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8
Atmospheric Observations of Voc, Nox, and Ozone

Introduction

Chapter 6 contains a discussion of the central role of the VOC/NOx ratio (the ratio of volatile organic compounds to oxides of nitrogen) in determining the chemical character of the VOC-NOx-ozone system. Two important points were made about the VOC/NOx ratio: First, the atmospheric boundary layer (defined here as a well-mixed layer extending from the surface to a height of about 1 or 2 km during the day) cannot be characterized by a single ratio because this ratio varies significantly with location and time of day. Second, ambient ratios often exceed by a substantial mount those calculated from emissions inventories. The goal of this chapter is to examine data gathered from atmospheric observations to determine if ambient VOC, NOx, and O3 concentrations follow a regular pattern as one moves from an urban or suburban area to a rural area and then to a remote area. By comparing these patterns with those observed in smog-chamber experiments, it may be possible to establish to what degree smog-chamber experiments, and the chemical models based on these experiments can be applied to the atmospheric VOC-NOx-ozone system. By comparing patterns found in urban and suburban areas with those found in rural and remote locations, it may be possible to infer the relative effectiveness of controlling VOC versus NOx in different parts of the country.

The second point above—that discrepancies between ambient and emission-inventory-derived VOC/NOx ratios have major implications concerning the accuracy of emissions inventories—is addressed in Chapter 9.



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Page 211 8 Atmospheric Observations of Voc, Nox, and Ozone Introduction Chapter 6 contains a discussion of the central role of the VOC/NOx ratio (the ratio of volatile organic compounds to oxides of nitrogen) in determining the chemical character of the VOC-NOx-ozone system. Two important points were made about the VOC/NOx ratio: First, the atmospheric boundary layer (defined here as a well-mixed layer extending from the surface to a height of about 1 or 2 km during the day) cannot be characterized by a single ratio because this ratio varies significantly with location and time of day. Second, ambient ratios often exceed by a substantial mount those calculated from emissions inventories. The goal of this chapter is to examine data gathered from atmospheric observations to determine if ambient VOC, NOx, and O3 concentrations follow a regular pattern as one moves from an urban or suburban area to a rural area and then to a remote area. By comparing these patterns with those observed in smog-chamber experiments, it may be possible to establish to what degree smog-chamber experiments, and the chemical models based on these experiments can be applied to the atmospheric VOC-NOx-ozone system. By comparing patterns found in urban and suburban areas with those found in rural and remote locations, it may be possible to infer the relative effectiveness of controlling VOC versus NOx in different parts of the country. The second point above—that discrepancies between ambient and emission-inventory-derived VOC/NOx ratios have major implications concerning the accuracy of emissions inventories—is addressed in Chapter 9.

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Page 212 It is useful to focus on four regions of the atmospheric boundary layer, each of which has a distinct mix of anthropogenic and natural VOC and NOx emissions: • The urban-suburban atmosphere, which is the area most strongly affected by anthropogenic emissions • The rural atmosphere, which is somewhat less affected by anthropogenic emissions and more affected by natural emissions than is the urban-suburban atmosphere • The atmosphere over remote tropical forests, which is essentially free of anthropogenic VOC and NOx emissions and strongly affected by natural emissions • The remote marine atmosphere, which is not only free of anthropogenic emissions, but also has relatively small biogenic sources of VOCs and NOx Because we are most interested in the conditions that foster episodes of high concentrations of ozone, our discussion concentrates on observations made during the daylight hours of the summer months. In the sections below, we first briefly examine the typical concentrations of ozone in these four regions and then turn to the more complex issues of NOx and VOC concentrations and their variability. Observations of Ozone Compared with those for NOx and VOCs, the data base of ozone observations is fairly extensive, especially for urban and suburban areas. At most rural surface sites, ozone concentrations have been found to vary over a diurnal cycle with a minimum in the early morning hours before dawn and a maximum in the late afternoon (Figure 8-1). This pattern is believed to result from daytime photochemical production or downward transport of ozone-rich air from above, combined with ozone loss by dry deposition and reaction with nitric oxide (NO) at night, when photochemical production ceases and vertical transport is inhibited by an inversion of the normal temperature profile. In locations near large sources of NO, the nighttime minimum in ozone can be quite pronounced because of the rapid reaction between ozone and NO. In fact, in many urban areas the NO source is strong enough to cause the complete nighttime disappearance of ozone. A somewhat different pattern has been observed at high-altitude sites (i.e., sites located 1 km or more above the local terrain). At these sites, ozone often exhibits a shallow maximum rather than a minimum at night (Figure 8-1b). This diurnal pattern is thought to

