American Meteorological Society corroborates
Mid-Atlantic regional PM2.5 episode analysis
The regional analysis describes the mid-July 2002 Mid-Atlantic haze episode in terms of regional transport and secondary particulate formation.
Regional secondary PM Possible
2002-07-18 to 2002-07-19 Baltimore-Columbia-Towson, MD, Harrisburg-Carlisle, PA, and 2 other metros regional secondary-particle pollution episode
Daily PM2.5 peaked at 58.9 µg/m³ near Philadelphia across 4 covered metros home to 16 million people.
Counting the extra PM2.5 above each metro's typical day, this event handed the average affected resident ≈ 2.7 cigarettes. Across everyone living in the affected metros, that is the collective equivalent of ≈ 44 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”
Regional episode analyses describe mid-July 2002 Mid-Atlantic haze as transported secondary aerosol. The available page did not provide a city-by-city causal table, so confidence remains possible.
Possible This is a transparent hypothesis supported by limited evidence or an explicit inference, not a definitive attribution.
See every retained source and limitation ↓
Sequence: Philadelphia (Jul 18) → Harrisburg (Jul 18) → Washington (Jul 18) → Baltimore (Jul 18)
Evidence path
From source to city
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.
- 1Documented
Source or mechanism
Regional secondary PM
The regional analysis describes the mid-July 2002 Mid-Atlantic haze episode in terms of regional transport and secondary particulate formation.
View supporting source ↗ - 2Modeled
Air-mass movement
The 72-hour air-mass history is reconstructed
NOAA HYSPLIT modeled 48 backward paths across multiple arrival times and heights. They show where the air traveled, not which source polluted it.
Explore the modeled paths ↓ - 3Documented
Local response
58.9 µg/m³ near Philadelphia
EPA monitors recorded the event across 4 metros. The peak daily concentration is roughly 2.7 cigarette-equivalents at the outdoor daily rate.
Inspect the pollutant and weather signals ↓
Event autopsy
What the instruments saw
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.
Baltimore-Columbia-Towson, MD
Monitor comparison window 2002-07-15 to 2002-07-22 · PM2.5 target: FMC-Fairfield
77% above nearby days
No adjacent-day comparison
Pollution fingerprint
Metro-wide median of reporting monitors PM2.5 47.1 µg/m³
77% higher than surrounding days
Before18.5 µg/m³ Event47.1 µg/m³ After28 µg/m³
PM10 58 µg/m³
No adjacent-day comparison
Before— Event58 µg/m³ After—
Carbon monoxide 0.299 ppm
5% lower than surrounding days
Before0.335 ppm Event0.299 ppm After0.312 ppm
Nitrogen dioxide 15.1 ppb
5% higher than surrounding days
Before15.3 ppb Event15.1 ppb After12.5 ppb
Sulfur dioxide 6.94 ppb
31% higher than surrounding days
Before4.48 ppb Event6.94 ppb After3.46 ppb
Ozone 0.059 ppm
About the same as surrounding days
Before0.056 ppm Event0.059 ppm After0.057 ppm
Atmospheric conditions
Nearby EPA weather observationsAbout the same as surrounding days
About the same as surrounding days
About the same as surrounding days
Harrisburg-Carlisle, PA
Monitor comparison window 2002-07-15 to 2002-07-22 · PM2.5 target: Carlisle
65% above nearby days
45% higher than surrounding days
Pollution fingerprint
Metro-wide median of reporting monitors PM2.5 47.2 µg/m³
65% higher than surrounding days
Before21.4 µg/m³ Event47.2 µg/m³ After32.9 µg/m³
PM10 56 µg/m³
45% higher than surrounding days
Before23 µg/m³ Event56 µg/m³ After41 µg/m³
Carbon monoxide 0.422 ppm
17% higher than surrounding days
Before0.412 ppm Event0.422 ppm After0.217 ppm
Nitrogen dioxide 10.1 ppb
26% higher than surrounding days
Before9.48 ppb Event10.1 ppb After6.48 ppb
Sulfur dioxide 5.89 ppb
83% higher than surrounding days
Before2.10 ppb Event5.89 ppb After3.19 ppb
Ozone 0.051 ppm
About the same as surrounding days
Before0.050 ppm Event0.051 ppm After0.053 ppm
Philadelphia-Camden-Wilmington, PA-NJ-DE-MD
Monitor comparison window 2002-07-15 to 2002-07-22 · PM2.5 target: Air Management Services Laboratory (AMS LAB)
