Reactants that form ozone come from natural and human sources

Most emissions, such as volatile organic compounds (VOCs) and nitrogen oxides (NOx), come from both anthropogenic (human-caused) and natural sources.  Although it is useful to know how much comes from natural sources, only anthropogenic sources can be controlled.

VOCs and nitrogen oxides are both naturally occuring pollutants that combine in heat and sunlight to form ground-level ozone (O3), which is known to cause respiratory and cardiovascular problems.  Ozone pollution can also damage trees and other wildlife.

Contribution and uncertainty of global anthropogenic and natural N emissions in 1990
Img source: http://www.igac.noaa.gov/newsletter/22/natural.php

Forests are a large source of VOCs, and emissions increase with higher temperatues and more sunlight. Isoprene, a common natural emission, is highly reactive and greatly contributes to ozone pollution.

Nitrogen oxides are released from soil during the binding process with plant roots.  However, only 3% of nitrogen oxides are from natural sources.  Much of the rest is released from agricultural land that has been fertilized with nitrogen compounds.

Since natural sources of nitrogen oxides constitute a smaller proportion of the total NOx in the air than natural VOCs do, regulating nitrogen oxides is the most effective way to reduce ground-level ozone. See (Link to Politics and Regulation Feature) to see areas of the U.S. with high levels of nitrogen oxides.

Img source: http://www.eoearth.org/article/Air_pollution_emissions