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Page 213 Figure 8-1 Diurnal behavior of ozone at rural sites in the United States in July.  Sites are identified by the state in which they are located. (a) Western  National Air Pollution Background Network (NAPBN); (b) Whiteface  Mountain (WFM) located at 1.5 km above sea level; (c) easter NAPBN  sites; and (d) sites selected from the Sulfate Regional Air Quality study.  IN(R)refers to Rockport. Source: Logan, 1989.

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Page 214 reflect the contrasting dynamic conditions typically encountered at high altitude sites, with upslope flow bringing ozone-poor air from the boundary layer to the site during the day and downslope flow bringing ozone-rich air from the free troposphere to the site at night. In addition to variations over a diurnal cycle, ozone concentrations at a given location also can vary significantly from one day to the next. It is not uncommon for the daily maximum ozone concentration at an urban site, for instance, to vary by a factor of two or three from day to day as local weather patterns change. Despite the variable nature of ozone, the data base of ozone observations suggests a systematic pattern of decreasing daily maximum concentrations as one moves from urban-suburban locations to rural locations and then to remote locations. Table 8-1 shows that daily maximum ozone concentrations within the atmospheric boundary layer tend to be largest in the urban-suburban atmosphere, where 1-hour ozone concentrations most often exceed the National Ambient Air Quality Standard (NAAQS) concentration of 120 parts per billion (ppb), and maxima well above 200 ppb have been observed. Although the NAAQS can be exceeded in rural areas, ozone concentrations in these regions tend to be more moderate and rarely exceed 150 ppb. In remote locations, ozone concentrations tend to be quite low, typically ranging from 20 to 40 ppb. Table 8-1 Typical Summertime Daily Maximum Ozone Concentrations Region Ozone, ppb I Urban-suburban 100-400 II Rural 50-120 III Remote tropical forest 2040 IV Remote marine 20-40 Sources: Cleveland et al. (1977); Hov (1984b); Gregory et al. (19881990); Kirchoff (1988); LeFohn and Pinkerton (1988); Janach (1989); Logan (1989).

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Page 215 Observations of Nox There is a sizable body of data on the concentrations of NOx (the combined concentrations of NO and nitrogen dioxide (NO2)) in the atmosphere, but caution must be exercised in drawing conclusions from these measurements. As noted in Chapter 7, most measurements of NOx have been made by devices that convert NO2 to NO, which is then measured by chemiluminescence. Comparison of these measurements with more specific techniques suggests that all surface converters that can convert NO2 to NO also convert other reactive nitrogen oxide species, such as peroxyacetyl nitrate (PAN), to NO, thereby causing interference (Singh et al., 1985; Fehsenfeld et al., 1987, 1990; Gregory et al., 1990). Because PAN concentrations can vary considerably depending on other pollutant concentrations and the temperature, the potential error associated with PAN interference will depend strongly on the location, season, and altitude at which samples are taken. In urban locations, where the local NO sources are typically large, NO and NO2 are probably the dominant constituents of the total reactive nitrogen or NOy (NOx + HNO3 [nitric acid] + NO3 [nitrate radical] + N2O5 [dinitrogen pentoxide] + HONO [nitrous acid] + PAN + other organic nitrogen compounds). Thus, in urban areas, interference from PAN and other oxides of nitrogen is believed to be relatively small. In rural and remote locations, however, the interference can be substantial. For this reason, all nonurban NOx measurements made with surface converters must be considered upper limits (biased toward a high measurement); these measurements are so indicated in this discussion. Urban Nox Given the dominant role of anthropogenic emissions in the budget of atmospheric NOx and the fact that the sources of these emissions tend to be located in or near urban areas, elevated concentrations of NOx are to be expected in these locations. Observations of NOx support this expectation. During the summer of 1986, NOx measurements were made from 6:00 to 9:00 a.m. Daylight Saving Time (DST) at six locations in Philadelphia, Pennsylvania (Meyer, 1987). Four of the sites were downtown, where the average measured NOx concentration ranged from 40 to 99 ppb. At two suburban sites, the average concentrations were 33 ppb (upwind of the core city) and 65 ppb (downwind of the core city). The average for the six-station network was 60 ppb. However, the NOx concentrations at all locations exhibited a high degree of variability—standard deviations ranged from 37% to 50% of the average NOx mixing ratio measured at the site.

