Photochemical

VNU Journal of Science, Earth Sciences 24 (2008) 169-175
Photochemical smog introduction and episode selection for the ground-level ozone in Hanoi, Vietnam
Dam Duy An1, Hoang Xuan Co1,*, Nguyen Thi Kim Oanh2
1 College of Science, VNU
2 Asian Institute of Technology, Thailand
Received 18 September 2008; received in revised form 20 December 2008.
Abstract. Ozone (O3) is a secondary pollutantformed in the atmosphere throughout a complex non-linear chemical reaction involving two classes of precursors: the reactive volatile organic compounds (VOCs) and the oxides of nitrogen (NOx) in the presence of sunlight.
The rapid urbanization and industrialization in Vietnam have brought about high air pollutant emissions including the O3 precursors. Ground level O3 may already be high in largecities like Hanoi and Ho Chi Minh City. The O3 episode is very important for scene of view of photochemical smog in Hanoi. Ozone episodes are selected on the days which have a high concentration that lasts for at least two days time. During the episode selection, ozone concentrations larger than 46ppb were observed at two stations (the Lang and Lac Long Quan stations) in March. The maximum value of74ppb was measured at the Lang station at 14:00 on March 3. This episode was observed in a common meteorological condition for this time of the year.
Keywords: Photochemical smog; Ozone; Volatile organic compounds; Secondary pollutant.
1. Introduction*
Photochemical smog occurs in the troposphere, the lower portion of our atmosphere. Ground-level ozone, the primary component of photochemicalsmog, is the most prevalent pollutant that has been known to cause a serious air pollution problem in many developed countries over the past few decades. In this paper, only ground-level ozone is considered as a pollutant.
Ozone (O3) is a secondary pollutant formed in the atmosphere through a complex non-linear chemical reaction involving two classes of
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* Corresponding author. Tel.:84-913594443. E-mail: cohx@vnu.edu.vn
precursors: reactive volatile organic compounds (VOCs) and oxides of nitrogen (NOx) in the presence of sunlight. Ozone formation can be described as either VOC- or NOx- sensitive, depending on VOC/NOx ratios, VOC reactivity, and other factors [10].
A stagnant air mass, normally resulting from high atmospheric pressure and light winds, limits the pollutiondispersion leading to accumulation of the formed O3 to high levels. It should be noted that VOCs, NOx and ozone do occur naturally in the lower atmosphere, too. However, human activities - fossil fuel use, in particular - have greatly increased the amounts of ozone in urban areas.
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VOCs (also called hydrocarbons) are themost important constituents of oil and natural gas. The major man-made sources of VOC emissions are motor vehicles, evaporation of gasoline, solvents, oil-based paints, and petrochemical industry. NOx are mainly produced by burning coal, oil and gas. The exhaust from fossil fuel combustion in motor vehicles is the primary source, followed by fuel burning in homes, businesses, factories and powerplants.
The temperature also affects ozone formation through the change in reaction rates. In particular, a high temperature causes an increase in VOC evaporative emissions. The warming temperature is associated with increased natural emissions of VOCs. Higher outdoor temperature could also enhance energy consumptions produced by fossil fuel combustion, which lead to emissions of NOx - the majorpollutant from fuel combustion.
Ground-level ozone built up over the cities that produce large amounts of VOCs and NOx. But it can also migrate up to several hundred kilometers downwind. Topography and meteorological conditions may enhance ozone build-up. Modeling approach is a powerful tool to study the complex processes leading to O3 formation and build up.
2. Photochemical smog pollution...