Honolulu, HI Air Quality Index (AQI) Today

Honolulu’s normal state is ventilated. The northeast trades cross open Pacific for hundreds of miles before reaching Oʻahu, arrive with almost nothing in them, break against the Koʻolau Range and descend over the city on the leeward south shore. There is no upwind continent, no regional industrial corridor, and no neighboring metro whose pollution can drift in. What the monitors read locally is what fits between the ridge and the water: H-1 traffic through a narrow coastal plain, Honolulu Harbor, and sea salt aerosol that counts as particulate without anything having burned.

The exception arrives from about 200 miles southeast. Kīlauea releases sulfur dioxide continuously, and the USGS reports summit emissions typically running between 1,000 and 5,000 tonnes a day even during pauses between eruptive episodes. That SO2 converts to fine sulfate aerosol in sunlight and moisture, which is vog. Under trade wind flow it collects along the Big Island’s Kona side and blows out to sea. When the trades break down and southerly Kona winds take over, more common in winter, the vog moves up the island chain instead, and Oʻahu’s readings climb under a haze with no fire behind it.

One night a year is purely local. Fireworks lift PM2.5 at the state health department’s Oʻahu monitors, in Honolulu, at Sand Island and at Kapolei, in the hours after midnight on New Year’s. The trades usually clear it before morning.