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Page 216 Figure 8-2a NOx concentrations measured in urban locations in the United States during  the summer of 1984. All measurements were made between 6:00 a.m. and 9:00  a.m. daylight savings time. The triangles are the averages for each site, the  squares are the medians, and the bars show the standard deviations of the  averages. Adapted from Baugues, 1986. The range and variability found in the Philadelphia study's measurements are reflected in measurements made in 29 other cities across the eastern and southern United States during the summers of 1984 and 1985 (Baugues, 1986). NOx measurements were made in 10 of the cities in both years. The measurements were made during the morning rush hour, 6:00 a.m. and 9:00 a.m. DST. Figure 8-2a and 8-2b show the average and median NOx mixing ratios and the standard deviations for each city. The average for all the cities studied varied between 18 ppb in Texas City, Texas (1985), and 114 ppb in Cleveland, Ohio (1985).

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Page 217 Figure 8-2b NOx concentrations measured in urban locations in the United States during  the summer of 1984. All measurements were made between 6:00 a.m. and 9:00  a.m. daylight savings time. The triangles are the averages for each site, the  squares are the medians, and the bars show the standard deviations of the averages.  Adapted from Baugues, 1986. Because urban areas have concentrated sources of NOx, urban measurements allow study of the rate of temporal and spatial decline of the NOx concentration with distance downwind of a source. Spicer et al. (1982) observed NOx concentrations in the Boston, Massachusetts, pollution plume a short distance from the city ranged from 27 to 131 ppb—concentrations similar to those typically found in surface measurements in urban areas. However, concentrations declined rapidly as the plume traveled away from the urban core. NOx concentrations in air masses 4-7 hours downwind of Boston were found to be 5-10 ppb. From this and similar plume studies (Spicer, 1977, 1982; Spicer et al., 1978; Spicer and Sverdrup, 1981) made over several cities

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Page 218 in the United States, it has been estimated that the characteristic time for conversion of NOx to other NOy species is 4-20 hours. Nonurban Nox Only during the past 10 years have techniques been available with sufficient sensitivity and range of detectability to measure NOx in nonurban locales (NOx concentrations below 1.0 ppb), and as a result the size and reliability of the data base needed to define nonurban NOx concentrations are limited. Altshuller (1986) compiled and reviewed a series of NOx measurements made at a number of rural sites in industrial regions of the United States; the results of these measurements are summarized in Table 8-2. Because of the proximity of these sites to urban and industrial sources, the NOx concentrations usually exceeded 1 ppb and exhibited a high degree of short-term variability. Measurements taken at more isolated rural sites in the United States are listed in Table 8-3. NOx concentrations at these sites also can be dominated by anthropogenic NOx emissions when meteorological conditions favor rapid transport of pollutants from urban and industrial centers to the site, but nevertheless tend to be significantly lower than concentrations measured at less-isolated rural sites (Table 8-2) and generally range from a few tenths to 1 ppb. Measurements of NOx in the atmospheric boundary layer and lower free troposphere in remote maritime locations have generally yielded concentrations of 0.02-0.04 ppb (Bottenheim et al., 1986, Gregory et al., 1988, 1990; Montzka et al., 1989). Although the data base is still quite sparse, concentrations in remote tropical forests (not under the direct influence of biomass burning) appear to range from 0.02 to 0.08 ppb; the somewhat higher NOx concentrations found in remote tropical forests, as compared with those observed in remote marine locations, could result from biogenic NOx emissions from soil (Kaplan et al., 1988; Torres and Buchan, 1988). A summary of the NOx measurements made in the four areas considered here is presented in Table 8-4. It can be seen that, even more than is the case for ozone, NOx concentrations decrease sharply as one moves from urban and suburban to rural sites in the United States and then to remote sites over the ocean and tropical forests. The striking difference of three orders of magnitude or more between NOx concentrations in urban-suburban areas and remote locations is compelling evidence for the dominant role of anthropogenic emissions of NOx over North America and suggests that NOx concentrations in the United States would be significantly lower than their current concentrations in the absence of these emissions.

