Regional secondary PM Possible
Daily PM2.5 peaked at 55.1 µg/m³ near Birmingham across 1 covered metro home to 1.2 million people.
Counting the extra PM2.5 above each metro's typical day, this event handed the average affected resident ≈ 1.9 cigarettes. Across everyone living in the affected metros, that is the collective equivalent of ≈ 2.3 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
Birmingham-Hoover, AL's 7-monitor PM2.5 episode reached 39.15 µg/m³, 1.583 times the adjacent-window level, during 58% light-wind hours, 7 haze hours. Exact-date regional speciation sites measured 84.0% to 84.0% of reconstructed mass as ammonium sulfate plus ammonium nitrate. Together these measurements support regional secondary PM at possible confidence; the regional sites did not directly sample the city and no particular source region is assigned.
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 14, 1999 hypothesis
BIRMINGHAM MUNI recorded calm conditions during 25% of event hours and wind at or below three knots during 58%. Observations included 7 haze hours, 4 mist hours, minimum visibility 4 miles.
U.S. EPA AirData hypothesis
EPA records show median daily PM2.5 of 39.15 micrograms per cubic meter across 7 monitors, 1.583 times the adjacent-window level. Co-pollutants at least 10% above the adjacent-window comparison were pm10 (1.2x), sulfur dioxide (1.729x). Other measured comparisons were carbon monoxide (0.553x), nitrogen dioxide (1.087x).
U.S. EPA AirData Chemical Speciation · Jul 14, 1999 hypothesis
On the exact event date, 1 EPA speciation site within 250 km measured a majority of reconstructed PM2.5 mass as ammonium sulfate plus ammonium nitrate: Sipsey Wilderness (101.3 km; 84.0%; reconstructed mass 29.0 µg/m³).
Inference, not direct attribution
nearby_exact_date_secondary_inorganic_regional_context.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
nearby_exact_date_secondary_inorganic_regional_context.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 2.5 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.
Birmingham-Hoover, AL
Monitor comparison window 1999-07-11 to 1999-07-17 · PM2.5 target: Wylam
34% above nearby days
6 calm hours observed
Haze reported during 7 hours
34% higher than surrounding days
18% higher than surrounding days
42% lower than surrounding days
About the same as surrounding days
56% higher than surrounding days
40% higher than surrounding days
7 kt maximum · 6 of 24 observed hours calm
0 observed hours at or below 3 miles
69°F to 83°F
93% maximum
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 · 1999-07-14
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 | 55.1 µg/m³ | 2.5 | Jul 14, 1999 | #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
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.
U.S. EPA AirData Chemical Speciation
On the exact event date, 1 EPA speciation site within 250 km measured a majority of reconstructed PM2.5 mass as ammonium sulfate plus ammonium nitrate: Sipsey Wilderness (101.3 km; 84.0%; reconstructed mass 29.0 µg/m³).
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The usable composition site was 101.3 km from the affected-place PM2.5 target and did not directly measure the city air.
National Weather Service surface observations via Iowa Environmental Mesonet
BIRMINGHAM MUNI recorded calm conditions during 25% of event hours and wind at or below three knots during 58%. Observations included 7 haze hours, 4 mist hours, minimum visibility 4 miles.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
A single surface station documents weak ventilation and obscuration but not the full vertical profile or particle source mix.
U.S. EPA AirData
EPA records show median daily PM2.5 of 39.15 micrograms per cubic meter across 7 monitors, 1.583 times the adjacent-window level. Co-pollutants at least 10% above the adjacent-window comparison were pm10 (1.2x), sulfur dioxide (1.729x). Other measured comparisons were carbon monoxide (0.553x), nitrogen dioxide (1.087x).
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The pollution fingerprint does not uniquely apportion traffic, heating, industry, wood combustion, or secondary aerosol.
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.