Ozone downward transport from stratosphere

Ultraviolet photolysis of ozone (there is a small background level of ozone in the troposphere as a result of downward transport from the stratosphere) ... 

The movement of atmospheric constituents within a region and between regions is a key process in atmospheric chemistry. For example, the transport of chemicals from the troposphere to the stratosphere sets off the depletion of ozone. Conversely, the downward transport from the stratosphere increases ozone in the troposphere. The phenomenon that most distinguishes the troposphere from the stratosphere is the rate of vertical mixing. The time scale for the vertical transport of air and other chemical species in the troposphere can be of a few hours, whereas vertical transport in the stratosphere can last months or years. 

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. 

It is worthwhile noting here that Reactions (7.20) and (7.21) are the only definitely established chemical mechanism for producing ozone in the troposphere the other source for tropospheric ozone is downward transport from the stratosphere. Together these two sources maintain a background concentration of ozone in the troposphere of about 0.03 to 0.05 parts per million of the air by volume (ppmv). Ozone is of critical importance in the chemistry of the troposphere because, not only is it a powerful oxidant itself, it is the primary source of the hydroxyl radical (OH), which is highly reactive and of paramount importance in tropospheric chemistry. Also, as can be seen from Eq. (7.23), the concentration of ozone in the air determines the ratio of [N02(g)l to [NO(g)]. Nitric oxide, NO(g), is also a very reactive gas and of great importance in atmospheric chemistry. ... 

Johnston argued that, if ozone were formed in the stratosphere and downward transport occurred, there would be a positive concentration gradient extending from 0.5 km to 5 km in altitude. Most of the cases in Table 4-9 obey this situation however, for the seven high-ozone cases in Table 4-9, the reverse is true. This seems to indicate that the occur-... 

We have addressed several aspects of STE of ozone and the impact on tropospheric ozone levels. Using ozone observations in the upper troposphere and lower stratosphere from MOZAIC, we have examined the rdation between ozone and PV in the lower stratosphere. A distinct seasonality in the ratio between ozone and PV is evident, with a maximum in spring and minimum in fall associated with the seasonality of downward transport in the meridional circulation and of the ozone concentrations in the lower stratosphere. The ozone-PV ratio is applied in our tropospheric chemistry-climate model to improve the boundary conditions for ozone above the tropopause, to improve the representativity of simulated ozone distributions near synoptic disturbances and realistically simulate cross-tropopause ozone transports. It is expected that the results will further improve when the model is applied in a finer horizontal and vertical resolution. 

In the literature a wide range of estimates regarding the influx of ozone from the stratosphere into the troposphere is presented, derived by many different methods. Some studies report an annual flux between 200-870 Tg 03 yr 1 globally, whereas other studies report a flux between 500 and 1000 Tg 03 yr 1 for the NH only. With our model we estimate for the NH a net downward flux from the stratosphere of 580 Tg 03 yr1, which is partly balanced by an upward flux of ozone from photochemical production in the troposphere of 210 Tg 03 yr1, yielding a net downward cross-tropopause ozone transport of 370 Tg 03 yr 1. Globally, these values are 950, 370 and 580 Tg 03 yr 1, respectively. (Note that the troposphere-to-stratosphere ozone flux in the... 

Approximately 10% of atmospheric ozone resides in the troposphere. Since solar radiation of wavelengths less than 242 nm that photolyzes molecular oxygen (see Reaction (5.11)) is entirely absorbed above the tropopause, and hence cannot produce ozone in the lower atmosphere, it was believed for a long time that the presence of O3 in the free troposphere was due to downward transport of ozone-rich air masses from the stratosphere. Dry deposition of O3 on vegetation was believed to be the only significant loss process. 

FIGURE 20 Schematic of the mechanisms that affect ozone distribution in the stratosphere. The mean meridional circulation is denoted by the heavy arrows it transports ozone poleward and downward from the source region in the tropical middle stratosphere. The circulation also carried into the stratosphere compounds of anthropogenic origin that contribute to ozone loss. The effect of quasihorizontal mixing by breaking planetary waves is denoted by the dashed lines with arrows. [From R. R. Garcia (1994). Phys. World 7, 49-55.]... 

The distribution patterns shown in Fig. 11 can briefly be explained as follows. Stratospheric ozone formed by photochemical processes is transported in poleward direction by atmospheric motions. This circulation is particularly strong in winter and spring months when stratospheric air moves downward over polar regions. At the same time the lower stratosphere over the tropics is characterized by a slow updraft (Brewer, 1949). Thus, stratospheric dynamics lead to the accumulation of ozone rich air in the lower polar stratosphere. It should be recalled here that at this altitude 03 is a conservative property of the air. During the late spring and summer, especially, the stratospheric 03 reaches the troposphere first of all through the tropopause gaps. In the troposphere this species is removed from the air by various sinks, as this will be shown in the next section. 

Generally, an increase of ozone is found with increasing height above sea level (note that Oj is not a primary emission produced photochemically within the boundary layer and transported downwards from the stratosphere). In the first few 100 meters... 

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