15.4: Mitigating Tropospheric Ozone

Ozone (chemical formula O₃) has very different impacts, depending on where it is found in the atmosphere. Stratospheric ozone, produced naturally in the upper atmosphere through a reaction between oxygen molecules and solar ultraviolet radiation, is vital to human health and indeed to the existence of life on the Earth’s land surfaces. The stratospheric ozone layer, at a height of roughly 20 to 30 km, absorbs ultraviolet radiation that would otherwise be damaging or even fatal to life on land. In Section 15.6, we’ll see how the Montreal Protocol (1987) led to the phasing out of chemicals that damage the ozone layer.

While ozone in the stratosphere is beneficial to life, ozone near the Earth’s surface, called tropospheric ozone, has serious negative effects on both human health and agricultural crop yields. It is also a significant greenhouse gas.

As discussed in the Overview, human activities aren’t responsible for the direct emission of ozone. However, they do generate a range of precursor gases that can react in the presence of sunlight to form ozone. Methane is a key ozone precursor. Other precursor gases include nitrogen oxides (often referred to as NO_x), carbon monoxide, and volatile organic compounds (VOCs). NO_x and carbon monoxide are generated by combustion of fossil fuels in power plants, industrial processes, and vehicle engines. During combustion, NO_x is formed through the reaction of nitrogen and oxygen at high temperatures, and carbon monoxide is formed by incomplete fuel combustion. VOCs represent a whole range of carbon-based molecules, including gasoline, benzene, solvents, and other industrial and household chemicals. Ozone formed by reactions between NO_x and VOCs is a major component of photochemical smog in urban areas.

Catalytic converters are designed to significantly reduce vehicle emissions of NO_x, carbon monoxide, and VOCs. Air quality regulations, including requirements to equip cars with catalytic converters, have significantly reduced ozone levels in Los Angeles and other urban areas. As discussed in previous chapters, measures to replace fossil-fuelpowered internal combustion engines with electric motors, powered by batteries or fuel cells, would also significantly reduce emissions of ozone precursors. While such measures reduce ozone pollution regionally, they have a relatively small impact on ozone on a global scale.

In contrast, methane reduction has significant potential to reduce tropospheric ozone and its warming impact on a global scale. Since methane is a major ozone precursor, the methane reduction strategies discussed in Section 15.3 are also effective ozone mitigation measures.

Mitigating tropospheric ozone would have significant health and agricultural co-benefits. Ozone can promote asthma attacks and cause respiratory irritation, particularly in children, older adults, and those with existing respiratory conditions such as bronchitis and emphysema. Long-term exposure to ozone can cause permanent inflammation and scarring of the lungs, resulting in respiratory illnesses and premature deaths.

Ozone pollution is a dominant destroyer of agricultural crops. Ozone pollution is estimated to result in the loss of more than 110 million metric tons of crops per year, and it is responsible for 39% of crop losses in North America and 37% of losses in Asia. A UNEP study focused on the world’s four main staple crops (maize, rice, soybeans, and wheat) showed that full implementation of the methane reduction measures outlined above would also reduce ozone, avoiding 25 million metric tons of crop losses each year, relative to a scenario of unmitigated emissions; implementing black carbon reduction measures along with methane mitigation could double that figure to 50 million metric tons.