CIESIN Thematic Guides

Changes in Tropospheric Chemistry

Changes in atmospheric chemistry and air quality are one consequence of ozone depletion and the resultant increase in ultraviolet radiation. Tropospheric ozone variability in both urban and rural environments is expected, along with perturbations in other important species, such as hydrogen peroxide (H202), hydroperoxy radical (H02), and hydroxyl radical (OH). Enhanced levels of ultraviolet-B (UV-B) may be involved in the slowdown of rising methane concentrations observed over the past few years. Madronich (1993) provides a comprehensive overview of the chemical processes involved and the possible responses in the chapter "Tropospheric Photochemistry and Its Response to UV Changes" of the volume The Role of the Stratosphere in Global Change.

In "Ozone Precursor Relationships in the Ambient Atmosphere," Chameides et al. (1992) note that increases in surface-level ozone have been linked to higher levels of non-methane hydrocarbons (NMHCs) and nitrogen oxide compounds (NOx), as would typically be found in urban and industrial areas of higher fossil-fuel combustion. Increased UV-B radiation reaching the lower atmosphere may exacerbate this production at the ground. Elevated ozone levels have been associated with urban smog episodes, which can greatly reduce air quality and lead to detrimental effects on human health and agriculture. Conversely, in rural regions having low NOx concentrations, ozone in the lower atmosphere is expected to decrease under higher UV irradiance. Schnell et al. (1991) describe this effect in "Decrease of Summer Tropospheric Ozone Concentrations in Antarctica."

In the modeling study "Calculations of Hydrogen Peroxide in Chemically Coherent Regions," Thompson, Huntley, and Stewart (1991) predict hydrogen peroxide will increase in the troposphere of both urban and rural environments under the influence of elevated ultraviolet radiation. Increasing trends of hydroxyl radical are also predicted from modeling studies involving higher UV levels, according to Madronich and Grainer (1992) in "Impact of Recent Total Ozone Changes in Tropospheric Ozone Photodissociation, Hydroxyl Radicals, and Methane Trends." The authors suggest that such an increase on a worldwide scale may play a role in the observed slowdown of rising atmospheric methane levels in recent years. A slowdown would have implications on global warming scenarios, because methane is an effective greenhouse gas. This positive correlation of hydroxyl radicals with ultraviolet radiation levels assumes no other changes in the composition of chemically active compounds in the atmosphere. A decrease in the hydroxyl radical has been suggested, however, when considering variations in other reactive compounds. In "Calculations of Hydrogen Peroxide in Chemically Coherent Regions," Thompson, Huntley, and Stewart (1990) indicate that OH levels decrease under various scenarios of carbon monoxide, nitrogen oxide species (NOx), and methane emissions. Any changes in the atmospheric concentration of OH will affect the chemical lifetimes of many chloroflourocarbon (CFC) replacement compounds, because OH is the primary compound involved in the breakdown of the alternative hydrochlorofluorocarbons (HCFCs). Knowledge of OH concentration is important in analyzing the global warming potential and ozone depletion potential of HCFCs.