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BEIJING, Aug. 27 -- Beijing saw more days with good air quality and a record-low average PM 2.5 density in July, local authorities said Tuesday.

The average concentration of PM 2.5 in the city was 37 micrograms per cubic meter last month, down 15.9 percent year on year and the lowest in the same period since the data was first collected in 2013, according to the Beijing Municipal Ecological Bureau.

The bureau said Beijing saw 125 good air quality days in the Jan.-July period and only three heavily polluted days, down from eight over the same period last year.

The average PM 2.5 density in the first seven months dropped 13.5 percent year on year to 45 micrograms per cubic meter.

Since July, local authorities have required all heavy-duty natural gas vehicles and heavy-duty diesel vehicles in Beijing's public transport and sanitation sectors to meet the stricter State VI emission standards.

Local authorities have also intensified efforts to curb pollutants from dust and volatile organic compounds.