基于AERONET的东沙海域气溶胶光学模型【增强内容出版】
Marine aerosol is the most important natural aerosol source, and can significantly affect radiative budget, climate change, and air quality prediction. A precise numerical model representing the optical characters of local aerosol could help much in relevant research. Photoelectric observation equipment working in the sea area is susceptible to marine aerosol, and the evaluation of its detection ability relies on an accurate aerosol optical model. There are some aerosol models applicable for this purpose, such as the navy aerosol model (NAM) and Mediterranean extinction code (MEDEX), which are based on the data acquired primarily near the sea surface at some specific field sites. It is necessary to build a counterpart model using aerosol observation data from China's sea areas. Ground-based remote sensing mainly provides the column averaged aerosol parameters, which can expand the spatial observation coverage by acting as a collaborative network like an aerosol robotic network (AERONET). We propose a tentative aerosol model based on AERONET to explore the database source in building an aerosol optical model.
AERONET is a commonly employed data source in aerosol-related research, such as air pollution prediction, climate changing analysis, and aerosol physics. Observation sites of AERONET are distributed around the world, making the network suitable to characterize the aerosol parameters in different geographical locations. Level 2.0 products from an island site of AERONET, Dongsha_Island, are utilized because of its relatively long temporal covering range, and the island is far enough to minimize the influence of terrestrial aerosol. An aerosol optical model is proposed based on column averaged parameters, aerosol optical depth (AOD), and retrieved size distributions from spectral and angular AOD. AODs obtained originally at 440 nm and 675 nm by CE-318 sun photometer are spectrally converted to 550 nm using Angstrom exponent derived from the AOD spectrum. Size distributions are averaged to the corresponding month to form a monthly aerosol model. Combined with the sea salt refractive index from the HITRAN 2020 database, spectral AOD could be calculated by Mie theory. Comparisons are conducted between calculated AOD spectra and the observed ones to evaluate the accuracy of the proposed model. During calculating the AOD spectra, the relative distributions of AODs at different wavelengths are normalized according to the observed 550 nm AOD.
Our efforts prove that building an aerosol optical model using column aerosol parameters acquired by ground-based remote sensing apparatus is viable. Monthly size distributions of local aerosols in Dongsha_Island are fitted by the lognormal distribution functions of three modes. Fitting coefficients show that the mode radii of fine mode, intermediate mode, and coarse mode are approximately 0.1, 0.28, and 2.2 μm respectively (Table 1). Although the fine mode radius of the built size distribution model is different from that of NOVAM, the intermediate and coarse mode radii conform to the values of their counterparts. Regional AOD is also analyzed and exhibits two peaks in the spring and autumn while concentrating on lower than 0.5. Local Angstrom exponent has the same seasonal tendency as AOD. Error analysis is carried out and the key index indicating the accuracy of the proposed model is root mean square error (RMSE). RMSE of spectral AOD is listed in Table 2 while that of the transmittance expressed in percent is tabulated in Table 3. RMSE of AOD is around 0.01-0.02 in the visible band, and takes a bit large value in the infrared band at around 0.01-0.03, while RMSE of transmittance is 1%-2% and 2%-3% in the corresponding band. Employing the proposed model to estimate the transmittance of the band of 3-5 μm (medium wave) and 8-12 μm (long wave) would result in the error of 0.0090 and 0.0039 respectively. The monthly variations of infrared transmittance demonstrate two peaks in the spring and autumn and have the same seasonal trend as AOD in both medium and long wave bands.
Based on the long-term aerosol observation data of AERONET station Dongsha_Island, a local aerosol optical model that can be adopted for calculating atmospheric radiative transport characteristics is built. The monthly aerosol properties are analyzed, and the built model is verified using spectral AOD acquired at the same place. The error analysis results show that this model performs better in infrared and visible bands. The proposed model consists of aerosol size distribution, 550 nm AOD, and Angstrom exponent. The results indicate that the regional aerosol optical model could be developed in a relatively simple way based on ground remote sensing data, and the accuracy could meet the optical calculation requirements. This approach adopts observation data from solar photometers instead of in-situ surface experiments to expand the data source in modeling. This model can be utilized in estimating aerosol optical properties at wavelengths other than the ones leveraged by field observation apparatus. However, the proposed model is a column mean aerosol one and does not consider the vertical aerosol distribution. Errors may appear when the aerosol optical properties are calculated at a specific altitude. In the future, a layered model would be built based on the vertical lidar profile to improve the model description accuracy on aerosol microphysical status.
