What is tropospheric ozone?

Tropospheric ozone (also called ground level ozone or surface ozone) is an air pollutant and a greenhouse gas that occurs near the surface of the Earth. Ozone near the surface of the Earth is a minor greenhouse gas that contributes to about 7% of the total warming. Natural sources of tropospheric ozone include lighting during thunderstorms and downward mixing of O3 from the upper layers of the atmosphere. Anthropogenic derived ozone is formed through photochemical reactions that are driven by sunlight. Ozone is not emitted directly from a tailpipe or a smokestack; rather it is formed through reactions with volatile organic compounds (VOC) and oxides of nitrogen (NOx). Sunlight plays a very important role in the formation of ozone. Therefore there are large diurnal and seasonal changes in the concentrations of O3 in the lower atmosphere.

Typical summertime maximum levels of stratospheric ozone range from 100 to 400 ppb of O3 in urban and suburban areas. Levels in rural areas range from 50 to 120 ppb. Remote tropical forests and marine areas have ozone concentrations of 20 to 40 ppb. Due to natural sources of ozone, background concentrations range from 20 to 30 ppb globally (Krupa et al. 2001).

http://www.epa.gov/air/ozonepollution/pdfs/ozonegb.pdf

Tropospheric ozone vs. Stratospheric Ozone

Tropospheric ozone is O3 gas that is found near the Earth’s surface. It is often referred to as “bad ozone” because levels are increasing due to human activity and because it cause detrimental health and environmental effects.

Stratospheric ozone occurs in the upper atmosphere and is often called “good ozone.” Ozone in the stratosphere protects the Earth’s surface from harmful ultraviolet radiation. Currently, the stratospheric ozone layer is being depleted by chlorofluorocarbons (CFCs) and other ozone-depleting substances. The stratospheric ozone layer is thinning slightly globally but is most notably thinning over the polar regions. The ozone hole in the Antarctic is a region of substantial ozone thinning (EPA 2003).

The 2007 Intergovernmental Panel on Climate Change (IPCC) report estimates the amount of warming and cooling associated with anthropogenic ozone. The report suggests that tropospheric ozone currently contributes to warming with a globally averaged radiative forcing of +0.35 watts/sq. meter (0.25 to 0.65). The net radiative forcing of stratospheric ozone is slightly negative with a forcing value of -0.05 watts/sq. meter (-0.15 to 0.05).