  The so-called unknown absorber in the clouds of Venus is an important
absorber of solar energy, but its vertical distribution remains poorly
quantified. We analyze the 283 and 365-nm phase curves of the disk-integrated
albedo measured by Akatsuki. Based on our models, we find that the unknown
absorber can exist either well-mixed over the entire upper cloud or within a
thin layer. The necessary condition to explain the 365-nm phase curve is that
the unknown absorber must absorb efficiently within the cloud scale height
immediately below the cloud top. Using this constraint, we attempt to extract
the SO$_2$ abundance from the 283-nm phase curve. However we cannot disentangle
the absorption by SO$_2$ and by the unknown absorber. Considering previous
SO$_2$ abundance measurements at mid-infrared wavelengths, the required
absorption coefficient of the unknown absorber at 283~nm must be more than
twice that at 365~nm.
