Temperature inversion Possible
Daily PM2.5 peaked at 48.7 µg/m³ near Denver across 1 covered metro home to 3.1 million people.
Counting the extra PM2.5 above each metro's typical day, this event handed the average affected resident ≈ 1.8 cigarettes. Across everyone living in the affected metros, that is the collective equivalent of ≈ 5.6 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
Denver's six-monitor PM2.5 rise coincided with increases in PM10, carbon monoxide, nitrogen dioxide, and sulfur dioxide, plus 12 hours of haze and restricted visibility. The exact-date NWS discussion independently documented a Denver inversion, light winds, and shallow haze that later ventilation was expected to erode. Together these support temperature-inversion trapping at possible confidence while leaving the emissions mixture unidentified.
Possible This is a transparent hypothesis supported by limited evidence or an explicit inference, not a definitive attribution.
U.S. EPA AirData hypothesis
EPA records show median daily PM2.5 of 45.1 micrograms per cubic meter across 6 monitors, 2.752 times the adjacent-window level. Compatible co-pollutant rises were pm10 (2.524x), carbon monoxide (1.551x), nitrogen dioxide (1.425x), sulfur dioxide (1.886x).
National Weather Service Denver/Boulder via Iowa Environmental Mesonet · Mar 2, 2015 hypothesis
On the event day, NWS Denver/Boulder reported that parts of Denver were under an inversion and that light winds and shallow haze persisted until showers and downslope flow began to erode the layer that evening.
National Weather Service surface observations via Iowa Environmental Mesonet · Mar 2, 2015 hypothesis
BROOMFIELD/JEFFCO recorded calm conditions during 46% of event hours and wind at or below three knots during 46%. Observations included 12 haze hours, 8 mist hours, minimum visibility 1.5 miles.
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 2.2 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.
Denver-Aurora-Lakewood, CO
Monitor comparison window 2015-02-27 to 2015-03-05 · PM2.5 target: I-25
162% above nearby days
11 calm hours observed
Haze reported during 12 hours
162% higher than surrounding days
83% higher than surrounding days
52% higher than surrounding days
41% higher than surrounding days
29% higher than surrounding days
8% lower than surrounding days
14 kt maximum · 11 of 24 observed hours calm
12 observed hours at or below 3 miles
15.8°F to 32°F
100% 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 |
|---|---|---|---|---|
| Denver-Aurora-Lakewood, CO | 48.7 µg/m³ | 2.2 | Mar 2, 2015 | #20 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 Denver/Boulder via Iowa Environmental Mesonet
On the event day, NWS Denver/Boulder reported that parts of Denver were under an inversion and that light winds and shallow haze persisted until showers and downslope flow began to erode the layer that evening.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The NWS discussion identifies the inversion and haze mechanism but does not apportion traffic, heating, industry, dust, or secondary aerosol.
National Weather Service surface observations via Iowa Environmental Mesonet
BROOMFIELD/JEFFCO recorded calm conditions during 46% of event hours and wind at or below three knots during 46%. Observations included 12 haze hours, 8 mist hours, minimum visibility 1.5 miles.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
A single surface station documents 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 45.1 micrograms per cubic meter across 6 monitors, 2.752 times the adjacent-window level. Compatible co-pollutant rises were pm10 (2.524x), carbon monoxide (1.551x), nitrogen dioxide (1.425x), sulfur dioxide (1.886x).
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, dust, 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.