132% above nearby days
5% higher than surrounding days
112% higher than surrounding days
Pollution fingerprint
Metro-wide median of reporting monitors PM2.5 51.5 µg/m³
132% higher than surrounding days
Before18.5 µg/m³ Event51.5 µg/m³ After22.4 µg/m³
PM10 71 µg/m³
112% higher than surrounding days
Before32 µg/m³ Event71 µg/m³ After33 µg/m³
Carbon monoxide 0.417 ppm
31% higher than surrounding days
Before0.313 ppm Event0.417 ppm After0.281 ppm
Nitrogen dioxide 24 ppb
19% higher than surrounding days
Before20.8 ppb Event24 ppb After9.65 ppb
Sulfur dioxide 6.70 ppb
62% higher than surrounding days
Before4.21 ppb Event6.70 ppb After1.23 ppb
Ozone 0.066 ppm
18% higher than surrounding days
Before0.058 ppm Event0.066 ppm After0.055 ppm
Atmospheric conditions
Nearby EPA weather observationsAbout the same as surrounding days
About the same as surrounding days
About the same as surrounding days
5% higher than surrounding days
Washington-Arlington-Alexandria, DC-VA-MD-WV
Monitor comparison window 2002-07-15 to 2002-07-22 · PM2.5 target: RIVER TERRACE
41% above nearby days
No adjacent-day comparison
Pollution fingerprint
Metro-wide median of reporting monitors PM2.5 44.9 µg/m³
41% higher than surrounding days
Before18 µg/m³ Event44.9 µg/m³ After33 µg/m³
PM10 39.5 µg/m³
No adjacent-day comparison
Before— Event39.5 µg/m³ After—
Carbon monoxide 0.531 ppm
9% higher than surrounding days
Before0.483 ppm Event0.531 ppm After0.485 ppm
Nitrogen dioxide 15.4 ppb
12% higher than surrounding days
Before21.8 ppb Event15.4 ppb After13 ppb
Sulfur dioxide 4.27 ppb
13% lower than surrounding days
Before4.45 ppb Event4.27 ppb After5.54 ppb
Ozone 0.056 ppm
About the same as surrounding days
Before0.047 ppm Event0.056 ppm After0.058 ppm
Atmospheric conditions
Nearby EPA weather observationsAbout the same as surrounding days
About the same as surrounding days
About the same as surrounding days
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.
Air arriving near Baltimore-Columbia-Towson
Peak-date arrivals · 2002-07-18
Broad history Mostly northwest of the metro
72-hour reach 700–1,450 km
Sensitivity runs 12 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.
Air arriving near Harrisburg-Carlisle
Peak-date arrivals · 2002-07-18
Broad history Mostly west of the metro
72-hour reach 700–1,100 km
Sensitivity runs 12 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.
Air arriving near Philadelphia-Camden-Wilmington
Peak-date arrivals · 2002-07-19
Broad history Mostly west of the metro
72-hour reach 650–1,200 km
Sensitivity runs 12 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.
Air arriving near Washington-Arlington-Alexandria
Peak-date arrivals · 2002-07-18
Broad history Mostly northwest of the metro
72-hour reach 650–1,550 km
Sensitivity runs 12 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.
Views from the selected metro
Camera views near
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.
Affected metros
| Metro | Peak PM2.5 | Cigarettes, whole event | Peak day | All-time standing |
|---|---|---|---|---|
| Philadelphia-Camden-Wilmington, PA-NJ-DE-MD | 58.9 µg/m³ | 5.2 | Jul 19, 2002 | #17 of its top days |
| Harrisburg-Carlisle, PA | 53.2 µg/m³ | 2.4 | Jul 18, 2002 | #24 of its top days |
| Washington-Arlington-Alexandria, DC-VA-MD-WV | 52.2 µg/m³ | 2.4 | Jul 18, 2002 | #22 of its top days |
| Baltimore-Columbia-Towson, MD | 50.5 µg/m³ | 2.3 | Jul 18, 2002 | #22 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
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.
Mid-Atlantic regional PM2.5 episode analysis
American Meteorological Society
The regional analysis describes the mid-July 2002 Mid-Atlantic haze episode in terms of regional transport and secondary particulate formation.
What it supports
A retained source supports the attributed cause for the stated place and dates.
What remains uncertain
- The retained evidence is scoped to the places and dates stated; it is not a catalog promotion.
Status and provenance
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.