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September 26, 2018

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Typhoons purify the summer sky

SKIES may have been heavy and overcast in August but the people breathed easily compared to last year, thanks to a drop in ozone, according to the environmental protection bureau.

Excellent or good air quality — an air quality index no higher than 100 — was recorded on 28 days last month. The three slightly polluted days were the 23rd, 24th and 31st with ozone the major pollutant. In August 2017, 12 polluted days were recorded.

City air is usually cleanest in summer, especially in August, but last year August was exceptionally hot.

Ozone is usually generated on the hottest of days, as it is created by intense ultra violet rays. Pollutants from volatile organic compounds and nitrogen oxide can also create ozone.

This year, frequent typhoons helped to ease the heat as well as the accumulation of local pollutants. Apart from ozone, the average density of other pollutants like PM2.5, PM10, SO2 and NO2 also dropped of 25 to 48 percent.

The average daily density of PM2.5 was 16 micrograms per cubic meter last month, well within the World Health Organization recommended standard for PM2.5 of 25 micrograms.

Good air quality has continued into this month except for September 5 and 6 when the average AQI was around 160 with ozone the major pollutant.

There are new targets for clean air, aiming to bring down the year’s PM2.5 average density to 37 micrograms per cubic meter by 2020 and below 35 by 2022. The figure was 39 micrograms last year.


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