Covalent molecules of low molecular weight are gases and would continue to build up in the atmosphere indefinitely if they were transformed to higher oxidation states. The highest oxidation states of most elements lead to ionic solids. These atmospheric transformations for carbon, nitrogen and sulfur are shown below. Successive oxidations of carbon, sulfur and nitrogen in the atmosphere. Atmospheric particulate matter comes from several sources.
Physical processes can breakdown larger material into smaller particles.
Coal grinding, fugitive road dust and dust from rock quarries are examples of physical processes that release particulate matter to the atmosphere. The smallest particles in the atmosphere are the result of high temperature combustion and gas to aerosol conversions. The atmospheric conversion of sulfur dioxide to ammonium sulfate is an example of a gas to aerosol conversion process. Depending on the relative humidity and the presence of other atmospheric gases, the sulfuric acid formed will be removed directly in precipitation, or neutralized and converted to ammonium sulfate. These particles will range in size from 50 to as many as 10,000 molecules.