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Page 219 TABLE 8-2 Average Concentrations Measured at Nonurban Monitoring Locations Reference Location NO, ppb NO2, ppb NOx, ppb Research Fort McHenry, Maryland NDa 6b ND Triangle Dubois, Pennsylvania ND 10b ND Institute, McConnelsville, Ohio ND 6b ND 1975 Wilmington, Ohio ND 6b ND   Wooster, Ohio ND 6b ND Decker et Bradford, Pennsylvania 2 3b 5b al., 1976 Creston, Louisiana 4 2b 6b   Deridder, Louisiana 1 3b 4b Martinez Montague, Massachusetts 2 3b 5b and Singh, Scranton, Pennsylvania 3 11b 14b 1979 Indian River, Delaware 3 5b 8b   Research Triangle Park, North Carolina 10 13b 23b   Lewisburg, West Virginia 1 5b 6b   Duncan Fails, Ohio 1 8b 9b   Fort Wayne, Indiana 3 7b 10b   Rockport, Indiana 3 7b 10b   Giles County, Tennessee 3 10b 13b   Jetmore, Kansas 1 4b 5b Pratt et al., Lamoure County, North 2.4 1.7b 4.1b 1983 Dakota 4.8 1.5b 6.3b     3.3 2.8b 6.1b     2.7 2.1b 4.8b   Wright County, Minnesota 3.2 5.4b 8.6b     3.0 6.7b 9.7b     3.5 5.8b 93b     2.9 4.7b 7.6b Pratt et al., Traverse County, Minnesota 3.6 3.7b 7.3b 1983   4.8 3.6b 8.4b     4.0 2.9b 6.9b     2.0 2.2b 4.2b (Table continued on next page)

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Page 220 (Table continued from previous page) Parrish et al., 1986 Scotia, Pennsylvania     3.0b Parrish et al., 1988 Scotia, Pennsylvania     3.1b aNo data. bUpper limit for NO2 and NOx TABLE 8-3 Average Mixing Ratios Measured at Isolated Rural Sites and Coastal Inflow Sites References Location NO, ppb NO2, ppb NOx, ppb Kelly et al., 1980 Niwot Ridge, Colorado     0-2a Kelly et al., 1982 Pierre, South Dakotab   1.2a   Kelly et al., 1984 Schaeffer Observatory Whiteface Mountain, New York < 0.2   1.1a Bollinger et al., 1984 Niwot Ridge, Colorado     0.80 Fehsenfeld et al., 1987 Niwot Ridge, Colorado     0.56 Parrish et al., 1985 Point Arena, California     0.37 aUpper limit for NO2 and NOx bMeasurement site located 40 km WNW of Pierre.