1 引言
气溶胶粒子是悬浮在大气中的微粒,对可见、红外波段的大气辐射传输具有显著影响[1],是环境污染监测、大气-海洋参数遥感、光电工程应用[2]等领域的重要研究对象。海洋气溶胶产生于海浪破碎溅射和气粒转换过程[3],主要由海盐粒子、硫酸盐粒子构成,是最重要的气溶胶自然源。海洋气溶胶对大气辐射传输的影响评估,依赖于准确的数值模型。因此,在光电工程应用中,亟需建立基于我国海域观测数据的海洋气溶胶光学特性模型[4]。
目前常用的海洋气溶胶模式有navy aerosol model(NAM)[5-6]、mediterranean extinction code(MEDEX)[7]等。NAM是Gathman[6]于1983年基于地中海、大西洋、太平洋沿岸的粒子谱观测数据建立的。1993年,Gathman等[8]总结多年实验积累的海洋气溶胶垂直分布数据,提出了包含垂直廓线模型的海军海洋垂直气溶胶模式(navy oceanic vertical aerosol model)。van Eijk等[5]将适用于开阔海域的NAM拓展到沿海[9],Piazzola等[7]则将NAM的参数化方案修正为符合地中海区域气溶胶特征的模态参数依赖于离岸距离的形式(即MEDEX)。
上述海洋气溶胶模式均基于国外海域的观测数据而建立,在光电工程应用领域亟需构建基于国内区域观测数据的相应模式,已有不少学者在该领域开展了研究工作,如:Chu等[10]依据南海走航观测数据,搭建了海洋气溶胶通量函数和两模态谱分布模型;Pan等[11]根据青岛、茂名的观测数据,修正了沿海气溶胶谱分布参数;许华团队[12]则基于气溶胶自动观测网(AERONET)的数据对南海区域的气溶胶来源及参数特征进行分析。走航观测和外场实验的周期一般较短,从数周到数月不等,尽管在时间选择上尽量考虑了实验地点的气候特征,但仍然缺乏基于国内典型海域长期观测数据的气溶胶光学特性模型。
AERONET作为站点遍布全球的长期地基气溶胶遥感监测网络,可以提供多种气溶胶微物理参数的观测(反演)数据,包括气溶胶光学厚度(AOD)、Angstrom指数(AE)、谱分布等。基于该数据集,可以对局地气溶胶类型进行区分[13],探究其时空分布规律[14],研究污染物长期变化行为[15],建立局地PM2.5污染物的预报模型[16]等。此类研究侧重于对局地气溶胶微物理参数的分析,较少考虑光学特性建模。AERONET基于太阳光度计的太阳直射和小角度散射观测,测量精度较高、结果可靠,能够反映局地气溶胶光学性质的长期变化规律。因此,探索AERONET数据资料在气溶胶光学模型搭建中的使用,对光电设备的原位测量、实时应用有重要意义。
本文基于AERONET长期观测的我国东沙地区气溶胶光学特性资料,初步建立了中国南海东沙岛区域气溶胶光学模型,主要参数包括东沙地区逐月平均的气溶胶谱分布模型和AOD。结合Mie散射理论,计算了该地区光学波段的AOD光谱,并使用一年期观测数据验证了该模型在计算AOD时的准确性。
2 数据及处理方法
2.1 AERONET数据
AERONET是由NASA主导的地基气溶胶遥感网络,收集数据资料时长超过25年,是气溶胶研究领域的重要公开数据源,主页网址为https://aeronet.gsfc.nasa.gov/。AERONET使用的CE-318太阳光度计(CIMEL公司)可进行太阳直射辐射测量和天空扫描测量[17],其中用于直射测量的通道共8个(中心波长分别为340、380、440、500、670、870、940、1020 nm),用于天空扫描测量的通道共4个(440、670、870、1020 nm)。CE-318的天空扫描模式测量太阳平纬圈和主平面的散射辐射,用于反演谱分布、相函数等气溶胶微物理参数。AERONET的二级产品(Level 2.0数据)经过除云和质量保证,一般作为机载、星载遥感校正[18]等研究中的地面基准。相关研究[18]表明,该产品所提供的气溶胶参数,如柱平均粒径谱等反演结果,与其他原位测量所得结果一致,数据质量较为可靠。
