Chemistry in the


Ozone, Space, and Time
Ozone concentrations in the troposphere vary widely over the Earth’s surface. The more direct the angle of sunlight, the greater its intensity. Where ozone’s precursors exist, more ozone tends to occur in regions closer to the Equator (lower latitudes) than in regions at the poles (higher latitudes).

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.

of Smog over Upstate New York
The STS-92 Space Shuttle astronauts photographed upstate New York at sunset on October 21, 2000. The view looks toward the southwest from southern Canada, and captures a regional smog layer extending across central New York, western Lake Erie and Ohio, and further west. Winds bring ozone and some chemicals that participate in its formation to rural areas downwind of emission sources. Ozone itself is invisible. (Photograph courtesy NASA JSC Gateway to Astronaut Photography of Earth)

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.

Graph of NO2 and O3 over

Measurements of nitrogen dioxide (NO2) [in blue] and ozone (O3) [in green] indicate rise and fall over a 48-hour period. Nitrogen dioxide participates in ozone formation, so after its concentrations peak, so do concentrations of ozone. Ozone concentrations peak during hours of maximum sunlight, around the middle of the day. (Graph ccourtesy William Brune, Penn State Earth Systems Science Center)

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  Ozone concentrations reach maximum shortly after the peak in
vehicle traffic, about noon or soon thereafter.