基于AERONET的东沙海域气溶胶光学模型

Aerosol Optical Model of Dongsha Area Based on AERONET

使用气溶胶自动观测网(AERONET)东沙站的长期观测资料,初步建立逐月的南海东沙海域气溶胶光学特性模型。长期观测数据表明,东沙海域气溶胶光学厚度(AOD)基本低于0.5,春秋两季达到峰值,夏季最低。气溶胶粒子的有效半径在春秋两季较小,其余月份在0.5μm左右。使用三模态对数正态函数拟合区域气溶胶粒径谱,得到细模态半径为0.1μm,中间模态半径为0.28μm,粗模态半径为2.2μm。基于多波段AOD观测数据,评估该模型计算所得AOD光谱和透过率误差,可见和近红外波段透过率的均方根误差(RMSE)为1%~2%,AOD的RMSE为0.01~0.03。结果表明,所建气溶胶模型可以准确描述东沙海域的气溶胶光学特性,满足工程计算的精度要求。

Objective 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.Methods 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.Results and Discussions 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 2.2μm 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 3-5μm(medium wave)and 8-12μm(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.Conclusions 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.