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3 Ambient Ozone and Related Pollutants
Pages 48-74

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From page 48...
... More important, however, tropospheric ozone is responsible for various deleterious effects on humans and other organisms that are exacerbated by increased ambient ozone concentrations. Preindustrial tropospheric background ozone concentrations were about 10 ppb, but they have increased to about 30-40 ppb (e.g., Volz and Kley 1988; NRC 1991; Finnlayson-Pitts and Pitts 2000)
From page 49...
... . The specific reactions that form most of the tropospheric ozone are the photolysis of NO2 followed by the combination of the released oxygen atom with the abundant oxygen molecules (O2)
From page 50...
... The long effective atmospheric lifetime of ozone, measured in weeks (IPCC 2001) , leads to substantial regional and intercontinental transport, and increased global NOx emissions result in steadily increasing background ozone concentrations (e.g., Finnlayson-Pitts and Pitts 2000)
From page 51...
... (b) FIGURE 3-3 Ozone concentrations in Atlanta in 2006: (a)
From page 52...
... 0.040 0.020 0.000 -0.020 -0.040 00 00 00 00 00 0 0 0 0 0 0 0 :0 :0 :0 :0 :0 :0 :0 0: 2: 4: 6: 8: 10 12 14 16 18 20 22 Local Time FIGURE 3-4 Simulated ozone concentration and sensitivity of ozone to Atlanta-area NOx and VOC emissions for downtown Atlanta and a suburban location downwind. Bars in each graph illustrate how much local emissions change ozone concentrations in downtown and suburban Atlanta.
From page 53...
... Local VOC and NOx emissions cause more ozone to be added (e.g., NRC 1991, 2004a and references therein) , and this leads to regionally high ozone concentrations with a noticeable increase in and downwind of cities (Figure 3-4; also see Box 3-1)
From page 54...
... 2003) , with U.S., Cana dian and Mexican anthropogenic emissions removed, to provide temporally and spatially varying PRB ozone levels (EPA 2006a)
From page 55...
... . Early ozone-control programs concentrated on VOC-emission reductions because they were thought to be cost-effective, decreased concentrations of many additional air toxicants, and, unlike control of NOx, were unlikely to raise ozone concentrations.
From page 56...
... RELATED POLLUTANTS In assessments of the mortality risks associated with ozone exposure, it is important to consider how emission sources, meteorology, and the chemistry of ozone formation affect the formation of other pollutants of concern, including components of particulate matter (for example, the sulfate fraction of PM2.5) and other gases, such as aldehydes, and acids (Figure 3-1)
From page 57...
... FIGURE 3-6 Left, ozone and PM2.5 nonattainment areas in the eastern United States in 2006. Right, implementation of CAIR and other controls is expected to bring future ozone concentrations in many areas in the eastern United States into attainment of the previous 0.08-ppm ozone NAAQS.
From page 58...
... . The strong linkages and the fact that the other pollutants may have health effects are captured in a regulatory context in TABLE 3-1a Day-to-Day Correlation Between Air Pollutants in Boston, MA, by Season (Summer Nonshaded, Winter Shaded)
From page 59...
... .) Similarly, epidemiologic studies of ozone-health associations tacitly include the potential exposure to co-occurring pollutants unless they control for such additional exposure.
From page 60...
... . That also means that ozone concentrations can be lower in areas that have pollutants coming from substantial NOx sources.
From page 61...
... TABLE 3-2 Pollutant Correlations at Jefferson Street SEARCH Site, Atlanta, GA, 1999-2006a 24-h O3 1-h max O3 8-h max O3 24-h O3 1-h max O3 8-h max O3 (O3 season) (O3 season)
From page 62...
... . As shown above, ambient ozone concentrations tend to correlate strongly and positively with ambient sulfate but not with ambient traffic-related elemental carbon, where a negative correlation is often found.
From page 63...
... OZONE CONTROL EPA uses morbidity and mortality risk estimates to assess the benefits of its pollutant-control programs and thus the benefits of reductions in specific emissions from specific sources. That approach has a subtle but profound effect on the metric chosen to link ambient ozone concentrations with mortality risk.
From page 64...
... However, the choice of metric can be influential in determining benefits of control programs. For example, a program that lowers NOx emissions could reduce peak ozone concentrations but raise average concentrations.
From page 65...
... and are given as parts per billion per 1% change in domainwide NOx emissions. As shown, annual average ozone goes up in each location, although fourthhighest 8-h averages typically are reduced.
From page 66...
... will affect ozone concentrations and (as in the case of control of fly ash) could reduce metal-associated ROS.
From page 67...
... Indoor ozone generally originates outdoors; indoor sources -- such as photocopiers, laser printers, and some air cleaners -- are not present in most homes. Indoor residential ozone concentrations are thus determined primarily by outdoor concentrations and by factors that affect the ability of ozone to penetrate into and persist in the home, including removal by reaction on surfaces, air exchange between indoors and outdoors, air filtration, and reactions between ozone and other indoor pollutants.
From page 68...
... are generally low relative to outdoor concentrations averaged over the same period regardless of geographic location, with measured personal ozone concentrations generally below the limit of detection of some measurement methods. Like 24-h outdoor and indoor concentrations, 24-h personal ozone exposures are higher in summer than in winter, although summertime concentrations to which individuals are exposed (averaged over 24-h)
From page 69...
... The seasonal variability probably reflects increased home ventilation in the hotter summer months. Consistently with that, the slope of the regression between 24-h ambient ozone concentrations and corresponding personal exposures in Steubenville, OH, was 100% higher for elderly people spending time in well-ventilated indoor environments (slope, 0.18 ± 0.03; t value, 7.34)
From page 70...
... For shorter averaging periods, results of the one scriptedexposure study suggest that hourly or peak ambient ozone concentrations may be appropriate proxies for corresponding hourly or peak personal exposures. Additional short-term personal-ozone exposure studies are needed, given ozone's large contribution to uncertainty in ozone-mortality risk estimates; however, these studies will require the development of new measurement methods that have sufficient sensitivity to measure these likely low, short-term exposures.
From page 71...
... The breathing, or ventilation, rates are derived from energy-expenditure estimates for each activity included in CHAD and are adjusted for age-specific and sexspecific physiologic characteristics associated with each simulated individual. Ozone concentrations in each microenvironment are estimated with massbalance or transfer factors, and the user specifies prior probability distributions for the parameters to be used in the model; the prior distributions are used to model the uncertainties and variabilities in the parameters.
From page 72...
... Correlations between ozone and PM2.5 vary spatially, seasonally, and with PM2.5 components, all of which can confound the interpretation of epidemiologic analyses. Furthermore, exposure studies which typically relied on passive ozone monitors found that ambient ozone concentrations are not highly correlated with concentrations indoors, where people spend most of their lives, or with observed
From page 73...
... However, findings on exposure in the afternoon, when both outdoor activity and ozone concentrations can peak, suggest that exposures to higher ozone concentrations are better captured by ambient monitoring. Control programs based on reducing peak afternoon ozone concentrations on days when they are greater than the standard can result in an increase in the 24-h concentrations on days that are conducive to ozone formation and can also increase afternoon concentrations on days less conducive to ozone formation.
From page 74...
... ozone-mortality relationships at lower ozone concentrations. In recognition that ozone is a regional pollutant, winter measurements need not be collected in all the summer locations but, when they are collected, should be collected at the frequency of summer measurements.


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