Fig. 3.

Climate relevance of ice nucleation by atmospheric aerosols via altered cloud radiative properties and precipitation (41). Heterogeneous INP are responsible for ice crystal initiation in clouds entirely warmer than –38 °C (midlevel clouds here), and INP compete with spontaneous freezing of wet aerosols in cirrus clouds in a nonintuitive manner. Various possible aerosol sources of INP are indicated, although the contributions of many aside from mineral dust are poorly constrained. Since 2010, the potential contribution of arable soil dust is also now recognized. The likely but uncertain change in the magnitude of the general cooling impact (blue arrows) of midlevel clouds and warming impact (red arrows) of high cirrus clouds in response to increases in the relative number concentrations of INP is suggested.