Temperature inversion Possible
Daily PM2.5 peaked at 68.3 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 ≈ 2.5 cigarettes. Across everyone living in the affected metros, that is the collective equivalent of ≈ 16 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
Atlanta's four-monitor PM2.5 rise coincided with elevated PM10, carbon monoxide, and sulfur dioxide. The nearby local airport was calm during 54 percent of the event hours and at or below three knots during 67 percent. A Peachtree City upper-air profile 46.8 kilometers away measured a strong morning surface inversion, with temperature rising 13.7 degrees Celsius through the lowest 559 meters. Temperature-inversion accumulation is therefore a possible atmospheric mechanism, while the evidence does not apportion the trapped mixture among traffic, combustion, industry, or secondary aerosol.
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 · Jan 16, 1999 hypothesis
ATLANTA/DEKALB recorded calm conditions during 54% of event hours and wind at or below three knots during 67%. Observations included 2 mist hours, minimum visibility 5 miles.
U.S. EPA AirData hypothesis
EPA records show median daily PM2.5 of 45 micrograms per cubic meter across 4 monitors, 2.804 times the adjacent-window level. Co-pollutants at least 10% above the adjacent-window comparison were pm10 (1.314x), carbon monoxide (1.557x), sulfur dioxide (1.429x). Other measured comparisons were nitrogen dioxide (0.915x).
NOAA IGRA / University of Wyoming upper-air archive · Jan 16, 1999 hypothesis
The Peachtree City 12 UTC upper-air profile measured a strong surface-based inversion: temperature rose 13.7 degrees Celsius from -4.1 degrees at 244 meters elevation to 9.6 degrees at 803 meters. The preceding evening profile had only a slight 0.8-degree surface temperature rise, so the evidence supports a strong morning retention layer rather than a continuously strong inversion across the whole day.
Inference, not direct attribution
local_stagnation_plus_regional_surface_inversion_possible.v1: This inference is recorded in the catalog methodology.
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
local_stagnation_plus_regional_surface_inversion_possible.v1. This inference is recorded in the catalog methodology.
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 1 metro. The peak daily concentration is roughly 3.1 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 “Temperature inversion” explanation; it does not assign the cause by itself.
Atlanta-Sandy Springs-Roswell, GA
Monitor comparison window 1999-01-13 to 1999-01-19 · PM2.5 target: Doraville
136% above nearby days
13 calm hours observed
0 hours at or below 3 miles
136% higher than surrounding days
15% higher than surrounding days
80% higher than surrounding days
About the same as surrounding days
43% higher than surrounding days
About the same as surrounding days
9 kt maximum · 13 of 24 observed hours calm
0 observed hours at or below 3 miles
30°F to 61°F
96% maximum
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 |
|---|---|---|---|---|
| Atlanta-Sandy Springs-Roswell, GA | 68.3 µg/m³ | 3.1 | Jan 16, 1999 | #7 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.
National Weather Service surface observations via Iowa Environmental Mesonet
ATLANTA/DEKALB recorded calm conditions during 54% of event hours and wind at or below three knots during 67%. Observations included 2 mist hours, minimum visibility 5 miles.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
A single surface station documents ventilation and obscuration but not the full vertical profile or particle source mix.
NOAA IGRA / University of Wyoming upper-air archive
The Peachtree City 12 UTC upper-air profile measured a strong surface-based inversion: temperature rose 13.7 degrees Celsius from -4.1 degrees at 244 meters elevation to 9.6 degrees at 803 meters. The preceding evening profile had only a slight 0.8-degree surface temperature rise, so the evidence supports a strong morning retention layer rather than a continuously strong inversion across the whole day.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
Peachtree City is 46.8 kilometers from the Atlanta analysis point, so the sounding describes the regional lower atmosphere rather than conditions at the PM2.5 monitors themselves.
U.S. EPA AirData
EPA records show median daily PM2.5 of 45 micrograms per cubic meter across 4 monitors, 2.804 times the adjacent-window level. Co-pollutants at least 10% above the adjacent-window comparison were pm10 (1.314x), carbon monoxide (1.557x), sulfur dioxide (1.429x). Other measured comparisons were nitrogen dioxide (0.915x).
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The pollution fingerprint does not uniquely apportion traffic, heating, industry, wood combustion, or secondary aerosol.
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.