2001-07-18 Richmond, VA regional secondary-particle pollution episode
July 18, 2001
Daily PM2.5 peaked at 39.1 µg/m³ near Richmond across
1 covered metro home to 1.4 million people.
Counting the extra PM2.5 above each metro's typical day, this event handed the
average affected resident ≈ 1.4 cigarettes.
Across everyone living in the affected metros, that is the collective equivalent of
≈ 1.9 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 “Regional secondary PM”
Richmond, VA's 5-monitor PM2.5 episode reached 34.8 µg/m³, 2.249 times the adjacent-window level, with 6 coded haze hours. 4 exact-date regional chemistry sites agreed that 66.4% to 82.1% of reconstructed mass was ammonium sulfate plus ammonium nitrate, and the largest reconstructed mass was 70.7% of the affected-place PM2.5 value. This supports regional secondary PM at possible confidence; the chemistry sites did not directly sample the city and no particular source region is assigned.
PossibleThis is a transparent hypothesis supported by limited evidence or an explicit inference, not a definitive attribution.
U.S. EPA AirData Chemical Speciation · Jul 18, 2001hypothesis
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.
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.
Observed context · not attribution
Richmond, VA
Monitor comparison window 2001-07-15 to 2001-07-21
6 pollutant signals
Fine-particle pulse34.8 µg/m³
60% above nearby days
Air movement6 kt median wind
7 calm hours observed
What observers saw1.5 mi minimum visibility
Haze reported during 6 hours
Pollution fingerprint
Metro-wide median of reporting monitors
PM2.534.8 µg/m³
60% higher than surrounding days
Before21.7 µg/m³ Event34.8 µg/m³ After9.25 µg/m³
PM1041.5 µg/m³
No adjacent-day comparison
Before— Event41.5 µg/m³ After—
Carbon monoxide0.817 ppm
42% higher than surrounding days
Before0.579 ppm Event0.817 ppm After0.537 ppm
Nitrogen dioxide12.1 ppb
29% higher than surrounding days
Before11.9 ppb Event12.1 ppb After4.67 ppb
Sulfur dioxide2.60 ppb
About the same as surrounding days
Before4.33 ppb Event2.60 ppb After1.02 ppb
Ozone0.023 ppm
40% lower than surrounding days
Before0.048 ppm Event0.023 ppm After0.036 ppm
Atmospheric conditions
Chesterfield, 11.8 km from the PM2.5 target
Wind6 kt median
10 kt maximum · 7 of 24 observed hours calm
Visibility1.5 mi minimum
12 observed hours at or below 3 miles
Temperature76°F median
73°F to 80°F
Humidity85% median
100% 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.
Air-mass reconstruction
Where the air had been
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.
Transport context · not source attribution
Air arriving near Richmond
Peak-date arrivals · 2001-07-18
Broad historyMostly southwest of the metro
72-hour reach250–750 km
Sensitivity runs12 modeled paths
Near surface · 100 mLower atmosphere · 500 mHigher layer · 1,000 m
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.
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.
1 retained source
Official record
Richmond, VA regional PM2.5 composition context, 2001-07-18
U.S. EPA AirData Chemical Speciation
On the exact event date, 4 EPA speciation sites within 175 km agreed that ammonium sulfate plus ammonium nitrate comprised a majority of reconstructed PM2.5: Shenandoah NP (139.3 km; 77.5% secondary inorganic share; 15.9 µg/m³ reconstructed mass); HAINS POINT PM-2.5 AND IMPROVE SITE ON ROOF OF PARK POLICE BLDG. (157.2 km; 66.4% secondary inorganic share; 24.6 µg/m³ reconstructed mass); Natural Bridge Ranger Station (178 km; 67.2% secondary inorganic share; 21.8 µg/m³ reconstructed mass); Dolly Sods Wilderness (244.4 km; 82.1% secondary inorganic share; 11.1 µg/m³ reconstructed mass).
What it supports
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
Limit
The chemistry sites were 139.3 km and 157.2 km and 178 km and 244.4 km from the affected-place PM2.5 target and did not directly measure the city air.
What remains uncertain
Composition supports an aerosol mechanism but cannot identify a particular emitting facility or source region.
No exact or same-window source with enough causal and geographic specificity was recovered from the accessible web record.
Same-date regional composition and distance do not prove that every site sampled one shared air mass.
The 50% chemistry share, 175 km distance, and regional-mass comparability gates are transparent IAQng catalog conventions, not EPA attribution thresholds.
The chemistry sites were 139.3 km and 157.2 km and 178 km and 244.4 km from the affected-place PM2.5 target and did not directly measure the city air.
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.