Numerical Simulation of Dust Weather,and the Transportation and Sources of Dust Aerosol in Different Seasons in Northwest China
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【Author in Chinese】 彭路；
【Author's Information】 兰州大学， 环境科学与工程·环境工程， 2020， 硕士
【Abstract in Chinese】 沙尘天气是干旱地区特有的一种灾害性天气,使西北地区脆弱的生态环境进一步恶化,还会对空气质量、人体健康和社会经济造成严重影响。西北地区位于我国西北内陆,气候特征复杂,土地类型多样,是我国沙尘天气发生频率较高的地区,同时也是中亚沙尘暴区的一个重要贡献源。沙尘天气多发于春季,在2017年冬季也出现了沙尘天气,因此,研究该地区不同季节沙尘天气特征、沙尘气溶胶的运输和潜在源区具有现实意义。本文首先使用WRF-Chem模型和NCEP资料,耦合MOZART-MOSAIC化学-气溶胶方案和AFWA起沙方案,对2017年发生在西北地区春季、冬季两次沙尘过程进行模拟,通过气象站点观测值验证了模型对沙尘天气的模拟效果,展示了沙尘天气过程和沙尘浓度的变化过程。其次,利用HYSPLIT模式、潜在源贡献因子分析法（PSCF）和权重浓度法（CWT）,追溯两次沙尘天气的气团移动轨迹,定量计算潜在源区对西北典型城市沙尘气溶胶浓度的影响程度。最后,利用逐小时监测数据,分析不同季节沙尘天气颗粒物(PM10、PM2.5)浓度变化特征和沙尘发生前期、中期、后期的污染特点,然后进一步分析大气颗粒物与风速和气温的关系。主要结果如下:（1）WRF-Chem模式较好地再现了沙尘天气变化过程。春季（5月）沙尘过程的主要影响因素为气旋与地面冷锋,冬季（12月）则为地面冷锋。模式识别出了春季沙尘主要源区为新疆南部塔里木盆地及周边地区、内蒙古西北部、陕西北部,冬季为吐鲁番盆地和库姆塔格沙漠。同时,也较好地捕捉到塔里木盆地、甘肃北部沙尘气团的移动。模式对于气温的模拟效果较风速更为理想,相关系数为0.89。不同季节的模拟效果也存在差异,对于春季气温与风速模拟效果较好,对于冬季极端气象条件下的天气过程模拟效果不甚理想。（2）WRF-Chem模拟结果显示,春季与冬季沙尘强度不同。春季沙尘天气强度较大,地面沙尘浓度最高超过400 mg/m3,沙尘排放通量最高为128μg/（m2·s）;冬季沙尘天气强度较小,地面沙尘浓度最高为162 mg/m3,沙尘排放通量最高为86μg/（m2·s）。（3）利用HYSPLIT模式对沙尘期间气团进行后向轨迹研究,不同季节沙尘的输送路径不同。春季沙尘为偏北路径,冬季为西北路径,且都有来自于境外的气团;CWT与PSCF方法的结果存在差异,CWT结果显示的潜在源区更为全面和细致,且对于目标城市的贡献大小有很好的体现;不同季节沙尘的潜在源区不同,春季沙尘的潜在源区为西北方向的塔里木盆地、吐鲁番盆地、河西走廊、柴达木盆地和内蒙古西部戈壁滩;冬季为塔里木盆地、吐鲁番盆地、河西走廊。（4）沙尘天气对城市空气质量有严重影响。AQI在沙尘发生时明显升高,在沙尘期间保持高值,个别城市AQI持续为500;沙尘天气对粗颗粒物PM10的影响较PM2.5大,在沙尘中期,各个兰州、西安、西宁、乌鲁木齐和银川的PM10的平均浓度分别为797.42μg/m3、991.20μg/m3、193.55μg/m3、231.07μg/m3和951.36μg/m3,PM2.5的平均浓度为174.91μg/m3、305.80μg/m3、42.37μg/m3、41μg/m3和239.28μg/m3。（5）无论春季还是冬季,在整个沙尘过程中,PM2.5/PM10的突降时刻较各地PM10或PM2.5浓度突增时刻较为提前。春季颗粒物与风速和气温的相关性较冬季更强,沙尘前期颗粒物与风速和气温的相关性较中期和后期强。综上所述,西北地区不同季节沙尘天气、沙尘气溶胶的时空分布不同,沙尘通量和浓度存在差异,春季沙尘为偏北路径,冬季为西北路径,且春季沙尘较冬季的潜在源区范围更大。沙尘天气对空气质量有较强负面影响,对PM10影响更大,PM2.5/PM10可作为沙尘天气预警的指标,需引起重视。
【Abstract】 Dust weather is a kind of special disastrous weather in arid areas,which makes the fragile ecological environment of Northwest China worsen further,and has a serious impact on air quality,human health and social economy.The Northwest District is located in the inland of Northwest China,with complex climate characteristics and diverse land types.It is an area with high frequency of dust weather in China,and an important contribution source of dust storm area in Central Asia.Dust weather occurs mostly in spring and appeared in winter in 2017.Therefore,it is of practical significance to study the characteristics of dust weather in different seasons,transport of particles and potential source areas in this area.In this paper,firstly,WRF-Chem model,NCEP data,coupled with MOZART-MOSAIC chemical aerosol scheme,AFWA sand scheme were used to simulate the two dust processes in spring and winter in the northwest of China in 2017.The meteorological station observations verified the simulation effect of the model on sand and dust weather,meanwhile this paper showed the changing process of sand and dust weather and sand and dust concentration.Secondly,by using HYSPLIT model,potential source contribution factor analysis（PSCF）and weight concentration method（CWT）,the movement track of two dust weather air masses was traced,and the influence degree of potential source area on the concentration of PM10 in Northwest typical cities was quantitatively calculated.Finally,by using hourly monitoring data,the characteristics of PM10 and PM2.5 concentration changed and the pollution characteristics in the early,middle and later stages of dust occurrence were analyzed,and then the relationship between atmospheric particles and