Environ. Sci. Technol. 1997, 31, 2797-2804
Climatology of Ozone Exceedences in the Atlanta Metropolitan Area: 1-Hour vs 8-Hour Standard and the Role of Plume Recirculation Air Pollution Episodes JAMES C. ST. JOHN* AND WILLIAM L. CHAMEIDES School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia 30332-0340
This work compares the relative frequencies of and meteorological conditions most conducive to exceedences of the current 1-h/120-ppbv national ambient air quality standard for ozone and the proposed revised standard based on daily maximum 8-h averages. Chemical and meteorological data gathered from 1987 through 1993 in Atlanta, GA, are used to determine the daily maximum 1-h and 8-h average ozone mixing ratios at each local monitoring site, during the ozone season for each year. The nature of the highest among the maxima and the concurrent local meteorological conditions are examined. We find a high degree of correlation between the daily maximum 1-h and 8-h averages (R 2 ) 0.92) in Atlanta, with an 8-h daily maximum concentration of 98 ppbv being climatologically equivalent to a 1-h daily maxima of 120 ppbv. Over the 7-year period investigated, exceedences of 8-h-averaged concentrations of 70, 80, and 90 ppbv were about 5, 3, and 2 times more frequent in Atlanta than exceedences of the current 1-h/120-ppbv standard. Meteorological conditions that fostered Atlanta’s most severe 1-h and 8-h maximum ozone pollution events over the period were quite similar. The majority of the most extreme 1-h and 8-h events were associated with multiple-day episodes with very stagnant meteorological conditions and a recirculation and recooking of Atlanta’s plume on consecutive days. These events, called “plume recirculation” episodes, are distinct from other multiple-day episodes that were characterized by fairly steady winds, little or no recirculation, and more modest O3 concentrations. Plume recirculation episodes are among Atlanta’s most severe O3 exceedences and therefore were chosen by the State of Georgia for urban airshed modeling in its most recent State Implementation Plan (SIP) and will likely remain the focus in future SIPs even if a new 8-h standard is promulgated.
Introduction Under the Clean Air Act and its associated amendments, the U.S. Environmental Protection Agency (EPA) is required to establish National Ambient Air Quality Standards (NAAQS) for the so-called criteria pollutants in order to protect human health. In the case of ozone (O3), this standard is currently set at a maximum, 1-h average concentration of 120 parts per * Author to whom all correspondence should be addressed. Telephone: 404-894-6180; fax: 404-894-1106; e-mail: stjohn@eas. gatech.edu.
S0013-936X(96)01068-1 CCC: $14.00
1997 American Chemical Society
billion by volume (ppbv). If a metropolitan area exceeds this standard, on average, more than one time per year over a 3-year period, it is deemed an “ozone non-attainment area” and is required to adopt various pollution control measures designed to bring the area into compliance with the NAAQS. Since the establishment of the 120 ppbv NAAQS for O3 in 1977, the number of non-attainment areas in the United States has shown a generally decreasing trend with spikes associated with hot, dry summers (e.g., 1988) when ozone pollution tends to be most frequent and severe. As of 1994, the U.S. EPA listed 77 metropolitan areas, with a combined population of about 50 million people, as O3 non-attainment areas (1). The U.S. EPA has recently proposed a major revision to the NAAQS for O3, replacing the current 1-h/120-ppbv standard with an 8-h standard to more appropriately reflect the human health effects of ozone (2-4). Under this revised standard, an area would be deemed to be in non-attainment “... when the 3-year average of the annual third-highest daily maximum 8-h average O3 (mixing ratio) exceeds 0.08 ppm (i.e., 80 ppbv)” (4). Consideration is also being given to 8-h standards of 70 and 90 ppbv as well. Since the completion of this work, the U.S. EPA has finalized the revised ozone standard. Areas are designated as being in