On the transition between heterogeneous and homogeneous freezing
Box model simulations of an uplifting and adi- abatically cooling cloud of aerosol have been performed in order to study the transition between cirrus formation domi- nated by homogeneous nucleation of ice to that dominated by heterogeneous nucleation. The aerosol was assumed to con- sist of an internal mixture of sulfuric acid solution droplets with inclusions of soot. The parametrisation of DeMott et al. (1997) was used to simulate the heterogeneous nucleation of ice in such droplets with soot inclusions. The simulations show that the transition from heterogeneous to homogeneous nucleation occurs over a narrow range of soot concentration. Thus it seems to be possible to fix critical concentrations of heterogeneous ice nuclei which must be exceeded if hetero- geneous freezing dominates cirrus formation. A formula has been derived that allows to compute the critical concentra- tions of heterogeneous ice nuclei as a function of temper- ature, updraft speed, ambient pressure, and supersaturation at which heterogeneous freezing occurs. Generally, homoge- neous nucleation dominates in regions with updrafts stronger than 20 cms1, with the exception of heavily polluted areas which could be common in the northern hemisphere due to air traffic, where updrafts of the order 1ms1 may be nec- essary to render heterogeneous nucleation unimportant. Ac- cording to the present results it cannot be excluded that het- erogeneous nucleation plays a more important role for cirrus formation in the northern midlatitudes than anywhere else. A possible consequence of these results is that air pollution may lead to a higher coverage of cirrus clouds, but then these clouds will be optically thinner than clouds formed by homo- geneous freezing, with the exception of regions where con- densation trails are frequent.