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Page 221 TABLE 8-4 Typical Boundary Layer NOx Concentrations Region NOx, ppb Urban-suburban 10-1000 Rural 0.2-10 Remote tropical forest 0.02-0.08 Remote marine 0.02-0.04 Observations of Noy In addition to examining the measurements of atmospheric NOx, it is instructive to consider the observed concentrations of atmospheric NOy. The ratio of NOx to NOy reflects the chemical processing that occurs in an air mass after the initial introduction of NOx. Thus this quantity is indicative of the oxidant formation that has occurred in the air mass. Because urban areas have large sources of NOx and because it takes several hours to convert NOx to other NOy compounds, NOy. concentrations in urban locations are generally dominated by NO For this reason, the NOy concentrations in urban and suburban locations should be approximately represented by the urban and suburban NOx concentrations described above. Because the ability to measure NOy was developed only recently (see Chapter 7), the rural and remote NO data base is even more limited than that for NOx. However, there are enough data to establish a rough indication of the NOy distribution. During the summer of 1986, NOy was measured at several rural sites in North America: Brasstown Bald Mountain, Georgia; Whitetop Mountain, North Carolina; Bondville, Illinois; Scotia, Pennsylvania; Egbert, Ontario; and Whiteface Mountain, New York. These were all rural sites in the industrial regions of the eastern United States or southern Canada. The NOy concentrations recorded at these sites (c.f., Parrish et al., 1988), along with the period of the measurements at the various sites and their latitudinal locations, are shown in Figure 8-3. The concentrations covered a large range and were quite variable. In general, the low-elevation sites, where air from the atmospheric boundary layer was sampled, were closer to anthropogenic sources. These sites exhibited somewhat higher concentrations of NOy than did the mountain sites, which were more remote and where the samples usually were from the free troposphere. However, the average NOy concentrations observed at all the sites were quite similar; median values ranged from 3 to

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Page 240 Figure 8-7 Observed atmospheric concentrations ratios of trans-2-pentene to cis-2-butene, 2-methyl-2-pentene to  cyclohexene, isoprene to cis-2-butene, and isoprene to cyclohexene as a function of time of day. The data  from Pride, a suburb of Baton Rouge, were obtained from M.O. Rodgers (personal communication). The  data from Glendora, a suburb of Los Angeles, were obtained from R. Rasmussen and D. Lawson (personal  communication).

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Page 241 Figure 8-8 Isoprene concentrations as function of temperature at Pride, a suburb of  Baton Rouge, and at the Louisiana State University campus, in downtown  Baton Rouge. Data from M.O. Rodgers (personal communication). The solid  lines are the least-square linear-regression fits to the data. The dashed lines  are the normalized temperature variations predicted by Tingey's (1980) algorithm  for isoprene emissions from trees for solar insolations of 800 and 400 mEinsteins/m2-s.  The dashed line is the normalized temperature dependence of the isoprene vapor pressure.

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Page 242 VOCs, a total Propy-Equiv concentration for mobile sources and for stationary sources was obtained. The resulting Propy-Equiv concentrations from mobile, stationary, and biogenic sources for the urban-suburban midday data sets as well as the rural data sets are shown in Figure 8-9. In Figures 8-10 and 8-11, we illustrate how the contributions of the various sources to the total reactivity (Propy-Equiv concentrations) at single locations (downtown Baton Rouge and Glendora) vary as a function of time of day. Because the distinction between biogenic and anthropogenic VOCs in the urban-suburban and rural data sets is straightforward, the sum of the reactivity of VOCs apportioned to mobile and stationary sources in the figures is probably a fairly reliable estimate of the total anthropogenic contribution to the reactivity of VOCs concentrations at the various sites. However, the apportionment of these VOCs between mobile and stationary sources should be viewed only as a rough estimate because of the sizable uncertainties in the inventories used to make the apportionment (see Chapter 9). Furthermore, because isoprene was the only biogenic species measured in the urban samples and because it generally constitutes 30-50% of the total biogenic emissions in an area (Chapter 9), the total reactivity of VOCs assigned to biogenic sources should be viewed as a lower limit in these cases. With the exception of the Baton Rouge data sets, mobile and stationary sources make the largest contributions to the total reactivity of VOCs at the urban sites. In Baton Rouge, biogenic sources contribute most to the total reactivity, and in the eastern U.S. rural data sets, biogenic sources dominate over anthropogenic sources. The anthropogenic and biogenic contributions to the total reactivity vary significantly over the course of the day. In Baton Rouge and Glendora, anthropogenic VOCs peak in the early morning hours; biogenic VOCs tend to peak during the mid- and late afternoon. In Los Angeles, the contribution from biogenic VOCs never equals the contributions from anthropogenic sources. In Baton Rouge, however, a very different pattern emerges; biogenic VOCs surpass the contributions from mobile and stationary sources by 1,000 hours and remain dominant for the rest of the daylight period. Although mobile and stationary sources make the largest contributions to the reactivity of VOCs at most of the urban-suburban sites, the contribution from biogenic sources to these areas should not be discounted as negligible. In most of the urban-suburban data sets, a significant fraction of the total reactivity was found to arise from VOCs of biogenic origin. Except for downtown Detroit, Michigan, and Columbus, Ohio, where extremely low isoprene concentrations were reported, midday biogenic VOC concentrations in the urban-suburban data sets accounted for a Propylene-Equiv (reactivity) of at

