Study Of Visibility In Shanghai

Visibility is usually defined to mean the furthest distance one can see and identify an object in the atmosphere.As is known to all, as a complex atmospheric phenomenon, visibility is related to air quality and meteorological condition. Several recent research show that visibility is tied up to particle and relative humidity, especially play an important role in sulfate particle's deliquescence. As weather system is concerned, visibility results from the interaction between variable scale weather system and air pollution process including atmospheric chemistry actions. For special coastal cities, Aerosol transportation and photochemical smog formation shall be considered from the influence of urban heat island current and sea-land wind under different weather conditions.Based on 5-year's visibility data, meteorological data and pollutant data from 1998 to 2002, the paper presents a study for the visibility's characteristic and its impact, especially to second particle derived from gas-to-particle transformation. Then the author developed a horizontal visibility forecast mode with system identification. Detailed contents and results are as followed:1. There is a great relationship between visibility variety and weather condition or pollutant emissions in Shanghai. The diurnal variation of visibility in Shanghai was apparent. There was low visibility in the morning and high visibility in the afternoon. This phenomenon was correlated to meteorological conditions, especially mixed-layer height. There appears high visibility during Jul. to Oct. in one year as for monthly variation.2. Air pollution factors include sulfate or nitrate second particles derived from gas-to-particle transformation as well as original particles. It will be better/ appropriate to atmospheric physics and chemistry mechanism for visibility variety if gas-to-particle chemistry actions are taken into account.3. Based on gas-phase to second-particle theory, we developed a gas-to-particle transformation mechanism including two chemical actions: SO₂ to SO₄= and NO_X to NO₃.The results show that the transformation rate of SO₂ and NO_X or the formation rate of nitric acid is increasing with temperature. We got simulated rates 2.3%/h in Beijing and 2.9%/h in Guangzhou in summer which are consistent to monitoring values/observations.4. The factors which impact visibility are induced into 6 species by factor analysis:F1-general meteorological factor; F2-pollutant factor; F3-gas to particle transformation rate; F4-absolute humidity; F5-cloudage/cloud cover; F6-wind direction, wind speed/velocity and atmospheric stability.5. The horizontal visibility forecast mode based on/with system recognition theory took into account the influence of meteorological factor and pollutant factor to visibility as well as the influence of forecast history to current forecast outcome in visibility forecast functions. According to three-year's forecast results, the forecast precision reaches 68.0%, while the primary forecast precision is 81.0%. The mode has a simple structure and high forecast precision, which shall be used in visibility forecast operation.This research is significant not only to help us make it clear the physical and chemical mechanism of coastal city's visibility variety but also to improve urban atmospheric environment and build-up favorable urban visualize during EXPO 2010 Shanghai.