Analysis On Variation Of Atmospheric Visibility In Shanghai Pudong 2009

Visibility is an important index sign to measure the atmospheric transparent condition. It reflects not only the quality of atmospheric environment, but also is bound up with human beings' social life. With water on three sides, Shanghai is located in the mid-latitudes and such environment brings plenty of water to form low visibility events (such as fog). By now, fog has become a major factor which endangers Pudong area.Using the continuous observations'data (visibility, relative humidity, temperature, wind et al.) from Shanghai Pudong automatic weather station and daily API data in 2009, the multi-timescales variation characteristics and the relations between visibility and meteorological elements are studied in this paper. With these data, the variation characteristics of low visibility are also discussed. What's more, the absorption of water by ambient aerosols and the influence of ambient relative humidity on the extinction coefficient are investigated under the assumptions that the atmosphere is in static stability during low visibility processes, which are caused by radiative cooling. Furthermore, the subtle features of meteorological elements are also studied with such high temporal resolution (1min) data. The results are as follows:(1) There are obviously daily, synoptic-scale, quasi-two-weekly, intra-seasonal and seasonal variations of visibility in Pudong 2009 by statistical method and wavelet analysis method. The frequency of low visibility events is higher in the night than in the daytime and accounts for 52.5% from 4a.m. to 8a.m. In seasonal variation, the frequency of low visibility events is high in winter, spring and early summer. It is easier to form long-lasting time low visibility enents in later autumn and early winter.(2) The variations of visibility in Pudong are associated with meteorological elements, weather phenomenons, and air pollution load. There is positive relationship between visibility and wind speed. From Beaufort Wind Scale 0~3, the visibility grows with the increase of wind speed, but the visibility is low when the wind is at Beaufort Wind Scale 4. The anti correlation exists between seasonal variation of visibility and mist (including fog) days. The pertinence of visibility and air pollutants load, or weather condition illustrates that relative humidity is the main influencing factor to visibility, then the wind speed, PM10 concentration, SO2 concentration, air temperature. The relative humidity is the most important influencing factor to visibility in spring, autumn and winter. While in summer, the main factor is PM10 concentration. It seems that high humidity ( relative humidity≥80 %), light wind (calm and light air) and fog are the main meteorological conditions which influence low visibility events.(3) The hygroscopic growth factor of the extinction coefficient f(RH) increases continuously with RH and features a first-slow-then-quick uptrend. The curves obtained from different seasons show that f(RH) is relatively higher in summer and autumn than that in winter and spring. Overall, the average f(RH) in 7 cases could reach to 6.6 when RH increases from less than 40% to 95%. We also present comparisons with other researches in the world, and the result shows the behavior of the average of hygroscopic growth factor of the extinction coefficient in the cases is similar to the calculated growth factor of the scattering coefficient for ammonium sulfate by IMPROVE.(4) Further study also shows that the decrease of relative humidity is lagging about 1~2 hours behind the increase of temperature and visibility, which accounts for 95.65% of all year cases. We also find that the vapor pressure is faced a turning point in post-fog period, which accounts for 91.3%. For 82.6% cases, the wind could increase to over 2m/s during the post-fog period.The research in this paper is significant not only to the study of the extinction of aerosols, but also to help to forecast the low visibility events, which greatly contributes to the development of Shanghai Pudong area and provides information for the government to reduce the damages by fog and haze, as well as but control atmospheric pollution. Furthermore, this research is also a FYI (for your information) to use high temporal resolution (1min) data for the staff working in meteorological stations.