Figure 1.

Ozone in the stratosphere can move downward to the troposphere, contributing to the “background” level of ground-level ozone. However, high levels of ozone in the troposphere are due to photochemical reactions involving volatile organic compounds (VOCs) and oxides of nitrogen (NO_x: NO, and NO₂). Anthropogenic emissions (e.g., fossil fuel combustion) are responsible for NO_x and mainly responsible for VOCs and CO. Trees also emit certain VOCs (e.g., isoprene). PM_2.5 from primary emission sources can react with (consume) free radicals (e.g., HO₂) responsible for ozone formation, which partly explains the observations in certain areas where ozone level increased while PM_2.5 level decreased. hv, photon; VOCs, volatile organic compounds; CO, carbon monoxide; NO, nitric oxide; NO₂, nitrogen dioxide; NO_x, NO and NO₂; HO, the hydroxyl radical; HO₂, The hydroperoxy radical; PM_2.5, Particulate matter with a diameter of 2.5 μm or less.