基于AERONET和OMI数据分析中国北方典型地区气溶胶单次散射反照率的变化趋势

气溶胶单次散射反照率（SSA）表征气溶胶吸收特性,很大程度上决定了气溶胶辐射强迫的正负,对准确评估气候变化中的气溶胶辐射强迫具有重要意义。根据2004年10月至2016年12月地基AERONET（AERosol RObotic NETwork）和OMI（Ozone Monitoring Instrument）卫星数据,分析了中国北方典型地区（北京、香河、兴隆和兰州郊区）SSA的变化趋势以及两组数据的一致性。4个站AERONET和OMI 的SSA年均值分别为:北京0.89±0.04和0.90±0.04,香河0.89±0.04和0.91±0.04,兴隆0.92±0.04和0.91±0.04,兰州0.91±0.04和0.90±0.04。4个站点SSA季节变化一致,夏季高冬季低。SSA年际变化趋势分析时,因有效数据较少且为非正态分布,用月中位数代替月均值,同时需对数据进行筛选和去季节变化,结果显示北京和香河均有地基和卫星有效数据时间尺度不同的情况,而兴隆OMI和兰州AERONET满足趋势分析要求。在研究期间,4站SSA均呈上升趋势,说明近年来中国北方气溶胶的吸收性减弱,散射特性增强,尤其北京四季地基和卫星数据均呈上升趋势,但香河秋冬吸收性增强。同时,对AERONET和OMI两种反演算法得到的SSA一致性进行分析。香河结果差异较大,仅30%的数据在误差±0.03的范围内,55%在±0.05的范围内;北京分别为46%（±0.03）和68%（±0.05）,兴隆分别为50%（±0.03）和76%（±0.05）,兰州数据一致性较好,分别为51%（±0.03）和86%（±0.05）,总体来说是受人为活动影响比较明显的地区,数据一致性较差。     

Comparison of column-integrated aerosol optical and physical properties in an urban and suburban site on the North China Plain

The column-integrated optical properties of aerosol in Beijing and Xianghe, two AErosol RObotic NETwork(AERONET)sites situated on the North China Plain(NCP), are investigated based on Cimel sunphotometer measurements from October2004 to June 2012. The outstanding feature found is that the seasonal medians of aerosol optical depth(AOD) at the two stations are in good agreement. The correlation coefficients and the absolute differences between AOD at the two stations are larger than 0.84 and less than 0.05, respectively. Good agreement in AOD at these two sites(one urban and the other suburban; 70 km apart) indicates that aerosol pollution in the Greater Beijing area is regional in nature. However, we find significant differences in the absorption ?Angstr ¨om Exponent(AAE), the real and imaginary part of the refractive index, and thereby the single scattering albedo(SSA), and the difference is seasonally dependent. The feature is found to be more prominent in fall when the fine-mode fraction(FMF) and fine-mode effective radius are significantly different at the two stations,besides the parameters mentioned above. The SSA in Beijing at four wavelengths shows lower values as compared to those in Xianghe, although the difference is not significant in some cases. Significant differences in AAE and fine-mode effective radius indicate that there are differences in aerosol physical and chemical properties in urban and suburban regions on the NCP.     

利用AVHRR数据反演陆地气溶胶光学厚度

开发AVHRR可见光通道反演陆地气溶胶光学厚度 (AOD) 的算法对于研究长时间序列AOD的变化有重要意义。AVHRR由于缺少2.1 μm通道而不能采用MODIS的暗背景算法,该文利用背景合成算法进行陆地AOD反演。背景合成算法是指假设一段时间内地表反射率变化不大且会出现相对清洁大气, 采用最小值合成即可得到地表反射率,再通过辐射传输模式6S制作的查算表查算得到AOD的反演结果。将此算法应用到2009年AVHRR中国部分陆地区域 (15°~45°N,75°~135°E) 得到AOD的时空分布,将反演结果与同期Aqua/MODIS的MOD04 AOD产品进行对比分析表明,华北和华东地区的反演效果较好,西北地区结果较差。以长江三角洲地区为例可知,AVHRR AOD产品与MODIS AOD产品以及AERONET观测的AOD相比相关系数基本在0.6以上,从时间变化规律来看,AVHRR AOD和MODIS AOD产品年变化趋势具有很好的一致性。该文为建立长时间序列AVHRR AOD数据集提供了一个较为可行的方法。     

