Current Understanding Of Tropospheric Ozone Budget

The global distribution of tropospheric ozone presented earlier in this chapter illustrates its heterogeneity and underscores the difficulty of quantifying a global budget using the simplistic assumptions about its vertical distribution that had been employed when budgets neglecting photochemical processes were formulated. It is clear from the depiction in Figure 1 that local-scale photochemical generation of ozone has had a considerable impact on the global distribution as evidenced by the dominant plumes originating over North America, Europe, Asia, and Africa. A proper calculation of the tropospheric ozone budget must quantify these local- and regional-scale processes that feed into the global budget. Studies investigating photochemical processes from industrial emissions of volatile organic compounds and nitrogen oxides on scales of ~ 1000 km showed that the ozone generated on these scales should at least be comparable to the amount generated in the background through methane and carbon monoxide oxidation. In addition, the data now indicate that large quantities of ozone are generated in the tropics as emissions from widespread vegetation burning are oxidized efficiently in the intense tropical sunshine. Furthermore, some recent analyses of ozonesonde data have concluded that very little (perhaps as small as 5%) ozone near the ground had originated in the stratosphere and only —25% of the ozone observed at 300mbar had originated in the stratosphere. This analysis agrees with more recent estimates of stratosphere-troposphere mass exchange suggesting that the amount of ozone from the stratosphere is likely only ~30% of the amount determined from the earlier estimates determined in the 1970s.

Calculations from a general circulation model, which includes a complete set of photochemical reactions, have been used to evaluate the tropospheric ozone budget (Wang et al, 1998). The results from these model calculations are shown in the four seasonal panels in Figure 3. These calculations show how the chemical terms are both considerably larger than the input from the stratosphere and the amount of destruction at the ground. In addition, the amount of ozone produced photochemi-cally is generally greater than the amount destroyed. The largest amount of production is at northern middle latitudes in July. The Southern Hemisphere is also a sizable source in both July and October, when biomass burning is most prevalent in the southern tropics and subtropics.

Tropospheric And Stratospheric OzoneTropospheric And Stratospheric Ozone
Figure 3 Zonally averaged column budget for tropospheric ozone in different seasons including term from in situ photochemical production and loss, transport from the stratosphere, and deposition. The abscissa scale is linear in sine of latitude (from Wang et al, 1998).

These studies, as well as the documented increase in tropospheric ozone over time scales of decades provide fairly strong evidence that its distribution has changed significantly over the last century and that a large fraction of the tropospheric ozone budget is now likely controlled by anthropogenic pollution from both industrialized and tropical regions of the world. Studies are currently underway to provide more quantitative information, and our understanding of tropospheric ozone will greatly improve as more data are analyzed and more sophisticated global models are developed to study the problem.

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