1999-09-02 to 1999-09-03 Memphis, TN-MS-AR stagnant mixed-pollution episode
September 2, 1999 to September 3, 1999
Daily PM2.5 peaked at 42.9 µg/m³ near Memphis across
1 covered metro home to 1.3 million people.
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
≈ 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 “Stagnant pollution mixture”
Memphis experienced a coherent five-monitor PM2.5 rise on September 2-3, 1999, accompanied by stronger PM10, carbon monoxide, and nitrogen dioxide. Surface observations show very weak ventilation, haze, and mist. An official regional modeling assessment independently identifies the same dates as part of a Memphis pollution episode governed by day-to-day buildup, low wind, warm air aloft, and stability. The combined record supports stagnant anthropogenic mixture at possible confidence, while the ozone-focused assessment and absent PM2.5 source fractions keep the confidence ceiling low.
PossibleThis is a transparent hypothesis supported by limited evidence or an explicit inference, not a definitive attribution.
Southeast States Air Resource Managers and U.S. EPA archive · Mar 30, 2004official
The official ATMOS technical support document treats August 29-September 9, 1999 as a Memphis regional pollution episode. It reports elevated Memphis ozone on September 2-3 and characterizes high-pollution conditions by regional day-to-day buildup, lower surface and upper-air wind speeds, warmer air aloft, and greater atmospheric stability.
National Weather Service surface observations via Iowa Environmental Mesonet · Sep 2, 1999hypothesis
Memphis International Airport recorded calm conditions during 49% of event hours and wind at or below three knots during 62%. The observations included haze and mist, with visibility as low as 2.5 miles.
EPA records show median daily PM2.5 of 40.675 micrograms per cubic meter across five monitors, 1.664 times the adjacent-window level. PM10 rose to 1.761 times, carbon monoxide to 2.266 times, and nitrogen dioxide to 1.503 times their adjacent-window comparisons, while sulfur dioxide was nearly unchanged.
Inference, not direct attribution
exact_date_atmos_pollution_fingerprint_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 official ATMOS technical support document treats August 29-September 9, 1999 as a Memphis regional pollution episode. It reports elevated Memphis ozone on September 2-3 and characterizes high-pollution conditions by regional day-to-day buildup,…
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
42.9 µg/m³ near Memphis
EPA monitors recorded the event across 1 metro.
The peak daily concentration is roughly 1.9 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.
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.
"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.
3 retained sources
Official record
Arkansas-Tennessee-Mississippi modeling assessment of the August 29-September 9, 1999 episode
Southeast States Air Resource Managers and U.S. EPA archive
The official ATMOS technical support document treats August 29-September 9, 1999 as a Memphis regional pollution episode. It reports elevated Memphis ozone on September 2-3 and characterizes high-pollution conditions by regional day-to-day buildup, lower surface and upper-air wind speeds, warmer air aloft, and greater atmospheric stability.
What it supports
An official source supports the attributed cause for the stated place and dates.
Limit
The technical support document models ozone, not PM2.5 composition or source fractions.
Official record
Memphis surface conditions, September 2-3, 1999
National Weather Service surface observations via Iowa Environmental Mesonet
Memphis International Airport recorded calm conditions during 49% of event hours and wind at or below three knots during 62%. The observations included haze and mist, with visibility as low as 2.5 miles.
What it supports
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
Limit
A single surface station documents weak ventilation and obscuration but not the full vertical profile or particle source mix.
Official record
Memphis joint pollution fingerprint, September 2-3, 1999
U.S. EPA AirData
EPA records show median daily PM2.5 of 40.675 micrograms per cubic meter across five monitors, 1.664 times the adjacent-window level. PM10 rose to 1.761 times, carbon monoxide to 2.266 times, and nitrogen dioxide to 1.503 times their adjacent-window comparisons, while sulfur dioxide was nearly unchanged.
What it supports
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
Limit
The pollution fingerprint supports a mixed combustion and accumulation episode but cannot uniquely apportion local and regional sources.
What remains uncertain
Confidence is capped at possible because the regional assessment is ozone-focused and does not apportion Memphis PM2.5 mass.
The atmospheric mechanism is paired with local PM2.5 and co-pollutant measurements; it is not a direct particulate source-apportionment result.
The technical support document models ozone, not PM2.5 composition or source fractions.
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.