Taiyuan, capital of North China's Shanxi province, saw its ozone concentration decrease for the first time over six years of growth. 

The number of days with good air quality in urban Taiyuan was 171 from January to September, an increase of 23 over the same period last year. The number of days with heavily polluted air declined by seven year-on-year to four. The urban comprehensive pollution index decreased 2.4 percent year-on-year to 4.97. 

Since Feb 28, 12 companies in the city have completed the upgrading and transformation of pollution control facilities, while 318 have replaced activated carbon and finished various transformation and pollution control tasks. 

During the same period, the city's ecological environment department, meteorological department and ecological environment monitoring center carried out discussions every day and joint air quality evaluations every week, so as to reduce industrial and mobile source pollution under the conditions of adverse weather conditions as well as reduce ozone production. 

From May to September, the ozone concentration decreased every month except for June. In particular, the ozone concentration in August fell to 175 micrograms per cubic meter, down 15.5 percent year-on-year and realizing the largest decline in recent six years.