Answer:
Particles in the air can serve as nuclei of condensation hence favoring precipitation.
Differently sized particles rest at different altitudes, according to vertical pressure gradient. For this reason the term 'air pollution' is not accurate enough for it does not specify the kind of polluting particles. For a particle to be able to act as nucleus of condensation it has to be of considerable size: dust.
Often the main pollutants being monitored (but that cannot serve as nuclei of condesation since they only are of molecular size) are NOx species, such as NO or NO2.
Also ozone O3 is considered an air pollutant, since ozone is not supposed to be found in the biosphere. NOx and O3 can cause irritation of lungs and airways, worsening pre-existing breathing problems, such as ashma. Old people and persons affected by breathing difficulties are advised to stay indoors when levels of NOx and O3 rise.
Finally,
concentration of NOx and O3 is largely driven by upper-atmosphere chemistry, driven by photodissociation reactions (hence solar light). For this reason periodic changes of concentration of these pollutants are recorded in winter and summer, according to different solar exposure.