Ozone global sinks

As discussed in other chapters of this book and summarized in Chapter 16, the formation of tropospheric ozone from photochemical reactions of volatile organic compounds (VOC) and oxides of nitrogen (NC/) involves many reactions. Concentrations are therefore quite variable geographically, temporally, and altitudinally. Additional complications come from the fact that there are episodic injections of stratospheric 03 into the troposphere as well as a number of sinks for its removal. Because 03 decomposes thermally, particularly on surfaces, it is not preserved in ice cores. All of these factors make the development of a global climatology for 03 in a manner similar to that for N20 and CH4, for example, much more difficult. In addition, the complexity of the chemistry leading to O, formation from VOC and NOx is such that model-predicted ozone concentrations can vary from model to model (e.g., see Olson et al., 1997). 

Worth, and Jeffries (211) obtained emissions 2 to 10 times larger than any previous estimate. Rasmussen (199) has concluded that, while the identity of the emissions is well known, a quantitative estimate of the worldwide terpene emission rate is not yet possible. It would appear that the natural emissions are much larger than that estimated for man s activities, 27 x 10 tons yr Ripperton et al. have suggested that the reaction of ozone with terpenes provides an important, if not dominant, sink for both compounds in the troposphere. While large terpene mixing ratios (ppm or more) have been measured locally in isolated areas [Ripperton et al. (211)], no global estimate is available. 

Non-methane hydrocarbons (NMHCs such as ethane, ethene, propane, propene, and isoprene) are trace atmospheric constituents that play an important role in both providing a sink for hydroxyl radicals and in controlling ozone concentrations (Donahue and Prinn, 1990). The oceans are known to be a source of NMHCs to the atmosphere, although globally they are significantly smaller than terrestrial sources. However, the main marine-produced NMHCs, ethane and propene, may have an important local impact on atmospheric photochemistry (Plass-Dulmer et al., 1995), particularly in... 

The HS radical reacts with either oxygen or ozone, the latter giving the HSO radical. The oxidation processes ultimately yield SO2. The typical lifetime for atmospheric H2S is 3d. Global sources and sinks of H2S are estimated as 7.72 1.25 Tg a and 8.50 2.80 Tg a respectively, with an imbalance that was indefinable (Watts, 2000). 

If one accepts the classical viewpoint and assumes photochemical ozone production and loss reactions negligible on a global scale, the budget of ozone in the troposphere will be dominated by the injection of ozone from the stratosphere and its destruction at the ground surface. Clearly, this is a minimum budget. Injection and destruction rates are examined below. For steady-state conditions both rates must balance, and if they do not, one would have an indication for the importance of additional sources or sinks of tropospheric ozone. The following discussion will show, however, that within a rather wide margin of error, the two rates are indeed compatible. 

Can Tropospheric Ozone Levels Be Explained by Downward Transport from the Stratosphere Before we begin the study of tropospheric chemistry, we pose the question—can tropospheric 03 levels be explained solely on the basis of the 03 flux from the stratosphere to the troposphere We can address this question indirectly through the OH radical (Jacob 1999). The principal source of the OH radical in the troposphere is O3 photolysis (Section 6.1). The principal sink of tropospheric OH is reaction with CO and CH4. Given estimated global budgets for CO and CH4, we can first determine the quantity of OH in the atmosphere needed to oxidize the emitted CO and CH4. 

I 2 Chemical evolution Table 2.81 Global tropospheric sources and sinks of ozone (in Tg a ). 

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