Aerosol properties and air pollutants over an urban area

For a better understanding of urban aerosols, sun/sky photometry has been undertaken at the Kinki University campus in Higashi-Osaka as a NASA/AERONET station since 2002. A new instrument, the SPM-613D (Kimoto Electric), has been taking measurements at the same site since March 15, 2004. The relationship between aerosol properties obtained from radiometry with AERONET and the SPM measurements is examined. It is found that there is a linear correlation between SPM concentrations and aerosol properties, which indicates that aerosol characteristics can be estimated from SPM data, and vice versa.It is also shown that the air quality of the Higashi-Osaka site is poor due to not only the anthropogenic particles by local emissions, such as diesel vehicles and chemical industries, but also due to the dust particles coming from continental desert areas by large scale climatic conditions.     

Aerosol properties and air pollutants over an urban area

For a better understanding of urban aerosols, sun/sky photometry has been undertaken at the Kinki University campus in Higashi-Osaka as a NASA/AERONET station since 2002. A new instrument, the SPM-613D (Kimoto Electric), has been taking measurements at the same site since March 15, 2004. The relationship between aerosol properties obtained from radiometry with AERONET and the SPM measurements is examined. It is found that there is a linear correlation between SPM concentrations and aerosol properties, which indicates that aerosol characteristics can be estimated from SPM data, and vice versa.It is also shown that the air quality of the Higashi-Osaka site is poor due to not only the anthropogenic particles by local emissions, such as diesel vehicles and chemical industries, but also due to the dust particles coming from continental desert areas by large scale climatic conditions.     

An operational MODIS aerosol retrieval algorithm at high spatial resolution,and its application over a complex urban region

Aerosol retrieval algorithms for the MODerate Resolution Imaging Spectroradiometer (MODIS) have been developed to estimate aerosol and microphysical properties of the atmosphere, which help to address aerosol climatic issues at global scale. However, higher spatial resolution aerosol products for urban areas have not been well-researched mainly due to the difficulty of differentiating aerosols from bright surfaces in urban areas. Here, an aerosol retrieval algorithm using the MODIS 500-m resolution bands is described, to retrieve aerosol properties over Hong Kong and the Pearl River Delta region. The rationale of our technique is to first estimate the aerosol reflectances by decomposing the top-of-atmosphere reflectances from surface reflectances and Rayleigh path reflectances. For the determination of surface reflectances, a Minimum Reflectance Technique (MRT) is used, and MRT images are computed for different seasons. For conversion of aerosol reflectance to aerosol optical thickness (AOT), comprehensive Look Up Tables specific to the local region are constructed, which consider aerosol properties and sun-viewing geometry in the radiative transfer calculations. Four local aerosol types, namely coastal urban, polluted urban, dust, and heavy pollution, were derived using cluster analysis on 3 years of AERONET measurements in Hong Kong. The resulting 500 m AOT images were found to be highly correlated with ground measurements from the AERONET (r² = 0.767) and Microtops II sunphotometers (r² = 0.760) in Hong Kong. This study further demonstrates the application of the fine resolution AOT images for monitoring inter-urban and intra-urban aerosol distributions and the influence of trans-boundary flows. These applications include characterization of spatial patterns of AOT within the city, and detection of regional biomass burning sources.     

Himawari-8气溶胶光学厚度产品的验证分析

Himawari-8是由日本气象局发射的新一代静止气象卫星，其搭载的传感器AHI（Advanced Himawari Imager）可实现10 min/次的高时间分辨率对地观测。本文将2015年9月至2017年12月Himawari-8卫星Level-2气溶胶光学厚度（AOD）产品与AERONET（AErosol RObotic NETwork）70个站点的地基观测数据进行对比验证分析，结果表明Himawari-8卫星反演的气溶胶光学厚度产品的精度存在很大的空间上的差异性，其中有48个站点Himawari-8 AOD与AERONET AOD之间存在较好的相关性（R>0.5），有22个站点Himawari-8卫星反演气溶胶光学厚度产品存在明显的低估现象。在American_Samoa、Bandung、Birdsville、Bukit_Kototabang、Canberra、Fowlers_Gap、Jabiru以及QOMS_CAS等站点出现地基观测值很小而卫星反演的气溶胶光学厚度较大的情况。对绝对误差（Himawari-8 AOD与AERONET AOD的差）进行分析发现Himawari-8 AOD存在低估现象时绝对误差与AERONET AOD之间存在较好的相关性；在地基观测值很小而Himawari-8卫星反演AOD较大的地区绝对误差与Himawari-8 AOD之间存在较好的线性关系，这为Himawari-8 AOD反演算法的改进与完善提供了有用的研究发现。     

