FIGURE 1. Sensitivity of cloud LW emissivity to changes in droplet number concentration for fixed water path, SLW = (d
/dN)W.
From the following article:
Increased Arctic cloud longwave emissivity associated with pollution from mid-latitudes
Timothy J. Garrett and Chuanfeng Zhao
Nature 440, 787-789 (6 April 2006)
doi:10.1038/nature04636
The surface is theoretically based, assuming a cloud depth and temperature of 100 m and 253 K, respectively. Superimposed contours show quartiles in distributions of ground-based retrievals27 of low-level water cloud number concentration N and liquid water path W from near Barrow, Alaska, between 2000 and 2003. Relative pollution levels for the contours are indicated by mean values of 
with [lower,upper] quartiles ( is the 550 nm light scattering cross-section density of effloresced aerosol particles).