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Page 243 Figure 8-9 Total nonmethane VOC in propylene-equivalent concentrations in units  of ppb carbon observed at urban-suburban sites (midday) and rural sites  (daylight hours) and apportioned by source category. Atl is Atlanta, Det  is Detroit, LA is Los Angeles, Col is Columbus, BR is Baton Rouge, Scotia  is in Pennsylvania, Brasstown is Brasstown Bald in Georgia. Because of  uncertainties, the apportionment between source categories should be viewed  as an estimate. For instance, except for the Los Angeles sites, the splitting of  anthropogenic VOCs between mobile and stationary source VOCs was based on  a national rather than a local inventory. The assignment of biogenic VOCs is a  lower limit because isoprene was generally the only biogenic VOC identified in the  speciated data. least 10 ppbC—and in many cases, more than 20 ppbC. Even in Glendora, in an area not generally noted for large biogenic emissions, midday isoprene concentrations were about 10 ppbC Propy-Equiv during a moderately hot period (see Table 8-5) and more than 25 ppbC during a particularly hot 3-day period. Furthermore, given that isoprene, which typically amounts to 30-50% of the total VOC emissions from vegetation (Chapter 9), was generally the only VOC of biogenic origin identified in the urban-suburban data sets, it is possible that the actual contribution of biogenic emissions to the reactivity of

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Page 244 Figure 8-10 Total nonmethane VOC Propy-Equiv concentrations in units of ppb carbon  observed at the Louisiana State University campus as a function of time of  day and apportioned by source category. Sampling period was July 18-26, 1989. VOCs in urban areas is considerably larger than we estimate here. It is also important to note that the biogenic contribution is a background VOC blanket that cannot be removed from the atmosphere through emissions controls. If VOC emissions from anthropogenic sources are reduced in the future, this background will be a larger and more significant fraction of the total reactivity of VOCs. Our analysis suggests that in many cities, even if anthropogenic VOC emissions are totally eliminated, a background of reactive biogenic VOCs will remain; on hot summer days, this background can be equivalent to 10 or 20 ppbC, or perhaps more, of propylene. Without control of NOx emissions, this VOC background should be able to generate ozone concentrations that exceed the NAAQS concentration of 120 ppb (Chameides et al., 1988). Another interesting aspect of our results relates to the relative contributions of mobile- and stationary-source VOCs. In the NAPAP and CARB inventories, stationary-source VOC emissions are estimated to be significantly larger than are mobile-source emissions. For instance, in the CARB invento-

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Page 245 Figure 8-11 Total nonmethane VOC Propy-Equiv concentrations in units of ppb Carbon  observed at Glendora, a site near Los Angeles, as a function of time of day  and apportioned by source category. Sampling period was August 18-20,  1986, a period of extremely high temperatures. ry, daily stationary-source emissions in the Los Angeles area are estimated to total 1,881,000 kilograms (kg); mobile source emissions total only 732,000 kg/day. By contrast, our analysis based on the ambient concentrations of mobile-source and stationary-source VOCs indicates that mobile sources contribute as much as or perhaps somewhat more than stationary sources (Figures 8-9, 8-10, and 8-11). Moreover, as illustrated in Figure 8-12, this finding appears to be essentially independent of whether the NAPAP invento-

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Page 246 ry or the CARB inventory is used in the analysis. The fact that the apportionment of the observed anthropogenic VOCs yields a relatively larger role for mobile sources than implied by the emissions inventories suggests that mobile emissions could have been underestimated in these inventories. Figure 8-12 Nonmethane VOC Propy-Equiv concentrations in units of ppb Carbon  apportioned by source category using the 1985 National Acid Precipitation  Assessment Program (NAPAP) speciated VOC inventory for the nation and  the California Air Resources Board (CARB) speciated VOC inventory for the  Los Angeles area during an August day. Results are shown for Glendora  (data set I.C2b) and Claremont (data set I.C3). Data sets are described in  Table 8-5.

