Stagnant pollution mixture Possible
Daily PM2.5 peaked at 63 µg/m³ near Birmingham across 1 covered metro home to 1.2 million people. 1 metro 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 ≈ 2.7 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
A ten-monitor PM2.5 rise occurred with compatible PM10 and carbon monoxide increases. Exact-date NWS products kept Birmingham under the influence of an upper ridge, restored haze to the forecast as visibility struggled above three miles, and described hazy skies into the night; the surface station was calm during two-thirds of event hours. This supports stagnant mixed-pollution accumulation as a possible mechanism.
Possible This is a transparent hypothesis supported by limited evidence or an explicit inference, not a definitive attribution.
National Weather Service surface observations via Iowa Environmental Mesonet · Jul 26, 2005 hypothesis
Birmingham's official station recorded 15 hours of haze; 79.2% of event hours had wind at or below three knots and 66.7% were calm.
National Weather Service Birmingham via Iowa Environmental Mesonet · Jul 26, 2005 hypothesis
The exact-date NWS update restored haze to the forecast because visibility was struggling to rise above three miles at many locations; the broader discussion kept Alabama under the ridge's influence for one more day.
U.S. EPA AirData · Jul 26, 2005 hypothesis
EPA records show median daily PM2.5 of 59.15 micrograms per cubic meter across ten monitors, more than twice the adjacent-window level, with compatible PM10 and carbon monoxide increases.
Inference, not direct attribution
official_surface_weak_dispersion_multimonitor_accumulation.v1: This inference is recorded in the catalog methodology.
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
official_surface_weak_dispersion_multimonitor_accumulation.v1. This inference is recorded in the catalog methodology.
Air-mass movement
NOAA HYSPLIT modeled 12 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 1 metro. The peak daily concentration is roughly 2.9 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 “Stagnant pollution mixture” explanation; it does not assign the cause by itself.
Birmingham-Hoover, AL
Monitor comparison window 2005-07-23 to 2005-07-29 · PM2.5 target: North Birmingham
71% above nearby days
77% higher than surrounding days
71% higher than surrounding days
77% higher than surrounding days
63% higher than 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
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 Birmingham-Hoover
Peak-date arrivals · 2005-07-26
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 |
|---|---|---|---|---|
| Birmingham-Hoover, AL | 63 µg/m³ | 2.9 | Jul 26, 2005 | #3 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.
National Weather Service surface observations via Iowa Environmental Mesonet
Birmingham's official station recorded 15 hours of haze; 79.2% of event hours had wind at or below three knots and 66.7% were calm.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
One surface station cannot fully characterize metropolitan ventilation, and haze is not source-specific.
National Weather Service Birmingham via Iowa Environmental Mesonet
The exact-date NWS update restored haze to the forecast because visibility was struggling to rise above three miles at many locations; the broader discussion kept Alabama under the ridge's influence for one more day.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The discussion documents haze and the synoptic setting but does not identify particle composition or source shares.
U.S. EPA AirData
EPA records show median daily PM2.5 of 59.15 micrograms per cubic meter across ten monitors, more than twice the adjacent-window level, with compatible PM10 and carbon monoxide increases.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The fingerprint supports mixed accumulation but cannot distinguish local from regional source shares.
Remote Sensing
The regional analysis describes central Alabama summers as frequently hot, humid, and stagnant under the Bermuda High, with inversions that can trap particles near the surface. That mechanism is consistent with the event fingerprint for July 26, 2005.
Context for the event; this item does not establish the attributed cause by itself.
This source describes the regional summer mechanism and a later fire episode; it is not a direct attribution of Birmingham's July 26, 2005 PM2.5 event.
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.