nonattainment when the 3-year average of the fourth highest annual daily maximum 8-h average exceeds 0.08 ppm. Analysis of O3 air quality data indicates that the impact of this new 8-h standard on the total number of non-attainment areas in the United States will be highly dependent upon the exact form of the standard. Adopting the proposed 80 ppbv standard with two allowable exceedences per year will increase the number of counties in non-attainment by about a factor of 3. A less stringent form of the 8-h standard (i.e., 8-h/90ppbv, with five allowable exceedences per year) would actually decrease the number of counties in the United States in nonattainment by about 70%, while a more stringent form of the standard (i.e., 8-h/70-ppbv, with one allowable exceedence per year) would increase the number by about 500% (3). However, yet to be assessed is the extent to which the new 8-h standard would necessitate a redesign of the ozone pollution management strategies that have already been formulated to bring a given non-attainment area into compliance with the current 1-h/120-ppbv standard. Relevant questions include: (1) Would the promulgation of an 8-h standard significantly change the number of non-attainment episodes experienced by a non-attainment area? (2) Would the use of a metric to gauge the severity of an ozone pollution episode based on an 8-h-averaged concentration instead of a 1-h average alter the perception of the specific days and attendant meteorological conditions when the most severe non-attainment episodes occurred in an area? (3) Would the promulgation of the new standard alter the types of control strategies that would most effectively bring a non-attainment area into compliance with the NAAQS? To help address the first two questions listed above, we present here a case study of the climatology of O3 pollution episodes (using both the 1-h/120 ppbv and 8-h criteria) in one of the nation’s so-called “serious” non-attainment areas: Atlanta, GA. In a subsequent paper, we will address the issue of control strategies for Atlanta (i.e., question 3).
Data Analyzed Our analysis makes use of 1-h averaged O3 concentrations and meteorological data measured in the Atlanta Metropolitan Area during the months of June-September in 1987 and April-September from 1988 through 1993. While an analysis of 7-years’ data is insufficient for a true climatology, it
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FIGURE 2. Scatterplot of Cmax1-h and Cmax8-h, the maximum 1-h and 8-h averaged ozone concentrations, respectively, observed in the Atlanta Metropolitan Area for the monitoring period from 1987 to 1993. Closed circles indicate daily maxima for days that did not exceed the current 1-h/120-ppbv NAAQS, and open circles indicate daily maxima for days that did exceed the current NAAQS. FIGURE 1. Schematic of the Atlanta Metropolitan Area. Red lines indicate interstate highways, which are labled; gray lines are borders of counties. Note: I-285 is the irregular loop that circumscribes Atlanta. Ozone measurement sites and the airport are indicated by the filled circles. nevertheless provides considerable insight into the nature of O3 pollution episodes in Atlanta. The O3 measurements were made at five monitoring sites in and around Atlanta (see Figure 1) by the Georgia Department of Natural Resources and obtained from the Aerometric Information Retrieval System (AIRS) database. We have chosen to begin our study with data from 1987 since two of the five monitoring sitess Sweetwater Creek and Martin Luther King Drivesbecame operational in that year. We ended our analysis with data from 1993, because the locations for some the O3 monitoring sites changed in 1994. In 1988, the Georgia Department of Natural Resources first adopted the practice of initiating their annual O3 monitoring program in April instead of June, and thus the data for 1987 only covers the months from June to September. The meteorological data included in our study were measured at the Atlanta Hartsfield International Airport (see Figure 1) and acquired from the National Climatic Data Center.
