Dispersion (Eq. (18)) resulting solely from variations in the dust SEDs between two clouds along theLOS in the target2 sample. We quantify the level of SED variation by the parameter σ_ρ (Eq. (15)), finding that σ_ρ = 0.15 can account for the excess dispersion in the target sample. Thus, we estimate that the ratio of 353 to 217 GHz polarized intensity is varying at roughly the 15% level from cloud to cloud. The blue shaded regions indicate the observed range of estimated in Sect. 4 and the ±1σ confidence interval from 1000 realizations of γ₁, γ₂ , ρ₁ , and ρ₂ in each pixel. The red shaded region is the resultant constraint on σ_ρ.