2002-07-17 to 2002-07-18 Durham-Chapel Hill, NC and Greensboro-High Point, NC stagnant mixed-pollution episode
July 17, 2002 to July 18, 2002
Daily PM2.5 peaked at 47.5 µg/m³ near Durham across
2 covered metros home to 1.4 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 ≈ 2.4 cigarettes.
Across everyone living in the affected metros, that is the collective equivalent of
≈ 3.4 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 2 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.
Mid-Atlantic Regional Air Management Association / Maryland Department of the Environment · Sep 30, 2004official
MARAMA's official planning report identifies July 17-18 as a widespread high-PM episode. It documents a strong upper-level high over the Southeast, strong surface high pressure over Georgia, very light upper-level winds, haze, no precipitation, and simultaneous unhealthy particle days from Charlotte through the Mid-Atlantic.
National Weather Service Raleigh via Iowa Environmental Mesonet · Jul 18, 2002hypothesis
NWS Raleigh explicitly retained ongoing haze on July 18; later discussions said no air-mass change was expected while the Atlantic upper ridge continued to build.
EPA records show a median daily PM2.5 value of 41.8 micrograms per cubic meter across 1 monitors during the event, compared with 10.9 before and 17 after. Same-window context included PM10 Total 0-10um STP null Micrograms/cubic meter (25 C), Ozone 0.0575 Parts per million.
Inference, not direct attribution
official_regional_pm_episode_plus_local_haze_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.
MARAMA's official planning report identifies July 17-18 as a widespread high-PM episode. It documents a strong upper-level high over the Southeast, strong surface high pressure over Georgia, very light upper-level winds, haze, no precipitation, and…
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
47.5 µg/m³ near Durham
EPA monitors recorded the event across 2 metros.
The peak daily concentration is roughly 2.2 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
Durham-Chapel Hill, NC
Monitor comparison window 2002-07-15 to 2002-07-21
3 pollutant signals
Fine-particle pulse47.5 µg/m³
42% above nearby days
Air movement1.5 kt median wind
12 calm hours observed
What observers saw2.5 mi minimum visibility
Haze reported during 21 hours
Pollution fingerprint
Metro-wide median of reporting monitors
PM2.547.5 µg/m³
42% higher than surrounding days
Before33.4 µg/m³ Event47.5 µg/m³ After20.1 µg/m³
Carbon monoxide0.374 ppm
11% higher than surrounding days
Before0.352 ppm Event0.374 ppm After0.309 ppm
Ozone0.059 ppm
10% higher than surrounding days
Before0.070 ppm Event0.059 ppm After0.050 ppm
Atmospheric conditions
Raleigh / Durham, 15.1 km from the PM2.5 target
Wind1.5 kt median
9 kt maximum · 12 of 24 observed hours calm
Visibility2.5 mi minimum
3 observed hours at or below 3 miles
Temperature80.5°F median
72°F to 95°F
Humidity72% median
97% maximum
Greensboro-High Point, NC
Monitor comparison window 2002-07-14 to 2002-07-21 · PM2.5 target: Mendenhall School
2 pollutant signals
Fine-particle pulse41.8 µg/m³
82% above nearby days
Air movement4 kt median wind
16 calm hours observed
What observers saw2.5 mi minimum visibility
Haze reported during 32 hours
Pollution fingerprint
Metro-wide median of reporting monitors
PM2.541.8 µg/m³
82% higher than surrounding days
Before10.9 µg/m³ Event41.8 µg/m³ After17 µg/m³
Ozone0.058 ppm
19% higher than surrounding days
Before0.038 ppm Event0.058 ppm After0.046 ppm
Atmospheric conditions
GREENSBORO/PIEDMONT_(ASOS), 12.8 km from the PM2.5 target
Wind4 kt median
9 kt maximum · 16 of 48 observed hours calm
Visibility2.5 mi minimum
3 observed hours at or below 3 miles
Temperature81.5°F median
72°F to 89°F
Humidity64% median
85% 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.
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.
9 retained sources
Official record
Report for MARAMA on PM2.5 Forecasting Tools: July 17-18, 2002 regional episode
Mid-Atlantic Regional Air Management Association / Maryland Department of the Environment
MARAMA's official planning report identifies July 17-18 as a widespread high-PM episode. It documents a strong upper-level high over the Southeast, strong surface high pressure over Georgia, very light upper-level winds, haze, no precipitation, and simultaneous unhealthy particle days from Charlotte through the Mid-Atlantic.
What it supports
An official source supports the attributed cause for the stated place and dates.
Limit
The report directly tabulates Charlotte and Mid-Atlantic cities rather than Greensboro-High Point, North Carolina; the local NWS discussion supplies the geographic bridge.
Official record
Report for MARAMA on PM2.5 Forecasting Tools: July 17-18, 2002 regional episode
Mid-Atlantic Regional Air Management Association / Maryland Department of the Environment
MARAMA's official planning report identifies July 17-18 as a widespread high-PM episode. It documents a strong upper-level high over the Southeast, strong surface high pressure over Georgia, very light upper-level winds, haze, no precipitation, and simultaneous unhealthy particle days from Charlotte through the Mid-Atlantic.
