Alkaline precipitation

Alkaline precipitation occurs when either calcium oxide or sodium hydroxide is emitted into the atmosphere, absorbed by water droplets in clouds, and then falls as rain, snow, or sleet. Precipitation containing these compounds can increase the pH of soil or bodies of water and lead to increased fungal growth.
The principal cause of alkaline rain are emissions from factories and waste deposits. Mineral dust containing large amounts of alkaline compounds such as calcium carbonate can also increase the pH of precipitation and contribute to basic rain. Alkaline rain can be viewed as opposite to acid rain.
India.
Due to the increased usage of soil-derived aerosols in India, alkaline precipitation has become a distinct phenomenon. Typically in industrialized areas the rain will be acidic. The usage of soil-derived aerosols that are calcium-rich, causes the atmosphere here to be alkaline, or basic, instead. Rain water was sampled over the period of a decade starting in the year 1974 in Pune, a city near the coast that is rather free of industrial pollution. The rainwater (save nearest the industrial complex) was all tested to be alkaline and having an abundance of calcium cations. This brings attention to the fact that the aerosol is spreading and causing this increase in alkaline precipitation. If no action is taken against these calcium derived aerosols then there could be detrimental damage done to the environment.
Natural causes.
While most natural rains are weakly acidic, alkaline rain can also occur in natural conditions, without significant impact of pollutants. Natural alkaline rains from semiarid areas carry significant amount of mineral dust lifted from the desert soil by convection and transported by winds. Mixed with water vapor, they are carried by clouds and deposited to the ground in the form of rain dust. 
For example, measurements in the Tibetan capital Lhasa, distant from heavy industry and agricultural pollution sources, have shown mean pH of the rainwater to be 7.5 in the period between 1997 and 1999. The phenomenon is ascribed to soil‐borne continental dusts in this semiarid environment, bicarbonates being the dominant anions in the samples. It is believed that the natural precipitation of arid and semiarid regions in the world should be alkaline, due to the typical chemical composition of soil‐borne aerosols.