The anomalous spectral dependence of the aerosol optical depth of the atmosphere observed at intrusion of the Arctic air is discussed. This dependence is characterized by a minimum near 0.44 mkm and a diffuse maximum in the range of 0.6-0.8 mkm. Such a situation is quite rare for continental midlatitudes. Based on the analysis of experimental conditions and comparison with other similar data, it is supposed that the observed type of the spectral dependence can be explained by peculiarities in the microphysical composition of aerosol characteristic of clear air. The aerosol origin of the anomalous dependence is confirmed by the results of microphysical simulation that has shown the following: the diffuse maximum in the red part of the spectrum represents the optical contribution of a medium-disperse fraction of particles with the size corresponding to the first maximum of the extinction efficiency factor; the increase of the optical depth in the UV region reflects the contribution of fine particles; the effect is observed at a weak influence of accumulative and coarse particles. The quantitative characteristics of the aerosol disperse structure adequate to the observed spectral dependence are presented.