms per cubic meter, about 52 percent of the measured mass.
A retained source supports the attributed cause for the stated place and dates.
The source-apportionment monitor is at Jefferson Street; the catalog event uses a metro-level observed peak that may come from a different monitor.
Likely Applicable reporting or a supported inference makes this the best current causal explanation.
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.