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Page 247 Summary of Voc, Nox, and Ozone Observations The ranges of VOC. NOx, and ozone concentrations measured in the four atmospheric boundary layer regions are summarized in Figure 8-13. The format for this figure was chosen to resemble the traditional ozone isopleth diagram (Chapter 6). However, although the x-axis in the ozone isopleth typically adopts a concentration-based scale (total VOC concentration), an OH-reactivity-based scale (Propy-Equiv) is adopted in Figure 8-13. Figure 8-13 VOC, NOx and ozone concentrations in the atmospheric boundary layer at four  locations. VOC is shown as Propy-Equiv concentrations in units of ppb carbon. The position of the four regions in the diagram shows a strong relationship between observed ozone and NOx concentrations but little or no consistent relationship between ozone and VOC reactivity. Although ozone and NOx concentrations increase substantially as one moves from the tropical forest to rural areas and then to urban and suburban regions, VOC reactivity as measured in Propyl-Equiv remains essentially the same in all three. Similarly,

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Page 248 although VOC reactivity increases by more than an order of magnitude from the remote marine region to the tropical forest, ozone concentrations remain the same; the NOx concentrations in these two areas also are quite similar. Summary An analysis of observed concentrations of ozone, oxides of nitrogen (NOx), and volatile organic compounds (VOCs) in remote, rural, and urban-suburban areas implies the following: • A large gradient in NOx concentrations exists between remote, rural, and urban-suburban areas. This gradient suggests that anthropogenic sources dominate the NOx atmospheric budget in the continental United States and that the greatest domination usually occurs in urban centers. • On an OH-reactivity-based scale, the VOC concentrations observed at surface sites in the remote tropical forests of Brazil, in rural areas of the eastern United States, and in urban-suburban areas of the United States are comparable. All three regions tend to exhibit total VOC concentrations equivalent to 50-150 ppbC of propylene. • In urban-suburban and rural areas of the United States, VOCs from mobile and stationary sources contribute about equally to the total ambient VOC reactivity. Comparison of this observation with VOC inventories suggests that the inventories have underestimated the contribution of mobile sources. • In urban-suburban areas of the United States at midday, biogenic VOCs can account for a significant fraction of the total ambient VOC reactivity. Under some conditions in Atlanta and Los Angeles, isoprene alone was found to be present in near-surface air at concentrations equivalent to 25 ppbC of propylene on an OH-reactivity scale. In Baton Rouge, isoprene concentrations equivalent to 40 ppbC of propylene were observed. • In rural areas of the eastern United States, biogenic VOCs contribute more than 90% of the total ambient VOC reactivity in near-surface air and dominate over anthropogenic VOCs. • As one moves from remote forests to rural areas in the eastern United States and then to urban and suburban areas in the United States, ozone concentrations are found to correlate with NOx but not with VOC reactivity. This suggests that, in the gross average, NOx and not VOCs is the limiting factor in ozone photochemical production.

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Page 249 These conclusions have been drawn from an analysis of a limited data base. It is hard to establish that the data are representative, particularly in the case of the VOC measurements, and it is likely that there are specific urban, suburban, and rural areas in the United States that do not follow the trends implied by the data analyzed here. A more representative analysis would be possible with a more complete data base, including a more comprehensive set of VOC measurements that more accurately establishes the horizontal and vertical variability of these species in the troposphere. In addition, one cannot exclude the possibility that the VOC analysis has been significantly biased by the inability of current techniques to identify and quantify the concentrations of all VOCs in the atmosphere. For these reasons, an important research priority for the coming decade should be the development and application of accurate and reliable techniques for the measurement of VOCs that react to form ozone.

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