FIGURE 2. Surface-based observations of Arctic pollution, clouds, and their radiative properties, made near Barrow, Alaska.

a, Restricting cloud tops to below 1.5 km, we show probability distribution functions of values of liquid cloud emissivity within an atmospheric window at 10.68 m wavelength. Values are sorted according to the upper (polluted) and lower (clean) quartile thresholds in surface measurements of , and binned according to retrieved values of water path, W. Differences in mean W between each pollution cohort are shown in per cent. Mean cloud properties shown are for greybody clouds only (r_e, area-weighted effective radius of the droplet size distribution; N, number concentration of droplets). b, For single-layer liquid clouds with bases less than 4 km (those radiatively important to the surface LW balance¹⁵ and for which cloud properties could be retrieved), we show the monthly-averaged cycle in (shown as a quartile plot), cloud coverage A, and the fraction of time f that is in the upper quartile for the entire data set.