This study examines the cause of the blue colour of the sun as observed after the eruption of Krakatao in 1883 as well as other volcanic eruptions or massive forest fire events. Aerosol particles, e.g. volcanic ash or products of biomass burning, are believed to be able to modify the spectral distribution of transmitted solar radiation making it appear blue or green to a human observer.Previous studies already showed that narrow aerosol particle size distributions with radii on the order of about 500 nm can lead to anomalous scattering; i.e. scattering cross sections increasing with increasing wavelength in the visible spectral range. In this work we treat the effect of Rayleigh scattering on the shape of the transmitted solar spectrum correctly employing radiative transfer (RT) simulations with the SCIATRAN RT-model. The colour associated with solar transmission spectra is determined based on the CIE (International Commission on Illumination) colour matching functions and CIE chromaticity values. It is shown that a blue sun can be simulated for aerosol optical depths (at 550 nm) of about τ=0.5 (or higher) if Rayleigh scattering is taken into account. Without considering Rayleigh scattering – as in most of the previous studies – a blue sun is in principle produced with aerosol optical depths as low as about τ=0.1 (at 550 nm) if the aerosol particle size distribution is chosen to maximize anomalous scattering in the visible spectral range. It is demonstrated that Rayleigh scattering – as expected – has a strong impact on the transmission spectrum, particularly at low solar-elevation angles, and needs to be considered for a correct determination of the perceived colour of the sun. We also test the hypothesis that the blue sun after the eruption of Krakatao was caused by large abundances of water vapour in the atmosphere, as proposed in earlier studies. In addition, we present a case study on a particularly noteworthy blue sun event in the past, i.e. the one related to the large Canadian forest fires in September 1950.

中文翻译：

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关于蓝色太阳现象

这项研究调查了1883年喀拉喀托火山喷发以及其他火山喷发或大规模森林火灾后观察到的太阳蓝色的原因。人们认为，气溶胶颗粒（例如火山灰或生物质燃烧产物）能够改变透射的太阳辐射的光谱分布，从而使人类观察者看起来呈蓝色或绿色。先前的研究已经表明，半径较小的气溶胶粒径分布较窄。大约500 nm的量级会导致异常散射；即散射截面随着可见光谱范围内波长的增加而增加。在这项工作中，我们使用带有SCIATRAN RT模型的辐射转移（RT）模拟，正确地处理了瑞利散射对透射太阳光谱形状的影响。基于CIE（国际照明委员会）颜色匹配函数和CIE色度值确定与太阳透射光谱相关的颜色。结果表明，可以模拟蓝色太阳的气溶胶光学深度（在550 nm处）约为如果考虑瑞利散射，则τ = 0.5（或更高）。如先前的大多数研究一样，在不考虑瑞利散射的情况下，原则上会产生蓝太阳，其气溶胶光学深度低至τ = 0.1（在550 nm处），如果选择气溶胶粒径分布以使可见光谱范围内的异常散射最大化。事实证明，瑞利散射（如预期的那样）对透射光谱有强烈影响，尤其是在低太阳仰角时，需要考虑这一点，以便正确确定可感知的太阳颜色。我们还检验了这样的假说，如早期研究中所提出的那样，喀拉喀托火山喷发后的蓝太阳是由大气中大量水蒸气引起的。此外，我们还提供了一个案例研究，该案例研究了过去一个特别值得注意的蓝色太阳事件，即与1950年9月加拿大大森林大火有关的事件。