Temperature inversion Possible
Daily PM2.5 peaked at 38.6 µg/m³ near Little Rock across 1 covered metro home to 769,258 people.
Why we think this happened
Three Little Rock-area regulatory monitors recorded PM2.5 at 2.016 times the adjacent-window comparison. Surface observations recorded wind at or below three knots during 54% of event-day hours and haze during 18 hours. A nearby event-date radiosonde measured a surface-based inversion of 4.1 degrees Celsius through 264 meters above ground. Together, these observations support temperature-inversion accumulation as a possible mechanism, without identifying the trapped particle sources.
Possible This is a transparent hypothesis supported by limited evidence or an explicit inference, not a definitive attribution.
NOAA/NWS radiosonde observations via Iowa Environmental Mesonet · Oct 16, 1999 hypothesis
North Little Roc AR/US's 12 UTC radiosonde profile measured temperature rising 4.1 degrees Celsius through 264 meters above ground, a 264-meter surface-based inversion. The NOAA/IGRA station is 9.0 kilometers from the PM2.5 target monitor.
National Weather Service surface observations via Iowa Environmental Mesonet · Oct 16, 1999 hypothesis
Little Rock airport observations report haze during 18 of 24 event-day hours. Winds were calm during 12 hours and at or below three knots during 13 hours; visibility fell to 2.5 miles.
U.S. EPA AirData hypothesis
EPA records show median daily PM2.5 of 38.6 micrograms per cubic meter across 3 Little Rock-area monitors, 2.016 times the adjacent-window level. Carbon monoxide was 1.293, nitrogen dioxide 2.021, and sulfur dioxide 1.3 times their adjacent-window comparisons.
Inference, not direct attribution
exact_radiosonde_inversion_plus_multimonitor_accumulation_possible.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
exact_radiosonde_inversion_plus_multimonitor_accumulation_possible.v1. This inference is recorded in the catalog methodology.
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 1.8 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.
Little Rock-North Little Rock-Conway, AR
Monitor comparison window 1999-10-13 to 1999-10-19 · PM2.5 target: PARR
144% above nearby days
12 calm hours observed
Haze reported during 18 hours
144% higher than surrounding days
26% higher than surrounding days
66% higher than surrounding days
23% higher than surrounding days
50% higher than surrounding days
14 kt maximum · 12 of 24 observed hours calm
2 observed hours at or below 3 miles
61°F to 85°F
97% 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.
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 |
|---|---|---|---|---|
| Little Rock-North Little Rock-Conway, AR | 38.6 µg/m³ | 1.8 | Oct 16, 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.
NOAA/NWS radiosonde observations via Iowa Environmental Mesonet
North Little Roc AR/US's 12 UTC radiosonde profile measured temperature rising 4.1 degrees Celsius through 264 meters above ground, a 264-meter surface-based inversion. The NOAA/IGRA station is 9.0 kilometers from the PM2.5 target monitor.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The radiosonde station is 9.0 kilometers from the PM2.5 target monitor and may not represent every part of the metro airshed.
National Weather Service surface observations via Iowa Environmental Mesonet
Little Rock airport observations report haze during 18 of 24 event-day hours. Winds were calm during 12 hours and at or below three knots during 13 hours; visibility fell to 2.5 miles.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
One airport station cannot determine the vertical mixing depth or apportion the observed particles by source.
U.S. EPA AirData
EPA records show median daily PM2.5 of 38.6 micrograms per cubic meter across 3 Little Rock-area monitors, 2.016 times the adjacent-window level. Carbon monoxide was 1.293, nitrogen dioxide 2.021, and sulfur dioxide 1.3 times their adjacent-window comparisons.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The co-pollutant pattern supports mixed combustion and precursor accumulation but does not assign source-sector shares.
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.