Results and Discussion Relationship between 1- and 8-h Daily Maxima. Figure 2 presents a scatterplot of the daily maximum 1-h and maximum 8-h averaged O3 concentrations recorded in the Atlanta Metropolitan Area during the 1987-1993 monitoring period described above. The maximum 1-h averaged concentration was simply taken as the maximum concentration recorded among the five Atlanta monitoring sites during each 24-h period. Similarly, the daily maximum 8-h average for Atlanta was taken to be the highest 8-h concentration obtained from the five monitoring sites for each day. In general, there are 24 individual 8-h averages for each site for each 24-h period from the 1-h averaged data from AIRS (e.g., 01000800 h, 0200-0900 h, etc). However, if an hourly value during any of the 8-h periods at any of the sites was missing from the AIRS database, that specific 8-h average was not included in the analysis. The maximum 8-h averaged concentration was then selected from these individual 8-h averages. Inspection of Figure 2 indicates a high degree of correlation between the 1-h and 8-h maxima in Atlanta. Linear regression of the data yields the following relationship (with R2 ) 0.92):
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Cmax8-h ) 5.27 ppbv + 0.744 Cmax1-h
(1)
where Cmax8-h and Cmax1-h are the daily 8-h and 1-h averaged O3 concentrations in ppbv, respectively. This is in agreement with previous findings (5). The high degree of correlation between the daily maximum 8-h and 1-h averaged O3 concentrations suggests that eq 1 can be used to define an 8-h averaged daily maximum O3 concentration in Atlanta that is the climatological equivalent to the current 1-h/120ppbv standard. By substituting a value of 120 ppbv for Cmax1-h into eq 1 and solving for Cmax8-h, we find this equivalent 8-h averaged maximum concentration to be 98 ppbv. The strong linear relationship between Atlanta’s 1-h and 8-h daily maxima suggests that Atlanta’s most severe pollution episodes based on a 1-h metric would tend to correspond to its most severe episodes using the 8-h metric. On the other hand, the slope between the two metrics is somewhat larger than the ratio of 80:120, and thus there is a tendency for O3 maxima to exceed an 8-h/80-ppbv concentration without also exceeding a 1-h/120-ppbv concentration. This suggests that exceedences of the 8-h/80-ppbv concentration have been more frequent in Atlanta over the 1987-1993 monitoring period than have been exceedences of the 1-h/120-ppbv concentration. These inferences are examined in more detail in the next section. O3 Exceedences in the Atlanta Metropolitan Area: 19871993. From the database of 1-h and 8-h daily maxima taken from the AIRS database for the 1987-1993 monitoring period, we identified a total of 98 days with a 1-h averaged O3 concentration in excess of the 120 ppbv standard and 494 days with an 8-h averaged O3 concentration in excess of 70 ppbv. [The NAAQS is actually set at 0.12 parts per million by volume (ppmv), and as a result, an exceedence is technically defined as a 1-h averaged O3 concentration measurement of at least 125 ppbv; i.e., a concentration that when rounded to two significant figures yields a concentration of 0.13 ppmv.] In this study however, for simplicity, we consider exceedences to have occurred whenever the 1-h averaged O3 concentration was equal to or greater than 121 ppbv for the 1-h standard and 71, 81, and 91 ppbv for possible 8-h standards of 70, 80, and 90 ppbv, respectively. While the EPA has proposed a new 8-h standard of 80 ppbv, consideration was given to standards as low as 70 ppbv. For this reason, we include in this study all days with 8-h averaged concentrations greater than 70 ppbv. Table 1 lists each day during the period with a 1-h/120ppbv exceedence, with the days listed in descending rank