近十年北京气溶胶光学特性及直接辐射强迫研究

利用SBDART（Santa Barbara DISORT Atmospheric Radiative Transfer）辐射传输模式,结合AERONET（Aerosol Robotic Network）北京站观测的气溶胶光学特性数据,评估北京地区近十年气溶胶以及黑碳气溶胶的辐射强迫,主要研究结果如下:北京近十年气溶胶平均光学厚度（aerosol optical depth, AOD_440nm）为0.61±0.56,?ngstr?m波长指数均值为1.09,单次散射反照率（single scattering albedo, SSA_440nm）的均值为0.888±0.045;AOD呈现下降趋势,SSA呈上升趋势,表明该区域气溶胶污染有所改善。晴空条件下,大气层顶、地面和大气的气溶胶直接辐射强迫多年均值分别为?24.91±19.80 W m?2、?65.52±43.78 W m?2、40.61±28.62 W m?2,即气溶胶对大气层顶和地表为冷却效应,对大气产生加热作用。气溶胶和黑碳气溶胶的直接辐射强迫绝对值的年际变化表现为微弱的下降趋势,季节变化特征为春夏季高,冬季低,这与AOD的变化规律一致。并且黑碳气溶胶的直接辐射强迫下降趋势与SSA的上升趋势呈现较好的反位相关系。     

Identification of Aerosol Types and Their Optical Properties in the North China Plain Based on Long-Term AERONET Data

Characterization of aerosols is required to reduce uncertainties in satellite retrievals of global aerosols and for modeling the effects of these aerosols on climate.Aerosols in the North China Plain(NCP) are complex,which provides a good opportunity to study key aerosol optical properties for various aerosol types.A cluster analysis of key optical properties obtained from Aerosol Robotic Network(AERONET) data in Beijing and Xianghe during 2001-11 was performed to identify dominant aerosol types and their associated optical properties.Five dominant aerosol types were identified.The results show that the urban/industrial aerosol of moderate absorption was dominant in the region and that this type varied little with season.Urban/industrial aerosol of weak absorption was the next most common type and mainly occurs in summer,followed by that strong aerosols occurring mainly in winter.All were predominantly fine mode particles.Mineral dust(MD) and polluted dust(PD) occurred mainly in spring,followed by winter,and their absorption decreased with wavelength.In addition,aerosol dynamics and optical parameters such as refractive index and asymmetry factor were examined.Results show that the size of coarse mode particles decreased with AOD indicating the domination of external mixing between aerosols.     

Grid-cell Aerosol Direct Shortwave Radiative Forcing Calculated Using the SBDART Model with MODIS and AERONET Observations:An Application in Winter and Summer in Eastern China

Taking winter and summer in eastern China as an example application, a grid-cell method of aerosol direct radiative forcing(ADRF) calculation is examined using the Santa Barbara DISORT Atmospheric Radiative Transfer(SBDART) model with inputs from MODIS and AERONET observations and reanalysis data. Results show that there are significant seasonal and regional differences in climatological mean aerosol optical parameters and ADRF. Higher aerosol optical depth(AOD)occurs in summer and two prominent high aerosol loading centers are observed. Higher single scattering albedo(SSA) in summer is likely associated with the weak absorbing secondary aerosols. SSA is higher in North China during summer but higher in South China during winter. Aerosols induce negative forcing at the top of the atmosphere(TOA) and surface during both winter and summer, which may be responsible for the decrease in temperature and the increase in relative humidity.Values of ADRF at the surface are four times stronger than those at the TOA. Both AOD and ADRF present strong interannual variations; however, their amplitudes are larger in summer. Moreover, patterns and trends of ADRF do not always correspond well to those of AOD. Differences in the spatial distributions of ADRF between strong and weak monsoon years are captured effectively. Generally, the present results justify that to calculate grid-cell ADRF at a large scale using the SBDART model with observational aerosol optical properties and reanalysis data is an effective approach.     

