2001-07-18 Charlotte-Concord-Gastonia, NC-SC, Greensboro-High Point, NC, and 1 other metros stagnant mixed-pollution episode
July 18, 2001
Daily PM2.5 peaked at 44.7 µg/m³ near Charlotte across
3 covered metros home to 4.4 million people.
Counting the extra PM2.5 above each metro's typical day, this event handed the
average affected resident ≈ 1.5 cigarettes.
Across everyone living in the affected metros, that is the collective equivalent of
≈ 6.7 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”
Manual research connected 3 source events to one stagnant mixed-pollution episode using the deterministic grouping rule.
PossibleThis is a transparent hypothesis supported by limited evidence or an explicit inference, not a definitive attribution.
North Carolina Division of Air Quality / U.S. Environmental Protection Agency · Feb 26, 2004official
North Carolina's meteorologist-reviewed exact-date trajectories assign the July 18 PM2.5 at Catawba and Davidson monitors primarily to North Carolina and South Carolina, with secondary influence from southeast Tennessee and north Georgia; the report identifies short trajectories, summertime recirculation, and prolonged high-pressure stagnation as the high-PM pattern.
U.S. Environmental Protection Agency · Sep 24, 2003hypothesis
EPA's Positive Matrix Factorization time series includes an actual sample at site 37-119-0041 on 2001-07-18; coal and oil combustion were the largest modeled factors, with mobile and other smaller factors also present.
Inference, not direct attribution
epa_pmf_exact_sample_factor_pattern.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.
North Carolina's meteorologist-reviewed exact-date trajectories assign the July 18 PM2.5 at Catawba and Davidson monitors primarily to North Carolina and South Carolina, with secondary influence from southeast Tennessee and north Georgia; the report…
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
44.7 µg/m³ near Charlotte
EPA monitors recorded the event across 3 metros.
The peak daily concentration is roughly 2.0 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
Charlotte-Concord-Gastonia, NC-SC
Monitor comparison window 2001-07-15 to 2001-07-21 · PM2.5 target: Garinger High School
Monitor comparison window 2001-07-15 to 2001-07-21
3 pollutant signals
Fine-particle pulse40.7 µg/m³
122% above nearby days
PM1043 µg/m³
No adjacent-day comparison
Pollution fingerprint
Metro-wide median of reporting monitors
PM2.540.7 µg/m³
122% higher than surrounding days
Before20.9 µg/m³ Event40.7 µg/m³ After16 µg/m³
PM1043 µg/m³
No adjacent-day comparison
Before— Event43 µg/m³ After—
Ozone0.044 ppm
About the same as surrounding days
Before0.051 ppm Event0.044 ppm After0.045 ppm
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.
5 retained sources
Official record
North Carolina PM2.5 boundary recommendation: July 18, 2001 trajectory analysis
North Carolina Division of Air Quality / U.S. Environmental Protection Agency
North Carolina's meteorologist-reviewed exact-date trajectories assign the July 18 PM2.5 at Catawba and Davidson monitors primarily to North Carolina and South Carolina, with secondary influence from southeast Tennessee and north Georgia; the report identifies short trajectories, summertime recirculation, and prolonged high-pressure stagnation as the high-PM pattern.
What it supports
An official source supports the attributed cause for the stated place and dates.
Limit
The state analysis supports a regional accumulation mixture and source regions, not a unique emitter or source-specific daily mass.
Official record
North Carolina PM2.5 boundary recommendation: July 18, 2001 trajectory analysis
North Carolina Division of Air Quality / U.S. Environmental Protection Agency
North Carolina's exact-date trajectories assign the July 18 PM2.5 at Catawba and Davidson primarily to North Carolina and South Carolina, with secondary influence from southeast Tennessee and north Georgia; the report identifies short trajectories, recirculation, and prolonged high-pressure stagnation.
What it supports
An official source supports the attributed cause for the stated place and dates.
Limit
The state analysis supports a regional accumulation mixture and source regions, not a unique emitter or source-specific daily mass.
Official record
North Carolina PM2.5 boundary recommendation: July 18, 2001 trajectory analysis
North Carolina Division of Air Quality / U.S. Environmental Protection Agency
North Carolina's meteorologist-reviewed exact-date trajectories assign the July 18 PM2.5 at Catawba and Davidson monitors primarily to North Carolina and South Carolina, with secondary influence from southeast Tennessee and north Georgia; the report identifies short trajectories, summertime recirculation, and prolonged high-pressure stagnation as the high-PM pattern.
What it supports
An official source supports the attributed cause for the stated place and dates.
Limit
The state analysis supports a regional accumulation mixture and source regions, not a unique emitter or source-specific daily mass.
Official record
Appendix F: Charlotte, North Carolina: exact-date PMF source strengths on 2001-07-18
U.S. Environmental Protection Agency
EPA's Positive Matrix Factorization time series includes an actual sample at site 37-119-0041 on 2001-07-18; coal and oil combustion were the largest modeled factors, with mobile and other smaller factors also present.
What it supports
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
Limit
The appendix reports modeled factor contributions at one chemical-speciation monitor, not the complete metro-wide emissions inventory.
Official record
Eight-Site Source Apportionment of PM2.5 Speciation Trends Data
U.S. Environmental Protection Agency
EPA's Charlotte site summary identifies coal and oil combustion, mobile, ammonium-nitrate, crustal, smelting, marine/industrial-salt, and vegetative-burning/fireworks factors. The report documents the site's modeled date range, factor interpretation, average apportioned mass, local source setting, and back-trajectory context.
What it supports
Context for the event; this item does not establish the attributed cause by itself.
Limit
The site summary defines and contextualizes the factors across the modeled period; the separate appendix supplies the event-date source strengths.
What remains uncertain
A PMF factor is a statistical source profile; it does not by itself identify a unique facility, fire, or transport path.
Period-average factor descriptions do not establish that every listed source category dominated this particular event day.
The appendix reports modeled factor contributions at one chemical-speciation monitor, not the complete metro-wide emissions inventory.
The exact-date result is a source-apportionment estimate at one monitor and remains capped at possible confidence in the IAQng catalog.
The qualitative ordering is read from EPA's exact-date source-strength plots after accounting for each factor's different mean-mass scale; no unsupported interpolated dates are used.
The site summary defines and contextualizes the factors across the modeled period; the separate appendix supplies the event-date source strengths.
The state analysis supports a regional accumulation mixture and source regions, not a unique emitter or source-specific daily mass.
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.