TABLE 1. Listing of 1-h/120-ppbv O3 Exceedences in Atlanta by Rank Severity and Episode Type date
1-h av
8-h av
7/31/87 7/8/88 8/28/90 7/23/93 6/25/88 8/1/87 7/24/87 6/6/88 7/21/93 6/13/88 7/31/87 8/2/87 7/27/93 7/23/87 7/25/87 6/21/88 7/30/87 8/2/88 6/24/88 6/9/87 6/22/88 6/16/88 7/26/87 6/18/90 8/3/87 9/7/90 9/6/90 6/16/92 6/10/87 5/31/88 8/15/90 6/15/88 8/21/87 6/5/90 7/9/90 7/8/93 9/8/90 7/9/88 8/3/88 7/22/93 6/20/90 8/4/87 8/5/87 8/25/93 6/17/88 8/29/90 7/29/87 6/14/88 6/28/90
201 186 181 174 172 169 168 166 162 160 159 157 156 155 155 155 155 153 153 150 149 149 149 148 146 146 146 144 144 143 143 141 140 140 140 139 139 138 137 136 136 136 136 135 135 135 135 135 134
140 153 148 148 135 134 132 135 138 130 133 113 123 124 107 134 135 108 126 127 118 131 103 121 87 125 105 120 119 119 104 117 102 109 119 101 107 121 108 122 109 85 98 106 103 101 115 114 106
1-h 8-h rank rank 1 2 3 4 5 6 7 8 9 10 11 12 13 15 15 15 15 19 19 20 23 23 23 24 25 25 25 28 28 30 30 32 35 35 35 36 36 38 39 41 41 41 41 45 45 45 45 45 51
4 1 2 2 6 9 12 6 5 14 11 32 19 18 46 9 6 44 16 15 27 13 69 21 215 17 57 23 24 24 64 28 74 40 24 81 46 21 44 20 40 239 105 51 69 81 29 30 51
type
date
1-h av
8-h av
plume recirculation plume recirculation plume recirculation missing data precluded typing normal plume recirculation (recovery) normal plume recirculation missing data precluded typing plume recirculation normal normal missing data precluded typing normal normal normal plume recirculation plume recirculation normal normal normal normal normal plume recirculation normal normal normal point source plume normal normal normal normal normal normal normal plume recirculation normal plume recirculation (recovery) plume recirculation (recovery) missing data precluded typing normal normal normal normal normal plume recirculation (recovery) normal plume recirculation (recovery) normal
8/1/88 6/11/90 9/3/87 8/20/93 7/1/91 6/11/87 8/10/92 8/18/88 9/2/87 6/2/87 5/2/87 9/16/91 8/7/87 7/9/89 6/24/87 8/31/92 7/28/93 6/8/88 7/25/88 7/9/93 9/5/90 8/1/93 6/29/90 7/8/90 9/15/91 6/3/87 5/7/93 7/2/91 8/16/91 7/19/87 8/14/90 6/27/88 5/30/88 8/12/93 9/14/88 8/23/87 8/3/89 7/24/91 6/1/89 9/10/91 5/14/92 7/12/92 6/20/88 6/11/93 6/21/90 5/11/92 9/23/93 7/29/93 8/27/90
134 134 133 133 133 133 132 132 132 132 131 131 130 130 130 130 130 130 129 129 129 129 128 128 128 128 128 126 126 126 125 125 125 125 125 125 124 124 124 123 123 123 123 123 123 123 122 121 121
97 107 103 101 110 99 92 96 104 105 105 87 107 100 99 102 114 110 110 107 101 108 110 112 90 82 106 100 99 102 95 103 105 106 98 101 99 71 92 93 99 98 90 108 113 111 98 97 102
order of the value of the 1-h O3 maximum. Also included in Table 1 are the values of the maximum 1-h and 8-h averaged O3 concentrations for each of the days, along with the rank order of the day on the basis of the 8-h averaged concentration. In Table 2, we list the 60 highest 8-h average mixing ratios in Atlanta that did not also correspond to an exceedence of the 1-h/120-ppbv standard, with days listed in descending rank order of the 8-h average and including the values of the maximum 8-h and 1-h averaged O3 concentrations. As expected from our discussion in the previous section, we find a significant increase in the number of historical exceedence days in Atlanta using the values of the 8-h concentration proposed for the new NAAQS. While there were 98 exceedences of the 1-h/120-ppbv concentration over the monitoring period, we find 494, 312, and 162 exceedences of an 8-h/70-ppbv, /80-ppbv, and /90-ppbv concentrations, respectively. These numbers yield averages of 71, 45, and 24 exceedences per year of the 8-h/70-ppbv, /80-ppbv, and /90ppbv concentrations, respectively, as compared to the 13 exceedences per year of the 1-h/120-ppbv concentration over the same 7 year period.
