The SPICAM UV spectrometer onboard Mars Express observed two regional dust storms: on October 24, 2005, near Argyre Planitia and on July 9, 2005, in Hellas Planitia. The measurements were performed in nadir mode. The spectral domain 210-310 nm is considered. A comparison with a prior orbit which passed above almost the same path 6 days before the storm near Argyre Planitia has shown a significant decrease of the radiance factor. The difference grows at shorter wavelengths and shows significant dust absorption in the UV. The data have been used to retrieve the dust optical thickness τ, single scattering albedo w and asymmetry factor g. The fitting of the spectra by a radiative transfer code has revealed a set of solutions which could be constrained by Mie modeling of the parameters τ, w, and g for a gamma particle size distribution. The following estimates of the dust optical parameters are obtained: w = 0.64 ± 0.04 at λ = 300 nm and decreases to 0.6 ± 0.045 at 213 nm, g = 0.86 ± 0.03 at 300 nm and slightly grows to 0.88 ± 0.04 at 213 nm, τ = 2.5 ± 0.5 for the first dust event and τ = 2.0 ± 0.5 for the second one. The obtained values of single scattering albedo are consistent with the Hubble Space Telescope and Mariner 9 results.