What it supports
An official source supports the attributed cause for the stated place and dates.
Limit
The report directly tabulates Charlotte and Mid-Atlantic cities rather than Durham-Chapel Hill, North Carolina; the local NWS discussion supplies the geographic bridge.
Official record
Area Forecast Discussion: ongoing haze under the persistent regional ridge
National Weather Service Raleigh via Iowa Environmental Mesonet
NWS Raleigh explicitly retained ongoing haze on July 18; later discussions said no air-mass change was expected while the Atlantic upper ridge continued to build.
What it supports
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
Limit
The discussion documents the local air mass and haze but does not independently apportion the measured PM2.5 mass.
Official record
Area Forecast Discussion: ongoing haze under the persistent regional ridge
National Weather Service Raleigh via Iowa Environmental Mesonet
NWS Raleigh explicitly retained ongoing haze on July 18; later discussions said no air-mass change was expected while the Atlantic upper ridge continued to build.
What it supports
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
Limit
The discussion documents the local air mass and haze but does not independently apportion the measured PM2.5 mass.
Official record
Area Forecast Discussion: surface and upper high pressure built over central North Carolina
National Weather Service Raleigh via Iowa Environmental Mesonet
NWS Raleigh reported on July 17 that high pressure was building over central North Carolina both aloft and at the surface while exceptional heat persisted.
What it supports
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
Limit
The discussion documents the local air mass and haze but does not independently apportion the measured PM2.5 mass.
Official record
Greensboro-High Point, North Carolina modeled aerosol fingerprint, 2002-07-17
U.S. EPA EQUATES CMAQ via Remote Sensing Information Gateway
EPA's retrospective 12-kilometer model reproduced an event-centered fine-mass increase of 2.5582 times adjacent modeled days. The modeled mixture was 45.2% organic matter, 45.9% secondary ions, 3.9% soil, and 2.6% elemental carbon.
What it supports
Context for the event; this item does not establish the attributed cause by itself.
Limit
EQUATES is a retrospective chemical-transport model, not a monitor observation or source-apportionment determination; it is compatible context only.
Official record
Greensboro-High Point, North Carolina measured pollution fingerprint, 2002-07-17
U.S. EPA AirData
EPA records show a median daily PM2.5 value of 41.8 micrograms per cubic meter across 1 monitors during the event, compared with 10.9 before and 17 after. Same-window context included PM10 Total 0-10um STP null Micrograms/cubic meter (25 C), Ozone 0.0575 Parts per million.
What it supports
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
Limit
The measured fingerprint establishes the local particle episode and co-pollutant pattern but cannot uniquely apportion its sources.
Official record
Durham-Chapel Hill, North Carolina modeled aerosol fingerprint, 2002-07-18
U.S. EPA EQUATES CMAQ via Remote Sensing Information Gateway
EPA's retrospective 12-kilometer model reproduced an event-centered fine-mass increase of 2.0277 times adjacent modeled days. The modeled mixture was 48.6% organic matter, 43.2% secondary ions, 3.7% soil, and 2.2% elemental carbon.
What it supports
Context for the event; this item does not establish the attributed cause by itself.
Limit
EQUATES is a retrospective chemical-transport model, not a monitor observation or source-apportionment determination; it is compatible context only.
Show 1 more retained source
Official record
Durham-Chapel Hill, North Carolina measured pollution fingerprint, 2002-07-18
U.S. EPA AirData
EPA records show a median daily PM2.5 value of 47.5 micrograms per cubic meter across 3 monitors during the event, compared with 33.4 before and 20.1 after. Same-window context included PM10 Total 0-10um STP null Micrograms/cubic meter (25 C), Carbon monoxide 0.3739 Parts per million, Ozone 0.0594 Parts per million.
What it supports
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
Limit
The measured fingerprint establishes the local particle episode and co-pollutant pattern but cannot uniquely apportion its sources.
What remains uncertain
EQUATES is a retrospective chemical-transport model, not a monitor observation or source-apportionment determination; it is compatible context only.
The classification identifies a broad regional accumulation regime, not a complete source allocation for the daily PM2.5 mass.
The discussion documents the local air mass and haze but does not independently apportion the measured PM2.5 mass.
The measured fingerprint establishes the local particle episode and co-pollutant pattern but cannot uniquely apportion its sources.
The report directly tabulates Charlotte and Mid-Atlantic cities rather than Durham-Chapel Hill, North Carolina; the local NWS discussion supplies the geographic bridge.
The report directly tabulates Charlotte and Mid-Atlantic cities rather than Greensboro-High Point, North Carolina; the local NWS discussion supplies the geographic bridge.
The report identifies the regional accumulation setting but does not apportion source sectors or chemical components for this metro.
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.