Clean troposphere

In a clean troposphere, ozone would react with NO molecules, resulting in no net generation of ozone. However, in a dirty atmosphere, excess ozone is generated, thereby resulting in the formation of an oxidant which can lead to health effects, an especially hazardous condition for children and the elders who suffer from asthma and other respiratory challenges. 

Air calculated lifetimes x = 4.6 h due to reaction with 03 in 24-h period, x = 4.6 h with OH radical during daytime, and x = 20 min for N03 radical during nighttime for clean atmosphere x = 1.4 h for reaction with 03 in 24-h period, x = 2.3 h with OH radical during daytime, and x = 2 min with N03 radical during nighttime in moderately polluted atmosphere (Atkinson et al.1984a, Winer et al. 1984) calculated atmospheric lifetimes of 5.6 h, 5.1 h and 11 min for reaction with 03, OH and N03 radicals respectively for clean tropospheric conditions at room temp. (Atkinson et al. 1986) calculated tropospheric lifetimes of 3.4 h, 4.6 h and 2.0 h due to reactions with OH radical, 03 and N03 radical respectively at room temp. (Corchnoy Atkinson 1990). 

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 ... 

Under clean troposphere conditions, however, an oxidation chain initiated by CH3S + 02 could give different products, and it is essential to understand the mechanism of SO2 formation at low NOx. It will be interesting to see whether the same radicals lead to SO2 formation in the high and low NOx cases. Since the reaction of CH3S with O2 appears to be so slow, it may be necessaiy to perform C.W. photolysis studies under very carefully controlled conditions in order to determine whether a slow reaction actually occurs, or whether secondary reactions lead to the observed products. 

Air atmospheric lifetimes of 2.3 h in clean troposphere and 1.2 h in moderately polluted atmosphere, based on gas-phase reaction with hydroxyl radical at room temp. atmospheric lifetimes of 15.0 d in clean troposphere and 5.0 d in moderately polluted atmosphere, based on gas-phase reaction with 03 at room temp. (Atkinson et al. 1987)... 

Air calculated lifetimes of 2.3 h, 1.4 min and 24 h due to gas-phase reactions with OH radical (concn of 1 x 106cnr3 during daylight hours), No3 radical (cone of 2.4 x 10s cnr3 during nighttime hours) and 03 (clean tropospheric cone of 7.2 x 10" molecule cnr3), respectively, at room temp. (Atkinson et al. 1985)... 

Oxidation rate constant k for gas-phase second order rate constants, koH for reaction with OH radical, kNQ3 with N03 radical and kQ3 with 03 or as indicated data at other temperatures see reference photooxidation t,/2 = 14.7-24.4 yr in water, based on measured rate data for the reaction with hydroxyl radical in aqueous solution (Dorfman Adams 1973 selected, Howard et al. 1991) k0H = (4.9 0.4) x 10 13 cm3 molecule1 s 1 with atmospheric lifetimes of 46 d in clean troposphere and 23 d in moderately polluted atmosphere kQ3 < 1.1 x 10-20 cm3 molecule1 s 1 with atmospheric lifetimes of > 4 yr in clean troposphere and > 1.3 yr in moderately polluted atmosphere at room temp, (relative rate method, Atkinson et al. 1987)... 

On the basis of the results of different authors Robinson and Robbins (1970) estimated concentrations of 2 ppb (NO) and 4 ppb (N02) for continental areas between 65° N and 65° S, while for other territories they proposed 0.2 ppb and 0.5 ppb, respectively. On the other hand, Soderlund and Svensson (1976) speculate that the sum of NO and N02 concentrations in clean tropospheric air, except temperate regions, is less than 1 ppb. Over temperate regions the NOx level is about 4 ppb. The above NOx levels for remote areas were confirmed recently by Cox (1977), who measured mean concentration of 0.12 ppb NO and 0.34 ppb N02 mean concentrations on the Irish West Coast. 

Sulfur compounds are very important atmospheric constituents, since in clean tropospheric air as well as in the stratosphere the majority of aerosol particles are composed of ammonium sulfate or sulfuric acid (see Chapter 4). This finding is particularly interesting since with the exception of sea salt sulfur, a predominant portion of sulfur emission is in gaseous form. 

Petrenchuk and Selezneva (1970) made model calculations to evaluate the removal both in the cloud and beneath the cloud. They found that, in a clean tropospheric environment, 55 % of the trace constituents found in precipitation collected at the ground level is due the rain-out processes, in agreement with Georgii s results discussed in Subsection 5.3.4. 

Cloud processes have been predicted to have a significant effect on the chemistry of the clean troposphere (Lelieveld and Crutzen, 1990, 1991 Warneck, 1991, 1992). For example, the uptake of HCHO, HOz radicals, and N2Os into cloud droplets can lead to a decrease in the production of ozone. Removal of HCHO reduces the rate of gas-phase production of HOz radicals, and N205 into cloud droplets can lead to a decrease in the production of ozone. Removal of HCHO reduces the rate of gas-phase production of H02 radicals [reactions (33)—(36)1, and consequent conversion of NO to N02. Also, aqueous-phase reactions of H2C(OH)2, the hydrated form of HCHO, lead to the formation of 02, which can react with dissolved 03 to enhance the rate of transfer of 03 to the liquid phase over that based solely on physical solubility. Absorption of N2Os into... 

The reaction of OH and CO is important for OH radical as the main reaction in the clean troposphere (Sect. 7.1), and for CO to determine its atmospheric lifetime. There are many studies on the rate constants of this reaction, and it has been revealed that the reaction proceeds via a process. 

Fig. 7.1 Photochemical reaction scheme in the clean troposphere (Levy 1971)...

Fig. 7.3 Production and loss rate of O3 as a function of NOx mixing ratio in the clean troposphere calculated by a model simulation (Adapted from Liu et al. 1992)...

From Fig. 7.3, it can be seen that there is a threshold of NO concentration under which net O3 destruction and above which net O3 production occurs in the clean troposphere. Putting P(03)=L(03) in Eq. (7.53), the threshold value of NO concentration, [NO]th> is calculated as. 

Various attempts have been made to model the behaviour of DMS in the troposphere using box models with a DMS flux into the box and diurnal variations in OH concentration (Saltzmann and Cooper, 1988). However it has proved difficult to reconcile observed and calculated DMS concentrations, suggesting that either higher concentrations of oxidants are present in the "clean troposphere or lower piston velocities have to be used to give lower calculated flux rates of DMS into the atmosphere. 

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