Glaciation activity of desert dust, air pollution and smoke from forest fires in convective clouds

The heavy air pollution, known as Atmospheric Brown Cloud (ABC), has been research mainly for its radiative effects and cloud condensation nuclei (CCN) activity that act to nucleate larger number of smaller cloud drops, and hence suppress coalescence and warm rain forming processes (Ramanathan et al., 2001). Precipitation in convective clouds with aerosol-induced suppressed coalescence is initiated nucleation of ice crystals that form ice hydrometeors that collect the supercooled cloud water. These ice particles are nucleated by ice forming nuclei (IFN) aerosols. Greater IFN activity of the ABC aerosols mean formation of more ice hydrometeors lower in the clouds, compensating to some extent for the precipitation suppression effect of the CCN activity of the ABC. The IFN activity of the ABC in clouds is little known. Documentation of the IFN activity of the ABC is challenging because of the wide possible sources of the aerosols composing the ABC, including industrial emissions, burning of low grade fuels and agricultural fires. Air pollution aerosols have been generally considered as having less IFN activity than desert dust aerosol, which are known to be major source of IFN. Szyrmer and Zawadzki (1997), stated in a review paper that "certainly, natural mineral particles dominate the atmospheric nucleation of ice in the conditions of low temperatures of -12° to -15°C or less." For testing whether this is really the case we focus our study over China and the vicinity, where both pollution and desert dust aerosol abound. Comparative measurements of IFN activity of heavy pollution haze and dust storms were conducted in China using the Bigg cloud chamber (1957), where activated IFN fall into supercooled sugar solution, grow and get counted there. You Laiguang and Shi An-Ying (1964) used the Bigg IFN counter in Beijing and showed that IFN activity of heavy pollution haze with visibility of ~1 km exceeds that of dust storm with similarly low visibility, especially at the higher temperatures (See Figure 1). Subsequent measurements in similar conditions (You Laiguang et al., 2002) had apparently greater detection efficiency of IFN and showed much greater overall concentrations, but air pollution and heavy desert dust had still comparable IFN activity, with desert dust exceeding the pollution activity by a factor of 2 at -15°C, whereas the opposite was the case in the earlier set of measurements. These measurements contradict the common belief that desert dust dominates the IFN activity of aerosols whereas air pollution mainly act as CCN, except for highly supercooling, e.g., <-30°C. This reexamined here using satellite measurements of cloud and aerosol properties over China and the vicinity.