基于光学特性测量的气溶胶吸湿增长因子观测研究

Hygroscopic growth of aerosols has an important impact on visibility and radiative forcings and poses very large uncertainty. Hygroscopic aerosols take up water as humidity increases. Aerosol water matters since water can affect both the size and refractive indices of atmospheric aerosols, thereby influencing the mass concentration, size distribution, and corresponding optical properties (e.g., scattering coefficient, backscattering coefficient, single scattering albedo, and asymmetry parameter. In particular, understanding the effect of relative humidity on aerosol light scattering is important to better estimate the radiative effect and evaluate the visibility impairment. This study will carry out the intensive field campaign at Lin'an station, which is a WMO GAW regional station (30.3 N, 119.73 E, 138m a.s.l.) located in the center of Yangtze River Delta, China. The hygroscopic growth factor is defined as the ratio of aerosol scattering coefficient at a given RH to that at a low reference RH (<40%). Correspondingly, the measurement system involves two nephelometers operating in series with a humidifier in the middle of them. Sample air enters the first nephelometerthrough an aerosol dryer, which controls the sample RH lower than 30%, then passes through the humidifier, where the sample RH is regulated to a higher RH ranging from 40% to 90%, and finally enters the second nephelometer (humidified nephelometer or WetNeph, TSI Inc., Model 3563) where the scattering coefficient of humidified aerosols are measured. We will study the variation of hygroscopic growth factor with different chemical, different wavelength. We will also investigate the systematic variation of the important parameters to radiative forcing. This study will help us understand more about the how the hygroscopic growth factor and optical properties effects the radiative forcing and visibility, and provide the fundamental data to improve the parameterization in the regional climate mode.

气溶胶吸湿增长对能见度和气溶胶的辐射强迫有重要的影响，而且具有很大的不确定性。目前国内有关基于气溶胶散射系数测量的吸湿增长因子的观测结果还较少，仅有的一些观测也多是基于单波段浊度仪的观测。本项目拟在我国经济比较发达的长三角地区的背景区域，使用三波段的积分浊度仪，同步观测干状态和可控相对湿度下气溶胶的散射特性，获得吸湿增长因子观测资料，分析气溶胶吸湿增长因子随相对湿度的变化趋势及其与测量波长的关系，探讨气溶胶吸湿增长因子的变化规律，获得气溶胶辐射强迫计算中关键参数如后向散射比、单次散射反照率等的变化，评估这些参数对辐射强迫计算的影响。结合气溶胶质谱测量的化学成分及其粒径分布资料，探讨气溶胶吸湿增长因子及其与化学成分的关系，为改进区域气候模式/辐射传输模式中的气溶胶辐射特性参数、减少辐射强迫计算中的不确定性提供科学基础。

气溶胶吸湿增长对能见度和气溶胶的辐射强迫有重要的影响，而且具有很大的不确定性。目前国内有关基于气溶胶散射系数测量的吸湿增长因子的观测结果还较少，仅有的一些观测也多是基于单波段浊度仪的观测。本项目使用三波段的积分浊度仪，在我国经济比较发达的长三角地区的背景区域和华北平原农村同步观测干状态和可控相对湿度下气溶胶的散射特性观测，同时也开展了气溶胶数谱分布和亚微米气溶胶化学成分的观测。获得了长三角背景地区和华北农村地区气溶胶的光学特性如散射系数、后向散射系数、后向散射比等评估气溶胶辐射强迫的关键因子的观测资料，以及气溶胶散射吸湿增长因子的变化特征，探讨不同气溶胶化学成分、测量波长对气溶胶吸湿增长因子的影响，以及气溶胶吸湿增长对气溶胶直接辐射强迫的贡献。研究表明，气溶胶散射吸湿增长因子具有明显的季节变化，在长三角背景地区气溶胶吸湿能力冬春季高、夏秋季低，而华北平原农村地区夏秋季高，冬春季低。吸湿增长因子与气溶胶化学成分关系密切，随着有机气溶胶所占比例增大而减小，随无机离子所占比例增大而增大，硝酸盐在气溶胶吸湿增长中起着重要作用；气溶胶散射吸湿增长的波长依赖性比较弱；评估了长三角背景地区气溶胶在环境相对湿度下吸湿增长吸收水分对大气总消光及气溶胶直接辐射强迫的贡献。气溶胶在环境相对湿度下吸湿增长吸收水分造成的气溶胶直接辐射强迫与干状态下气溶胶直接辐射强迫的月平均比值约1.3-1.9，气溶胶吸湿增长吸收水分对气溶胶直接辐射强迫的贡献约为23%-47%。本研究获得了大量的观测资料，这对气溶胶气候环境效应的评估、化学天气数值预报模式有关的参数方案的制定和模式结果验证、气溶胶-云相互作用的研究等至关重要。