Some pollutants are released directly into the
atmosphere. These include gases, such as SO2 , and
some particles, such as crustal material and elemental
carbon. Other pollutants are formed in the air.
Ground-level ozone forms when emissions of NOx
and VOCs react in the presence of sunlight. Similarly,
some particles are formed from other directly emitted
pollutants. For example, particle sulfates result from
SO2 and ammonia (NH3) gases reacting in the