Regional secondary PM Possible
Daily PM2.5 peaked at 72.8 µg/m³ near Pittsburgh across 10 covered metros home to 14 million people. 3 metros 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 ≈ 33 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
Manual research connected 3 source events to one regional secondary-particle pollution episode using the deterministic grouping rule.
Possible This is a transparent hypothesis supported by limited evidence or an explicit inference, not a definitive attribution.
Journal of the Air & Waste Management Association · Aug 1, 2003 corroborates
The study identifies July 15-19, 1999 as the strongest PM2.5/haze episode in its summer record. Ammoniated sulfate was the primary cause of the particle-mass buildup, while persistent high pressure, westerly transport, and recirculation slowed pollutant removal.
Northeast States for Coordinated Air Use Management corroborates
The regional haze assessment documents a Northeast-wide PM2.5 pattern on 17 July 1999, supporting grouping with the contemporaneous northeastern event; it does not establish the source or specifically attribute conditions in Scranton or Syracuse.
Environmental Fluid Mechanics · Jan 1, 2001 hypothesis
The study treats July 15-19 as an eastern U.S. ozone and PM2.5 episode under persistent high pressure, high temperatures, and westerly flow favorable for transporting pollutants from the Midwest toward the Northeast.
See every retained source and limitation ↓
Sequence: Indianapolis (Jul 15) → Cincinnati (Jul 15) → Akron (Jul 16) → Dayton (Jul 16) → Columbus (Jul 16) → Toledo (Jul 16) → Pittsburgh (Jul 17) → Scranton (Jul 17) → Buffalo (Jul 17) → Syracuse (Jul 17)
Evidence path
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.
Source or mechanism
The study identifies July 15-19, 1999 as the strongest PM2.5/haze episode in its summer record. Ammoniated sulfate was the primary cause of the particle-mass buildup, while persistent high pressure, westerly transport, and recirculation slowed…
View supporting source ↗Air-mass movement
NOAA HYSPLIT modeled 120 backward paths across multiple arrival times and heights. They show where the air traveled, not which source polluted it.
Explore the modeled paths ↓Local response
EPA monitors recorded the event across 10 metros. The peak daily concentration is roughly 3.3 cigarette-equivalents at the outdoor daily rate.
Inspect the pollutant and weather signals ↓Event autopsy
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.
Akron, OH
Monitor comparison window 1999-07-13 to 1999-07-20 · PM2.5 target: East HS
46% above nearby days
45% higher than surrounding days
46% higher than surrounding days
45% higher than surrounding days
9% higher than surrounding days
14% higher than surrounding days
7% higher than surrounding days
Buffalo-Cheektowaga-Niagara Falls, NY
Monitor comparison window 1999-07-13 to 1999-07-20 · PM2.5 target: BUFFALO
26% above nearby days
72% higher than surrounding days
No adjacent-day comparison
26% higher than surrounding days
No adjacent-day comparison
About the same as surrounding days
About the same as surrounding days
68% higher than surrounding days
22% higher than surrounding days
About the same as surrounding days
About the same as surrounding days
5% higher than surrounding days
72% higher than surrounding days
Pittsburgh, PA
Monitor comparison window 1999-07-13 to 1999-07-20
85% above nearby days
85% higher than surrounding days
85% higher than surrounding days
85% higher than surrounding days
90% higher than surrounding days
27% higher than surrounding days
19% higher than surrounding days
14% higher than surrounding days
Scranton--Wilkes-Barre--Hazleton, PA
Monitor comparison window 1999-07-14 to 1999-07-21 · PM2.5 target: Scranton
119% above nearby days
About the same as surrounding days
119% higher than surrounding days
About the same as surrounding days
17% lower than surrounding days
12% higher than surrounding days
20% higher than surrounding days
Syracuse, NY
Monitor comparison window 1999-07-14 to 1999-07-21 · PM2.5 target: EAST SYRACUSE
174% above nearby days
About the same as surrounding days
About the same as surrounding days
174% higher than surrounding days
About the same as surrounding days
About the same as surrounding days
About the same as surrounding days
About the same as surrounding days
About the same as surrounding days
5% higher than surrounding days
About the same as surrounding days
Cincinnati, OH-KY-IN
Monitor comparison window 1999-07-12 to 1999-07-19 · PM2.5 target: VERITY
72% above nearby days
98% higher than surrounding days
72% higher than surrounding days
98% higher than surrounding days
47% higher than surrounding days
20% higher than surrounding days
45% higher than surrounding days
17% higher than surrounding days
About the same as surrounding days
Indianapolis-Carmel-Anderson, IN
Monitor comparison window 1999-07-12 to 1999-07-19 · PM2.5 target: Indpls- W. 18th St./ Ernie Pyle School 90
64% above nearby days
About the same as surrounding days
64% higher than surrounding days
About the same as surrounding days
About the same as surrounding days
54% higher than surrounding days
22% higher than surrounding days
About the same as surrounding days
About the same as surrounding days
Columbus, OH
Monitor comparison window 1999-07-12 to 1999-07-19 · PM2.5 target: Fairgrounds Garage
89% above nearby days
61% higher than surrounding days
