A simulation of the influence of organic and inorganic pollutants on the formation and development of warm clouds over Mexico City

The effects of anthropogenic aerosol particles on the growth by condensation of droplets in warm clouds over Mexico City are studied with a spectral microphysical air parcel model. The atmospheric aerosol introduced in the model is an external mixture of inorganic and organic particles. It is found that high concentrations of particles smaller than 2.5 μm are responsible for two phenomena which may have a negative effect on warm precipitation development. The first phenomenon is a suppression of the supersaturation maximum near the base of the cloud and, as a consequence, the absence of a mode in a droplet spectrum which normally is formed by droplets nucleated near the cloud base and is a source of mass for growth by coalescence. The second phenomenon, caused by high concentrations of pollutants with diameters smaller than 2.5 μm, is the delay in the growth of the largest droplets due to a reduction in supersaturation and the competition for water vapor. It is also shown that the shape of droplet spectra depends on the chemical composition of aerosol particles: a mixture of organic and inorganic particles favors the formation of bimodal spectra, whereas aerosol particles composed exclusively of inorganic particles tend to produce droplet spectra with only one peak.