volcanic_emissions Possible
Daily PM2.5 peaked at 25.6 µg/m³ near Urban Honolulu across 1 covered metro home to 998,747 people.
Why we think this happened
An exact-date NWS Honolulu discussion used HYSPLIT transport modeling to place Big Island vog over Oahu and added haze to the Oahu forecast. That is affirmative official transport evidence for volcanic emissions, but it is a forecast rather than monitor-level source apportionment, so the estimate remains possible.
Possible This is a transparent hypothesis supported by limited evidence or an explicit inference, not a definitive attribution.
National Weather Service Honolulu via Iowa Environmental Mesonet · Dec 16, 2009 official
Early on December 16, NWS Honolulu reported that a recent HYSPLIT run showed southwesterly flow capable of carrying vog north from the Big Island and said haze had been added to the Oahu forecast for that day.
State of Hawaii environmental review document hypothesis
The document records hazy skies due to vog on December 15, 2009 in West Hawaii.
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
Early on December 16, NWS Honolulu reported that a recent HYSPLIT run showed southwesterly flow capable of carrying vog north from the Big Island and said haze had been added to the Oahu forecast for that day.
View supporting source ↗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 1.2 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 “volcanic_emissions” explanation; it does not assign the cause by itself.
Urban Honolulu, HI
Monitor comparison window 2009-12-13 to 2009-12-19 · PM2.5 target: Kapolei
184% above nearby days
9 calm hours observed
Haze reported during 2 hours
184% higher than surrounding days
43% higher than surrounding days
About the same as surrounding days
About the same as surrounding days
86% higher than surrounding days
40% lower than surrounding days
9 kt maximum · 9 of 24 observed hours calm
0 observed hours at or below 3 miles
65°F to 81°F
90% 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 |
|---|---|---|---|---|
| Urban Honolulu, HI | 25.6 µg/m³ | 1.2 | Dec 16, 2009 | #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.
National Weather Service Honolulu via Iowa Environmental Mesonet
Early on December 16, NWS Honolulu reported that a recent HYSPLIT run showed southwesterly flow capable of carrying vog north from the Big Island and said haze had been added to the Oahu forecast for that day.
An official source supports the attributed cause for the stated place and dates.
The NWS statement is a modeled transport assessment and forecast, not a retrospective source-apportionment result for the Honolulu PM2.5 monitor.
State of Hawaii environmental review document
The document records hazy skies due to vog on December 15, 2009 in West Hawaii.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
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.