Abstract. Uncertainties in the mineral dust’s direct radiative effect arise from the variability in its optical properties. The optical properties can also be influenced by mixing processes with anthropogenic aerosols, such as black carbon or fine particles (called "pollution" in this study). We aimed to investigate the effect of mixing pollution with mineral dust aerosols from different source regions on the intensive aerosol optical properties. Thus, the Ångström exponents of scattering and absorption (i.e., their wavelength dependence), the single scattering albedo, and the asymmetry parameter were determined from direct optical measurements performed during the A-LIFE aircraft field experiment over the Eastern Mediterranean. This location provided access to Arabian and Saharan dust layers mixed with pollution. Our findings indicated significant changes in all the intensive aerosol optical properties with increasing pollution content within mineral dust layers. Interestingly, the differences between Arabian and Saharan dust’s intensive aerosol optical properties were negligible. We discussed the implications of these results for identifying mineral dust events and for their direct radiative effect. First, the mixing with pollution masked the mineral dust signal, suggesting that caution is needed when using the Ångström exponents for identifying mineral dust events. However, the Ångström exponents can help estimate the amount of pollution once a mineral dust event is confirmed. Second, our measurements of the asymmetry parameter and single scattering albedo changed from pure to polluted mineral dust layers (e.g., at 525 nm, the median values decreased from 0.67 to 0.56 and from 0.96 to 0.89, respectively). These changes have 15 opposing effects on the short-wave direct radiative effect efficiency (i.e., the direct radiative effect per unit of aerosol optical depth) and may partly cancel out each other. Nevertheless, the impact of mixing with pollution on the mineral dust’s direct radiative effect efficiency can differ depending on the surface albedo. In conclusion, accurate quantification of the pollution content within mineral dust layers is crucial. The pollution significantly impacts mineral dust event identification, its optical properties, and the local direct radiative effect.

中文翻译：

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污染影响东地中海阿拉伯和撒哈拉尘埃的光学特性

摘要。矿尘直接辐射效应的不确定性源于其光学特性的变化。光学特性也会受到人为气溶胶的混合过程的影响，例如黑碳或细颗粒（在本研究中称为“污染”）。我们的目的是研究来自不同来源地区的矿物粉尘气溶胶的混合污染对强气溶胶光学特性的影响。因此，散射和吸收的 Ångström 指数（即它们的波长依赖性）、单次散射反照率和不对称参数是根据 A-LIFE 飞机在东地中海进行现场实验期间进行的直接光学测量确定的。这个位置提供了与污染混合的阿拉伯和撒哈拉尘埃层的通道。我们的研究结果表明，随着矿物粉尘层内污染含量的增加，所有密集气溶胶光学特性发生了显着变化。有趣的是，阿拉伯和撒哈拉沙尘的强烈气溶胶光学特性之间的差异可以忽略不计。我们讨论了这些结果对于识别矿物尘埃事件及其直接辐射效应的影响。首先，与污染物的混合掩盖了矿物粉尘信号，这表明在使用 Ångström 指数识别矿物粉尘事件时需要谨慎。然而，一旦矿尘事件得到确认，Ångström 指数可以帮助估计污染量。其次，我们对不对称参数和单散射反照率的测量从纯净的矿物尘埃层变为受污染的矿物尘埃层（例如，在525 nm处，中值分别从0.67下降到0.56和从0.96下降到0.89）。这些变化对短波直接辐射效应效率（即每单位气溶胶光学深度的直接辐射效应）有15种相反的影响，并且可能部分相互抵消。然而，与污染物混合对矿尘直接辐射效应效率的影响可能因表面反照率而异。总之，准确量化矿尘层内的污染含量至关重要。污染严重影响矿物粉尘事件的识别、其光学特性以及局部直接辐射效应。