58% above nearby days
Fireworks Possible
1999-07-05 Los Angeles-Long Beach-Anaheim, CA fireworks episode
Daily PM2.5 peaked at 85.6 µg/m³ near Los Angeles across 1 covered metro home to 13 million people.
Counting the extra PM2.5 above each metro's typical day, this event handed the average affected resident ≈ 3.2 cigarettes. Across everyone living in the affected metros, that is the collective equivalent of ≈ 41 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
The basis for “Fireworks”
The Los Angeles one-day peak falls immediately after Independence Day and the versioned holiday-fireworks rule applies. No exact local attribution was recovered.
Possible This is a transparent hypothesis supported by limited evidence or an explicit inference, not a definitive attribution.
Inference, not direct attribution
Holiday timing: The event falls in the July 3–5 or December 31–January 1 fireworks window. That makes fireworks a reasonable hypothesis, but no retained source directly attributes this event.
Evidence path
From source to city
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.
- 1Inferred
Source or mechanism
Pattern-based explanation
Holiday timing. The event falls in the July 3–5 or December 31–January 1 fireworks window. That makes fireworks a reasonable hypothesis, but no retained source directly attributes this event.
- 2Research gap
Air-mass movement
The transport path has not been reconstructed
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.
- 3Documented
Local response
85.6 µg/m³ near Los Angeles
EPA monitors recorded the event across 1 metro. The peak daily concentration is roughly 3.9 cigarette-equivalents at the outdoor daily rate.
Inspect the pollutant and weather signals ↓
Event autopsy
What the instruments saw
Before, during, and after the episode: nearby pollutant monitors and surface-weather stations show what changed. This context tests the “Fireworks” explanation; it does not assign the cause by itself.
Los Angeles-Long Beach-Anaheim, CA
Monitor comparison window 1999-07-02 to 1999-07-08
37% lower than surrounding days
About the same as surrounding days
Pollution fingerprint
Metro-wide median of reporting monitors PM2.5 36.1 µg/m³
58% higher than surrounding days
Before11.1 µg/m³ Event36.1 µg/m³ After22.9 µg/m³
PM10 26 µg/m³
About the same as surrounding days
Before20 µg/m³ Event26 µg/m³ After29 µg/m³
Carbon monoxide 0.806 ppm
About the same as surrounding days
Before0.504 ppm Event0.806 ppm After0.965 ppm
Nitrogen dioxide 24.4 ppb
About the same as surrounding days
Before16.4 ppb Event24.4 ppb After31.2 ppb
Sulfur dioxide 3.48 ppb
36% higher than surrounding days
Before2.30 ppb Event3.48 ppb After5.39 ppb
Ozone 0.038 ppm
25% higher than surrounding days
Before0.030 ppm Event0.038 ppm After0.028 ppm
Atmospheric conditions
Nearby EPA weather observations9% lower than surrounding days
About the same as surrounding days
About the same as surrounding days
37% 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.
Measured daily PM2.5 at EPA monitors, as cigarette-equivalents
EPA monitors sit in metro areas, so the colour clusters there. Land between and beyond
them is unmeasured, not clean: this map shows where IAQng has readings, not the extent
of the smoke.
—
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.
Affected metros
| Metro | Peak PM2.5 | Cigarettes, whole event | Peak day | All-time standing |
|---|---|---|---|---|
| Los Angeles-Long Beach-Anaheim, CA | 85.6 µg/m³ | 3.9 | Jul 5, 1999 | #16 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
What supports this account
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.
Holiday timing pattern
IAQng deterministic inference policy
The event peak falls on 1999-07-05 and the event lasts no more than two days. Under holiday_fireworks_possible.v1, that timing is consistent with a possible fireworks contribution.
What it supports
A possible fireworks contribution; this is a reproducible inference, not direct occurrence or causal evidence.
Limit
Pre-event and post-event temporal-shape data were not supplied.
No news search has been run for this episode. Its evidence is agency records only, and an absent article is not a finding that none exists.
What remains uncertain
- The retained evidence is scoped to the places and dates stated; it is not a catalog promotion.
Status and provenance
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.