| The Xi'an area of the ash haze weather was very frequency high in autumn and winter every year since 2013, and has posed a threat to daily activities and even survival of residents. How to monitoring the development of ash haze quickly, finding out the fundamental cause of its generation and establishing effective controls of the ash haze pollution have become a problem that we need to solve. Using satellite remote detection technology on a large scale and long air probe have made considerable development around the world. In this paper, the aerosol optical thickness was calculated during a gray haze in Xi'an area by using remote sensing data, and the inversion results were analyzed accurately. By combining the HYSPLIT back propagation model, the ERA-Interim analysis data and the conventional weather conditions, the typical space-time evolution characteristics and the cause of this ash haze aerosol were analyzed. The results showed that:this gray haze pollution was a gradual accumulation of fouling process of the main source of aerosol pollutants to the atmosphere in the process of regional transport. The specific conclusions were as follows:(1) According to the dark pixel method which excluding energy surface reflectance in remote sensing information, the results of aerosol optical thickness were obtained by using MODIS sensor data, and the results were compared with the mature aerosol optical thickness products of NASA.It was showed that independent product has certain accuracy and the MODIS sensor data could be used in Xi'an area of gray haze process analysis. Besides, the spatial resolution of the aerosol optical thickness which we calculated was 1 km, while the aerosol products which foreign public released was at least 10 km, the independent inversion results have more detailed description of continuous gray haze in Xi'an region during the spatial distribution and temporal variation characteristics of aerosol.(2) According to the remote sensing inversion results and the simulation results of the HYSPLIT back propagation model, the underlying transport pollutants in Xi'an region which come from Shanxi, Henan and other regions was the internal cause of this ash haze.(3) The occurrence of ash haze development were also related with weather conditions. During the ash haze, the surface pressure field in Xi'an region was weak high pressure and temperature-pressure field, and Xi'an was located in China mainland hinterland which has special geomorphology features. The flow of air which was close to the surface was to the northeast for days, and the temperature inversion phenomenon of air appeared. These factors were the main meteorological reasons that lead to the pollution of long duration and increasing degree. In addition, the occurrence of negative pressure and positive variable temperature before the warm front, lower boundary layer, lower wind speed in surface layer, adverse dispersion conditions in high humidity were the important conditions of occurrence in this continuous gray haze, which have promoted further pollution of the ash haze. Simultaneously, the ground horizontal visibility was very low because of the large air humidity during the ash haze.(4) During the ash haze, the positive and negative changes over time of vertical speed, potential eddy degree and horizontal divergence of air mass close to the surface height below 850 hpa were the internal momentum which caused the divergence of the ash haze. The absolute value of three kinds of physical quantities were small, which caused the continuous gray haze. Gray haze evolution was the result of the interaction of vertical speed, potential eddy degree and horizontal divergence of the air mass. The physical mechanism of weather conditions and aerosols were interrelated during the ash haze. |
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