Analysis and evaluation of the global aerosol optical properties simulated by an online aerosol-coupled non-hydrostatic icosahedral atmospheric model

Aerosol optical properties are simulated using the Spectral Radiation Transport Model for Aerosol Species(SPRINTARS)coupled with the Non-hydrostatic ICosahedral Atmospheric Model(NICAM). The 3-year global mean all-sky aerosol optical thickness(AOT) at 550 nm, the ngstr m Exponent(AE) based on AOTs at 440 and 870 nm, and the single scattering albedo(SSA) at 550 nm are estimated at 0.123, 0.657 and 0.944, respectively. For each aerosol species, the mean AOT is within the range of the Aero Com models. Both the modeled all-sky and clear-sky results are compared with observations from the Moderate Resolution Imaging Spectroradiometer(MODIS) and the Aerosol Robotic Network(AERONET). The simulated spatiotemporal distributions of all-sky AOTs can generally reproduce the MODIS retrievals, and the correlation and model skill can be slightly improved using the clear-sky results over most land regions. The differences between clear-sky and all-sky AOTs are larger over polluted regions. Compared with observations from AERONET, the modeled and observed all-sky AOTs and AEs are generally in reasonable agreement, whereas the SSA variation is not well captured. Although the spatiotemporal distributions of all-sky and clear-sky results are similar, the clear-sky results are generally better correlated with the observations. The clear-sky AOT and SSA are generally lower than the all-sky results, especially in those regions where the aerosol chemical composition is contributed to mostly by sulfate aerosol. The modeled clear-sky AE is larger than the all-sky AE over those regions dominated by hydrophilic aerosol, while the opposite is found over regions dominated by hydrophobic aerosol.     

Retrieval of aerosol optical properties over the Beijing area using POLDER/PARASOL satellite polarization measurements

Aerosol optical properties over Beijing and Xianghe under several typical weather conditions (clear sky, light haze, heavy pollution and dust storm) are derived from POLDER (POLarization and Directionality of the Earths Reflectances)/PARASOL (Polarization and Anisotropy of Reflectances for Atmospheric Sciences coupled with Observations from a Lidar) multi-directional, multi-spectral polarized signals using a more reliable retrieval algorithm as proposed in this paper. The results are compared with those of the operational retrieval algorithm of POLDER/PARASOL group and the ground-based AERONET (AErosol RObotic NETwork)/PHOTONS (PHOtometrie pour le Traitement Operational de Normalisation Satellitaire) measurements. It is shown that the aerosol optical parameters derived from the improved algorithm agree well with AERONET/PHOTONS measurement. The retrieval accuracies of aerosol optical thickness (AOT) and effective radius are 0.06 and 0.05 mu m respectively, which are close to or better than the required accuracies (0.04 for AOT and 0.1 mu m for effective radius) for estimating aerosol direct forcing.     

Modification of the SUNFLUX Solar Radiation Scheme with a New Aerosol Parameterization and Its Validation Using Observation Network Data

SUNFLUX is a fast parameterization scheme for determination of the solar radiation at the Earth's surface.In this paper,SUNFLUX is further modified in the treatment of aerosols.A new aerosol parameterization scheme is developed for five aerosol species.Observational data from Baseline Surface Radiation Network(BSRN),Surface Radiation Budget Network(SURFRAD) and Aerosol Robotic Network(AERONET) stations are used to evaluate the accuracy of the original and modified SUNFLUX schemes.General meteorological data are available at SURFRAD stations,but not at BSRN stations.Therefore,the total precipitable water content and aerosol data are obtained from AERONET stations.Fourteen stations are selected from both BSRN and AERONET.Cloud fraction data from MODIS are further used to screen the cloud.Ten-year average aerosol mixing ratios simulated by the CAM-chem system are used to calculate the fractions of aerosol optical depth for each aerosol species,and these fractions are further used to convert the observed total aerosol optical depth into the components of individual species for use in the evaluations.The proper treatment of multiple aerosol types in the model is discussed.The evaluation results using SUNFLUX with the new aerosol scheme,in terms of the BSRN dataset,are better than those using the original aerosol scheme under clear-sky conditions.However,the results using the SURFRAD dataset are slightly worse,attributable to the differences in the input water vapor and aerosol optical depth.Sensitivity tests are conducted to investigate the error response of the SUNFLUX scheme to the errors in the input variables.     

