Constructed Climates

Figure 4.13: Top two plots show the fraction of damaged leaf area on two plant species, milkweed and an introduced European shrub, Viburnum lantana, at three different ozone exposures lasting 7 hours per day, 5 days per week, from June through August. The average exposure measure includes only values above a threshold value 40 ppb (AOT40) (after Orendovici et al. 2003). These species were two of the most sensitive of 40 species examined. The bottom plots show reduced yields of wheat and potatoes with increased ozone uptake through the stomata. Measuring ozone uptake is a more complicated — and more relevant — measure for understanding ozone damage (after Pleijel et al. 2007).

I discuss human health implications of ozone in the next chapter, but ozone hurts vegetation, too. The plots in Figure 4.13 show data from a perennial milkweed, an introduced shrub, and two important crop plants.[65] Incredibly high amounts of leaf damage and 20-30% reductions in crop yields occur at ozone levels typically found in cities—levels sometimes exported downwind of cities and power plants feeding urban energy demands.

Ozone exposure measures for vegetation use a threshold ozone level, primarily relating to nighttime ozone levels when stomata are closed. Including those time periods by averaging over these subthreshold values obscures the mechanistically relevant ozone levels. Ozone entering the stomata provides an even better measure than exposure, but a much more difficult one. It’s better, in part, because high-ozone levels take place in the sunny afternoon heat when plants close their stoma to conserve water. Regardless of how it’s measured, ozone damages plants.

These nonattainment ozone regulations relate to average levels, not instantaneous ones. These averaged levels are like taking a 2-hour drive and during the first hour driving 100 miles per hour (mph) and during the second hour driving 20 mph, giving an average of 60 mph. If the speed limit is 65 mph, then all is well with the averaged speed, despite the instantaneous violation during the first hour.

How harsh should we be on the EPA’s allowance for variability? Clearly, averaging needs to be done with respect to nonattainment due to all the causes of ozone variability for which a city can’t really be held responsible. An extraordinarily high heat wave passing by one year in three gets averaged out of this nonattainment designation. Similarly, the emissions from an upwind wildfire shouldn’t produce penalties against a city. Yet, if climate change alters weather patterns over a decades-long time scale, then the definition of nonattainment (see the Figure 4.12 discussion) provides an appropriate change. An 8-hour average seems acceptable, but the devil’s in the variability.[66]

The counties around Durham constitute a noncompliance area. Of course, the forests of the southeastern United States provide a little cover for noncompliance because of their biogenic VOC levels (Figure 4.4), but, then again, North Carolina counties with the forests but without the urban areas are in compliance.[67] It’s hard to blame nature for the noncompliance.

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[65]Orendovici et al. (2003) provide the ozone damage data for milkweed, Asclepias incarnata, and Viburnum lantana (called the “Wayfaring tree”), along with reviewing damage to 38 other North American and Spanish shrubs and trees. Pleijel et al. (2007) discuss ozone damage to wheat and potatoes, along with a detailed discussion of stomatal ozone flux.

[66]I found little information regarding the health implications of, say, a 1-hour time span with excessive ozone, while the 8-hour limit is maintained. Making informed regulatory decisions demands information on these potential health implications — if, for example, exceeding some ozone level for five minutes triggers asthma, rather than the exposure integrated over an entire day, then citizens must demand the shorter time scale average limits (see Figure 6.3).

[67]The EPA’s website shows continuously updated nonattainment areas across the United States.