Volcanic gas carrying sulfur dioxide released from Mount Sakurajima in Kagoshima Prefecture is partly responsible for spikes in the level of the air pollutant PM2.5 in the Kanto and Tokai regions, according to the government’s Meteorological Research Institute.

The institute ruled out pollutants from China as a factor. China along with factory smoke and car exhaust have so far been seen as the most likely sources of the hazardous fine particulate matter.

The Chiba Prefectural Government has also discounted pollutants from China as a factor behind the PM2.5 rise earlier this month in some parts of the prefecture. It points the finger at climate conditions that apparently drew PM2.5-laced air into the affected areas.

PM2.5 particles measure up to 2.5 microns, or 2.5 thousandths of a millimeter, in diameter and are feared to cause health problems.

An official with the Meteorological Research Institute said that while Sakurajima’s environmental impact has been reported in Kyushu, “it is the first time its impact has become apparent in Kanto and other places.”

The findings of the institute, part of the Japan Meteorological Agency, will be presented next Tuesday at the fall meeting of the Meteorological Society of Japan.

According to the institute, PM2.5 concentrations can be more accurately predicted by incorporating data on volcanic eruptions and sulfur dioxide emissions.

The institute said Sakurajima became more active in July, spewing a plume of smoke over 3,000 meters high.

A simulation showed that the level of sulfate aerosols from volcanic gas was highest in areas near Kagoshima Prefecture, where the volcano is located, and was also high along the south side of Honshu.

In Kanto and Tokai, as well as in the Kinki region, which includes Osaka, a particularly high level of PM2.5 was observed in early July.

Researchers compared the data observed in Shizuoka Prefecture, where PM2.5 concentration showed marked increases in July, with the data on sulfate aerosol levels obtained through the simulation. The result indicated a strong correlation between them, according to the institute.

The study found that the volcanic gas from Sakurajima impacted eastern and central Japan. Depending on which direction the wind blows, it could also affect western regions such as Kinki and Shikoku, according to the institute.

Satellite data from the U.S. space agency NASA also showed similar trends in atmospheric opacity in the period under observation.

As air pollutants from mainland China were flowing over the north side of the Japanese archipelago around that time, the likelihood was low that they caused pollutant levels to surge in Kanto and Tokai, according to the institute.

On Tuesday, Chiba Prefecture said the recent PM2.5 spike in locations including the city of Ichihara was due to higher humidity in the atmosphere that made airborne particles absorb moisture and become heavier.

“It was due to a combination of weather conditions. It is less likely the pollutants flew over from China,” an official with the prefecture said.

According to the prefecture, on the night of Nov. 3, humidity rose along with a stabilization of the atmosphere in the prefecture. As winds blew into areas near the cities of Ichihara and Chiba the following day from the early hours to later in the morning, air containing high levels of PM2.5 is thought to have gathered in these areas, according to the prefecture.

On Nov. 4, the prefecture issued alerts on all parts of the prefecture saying that the pollutant level might exceed the daily average limit of 70 micrograms per cu. meter set under the provisional national guidelines, but the actual level came to 57 micrograms at most — observed at a monitoring station in Ichihara.

A prefectural official at that time said the cause of the anticipated spike was “unknown.”