Formation of NLTE circular polarization with atomic polarization and velocity gradients in the Na I D₁ line. The figure indirectly explains why the double-peak and Q-like V signals of the solar Na I D lines are more often found near the solar limb (black profiles) than at the disk center (yellow). The calculation is done with μ = 0.1, T_ν0 = 4, α = 3.14 (with T = 6.5 kK and microturbulent velocity υ_micro = 6.5 km s⁻¹), I_c = 2 × 10⁻⁵ (cgs units), β = 0.01 (B = 154 G with B parallel to LOS vector). Violet lines: critical intensity .