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Rain vs. Pollution in June

Thought clean air was the silver lining in rainy June? Think again

Elaine Wu – Updated on Jul 13, 2008 – SCMP

Many thought that if there was something worth celebrating about the 24 days of rain last month, it would be that the air would be cleaner.

Rain normally washes pollutants from the air. Not last month.

Although June was Hong Kong’s wettest month in 125 years of recorded weather history, Environmental Protection Department figures show pollution was far worse than in the same month in 2007 and 2006.

The department recorded 47 hours of high air pollution levels, compared to 16 hours in June last year and only 3.5 hours in June 2006.

This has everyone – from meteorologists to clean-air advocates – scratching their heads.

“I don’t have a clue as to why we would have more high-air-pollution index [API] days,” Leung Wing-mo, senior scientific officer at the Hong Kong Observatory, said.

“When it is raining, the suspended particulates and other pollutants will normally be washed away. We have had only six days without rain for the whole month.”

Air pollution index readings are categorised as low, medium, high, very high and severe. Levels last month were low to high.

Christian Masset, chairman of Clear the Air, said his organisation had already sought expert help to understand the phenomenon.

“I can’t recall [this happening before],” he said. “There is something we don’t know that we need to understand. There must be a link with heavier rain and air pollution.”

A departmental spokeswoman said that while rain does affect air quality in the short run, other factors such as wind direction, wind speed, solar radiation, cloud cover, humidity and temperature played a role.

She said it was not appropriate to gauge air quality by comparing data from a certain month with that from the same month a year earlier.

“A more scientific way to assess whether air quality is improving or deteriorating is to look at the changes in average pollutant concentrations on an annual basis,” she said.

The department found that average air pollutant concentrations last year were comparable to levels in 2005 and 2006, and better than those in 2004, she said.

Christine Loh Kung-wai, chief executive of think-tank Civic Exchange, said: “When there is a lot of rain, the rain does pat down emissions temporarily. But we must not forget that pollution is being emitted all the time.

“As for wind, if the wind is strong and blows away the pollution, that’s when we have clearer days. The summer months have the best visibility due to wind direction. This does not mean pollution is not being emitted, it is just being carried elsewhere faster.”

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