Flexi Says: The two main types of smog are sulfurous smog and photochemical smog. Sulfurous smog forms where sulfur-rich smoke particles react with water droplets in fog or clouds. This fog is typically associated with coal burning, which has a high sulfur content. Photochemical smog forms when car exhaust is exposed to sunlight. Nitrogen oxide is created by gas combustion in cars and is released into the air. In the presence of sunshine, the NO2 splits and releases an oxygen ion (O). The O then combines with an oxygen molecule (O2) to form ozone (O3).