Page:Handbook of Meteorology.djvu/142

 dust in the air, while the earth is receiving a lessened amount of heat from the sun it is radiating into space about thirty times as much.

The products of combustion must also be taken into consideration as having a similar effect on absorption and radiation of heat. The world’s fuel consumption each year is the equivalent of about 1,500,000,000 tons of coal. Forest fires and grass fires add to the total of combustion whose products in part escape into the air. By their means an enormous number of dust particles are projected into the air and distributed through it. As a rule, the dust particles of fuel combustion are nuclei favorable to condensation. One cannot estimate even broadly the extent of air pollution from this source; it can be measured chiefly in terms of city fogs.

The suspended matter of combustion products has been measured at times. Systematic measurements both of suspended matter and of matter which is brought to the ground by rainfall have been made in various parts of England, at regular stations. The insoluble matter caught in gauges consisted chiefly of smoke carbon, a mixture of free carbon and heavy hydrocarbons, minute globules of liquid tar and insoluble ash. The soluble matter consisted of various sulphates, chlorine, ammonia, and soluble ash. The amount varied from a few hundred tons per year on each square mile to nearly 6000 tons per square mile. Measurements in several manufacturing districts of Pennsylvania showed an average of about 1900 tons per square mile per year falling to the ground.

In regions where smokeless fuel is used there practically is no smoke problem, and the pollution of the air is confined almost wholly to wind-blown dust and to local sources of pollution. In localities swept by sea winds, salt derived from wind-whipped spray is usually a factor in the floating dust. In most of the large seaports of the United States the chlorine content from this source is made a matter of systematic measurement. The tendency of tools and polished steel articles to become rusty in the vicinity of the sea is probably due as much to the chlorine content of the air as to the presence of excessive moisture.