Results for the chemical evolution model in which we consider the density fluctuation associated with the analytical spiral arm formulation. Upper panel: azimuthal distribution of the residual of the oxygen abundances computed with our chemical evolution model at 4, 8, 12, 16, and 20 kpc (after subtracting the average radial gradient for a model with R_S = 7, R_D = 3.5, Σ₀ = 20, ν = 1.1, Ω_s = 20 km s⁻¹ kpc⁻¹, and m = 2 spiral arm; model S2A in Table 1). Lower panel: time evolution of the [O/H] abundance as a function of the azimuthal co-ordinate computed at 8 kpc.