1-h 8-h rank rank 51 51 52 52 52 52 57 57 57 57 61 61 64 64 64 64 64 64 70 70 70 70 75 75 75 75 75 77 77 77 82 82 82 82 82 82 88 88 88 93 93 93 93 93 93 93 96 97 97
112 46 69 81 36 93 148 120 64 57 57 215 46 86 93 74 30 36 36 46 81 44 36 34 163 282 51 86 93 74 127 69 57 51 105 81 93 476 148 135 93 105 163 44 32 35 105 112 74
type normal normal point source plume missing data precluded typing normal normal normal normal normal normal normal normal plume recirculation normal normal normal plume recirculation (recovery) normal normal missing data precluded typing normal normal normal normal plume recirculation normal missing data precluded typing normal plume recirculation (recovery) normal plume recirculation normal plume recirculation missing data precluded typing normal normal normal normal normal normal normal normal normal normal normal normal missing data precluded typing normal normal
Inspection of Figure 2 indicates that the O3 concentrations for both metrics during 1-h/120-ppbv exceedence days remain well-correlated (i.e., R 2 ) 0.66 for all days with a 1-h exceedence), and as a result, there is a high degree of correspondence between Atlanta’s most severe 1-h and 8-h exceedences. For example, note in Table 1 that the first, second, and third most severe 1-h exceedences, with 1-h maxima of 201, 186, and 181 ppbv, were the fourth, first, and second most severe of the 8-h exceedences, respectively, with 8-h maxima of 140, 153, 148 ppbv. The fourth most severe 1-h exceedence (174 ppbv) was tied with another day for the third most severe 8-h exceedence (148 ppbv). While there were a significant number of 8-h exceedence days without a corresponding 1-h exceedence, these episodes were not among Atlanta’s most severe 8-h pollution events. Specifically, note that the highest 8-h maxima without a corresponding 1-h exceedence occurred on June 7, 1988, and July 12, 1992, with maximum 8-h average O3 mixing ratios of 106 ppbv. These two days were tied for the 51st position in terms of their 8-h O3 maximum rank-order out of a total of 494 days
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TABLE 2. Listing of 60 highest 8-h/70-, /80-, and /90-ppbv O3 Exceedences That Did Not Exceed the 1-h Standard in Atlanta date
1-h av
8-h av
1-h rank
8-h rank
07/07/90 08/16/90 07/20/87 06/06/87 08/17/93 07/22/87 07/27/87 05/21/88 07/22/91 05/29/88 05/23/92 08/11/93 06/23/93 08/18/93 07/23/91 09/04/90 07/31/88 07/18/93 07/10/90 06/14/90 08/20/90 07/07/93 07/03/90 07/06/90 08/03/92 08/11/90 06/15/93 08/03/90 07/01/88 08/26/93 08/03/91 09/08/93 06/02/88 06/07/87 08/19/89 06/07/88 05/24/92 08/18/90 08/19/90 05/14/88 06/24/92 06/12/90 08/20/87 06/01/88 06/26/88 06/12/88 06/19/90 07/29/89 07/01/90 06/10/91 07/07/92 08/25/90 06/29/87 07/17/93 06/26/90 06/13/90 08/14/93 05/28/88 05/10/92
114 118 120 118 111 120 120 113 108 113 111 114 115 116 108 112 105 112 106 104 113 108 109 103 111 112 113 118 119 84 108 119 108 108 111 103 102 111 111 105 109 102 120 105 105 103 101 110 111 111 104 101 113 102 103 103 109 98 102
106 106 105 105 105 104 104 103 102 102 102 100 100 100 100 100 100 99 99 99 99 99 99 99 98 98 98 97 97 97 97 97 97 96 96 96 96 96 96 95 95 95 94 94 94 94 93 93 93 93 93 93 93 93 93 93 93 93 92
125 110 102 110 144 102 102 128 166 128 144 125 122 117 166 136 205 136 190 214 128 166 160 227 144 136 128 110 105 472 166 105 166 166 144 227 243 144 144 205 160 243 102 205 205 227 258 156 144 144 214 258 128 243 227 227 160 294 243
51 51 57 57 57 64 64 69 74 74 74 86 86 86 86 86 86 93 93 93 93 93 93 93 105 105 105 112 112 112 112 112 112 120 120 120 120 120 120 127 127 127 132 132 132 132 135 135 135 135 135 135 135 135 135 135 135 135 148