89% higher than surrounding days
61% higher than surrounding days
28% higher than surrounding days
About the same as surrounding days
16% higher than surrounding days
Dayton, OH
Monitor comparison window 1999-07-12 to 1999-07-19
82% above nearby days
9% higher than surrounding days
82% higher than surrounding days
9% higher than surrounding days
33% higher than surrounding days
14% higher than surrounding days
Toledo, OH
Monitor comparison window 1999-07-12 to 1999-07-19 · PM2.5 target: RAPS
51% above nearby days
51% higher than surrounding days
29% higher than surrounding days
12% higher than 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
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 Cincinnati
Peak-date arrivals · 1999-07-16
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 Columbus
Peak-date arrivals · 1999-07-16
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 Dayton
Peak-date arrivals · 1999-07-16
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 Indianapolis-Carmel-Anderson
Peak-date arrivals · 1999-07-15
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 Toledo
Peak-date arrivals · 1999-07-16
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 Akron
Peak-date arrivals · 1999-07-16
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 Buffalo-Cheektowaga-Niagara Falls
Peak-date arrivals · 1999-07-17
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 Pittsburgh
Peak-date arrivals · 1999-07-17
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 Scranton--Wilkes-Barre--Hazleton
Peak-date arrivals · 1999-07-17
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 Syracuse
Peak-date arrivals · 1999-07-17
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.
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
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.
| Metro | Peak PM2.5 | Cigarettes, whole event | Peak day | All-time standing |
|---|---|---|---|---|
| Pittsburgh, PA | 72.8 µg/m³ | 3.3 | Jul 17, 1999 | #16 of its top days |
| Scranton--Wilkes-Barre--Hazleton, PA | 58 µg/m³ | 4.9 | Jul 17, 1999 | #10 of its top days |
| Akron, OH | 56.6 µg/m³ | 4.8 | Jul 16, 1999 | #9 of its top days |
| Indianapolis-Carmel-Anderson, IN | 56.1 µg/m³ | 2.6 | Jul 15, 1999 | #13 of its top days |
| Dayton, OH | 53.1 µg/m³ | 2.4 | Jul 16, 1999 | #8 of its top days |
| Cincinnati, OH-KY-IN | 51.4 µg/m³ | 4.5 | Jul 16, 1999 | #13 of its top days |
| Columbus, OH | 47.1 µg/m³ | 2.1 | Jul 16, 1999 | #22 of its top days |
| Buffalo-Cheektowaga-Niagara Falls, NY | 46.8 µg/m³ | 2.1 | Jul 17, 1999 | #12 of its top days |
| Toledo, OH | 45 µg/m³ | 2.0 | Jul 16, 1999 | #25 of its top days |
| Syracuse, NY | 41.4 µg/m³ | 1.9 | Jul 17, 1999 | #13 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
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.
Journal of the Air & Waste Management Association
The study identifies July 15-19, 1999 as the strongest PM2.5/haze episode in its summer record. Ammoniated sulfate was the primary cause of the particle-mass buildup, while persistent high pressure, westerly transport, and recirculation slowed pollutant removal.
A retained source supports the attributed cause for the stated place and dates.
Composition measurements were centered on the Baltimore-Washington corridor rather than the five catalog metros; the extension is therefore capped at possible.
Journal of the Air & Waste Management Association
The paper identifies July 15-19, 1999 as the strongest PM2.5/haze episode in its record and attributes the particle buildup primarily to ammoniated sulfate under high pressure, westerly transport, and recirculation.
A retained source supports the attributed cause for the stated place and dates.
The composition site was downwind of these metros, so the corridor match cannot exceed possible confidence.
Journal of the Air & Waste Management Association
The study identifies July 15-19, 1999 as the summer's strongest regional PM2.5/haze episode and attributes the mass buildup primarily to ammoniated sulfate under persistent high pressure, westerly transport, and recirculation.
A retained source supports the attributed cause for the stated place and dates.
The intensive composition measurements were south of these metros, so confidence remains possible.
Environmental Fluid Mechanics
The study treats July 15-19 as an eastern U.S. ozone and PM2.5 episode under persistent high pressure, high temperatures, and westerly flow favorable for transporting pollutants from the Midwest toward the Northeast.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
This source establishes the regional meteorology and transport corridor, not the particle composition at each catalog metro.
Environmental Fluid Mechanics
The meteorological study describes the exact period as an eastern U.S. PM2.5 episode with persistent high pressure and westerly pollutant transport from the Midwest toward the Northeast.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
It documents the regional transport mechanism, not a unique emitting source.
Environmental Fluid Mechanics
The paper describes the exact period as an eastern U.S. PM2.5 episode under persistent high pressure and westerly transport from the Midwest into the Northeast.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The paper supplies regional episode meteorology rather than city-specific source apportionment.
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.