Ozone atmospheric concentration

Measurements of ozone (O3) concentrations in the atmosphere are of particular importance. Ozone absorbs strongly in the ultraviolet region and it is this absorption which protects us from a dangerously high dose of ultraviolet radiation from the sun. The vitally important ozone layer lies in the stratosphere and is typically about 10 km thick with a maximum concentration about 25 km above the surface of the earth. Extreme depletion of ozone in a localised part of the atmosphere creates what is known as an ozone hole. 

In this sequence the Cl also acts as a catalyst and two molecules are destroyed. It is estimated that before the Cl is finally removed from the atmosphere in 1—2 yr by precipitation, each Cl atom will have destroyed approximately 100,000 molecules (60). The estimated O -depletion potential of some common CFCs, hydrofluorocarbons, HFCs, and hydrochlorofluorocarbons, HCFCs, are presented in Table 10. The O -depletion potential is defined as the ratio of the emission rate of a compound required to produce a steady-state depletion of 1% to the amount of CFC-11 required to produce the 1% depletion. The halons, bromochlorofluorocarbons or bromofluorocarbons that are widely used in fire extinguishers, are also ozone-depleting compounds. Although halon emissions, and thus the atmospheric concentrations, are much lower than the most common CFCs, halons are of concern because they are from three to ten times more destmctive to O, than the CFCs. 

The reaction of volatile chlorinated hydrocarbons with hydroxyl radicals is temperature dependent and thus varies with the seasons, although such variation in the atmospheric concentration of trichloroethylene may be minimal because of its brief residence time (EPA 1985c). The degradation products of this reaction include phosgene, dichloroacetyl chloride, and formyl chloride (Atkinson 1985 Gay et al. 1976 Kirchner et al. 1990). Reaction of trichloroethylene with ozone in the atmosphere is too slow to be an effective agent in trichloroethylene removal (Atkinson and Carter 1984). 

Tropospheric chemistry is strongly dependent on the concentration of the hydroxyl radical (OH), which reacts very quickly with most trace gases in the atmosphere. Owing to its short boundary layer lifetime ( 1 s), atmospheric concentrations of OH are highly variable and respond rapidly to changes in concentrations of sources and sinks. Photolysis of ozone, followed by reaction of the resulting excited state oxygen atom with water vapour, is the primary source of the OH radical in the clean troposphere ... 

Approximately the first third of this report is concerned with the origins and measurement of ozone and other photochemical oxidants and the relationship of atmospheric concentrations to emissions. The middle third deals with toxicologic studies and effects on humans, and the last with effects on plants, ecosystems, and materials. 

A critical question concerning atmospheric concentrations of ozone and other photochemical oxidants is What fraction of the observed values in each locale can be controlled by reduction of emissions Some contend that natural background concentrations exceed the federal ambient air quality standard (0.08 ppm). Another point of view is that background ozone concentrations rarely exceed about 0.05-0.06 ppm at the surface and that higher concentrations are caused by man-made sources. 

The main purpose of this chapter is to survi atmospheric concentrations of photochemical oxidants, with emphasis on surface concentrations and the distribution patterns associated with them. The reason for that em> phasis is that the photochemical oxidants that affect public health and welfare are largely concentrated in this region. The whole subject of stratospheric ozone (and its filtering of ultraviolet light and interactions with supersonic-transport exhaust products), nuclear weapon reaction products, and halogenated hydrocarbon decomposition pr ucts is not treated here. 

This review begins with a summary of the sources of monitoring data operated primarily by public agencies. The spatial and temporal patterns of oxidant concentrations are then discussed—urban versus rural and indoor versus outdoor relationships, diurnal and seasonal patterns, and long-term trends. The chapter includes brief discussions of photochemical oxidants other than ozone and of data quality and concludes with a set of recommendations for guidelines in future monitoring of atmospheric concentrations of ozone and other photochemical oxidants. 

Destruction of stratospheric ozone caused by relatively small atmospheric concentrations of chlorofluo-rocarbons has vividly illustrated the capacity of human activity to alter our atmosphere in a manner that has significant and far-ranging effects. There is similar concern for the effects of greenhouse gases on the earth s climate. 

Despite the growth of HCFCs, the National Oceanic Atmospheric Administration (NOAA) researchers data show that ozone-depleting chemicals in the atmosphere have declined overall about 3% since 1994. But that decrease is due almost entirely to a dramatic drop in the amount of methylchloroform, which was used as a cleaning solvent. Natural oxidation processes in the atmosphere remove methylchloroform so efficiently that its atmospheric concentration has been falling rapidly since production stopped. 

