Top panel: aerosol extinction curves at the zenith for the wavelength range of X-shooter. These are the most likely, optimal aerosol extinction curves for each run discussed in Sect. 5.2. For Run A, an alternate optimal curve (dashed) is also shown, which is the same calculation but excluding the 90 and 110° observations. The default curve is the one currently used in the ESO sky background model. Lastly, the uniform probability curve is derived with Eq. (6) but with all L_i = 1. Bottom panel: relative error (1σ_τ/τ). The values and errors for the extinction curves are also given in Table 5.