Ozone, Space, and Time
Wind directions and speeds, high or low concentrations of NOx and VOCs, precipitation, and air temperatures influence ozone concentrations throughout the troposphere. Because ozone formation takes place over time, and winds can carry air parcels far downwind of NOx and VOC sources, people in some rural areas breathe more ozone than people in some urban areas.
Ozone and some of its precursors are intercontinental travelers. Some air pollution from North American reaches Europe, and pollution from Asia reaches western North America. Extensive biomass burning in South America raises ozone levels in Australia, and the same activity in Africa degrades air quality over the Pacific Ocean.
Ozone concentrations also vary through time, throughout the day and through the year. The highest ozone concentrations of the year generally occur during summer, when sunlight is most intense. On a daily cycle, as industrial and motor vehicle activity rises throughout the morning, concentrations of NOx and VOCs also rise. Ozone concentrations consequently reach maximum shortly after the peak in vehicle traffic, about noon or soon thereafter. Downwind from urban areas, ozone may peak later in the afternoon or even after dark. After sunset, when no more sunlight initiates ozone formation, ozone concentrations fall as ozone reacts with other chemicals and rapidly settles onto various surfaces. NOx and VOC concentrations drop as they too participate in other reactions.