Study On Atmospheric Pollution In Shanghai Using Zenith-sky Passive DOAS Technique

Caused by the rapid urbanization and motorization in recent years, Shanghai has been suffering from much more serious air pollution. Nitrogen oxides and particle pollution in this distinct have become the research focus and drawn much attention. Zenith-sky Passive DOAS (Differential Optical Absorption Spectroscopy) has the advantages of on-line,quick,sensitive and high-resolution. Besides, it can measure many kinds of trace gases at the same time and also can reach the whole atmospheric area, which is an incomparable advantage over ground routine measurement. Based on Zenith-sky Passive DOAS technique, this paper retrieved the VCD (Vertical Column Densities) of total O3 and tropospheric NO2, analyzed their time variation characteristics, researched city NO2 pollution type, and primary explored the possibility of O4 and H2O correlation slope providing the characteristic signature for the occurrence of haze and dust. The main studies and conclusions are shown as follows:Through retrieving the Zenith-sky Passive DOAS spectroscopy at fudan observation (Jun 2009 to Oct 2010) and foho observation (Jun 2010 to Oct 2010), we got VCD of total O3 and tropospheric NO2 and analyzed their time variation characteristics. We found that:tropospheric NO2 at fudan was obviously influenced by traffic and researched high peak in morning and evening; NO2 VCD showed obvious weekend effect; the highest NO2 VCD appeared in December (winter) and the lowest in August (summer). Daily NO2 VCD during the same experimental days at fudan and foho presented the same variation trend, and the value and fluctuation of NO2 VCD at foho which was located in suburb, was small. Morning and evening O3 VCD at foho was higher than that at fudan in the middle and end of June, July, August and September, and lower in the beginning of June and October, which was because the wind direction in most days in that period was east and much atmospheric pollution gases was transported to foho from shanghai. That induced the high NO2 VCD in foho and finally restrained the O3 generation.Tropospheric NO2 vertical column density (VCDTotal) and NO2 surface concentration (CSurface) of 93 days in 2007 were obtained using Zenith-sky Passive DOAS and long-path Active DOAS techniques. With CSurface≥8.0×10-2 mg·m-3(API =50) assumed as low-altitude NO2 pollution, and VCDTotal≥6.03×1016 molecule·cm-2 (VCD, corresponding to API=50) as upper-altitude NO2 pollution, the 93 days were categorized into light NO2 pollution, severe NO2 pollution, upper-altitude NO2 pollution and low-altitude NO2 pollution, the causes of which were analyzed considering mixing height and meteorological condition. The results indicated that with the relatively stable sources of NO2, mixing height, wind speed and wind direction were the main reasons for the change of NO2 surface concentration. NO2 vertical column density was mostly influenced by the transportation of inland polluted air, local emissions and mixing height. The research found NO2 surface data had not so good relation with the NO2 concentration of the whole atmosphere. Only when considering NO2 vertical column density, NO2 surface concentration, meteorological condition, mixing height and other factors can we reasonably analyze urban NO2 pollution.A total of 347 days'valid zenith-sky DOAS data from Oct 1,2009 to Sep 30,2010 in Shanghai were used to conduct linear regression analysis between O4 DSCD (Differential Slant Column Densities) and H2O DSCD and the dependence of their correlation slope on the pollution type. O4 DSCD and H2O DSCD had a very high correlation coefficient, larger than 80%(H2O DSCD and O4 DSCD as X and Y, respectively), no matter it was sunny, cloudy, overcast or rainy. The slope values showed characteristic pattern in. different month and season:small value and little fluctuation were observed in the most days of July, Aug and Sep (summer), while large value and wide-range fluctuation in other seasons, presented in the order of winter>spring>autumn. Through studying on two typical pollution period, it was found that different pollution type showed much discrepant pattern of slope variation: dust pollution happened with the sudden increase of slope value during one period and haze with the sudden decrease. And thus, O4 and H2O correlation slope can provide a characteristic signature for the occurrence of different pollution (especially dust and haze).