A regional pollutant is one that may impact areas far away from the sources of emissions. The impacts of ozone are usually not found directly at sources of VOCs or NOx because it takes time for them to combine in the atmosphere. Ozone isn't directly emitted from sources, instead VOCs and NOx turn into ozone in the atmosphere. This causes higher measurements of ozone in areas downwind of emission sources and can lead to emissions from sources far away creating ozone in different areas than where pollutants were emitted, depending on the weather.