Temperature inversion Possible
Daily PM2.5 peaked at 41.7 µg/m³ near Raleigh across 1 covered metro home to 1.6 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 ≈ 2.2 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-date NWS discussion documents a capped inversion over the central North Carolina air mass during a short, certified Raleigh particle episode. The evidence supports a possible accumulation mechanism, not a specific emissions source.
Possible This is a transparent hypothesis supported by limited evidence or an explicit inference, not a definitive attribution.
National Weather Service Raleigh via Iowa Environmental Mesonet · Aug 6, 2004 hypothesis
On August 6, NWS Raleigh reported lingering low-level moisture beneath a capped inversion shown by the Greensboro sounding, with clouds expected to limit daytime heating across central North Carolina.
U.S. EPA AirData hypothesis
EPA monitor records show a short Raleigh particle episode: continuous PM2.5 reached 38.2 µg/m³ on August 4, the Millbrook FRM sample reached 41.7 µg/m³ on August 6, and continuous PM2.5 fell to 12.8 µg/m³ on August 7. PM10 also fell from 44 µg/m³ on August 4 to 12 µg/m³ on August 6, while event-day ozone, CO, and SO2 were not similarly elevated.
North Carolina Division of Air Quality · Jan 1, 2005 hypothesis
North Carolina's annual monitoring report lists 41.7 µg/m³ as the highest of 330 valid 24-hour PM2.5 samples at Raleigh's Millbrook monitor in 2004, corroborating that the August 6 value was a real local annual maximum.
Inference, not direct attribution
Documented inversion overlap: The event overlaps a documented inversion period, but the retained material does not directly attribute the measured PM2.5 episode.
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
Documented inversion overlap. The event overlaps a documented inversion period, but the retained material does not directly attribute the measured PM2.5 episode.
Air-mass movement
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.
Local response
EPA monitors recorded the event across 1 metro. The peak daily concentration is roughly 1.9 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 “Temperature inversion” explanation; it does not assign the cause by itself.
Raleigh, NC
Monitor comparison window 2004-08-03 to 2004-08-09 · PM2.5 target: Millbrook School
83% above nearby days
43% lower than surrounding days
83% higher than surrounding days
43% lower than surrounding days
22% lower than surrounding days
52% lower than surrounding days
34% lower than surrounding days
About the same as surrounding days
8% lower 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.
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 |
|---|---|---|---|---|
| Raleigh, NC | 41.7 µg/m³ | 1.9 | Aug 6, 2004 | #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.
North Carolina Division of Air Quality
North Carolina's annual monitoring report lists 41.7 µg/m³ as the highest of 330 valid 24-hour PM2.5 samples at Raleigh's Millbrook monitor in 2004, corroborating that the August 6 value was a real local annual maximum.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The report validates the monitored value but does not attribute the episode to an emissions source or meteorological mechanism.
National Weather Service Raleigh via Iowa Environmental Mesonet
On August 6, NWS Raleigh reported lingering low-level moisture beneath a capped inversion shown by the Greensboro sounding, with clouds expected to limit daytime heating across central North Carolina.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The discussion documents an atmospheric trapping layer, not a particular emitting source.
U.S. EPA AirData
EPA monitor records show a short Raleigh particle episode: continuous PM2.5 reached 38.2 µg/m³ on August 4, the Millbrook FRM sample reached 41.7 µg/m³ on August 6, and continuous PM2.5 fell to 12.8 µg/m³ on August 7. PM10 also fell from 44 µg/m³ on August 4 to 12 µg/m³ on August 6, while event-day ozone, CO, and SO2 were not similarly elevated.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The scheduled FRM and continuous measurements are not interchangeable and were not available on every date; together they establish episode timing, not a daily chemical composition series.
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.