Constructed Climates

Figure 4.9: At left, the plot shows how ozone production changes with VOC and NOx concentrations, the two important emissions ingredients in the reactions of Figure 4.7. Lines of equal ozone production, with rates of 1, 2.5, 5, . . . , 30 ppb/hr, form curves on this logarithmic plot of emissions concentrations (after Sillman 1999). As the reactions produce ozone during an 8-hour sunlit day, the arrows depict how the VOC and NO_x concentrations would change from the dot to the arrow’s head. Below the “ridge-line,” increasing NO_x concentration increases ozone production, and increasing VOC concentrations increases it above the line. At right, data from Essen, Germany, show that ozone production variations lead to fall and winter ozone levels that differ greatly in diverse city areas, with greener areas having counterintuitively higher ozone levels (after Kuttler and Strassburger 1999).

The graph at left in Figure 4.9 summarizes complicated ozone production dynamics in an amazingly efficient and content-rich way, also making it a challenge to understand.[51] Specifically, it shows how ozone production depends on the concentrations of two important pollutants — VOCs and NO_x — defined earlier in conjunction with Figure 4.2. Solid lines that curve through the plot indicate ozone production (in units of parts per billion per hour), and the arrows indicate the direction the pollutant concentrations change over an 8-hour, sunlit period from 9 AM to 5 PM.

So what does it all mean? Here is a detailed description. Take note of the ridge-line that roughly follows the ozone production numbers 30 down to 10 and then toward the lower left corner, as if this plot were a topographic map. Reactive nitrogen and VOCs compete for OH, and the relative amounts of each determine which reaction dominates the competition (see Figure 4.7). Typically, a ratio of VOCs to reactive nitrogen of 5.5 to one balances the reactions of the two groups with OH, though that number depends on the VOC species involved. Above this ridge-line, increasing levels of NO_x reduce ozone formation because NO₂ pulls out OH, slowing down the VOC-fueled process that converts NO to NO₂. However, also above this ridge-line, increasing levels of VOCs increase ozone formation because the reaction between the VOC (methane in Figure 4.7) and NO is limited by VOC concentration. Below the ridge-line, the scarcity of NO_x limits the reaction between the VOC and NO, and variation in the VOC concentration has relatively little effect on ozone formation. At the same time, increasing levels of NO_x directly increases ozone formation.

The graph at right shows what the details mean. These results, from Essen, Germany, show the cool weather variation of ozone and NO₂ within a city, again demonstrating how reactive nitrogen depletes ozone.[52] In contrast to the actual mechanisms, the graph gives the appearance that green areas might be responsible for high-ozone levels. Of course, that’s not correct, so don’t go cut down all your city trees.

Suppose you’re the one tasked with writing nationwide regulations for automakers regarding emissions. These complexities demonstrate the difficulties of controlling ozone levels by regulating these two emissions. How do you deal with such details as whether or not there’s a power plant upwind affecting ozone formation in a downwind city, and if so, NO_x emissions in cars may either increase or decrease ozone formation? Do you worry whether the car operates in a city with high- or low-biogenic VOCs?

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[51]The plot showing how ozone production depends on VOCs and NO_x was adapted from Sillman (1999), which has an excellent discussion of ozone chemistry. The plot depicts results from model calculations discussed therein. The book by Seinfeld and Pandis (2006) and an article by Chameides et al. (1992) also provide good backgrounds.

[52]Kuttler and Strassburger (1999) measured ozone and nitrogen levels in different parts of the city of Essen, Germany.