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June 3rd, 2011:

Tung Chung/Yuen Long Ozone levels

—–Original Message—–
From: []
Sent: 03 June, 2011 15:26
To: Hotmail
Cc: Edwin Town; Christian Masset;; James Middleton
Subject: Tung Chung/Yuen Long Ozone levels

Dear Mr. Furner,

Ozone is not a pollutant directly emitted into the air from particular
activities characteristic of urban or industrial areas, and are usually
referred to as secondary pollutant. It is formed by the photochemical
reaction of oxygen, nitrogen oxides (NOx) and volatile organic compounds
(VOCs) in the air under sunlight. It is the main component of photochemical
smog. Its production and concentration is dependent on the presence of the
pollutants mentioned as well as ultra-violet light intensity. In the
presence of volatile organic compounds, high concentrations of ozone are
formed. It usually takes a few hours for O3 to be formed and rise to its
peak level. Ozone and its precursors can be transported to rural areas
downwind of their sources during this period.

On the few days you mentioned, the weather in the region was generally fine
and dry with light to moderate east to northeast wind. The intense sunshine
(i.e high UV intensity) enhanced photochemical smog activities and formation
of ozone while the low wind speed could not effectively disperse the
photochemical smog, causing the API at some general air monitoring stations
raised over 100 in the afternoon. As the light intensity diminished later in
the afternoon, the photochemical reaction slowed down and the ozone level
dropped to lower concentrations.

You may wish to visit our website at for further


WM Pun

02/06/2011 07:34
Subject : Tung Chung/Yuen Long Ozone levels

Dear Mr Wong
Could you please help explain why we are continuing to see afternoon peaks
in Ozone in Tung Chung and Yuen Long? Ozone levels rapidly rose yesterday
from 25.1 at 11am to 106 at midday to 299.1 by 4pm with a dip back down
overnight. This has been happening for the last few days. Can you identify
the source? Why is this not happening in other parts of Hong Kong?