Sulfur dioxide emissions at Kilauea volcano summit have dropped sharply in recent weeks compared to the nine months since a new vent formed there in March, scientists said Wednesday.

But those hopeful the decline will lead to a sharp decline in vog — or volcanic smog — in the Islands shouldn't get too optimistic.

Jim Kauahikaua, scientist-in-charge at Hawaiian Volcano Observatory on the Big Island, said he doesn't know if the vent's emissions will remain low or spike again.

"It could remain in a state of unrest," Kauahikaua said.

Sulfur dioxide forms vog when it combines with dust and sunlight. It creates acid rain when mixed with precipitation.

Last year's elevated emissions put Big Island residents and visitors under exceptionally heavy vog for months, leading to the destruction of protea, lettuce, and other crops on Big Island farms near the summit.

The vog also has irritated the throats and skin of many residents, especially those living close to Kilauea. Vog is known to aggravate asthma and other respiratory conditions.

When the vent at Kilauea's Halemaumau crater formed in March, the summit began belching as much as 2,200 tons of sulfur dioxide each day, up from about 150 tons before.

But since Dec. 4, the summit's gas plume has been wispy and translucent.

An infrared camera showed the vent was filled with rubble and rocky debris, while vent temperatures have dropped.

On Wednesday, sulfur dioxide emissions were measured at about 850 tons.

Kauahikaua said he would be looking to see whether the summit produces any more volcanic rock to determine whether activity there is ending.

The summit also has had explosions in the past year. Shortly after the vent opened in March, a pre-dawn explosion at the summit rained gravel-size rocks over 75 acres.

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Hawaiian Volcano Observatory: http://hvo.wr.usgs.gov/