500 μm emissivity excess (panel A), dust temperature (panel B), A parameter (panel C), and l_c parameter (panel D), as a function of Galactic longitudes. The dashed line shows the solar neighborhood value when normalized to c_Δ,s = 475. The vertical error bars correspond to the 1-σ standard deviation. The blue arrows show the tangent directions of the spiral arms at l =  −50°, –33°, –21°, and 31° (Vallée 2008) and of the 3 kpc ring at |l| = 23° (Dame & Thaddeus 2008).