AERONET东沙站(Dongsha_Island)位于中国南海北部,经纬度为(20.699°N,116.729°E),海拔高度约6 m,局地气溶胶受陆源气溶胶的影响相对较小,观测数据适用于海洋气溶胶光学特性研究。本文所用Level 2.0数据的时间范围为2003年12月15日至2021年11月22日,其中用于局地气溶胶光学模型搭建的数据时间范围为2003年12月15日至2020年10月14日(阶段1)。为检验所建气溶胶光学模型的准确性,使用2020年10月15日至2021年11月22日(阶段2)的多波段AOD光谱数据对所建模式进行检验,验证模型在该地区的计算精度。
2.2 数据处理方法
在气溶胶光学特性模式中[8],主要关注消光系数
式中:
气溶胶AE与荣格(Junge)谱分布之间存在密切关系,AE反映了使用幂函数拟合气溶胶数浓度谱的结果,AE越大,小粒子占比越高,反之大粒子占比较高。海洋气溶胶数浓度谱常用对数正态分布函数进行拟合[6, 11],即
式中:
在海洋气溶胶谱分布建模时进行归一化处理[6, 8-9],将粒子数浓度归一化到单位体积内仅有一个气溶胶粒子,能够消除不同测量时间点的数浓度差异,便于分析谱分布形态的变化。数浓度归一化并不影响
式中:
由于东沙站测量数据的时间分布极不均匀,为了避免因有效测量次数在不特定时间段内较高而提高该时间段在平均计算时的权重,进而影响平均结果,本研究所用月均值均为逐级平均所得,即首先计算日均值,然后由日均值计算各月均值,最后计算各月份的平均结果,用于研究气溶胶参数的月际分布。
3 结果与分析
3.1 气溶胶月际变化
AERONET基于太阳直射辐射的AOD测量时间间隔约为15 min[20],经除云和质量控制后,数据分布较为稀疏。根据
图 1. 东沙站550 nm AOD、AE月均值时间序列及频率分布。(a)AOD月均值;(b)AE月均值;(c)AOD和AE的月均值频率分布
Fig. 1. Sequences of 550 nm AOD and AE monthly mean, and their frequencies in Dongsha_Island. (a) AOD monthly mean; (b) AE monthly mean; (c) frequencies of AOD and AE
东沙站多波段AOD的月份均值光谱如
3.2 局地气溶胶光学模型
气溶胶消光系数
表 1. 东沙站谱分布三模态拟合参数
Table 1. Fitting coefficients of particle size distribution in Dongsha_Island
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图 4. 东沙站归一化谱分布拟合结果
Fig. 4. Fitting results of normalized aerosol size distribution in Dongsha_Island
东沙站谱分布的拟合结果提供了和NOVAM类似的气溶胶谱分布模型[8],但模态半径存在一些差异。NOVAM的细模态半径为0.03
NOVAM[5]和MEDEX[7]的建模数据均来自近海表面的观测实验,测量高度约为船体高度(10~20
3.3 模型精度评估
选取AERONET东沙站2020年10月15日至2021年11月22日(阶段2)的多波段AOD观测数据,对前文建立的气溶胶光学模式进行精度评估,阶段2共包含7649组AOD光谱。模型精度验证的方式为,对比由前述月份谱分布模型计算所得的多波段AOD光谱和太阳光度计的太阳直射测量结果之间的误差,统计其均方根误差(RMSE)。本研究所建模型以550 nm AOD为宏观约束参数,即先由前述模型计算各波段AOD之间的光谱相对分布,将相对分布归一化到550 nm波长处,再乘以太阳光度计的550 nm AOD测量结果,即可得到其他波段的模型AOD计算值。在误差统计时,以不确定度合成的方式叠加CE-318太阳光度计的AOD观测误差(0.01),得到模型在计算各波段AOD、透过率时的精度,结果如
表 2. 模型AOD光谱误差统计(Ntot=7649)
Table 2. Error statistics for model AOD spectra (Ntot=7649)
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表 3. 光谱透过率误差统计(Ntot=7649)
Table 3. Error statistics for spectrum transmittance (Ntot=7649)
