As part of efforts to improve the dismal air quality in Japan's major cities, the Environment Agency will announce today that diesel automobiles will have to meet stricter standards beginning in 2002, Agency officials said Monday.

The revised standards will obligate vehicles to reduce emissions of nitrogen oxides by between 25 percent and 30 percent and particulate matters by between 28 percent and 35 percent, the officials said.

Vehicles under 1.7 tons -- passenger cars and small size trucks and buses -- must comply with the new standards starting in October 2002, while vehicles between 1.7 and 12 tons must do so in 2003 and those over 12 tons in 2004, according to the officials.

In addition to the stiffer regulations, carmakers will also have to equip automobiles with a device to prevent "blow-by gas," which leaks from gaps between the pistons and contributes to air pollution.

The Transport Ministry will put the revised standards into effect in accordance with the Road Trucking Vehicle Law, they said.

The Environment Agency is planning to make the standards for smog-forming nitrogen oxides and fine sooty particle matters twice as stiff as the new regulations sometime around 2007, agency officials said.