塔克拉玛干沙漠地区气溶胶光学厚度卫星遥感产品验证

基于塔克拉玛干沙漠地区地基太阳光度计数据,系统验证2007～2008年星载多角度成像光谱仪(MISR)、中分辨率成像光谱仪(MODIS)和臭氧监测仪(OMI)气溶胶反演产品,旨在定量评估这些产品在我国沙漠地区的气溶胶光学厚度(AOD)反演精度。结果表明:MODIS/AOD的相关系数在4种产品中最高(0.91),OMI/AOD次之(0.87),其次为MISR/AOD(0.84),OMI/UVAI相关系数偏低(0.51)。MISR/AOD均方根误差(0.14)和平均偏差(-0.06)在4种反演产品中最低。与地基观测相比,MISR/AOD、MODIS/AOD系统偏低,OMI/AOD、OMI/UVAI系统偏高。在相同比较条件下(地基观测气溶胶光学厚度值限定在2.0以内),MISR的均方根误差和平均偏差在4种反演产品中最低,且相关系数也较高(0.84)。尽管存在诸多不同,但3种探测器气溶胶反演产品均能较好地展示该地区的气溶胶季节变化。塔克拉玛干沙漠春、夏季AOD较大,秋、冬季AOD相对较小。ngstrm波长指数的结果表明,春季(3～5月)最小(均值为0.11),夏季(6～8月)次之,秋季(9～11月)和冬季(12月至次年2月)较大(均值达到0.61),这表明在春、夏季气溶胶粒子偏大,秋、冬季气溶胶粒子偏小。此外,通过研究2000～2010年AOD年际变化表明,由于塔克拉玛干沙漠地区属于沙尘源区,气溶胶类型较为单一,所以总体来说,变化趋势不是较为明显。从反演结果来看,2003年的气溶胶含量为此10年中最高,年均值达到0.32;2005年的气溶胶含量在这10年中最低,年均值为0.28。     

Absorption properties of urban/suburban aerosols in China

The broadband diffuse radiation method is improved to retrieve the aerosol refractive index imaginary part （AIP） and broadband （400-1000 nm mean） single scattering albedo （SSA）. In this method, four sets of SSA selection criteria are proposed for quality control. The method is used to retrieve AIP, SSA and absorptive optical thickness （AbOT） from routine hourly-exposed pyrheliometer and paranometer measurements over 11 sites （meteorological observatories） in China during 1998-2003. Apart from one suburban site （Ejin Qi）, the other urban sites are all located around big or medium cities. As shown in the retrieval results, annual mean SSA during 1998-2003 changes from 0.941 （Wuhan） to 0.849 （Lanzhou）, and AIP from 0.0054 to 0.0203. The 11-site average annual mean SSA and AIP are 0.898 and 0.0119, respectively. SSA during winter is smaller for most sites. There is an evidently positive correlation between SSA and aerosol optical thickness （AOT） for all sites. There is also a positive correlation between SSA and relative humidity for most sites, but a negative correlation for a few sites, such as Kashi and ǚrǚmqi in Northwest China.     

近年来华东地区大气气溶胶的时空特征

利用2000年2月—2008年12月的AERONET（AErosol RObotic NETwork）地基观测数据对MODIS/TERRA Collection 005气溶胶光学厚度（aerosol optical thickness;AOT）在华东区域的适用性进行了验证,并利用验证后的MODIS气溶胶产品对华东区域气溶胶光学厚度和尺度分布特征进行了分析。结果表明,（1）通过验证比较,MODIS的AOT在华东区域与AERONET站陆基观测到的AOT具有非常好的一致性,满足美国NASA的设计要求。（2）华东区域的气溶胶光学厚度存在明显的时空分布特征。时间上,在春季和夏季达到最大,而在秋季和冬季最小,表现出明显的季节变化规律。空间上,气溶胶光学厚度受地形影响明显。其高值区主要分布在平原地区,而低值区主要在海拔较高的山区。（3）该区域的气溶胶尺度分布也存在显著的变化特征。在冬、春由于沙尘输送的影响,整个华东区域气溶胶粒子的尺度都比较大,主要以自然生成的沙尘粒子为主。而在夏、秋季由于夏季风和降水的影响,气溶胶粒子的尺度都比较小,以工业排放的人为气溶胶粒子为主。     

