SHANGHAI — Forty-one major cities in China's Yangtze River Delta have experienced a significant improvement in air quality, with a 31.4-percent decline in the average concentration of PM2.5 during the 13th Five-Year Plan period (2016-2020), a Shanghai environment official said at a news conference on Jan 15.

The cities' average density of PM2.5 was 35 micrograms per cubic meter in 2020, meeting the country's second-level air quality standard, said Bai Guoqiang, chief engineer of the Shanghai municipal bureau of ecology and environment.

Shanghai's average concentration of PM2.5 was 32 micrograms per cubic meter in 2020, 36 percent lower than the reading in 2015 and down from 35 micrograms per cubic meter in 2019.

The PM2.5 reading is a gauge monitoring airborne particles of 2.5 microns or less in diameter, which can penetrate deep into people's lungs.

In 2019, China unveiled an outline for the regional integrated development of the Yangtze River Delta, which includes Shanghai and the provinces of Jiangsu, Zhejiang and Anhui.

The delta region is one of the country's most economically active, open and innovative regions, and produces about one-fourth of the national GDP.