Effects of elevated ozone on photosynthetic CO2 exchange and chlorophyll a fluorescence in leaves of Quercus mongolica grown in urban area

The aim of this study was to determine the responses of photosynthesis in leaves of Quercus mongolica to elevated O(3) exposure in an urban area. The results showed that the photosynthesis parameters were all decreased by the elevated O(3) during the whole season. Especially, light-saturated net photosynthetic rate (A) was reduced about 72% after 45 days exposure, and over the whole growing season, the differences were significant (p<0.05) compared with control. The reduction is related to both stomatal and non-stomatal factors, including a decrease (about 9%) in the maximum quantum yield of PSII photochemistry (F(v)/F(m)) detected after 90 days O(3) exposure.

Changes in tropospheric composition and air quality

Reductions in stratospheric ozone (O3) cause increased penetration of ultraviolet-B (UV-B) radiation to the troposphere, and therefore increases in the chemical activity in the lower atmosphere (the troposphere). Tropospheric ozone levels are sensitive to local concentrations of nitrogen oxides (NOx) and hydrocarbons. Model studies suggest that additional UV-B radiation reduces tropospheric ozone in clean environments (low NOx), and increases tropospheric ozone in polluted areas (high NOx). Assuming other factors remain constant, additional UV-B will increase the rate at which primary pollutants are removed from the troposphere. Increased UV-B is expected to increase the concentration of hydroxyl radicals (OH) and result in faster removal of pollutants such as carbon monoxide (CO), methane (CH4), non-methane hydrocarbons (NMHCs), sulfur and nitrogen oxides, hydrochlorofluorocarbons (HCFCs), and hydrofluorocarbons (HFCs). Concentrations of peroxy radicals (both inorganic and organic) are expected to increase, leading to higher atmospheric levels of hydrogen peroxide (H2O2) and organic peroxides. The effects of UV-B increases on tropospheric O3, OH, methane, CO, and possibly other tropospheric constituents, while not negligible, will be difficult to detect because the concentrations of these species are also influenced by many other variable factors (e.g., emissions). Trifluoroacetic acid (TFA, CF3COOH) is produced in the atmosphere by the degradation of HCFC-123 (CF3CHCl2), HCFC-124 (CF3CHFCl), and HFC-134a (CF3CH2F), which are used as substitutes for ozone-depleting substances. The atmospheric oxidation mechanisms of these replacement compounds are well established. Reported measurements of TFA in rain, rivers, lakes, and oceans show it to be a ubiquitous component of the hydrosphere, present at levels much higher than can be explained by reported sources. The levels of TFA produced by the atmospheric degradation of HFCs and HCFCs emitted up to the year 2020 are estimated to be orders of magnitude below those of concern, and to make only a minor contribution to the current environmental burden of TFA. No significant effects on humans or the environment have been identified from TFA produced by atmospheric degradation of HCFCs and HFCs. Numerous standard short-term studies have shown that TFA has, at most, moderate toxicity.

The oxidizing capacity of the earth's atmosphere: probable past and future changes

The principal oxidants in the lower atmosphere are ozone (O3) and two by-products of O3 photodissociation, the hydroxyl radical (OH) and hydrogen peroxide (H2O2). A number of critical atmospheric chemical problems depend on the earth's "oxidizing capacity," which is essentially the global burden of these oxidants. There is limited direct evidence for changes in the earth's oxidizing capacity since recent preindustrial times when, because of industrial and population growth, increasing amounts of O3 precursor trace gases (carbon monoxide, nitrogen oxides, and hydrocarbons) have been released into the atmosphere. The concentrations of O3 and possibly H2O2 have increased over large regions. Models predict that tropospheric O3 will increase approximately 0.3 to 1% per year over the next 50 years with both positive and negative trends possible for OH and H2O2. Models and the observational network for oxidants are improving, but validation of global models is still at an early stage.