Aerosol Optical Properties Affected by a Strong Dust Storm over Central and Northern China

Aerosol observational data at 8 ground-based observation sites in the Chinese Sun Hazemeter Network(CSHNET)were analyzed to characterize the optical properties of aerosol particles during the strong dust storm of 16-21 April 2005.The observational aerosol optical depth(AOD)increased significantly during this dust storm at sites in Beijing city(86%),Beijing forest(84%),Xianghe(13%),Shapotou(27%),Shenyang(47%),Shanghai(23%),and Jiaozhou Bay(24%).The API(air pollution index)in Beijing and Tianjin also had a similar rise during the dust storm,while the Angstrm exponent(α)declined evidently at sites in Beijing city(21%),Beijing forest(39%),Xianghe(19%),Ordos(77%),Shapotou(50%),Shanghai(12%),and Jiaozhou Bay(21%),respectively.Furthermore,The observational AOD andαdemonstrated contrary trends during all storm stages(pre-dust storm,dust storm,and post-dust storm),with the AOD indicating an obvious"Valley-Peak-Valley"pattern of variation,whileαdemonstrated a"Peak-Valley-Peak"pattern. In addition,the dust module in a regional climate model(RegCM3)simulated the dust storm occurrence and track accurately and RegCM3 was able to basically simulate the trends in AOD.The simulation results for the North China stations were the best,and the simulation for dust-source stations was on the high side,while the simulation was on the low side for coastal sites.     

Study on Probability Distributions of Multi-Timescale Aerosol Optical Depth Using AERONET Data

The probability distribution analysis is per-formed for multi-timescale aerosol optical depth (AOD) using AErosol RObotic NETwork (AERONET) level 2.0 data.The maximum likelihood estimation is employed to determine the best-fit probability density function (PDF),and the statement that the fitting Weibull distribution will be light-tailed is proved true for these AOD samples.The best-fit PDF results for multi-site data show that the PDF of AOD samples with longer timescale in most sites tends to be stably represented by lognormal distribution,while Weibull distribution is a better fit for AOD samples with short timescales.The reason for this difference is ana-lyzed through tail characteristics of the two distributions,and an indicator for the selection between Weibull and lognormal distributions is suggested and validated.The result of this research is helpful for determining the most accurate AOD statistics for a given site and a given time-scale and for validating the retrieved AOD through its PDF.     

Preliminary Retrieval of Aerosol Optical Depth from All-sky Images

The relationship between the radiance ratio (radiance at wavelength 450 nm to 650 nm) and aerosol optical depth (AOD) is analyzed in this paper by numerical simulation and a "LUT" (look-up table) approach is then presented for the retrieval of AOD from the radiance ratio. In this LUT approach, the AOD retrieval error depends mainly on the assumption of aerosol types. From the preliminary simulation, a typical error of 15%–20% in AOD obtained from the radiance ratio is estimated, due to neglecting changes in...     

中国区域MODIS陆上气溶胶光学厚度产品检验

以我国MODIS共享网站积累的MODIS L1B数据和美国威斯康辛大学提供的IMAPP软件包气溶胶产品软件为基础, 经过产品运行本地化改进处理, 在国家卫星气象中心建立了气溶胶产品业务化生成和发布机制。为支持气溶胶遥感产品算法改进以及潜在用户对产品的合理应用, 给出对国家卫星气象中心运行的MODIS气溶胶遥感产品质量检验分析结果。利用2005年1月— 2007年5月AERONET地基气溶胶监测网的L2.0级气溶胶光学厚度产品作为真值, 用它匹配MODIS陆上气溶胶光学厚度产品开展检验。检验结果表明:以卫星过境前后30min地基观测时间平均值匹配地基站点位置10 km半径范围内的卫星反演结果空间平均值开展检验, 总体样本的气溶胶光学厚度均方根误差约为0.25;满足产品误差要求 (±0.05±0.20τ) 的样本占总样本数的44%; 气溶胶光学厚度反演结果精度具有季节和地域差异, 干季(秋、冬、春)的气溶胶光学厚度误差较小, 而雨季气溶胶光学厚度误差较大, 云是雨季气溶胶光学厚度反演结果误差较大的主要影响因素。