wind speed and temperature was further analyzed.The main results are as follows:（1）The WRF-Chem model can well reproduce the process of dust weather change.The main influencing factors of dust process in spring were cyclones and cold front on the ground,and in winterwere cold front on the ground.The main source areas of sand dust in spring are Tarim Basin and its surrounding areas in South Xinjiang,northwest Inner Mongolia and North Shaanxi,and Turpan Basin and Kumtag Desert in winter.At the same time,the movement of dust air mass in Tarim Basin and Northern Gansu Province was also well captured.The simulation effect of the model was better than that of the wind speed,and the correlation coefficient was0.89.The simulation results of different seasons were also different.The simulation results of spring temperature and wind speed were better,but the simulation results of winter extreme weather conditions were not ideal.（2）WRF Chem simulation results showed that the intensity of dust was different in spring and winter.In spring,the dust weather intensity was relatively high,the highest dust concentration on the ground was over 400 mg/m3,and the highest dust emission flux was 128μg/（m2·s）;in winter,the dust weather intensity was relatively small,the highest dust concentration on the ground is 162 mg/m3,and the highest dust emission flux was 86μg/（m2·s）.（3）Using HYSPLIT model to study the backward trajectory of the air mass during the dust period,the transport path of the dust was different in different seasons.Sand dust was a northerly path in spring and a northwestern path in winter,and all had air masses from outside the country.The results of CWT and PSCF were different.On the whole,CWT results showed that the potential source area was more comprehensive and detailed,and the contribution to the target city was well reflected.The potential sources of dust were different in different seasons.The potential source areas of sand dust in spring were Tarim Basin,Turpan Basin,Hexi Corridor,Qaidam Basin and Gobi Desert in the west of Inner Mongolia,and in winter are Tarim Basin,Turpan Basin and Hexi corridor.（4）Dust weather had a serious impact on urban air quality.AQI increased significantly during the occurrence of sand dust,and maintained a high value during the period of sand dust.In some cities,AQI continued to be 500.The effect of dust weather on PM10 was greater than that of PM2.5.In the middle period of sand dust,the average concentrations of PM10 in Lanzhou,Xi’an,Xining,Urumqi and Yinchuan were 797.42μg/m3,991.20μg/m3,193.55μg/m3,231.07μg/m3,951.36μg/m3,respectively,and the average concentrations of PM2.5 were 174.91μg/m3,305.80μg/m3,42.37μg/m3,41μg/m3,239.28μg/m3,respectively.（5）No matter in spring or winter,the time of PM2.5/PM10 value sudden drop was earlier than that of PM10 or PM2.5 concentration sudden increase.The correlation between particles and wind speed and temperature were stronger in spring than in winter,and the correlation between particles and wind speed and temperature were stronger in the early stage of dust than in the middle and late stage.To sum up,the temporal and spatial distribution of dust aerosols,dust flux and concentration were different in different seasons in Northwest China.The dust path was northward in spring and northwest in winter,and the potential source area of dust in spring was larger than that in winter.Dust weather had a strong negative impact on air quality,and had a greater impact on PM10.PM2.5/PM10 can be used as early warning index of sand dust weather,which should be paid attention to.