Methane is removed continually from the atmosphere by reaction with OH radicals (Section 8.3). In contrast, chlorofluorocarbons and related volatile compounds are inert under the conditions of the lower atmosphere (troposphere), so atmospheric concentrations of these refrigerants and solvents will tend to increase as long as releases continue. The chief concern over chlorofluorocarbons is that they are a major factor in destruction of the stratospheric ozone layer (Section 8.3). They have been banned under the Montreal Protocol of 1988, but it is important that whatever substitutes (inevitably greenhouse active) are introduced to replace them degrade relatively quickly in the troposphere to minimize any contribution they may be capable of making to greenhouse warming. 

The Montreal Protocol on Substances that Deplete the Ozone Layer requires each signatory nation to reduce its production and consumption of the CFCs 11, 12, 113, 114 and 115 to 80% of their 1986 levels by 1993 and to 50% by 1998. Figure 8 shows that production levels of the first three of these has indeed fallen dramatically since 1988, according to data reported in Reference 109 by the major industrial producers. However, the Montreal measures will have little effect on the current levels of stratospheric CFCs, which would still continue to rise for many years, as illustrated by Figure 9 for the example of CFC-12. It would be necessary to impose an 85% reduction in order to stabilize atmospheric concentrations at their 1989 level110. Even with a total cessation of CFC emission atmospheric concentrations will not be restored to their pre-1960 levels until well... 

In addition to OH radicals, unsaturated bonds are reactive towards O3 and NO3 radicals and reaction with these species is an important atmospheric degradation mechanism for unsaturated compounds. Table 4 lists rate constants for the reactions of 03 and NO3 radicals with selected alkenes and acetylene. To place such rate constants into perspective we need to consider the typical ambient atmospheric concentrations of O3 and NO3 radicals. Typical ozone concentrations in pristine environments are 20-40 ppb while concentrations in the range 100-200 ppb are experienced in polluted air. The ambient concentration of NO3 is limited by the availability of NO sources. In remote marine environments the NO levels are extremely low (a few ppt) and NO3 radicals do not play an important role in atmospheric chemistry. In continental and urban areas the NO levels are much higher (up to several hundred ppb in polluted urban areas) and NO3 radicals can build up to 5-100 ppt at night (N03 radicals are photolyzed rapidly and are not present in appreciable amounts during the day). For the purposes of the present discussion we have calculated the atmospheric lifetimes of selected unsaturated compounds in Table 4 in the presence of 100 ppb (2.5 x 1012 cm 3) of O3 and 10 ppt (2.5 x 108 cnr3) of NO3. Lifetimes in other environments can be evaluated by appropriate scaling of the data in Table 4. As seen from Table 4, the more reactive unsaturated compounds have lifetimes with respect to reaction with O3 and NO3 radicals of only a few minutes ... 

Benzene in the atmosphere exists predominantly in the vapor phase (Eisenreich et al. 1981). The most significant degradation process for benzene is its reaction with atmospheric hydroxyl radicals. The rate constant for the vapor phase reaction of benzene with photochemically produced hydroxyl radicals has been determined to be 1.3 10"12 cm3/molecule-second, which corresponds to a residence time of 8 days at an atmospheric hydroxyl radical concentration of 1.1 x 106 molecules/cm3 (Gaffney and Levine 1979 Lyman 1982). With a hydroxyl radical concentration of 1 x 108 molecules/cm3, corresponding to a polluted atmosphere, the estimated residence time is shortened to 2.1 hours (Lyman 1982). Residence times of 472 years for rural atmospheres and 152 years for urban atmospheres were calculated for the reaction of benzene with ozone (03) using a rate constant for 03 of 7 /1 O 23 cm3/molecule-second (Pate et al. 1976) and atmospheric concentrations for 03 of 9.6/1011 molecules/cm3 (rural) and 3/ 1012 molecules/cm3 (urban) (Lyman 1982). 

Future changes in atmospheric concentration of other greenhouse gases (methane N2O, chlorofluorocarbons, tropospheric ozone). 

Typically, stratospheric ozone (O3) concentrations are about 0.2-0.4 ppm (parts per million), compared with about 0.03 ppm in unpolluted situations close to ground level in the troposphere. Stratospheric ozone concentrations are also measured in Dobson units (DU). A Dobson unit is equivalent to the amount of ozone that, if accumulated from the entire atmosphere and spread evenly over the surface of the earth at a pressure of one atmosphere and a temperature of about 68°F (20°C), would occupy a thickness of 10 mm (0.01 m or 0.4 in). Typically, stratospheric zone occurs at a couceutratiou of about 350 DU, equivalent to a layer of only 3.5 mm (0.14 in). 

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