"Ground-level ozone is not emitted directly into the air, but is formed when the precursor pollutants, oxides of nitrogen (NOx) and volatile organic compounds (VOC), react in the presence of sunlight. The process of ozone formation is complex and highly non-linear with respect to precursor emissions, photochemistry and meteorology. Sources of ozone precursors include motor vehicles, oil refining, printing, petrochemicals, lawn mowing, aviation, bushfires and prescribed burning." (Quoted from paper)