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从阶段2的平均结果(
图 6. 模型计算结果与2021-07-14T10:09:15的东沙站测量结果对比。(a)AOD光谱对比;(b)透过率光谱对比
Fig. 6. Comparison of calculated results of aerosol optical model and measured results at 2021-07-14T10:09:15 in Dongsha_Island. (a) AOD spectrum comparison; (b) transmittance spectrum comparison
上述误差评估结果表明,所建模型可以正确地计算局地气溶胶的光谱消光特性,其精度满足工程计算需要。可以使用该模型估计气溶胶在无实际观测数据波段上的光学特性,如对于常用的红外3~5
3.4 气溶胶光学特性计算
所建模型应用于工程计算时,可通过由原位测量、遥感反演等手段获得的观测波长处的气溶胶光学特性参数来估算其他所需波长处的光学参数。例如,可通过能见度仪、太阳辐射计或激光雷达等设备测量550 nm处(或附近波长)的消光系数,再使用上述模型估算近红外等波段的消光系数。本节使用HITRAN 2020数据库[19]中的海盐粒子复折射率,结合前文所建局地气溶胶光学模型,计算东沙地区局地气溶胶消光系数
光学工程应用主要关注大气窗区的气溶胶消光系数
图 9. 3~5 、8~12 波段消光系数的月份变化。(a)3~5 ;(b)8~12
Fig. 9. Monthly tendency of extinction coefficient of 3-5 and 8-12 wavebands. (a) 3-5 ; (b) 8-12
图 10. 3~5 、8~12 波段平均透过率的月份变化
Fig. 10. Monthly tendency of averaged transmittance of 3-5 and 8-12 wavebands
4 结论
基于AERONET东沙站的长期气溶胶观测数据,建立了可用于大气辐射传输特性计算的局地气溶胶光学模型,分析了不同月份间气溶胶光学特性的时间分布,对所建立的模型进行了初步验证。阶段2验证数据集的误差分析结果表明,在AERONET的测量波段,所提模型的计算误差在红外和可见波段较好。模型AOD在可见波段的RMSE为0.01~0.02,红外波段为0.01~0.03,相应的可见透过率RMSE为1%~2%,红外透过率RMSE为2%~3%。对于3~5
所建局地气溶胶模型给出了各月份的550 nm AOD、AE和气溶胶谱分布拟合参数。三模态对数正态分布函数的拟合结果表明,细模态半径约为0.1
本研究结果表明,基于AERONET长期观测数据,可以通过较为简便的方式建立精度满足工程计算需要的局地气溶胶光学模型。该方式使用太阳光度计观测(反演)结果,拓展了建模数据来源,使之不再局限于近海表面观测。可以使用该模型来估计非实验观测波长处的气溶胶光学特性,其计算结果的精度依赖于准确的气溶胶谱分布、折射率等参数。使用AERONET监测数据,可以从更长时间尺度上分析局地气溶胶的演变,不断修正这些参数。但是,本文所建立的模型为柱平均气溶胶模型,未考虑气溶胶垂直分布,在计算具体某个海拔高度的气溶胶光学特性时,可能会引入误差,后续将基于激光雷达等垂直廓线观测数据建立分层模式,提高模型对气溶胶微物理状态描述的准确度。
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Article Outline
陈舜平, 戴聪明, 刘娜娜, 连文涛, 张聪, 吴凡, 张宇轩, 魏合理. 基于AERONET的东沙海域气溶胶光学模型[J]. 光学学报, 2023, 43(24): 2401002. Shunping Chen, Congming Dai, Nana Liu, Wentao Lian, Cong Zhang, Fan Wu, Yuxuan Zhang, Heli Wei. Aerosol Optical Model of Dongsha Area Based on AERONET[J]. Acta Optica Sinica, 2023, 43(24): 2401002.