Both react with oxygen and water in the atmosphere to form acids which cause acid rain.
Sulfur oxides come from sulfur contaminants in fuels.
However, most nitrogen oxides are formed by the reaction of atmospheric nitrogen with oxygen in high temperature flames. In essence, sulfur oxides are formed from things that we take from the ground and nitrogen oxides are formed from things that are in the air. The formation of nitrogen oxides in a flame can be controlled by altering temperature, oxygen content, or time. However, the same can't be done for sulfur oxides. The ultimate product of sulfur oxides removed in control equipment is CaSO 4 .