Regional secondary PM Likely
Daily PM2.5 peaked at 54.3 µg/m³ near Boston across 7 covered metros home to 11 million people. 4 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 ≈ 2.3 cigarettes. Across everyone living in the affected metros, that is the collective equivalent of ≈ 25 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
An exact-window scientific analysis identifies a coherent Northeast sulfate haze episode caused by long-range transport.
Likely Applicable reporting or a supported inference makes this the best current causal explanation.
CAPITA / Washington University corroborates
The episode analysis calls July 16-17 a major Northeast PM2.5 event and reports modeling that identified sulfate transported over long distances from the eastern U.S. and southeastern Canada as the most likely haze component.
Northeast States for Coordinated Air Use Management corroborates
The regional haze assessment includes a map of average 24-hour total PM2.5 concentrations across the Northeast on 17 July 1999, corroborating that this was a regionally documented fine-particle episode; it does not assign a source to this event.
See every retained source and limitation ↓
Sequence: Boston (Jul 17) → Albany (Jul 17) → Worcester (Jul 17) → Providence (Jul 17) → Springfield (Jul 17) → New Haven (Jul 17) → Hartford (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 episode analysis calls July 16-17 a major Northeast PM2.5 event and reports modeling that identified sulfate transported over long distances from the eastern U.S. and southeastern Canada as the most likely haze component.
View supporting source ↗Air-mass movement
NOAA HYSPLIT modeled 84 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
The regional haze assessment includes a map of average 24-hour total PM2.5 concentrations across the Northeast on 17 July 1999, corroborating that this was a regionally documented fine-particle episode; it does not assign a source to this event.
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.
Albany-Schenectady-Troy, NY
Monitor comparison window 1999-07-14 to 1999-07-21 · PM2.5 target: LOUDONVILLE
313% above nearby days
About the same as surrounding days
5% higher than surrounding days
313% higher than surrounding days
5% higher than surrounding days
28% lower than surrounding days
16% higher than surrounding days
About the same as surrounding days
About the same as surrounding days
14% higher than surrounding days
About the same as surrounding days
Boston-Cambridge-Newton, MA-NH
Monitor comparison window 1999-07-14 to 1999-07-21
344% above nearby days
9% higher than surrounding days
344% higher than surrounding days
9% higher than surrounding days
About the same as surrounding days
11% lower than surrounding days
6% lower than surrounding days
36% higher than surrounding days
10% lower than surrounding days
About the same as surrounding days
16% higher than surrounding days
Hartford-West Hartford-East Hartford, CT
Monitor comparison window 1999-07-14 to 1999-07-21 · PM2.5 target: McAuliffe Park
218% above nearby days
No adjacent-day comparison
218% higher than surrounding days
No adjacent-day comparison
About the same as surrounding days
14% lower than surrounding days
56% higher than surrounding days
27% higher than surrounding days
11% higher than surrounding days
About the same as surrounding days
13% higher than surrounding days
New Haven-Milford, CT
Monitor comparison window 1999-07-14 to 1999-07-21 · PM2.5 target: Meadow and Bank Streets
250% above nearby days
31% higher than surrounding days
250% higher than surrounding days
31% higher than surrounding days
About the same as surrounding days
About the same as surrounding days
30% higher than surrounding days
36% higher than surrounding days
About the same as surrounding days
10% higher than surrounding days
Providence-Warwick, RI-MA
Monitor comparison window 1999-07-14 to 1999-07-21
72% above nearby days
No adjacent-day comparison
72% higher than surrounding days
No adjacent-day comparison
30% lower than surrounding days
11% lower than surrounding days
26% lower than surrounding days
44% higher than surrounding days
7% lower than surrounding days
About the same as surrounding days
10% higher than surrounding days
Springfield, MA
Monitor comparison window 1999-07-14 to 1999-07-21 · PM2.5 target: LIBERTY ST PARKING LOT
249% above nearby days
No adjacent-day comparison
249% higher than surrounding days
No adjacent-day comparison
10% lower than surrounding days
About the same as surrounding days
40% higher than surrounding days
13% higher than surrounding days
About the same as surrounding days
About the same as surrounding days
13% higher than surrounding days
Worcester, MA-CT
Monitor comparison window 1999-07-14 to 1999-07-21
294% above nearby days
No adjacent-day comparison
294% higher than surrounding days
No adjacent-day comparison
About the same as surrounding days
22% lower than surrounding days
17% higher than surrounding days
30% higher than surrounding days
11% 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 Albany-Schenectady-Troy
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 Boston-Cambridge-Newton
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 Hartford-West Hartford-East Hartford
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 New Haven-Milford
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 Providence-Warwick
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 Springfield
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 Worcester
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 |
|---|---|---|---|---|
| Boston-Cambridge-Newton, MA-NH | 54.3 µg/m³ | 2.5 | Jul 17, 1999 | #7 of its top days |
| Albany-Schenectady-Troy, NY | 49.8 µg/m³ | 2.3 | Jul 17, 1999 | #9 of its top days |
| Worcester, MA-CT | 49.1 µg/m³ | 2.2 | Jul 17, 1999 | #6 of its top days |
| Providence-Warwick, RI-MA | 48.5 µg/m³ | 4.0 | Jul 17, 1999 | #7 of its top days |
| Springfield, MA | 47.7 µg/m³ | 2.2 | Jul 17, 1999 | #16 of its top days |
| New Haven-Milford, CT | 46.8 µg/m³ | 2.1 | Jul 17, 1999 | #25 of its top days |
| Hartford-West Hartford-East Hartford, CT | 43.9 µg/m³ | 3.9 | Jul 17, 1999 | #19 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.
CAPITA / Washington University
The episode analysis calls July 16-17 a major Northeast PM2.5 event and reports modeling that identified sulfate transported over long distances from the eastern U.S. and southeastern Canada as the most likely haze component.
A retained source supports the attributed cause for the stated place and dates.
This is an early scientific episode presentation preserved as a transcript rather than a regulator's formal event determination.
Likely Applicable reporting or a supported inference makes this the best current causal explanation.
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.