| As one of the most important constituents in the Earth's atmospheric environment,aerosols have substantial influence on climate change by interacting with solar radiation and clouds.Aerosols' spatial distribution are highly varied,and hence,their vertical distribution in the atmosphere not only makes radiative transfer more complicated,but also has a key role on local air quality.Aerosols accumulating near surface due to emission of pollutants and/or stagnant weather condition could lead to deterioration of air quality and low visibility,and further bring about inconvenience for human life and production activities and potential damage to human health.Beijing-Tianjin-Hebei(BTH)is one of the most populous region in China,where air pollution episodes occur frequently in recent years attracting extensive attention.Most previous studies associated with these pollution processes were conducted near surface.The vertical profiles of aerosols were obtained mainly by surface-based LIDAR or meteorological tower at a single site,which was subjected to the lack of knowledge about the aerosolvertical distribution on a regional scale.However,the Cloud-Aerosol Lidar with Orthogonal Polarization(CALIOP)instrument,onboard the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations(CALIPSO),has provided well developed and validated global,multiyear dataset,which is valuable for analyzing aerosol vertical profiles on a regional scale and investigating the vertical distribution of air pollutants in detail.In this paper,multi-satellite datasets(e.g.CALIOP and MODIS)and meteorological surface observations and radiosonde as well as environmental monitoring data were analyzed to obtain characteristics of long term vertical distribution of aerosols over BTH,and to investigate the role of aerosol vertical distribution in two heavy air pollution events.The main work and conclusions are as follows:1.Constructing the methods for processing CALIOP Level 2 Aerosol Profile product.The data quality control(QC)has been implemented based on literatures and CALIOP L3 standard product.Profiles in which,the surface elevation is higher than 250 m were screened out as well as data above ocean(DEM and ocean filter,DOF),because we focus only on the plain area over land.Using only QC procedures,our results showed the same seasonal variation and vertical distribution compared to CALIOP L3 data within the same grid.There are slightly low bias in the lowest layer near surface.Screening data using DOF could decrease the biases induced by ocean aerosol and high or low aerosol concentration over mountain area.Finally,we got the results more close to the real state over plain-land region over BTH.2.Over the plain area of BTH(36°~41°N,114°~119°E),aerosol extinction coefficient(AEC)showed the noticeable seasonal variation.Below 0.5 km,the order of AEC was: winter>autumn>summer>spring.However,the order shifted above 0.5 km as: summer>autumn>spring>winter,due to the fastest decreasing of AEC in winter and higher mixing layer in summer,AEC decreased fastest among other seasons.There is also a day-night variation for AEC below 2 km.For spring and summer,below 1 km,AEC was higher in the nighttime than that in the day with a positive bias of 0.2~0.3 km-1 at most near surface.On the contrary,in autumn,AEC was lower in the nighttime with a negative bias of 0.1 km-1 at an altitude of 0.8 km.Furthermore,in summer blow 0.5 km,the variation of AEC shifted from increasing trend in the day to decreasing trend at night.The distinct day-night difference of AEC may be related to the difference of relative humidity between day and night.3.CALIOP-aerosol optical depth(AOD),derived from above averaged profiles,was in the order of summer>autumn>winter>spring for daytime and summer>spring>autumn>winter for nighttime.CALIOP-AOD showed the similar seasonal trend compared to MODIS-AOD with the largest value in summer,but had a systematic low bias.The values of nighttime-CALIOPAOD were closer to MODIS-AOD,mainly due to the higher relative humidity in the night.Seasonal variation of column AOD was just the opposite to the surface particulate matter with aerodynamic diameter less than 2.5 ?m(PM2.5).The low level(below 0.5 km)CLAIOP-AOD may have a potential to improvethe relation between two of them.4.The vertical distribution of AEC varied with locations and seasons for each CALIPSO orbit through the BTH region.In spring,for most of the areas and heights,AECs are less than 0.5 km-1,but very low AEC can be observed at higher altitudes(6~7 km).In summer,a wide range of continuous high value(>0.5 km-1)of AEC presents in central and southern Hebei and Northern Shandong,and can be extended upward to the height of 1 km.In autumn,surface attached thick layer(1 km)with high AEC is still observed to the south of Shijiazhuang and Hengshui.In winter,continuous aerosol layer with high extinction coefficient mainly concentrated below 500 m.5.The North China Plain(NCP)was controlled by stagnant weather conditions during 5~13 October 2014.Heavy air pollution occurred mainly in the cities along the Taihang Mountain,e.g.,Beijing,Baoding,Shijiazhuang and Xingtai.The whole area was influenced by cap inversion due to high pressure system,making the top of aerosol layer more uniform.Smoke aerosols emitted from biomass burning in southern Hebei,northern Henan and Western Shandong were suppressed by the above cap inversion at 1.5~2 km.aerosol layers with AEC>0.5 km-1 extended below 2 km in altitude.Under the condition of large wind speed at about 1 km,smoke may be transported from south to north quickly.When planetary boundary layer developed after sun rise,air pollutants mixed vertically,and then contributed to the pollution event on the ground.6.Using vertical detection of CALIOP,the characteristics of vertical structure between fog and aerosol was identified during heavy pollution in winter.In January 2013,the heavy pollution frequently occurring over BTH was characterized by a vertical structure in which the fog layer overlapped with the high-concentration aerosol layer.In the case of heavy pollution from January 12 to 13,the height of the top of the fog was different,but it was distributed below 1 km.The AEC corresponding to the aerosol layer in close contact with the fog top was very high(1.5 km-1,which is equivalent to the value of heavy pollution on the ground;above the dense aerosol layer,there is still a higher concentration of aerosols with AEC of up to 0.5 km-1 could extended up to 2 km height,Beyond that,a region of a relatively low level of aerosols with AEC of less than 0.2 km-1 could extend up to an altitude of 5 km.Some of fog events were maintained until the afternoon,which increased the reflected solar radiation,making the radiation transfer process more complicated compared with the cases when only an aerosol layers were considered.The amplified heating effects of absorbing aerosol above fog layer may stabilize atmospheric stratification,thereby facilitating the establishment of an extremely high concentration of PM near the surface layer. |
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