with 8-h O3 concentrations above 70 ppbv over the 7-year period. While not specifically illustrated here, we also found a strong geographical correspondence vis-a-vis the two metrics. On 75% of the 1221 days used in the study, the 8-h and 1-h O3 maxima occurred at the same site. Missing data sometimes precluded the computation of the maximum 8-h averaged O3 concentration at the site of the maximum 1-h averaged O3 concentration. On these occasions, the 8-h maximum could only be assigned to a station other than the site of the 1-h maximum, and thus we could not find a collocation of the
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two maxima. For this reason, our estimate of the frequency of collocation is actually a lower limit, and the actual percentage may be somewhat higher than 75%. The results described above have important implications. The highest 1- h and 8-h averages tend to occur on the same days and, to a lesser degree, at the same sites, but there will be many more exceedences of the primary 8-h concentration. The current U.S. EPA guidelines have traditionally required local regulatory agencies to develop State Implementation Plans (SIPs) to bring areas into compliance with the NAAQS for O3 on the basis of an analysis of the historically most severe non-attainment episodes (6, 7). The correspondence between the most severe 1-h and 8-h exceedences suggests that a change in the standard may not require an analysis of a different set of pollution episodes. It also suggests that the meteorological conditions that tend to favor severe 1-h O3 exceedences also favor severe 8-h exceedences. The similarity of meteorological conditions between the two types of exceedences is examined in more detail in the next section. An Urban-Scale Analysis of Atlanta’s Most Severe Exceedences. Inspection of Table 1 indicates that many of the most severe exceedence days in Atlanta were actually part of so-called multiple-day episodes, in which high O3 concentrations were observed in Atlanta on two or more consecutive days. Most notable of these multiple-day episodes is the one from July 29 to August 5, 1987. This period included Atlanta’s most severe 1-h O3 exceedence of 201 ppbv and fourth most severe 8-h exceedence of 140 ppbv on July 31, 1987. We begin our analysis of the meteorological conditions of Atlanta’s most severe exceedences by focusing on this episode. Multiple-Day Episodes during July and August, 1987. Figure 3A,B shows the daily maximum 1-h and 8-h averaged O3 concentration measured at each of the five monitoring sites in Atlanta from July 19 to August 11, 1987. This period had two major multiple-day O3 episodes: 7/23-7/26 and 7/29-8/5. Comparison of Figure 3, panels A and B, indicates that on most days the site of 1-h averaged O3 maximum is the same as that for the 8-h averaged O3 maximum. However, there are a few exceptions (e.g., 7/19 and 8/04 being the most obvious). In these cases, the site of the highest 8-h O3 maximum corresponded the site of the 2nd highest 1-h O3 maximum, confirming the close spatial correlation between these two metrics. In analyzing these episodes, we will make use of an additional diagnostic parameter: ∆O3, defined as the difference between the highest 1-h averaged O3 maximum and the lowest 1-h averaged O3-maximum measured in Atlanta each day. This quantity has been used in a previous analysis of O3 pollution episodes to estimate the amount of O3 produced photochemically in the Atlanta Metropolitan Area each day (8). Using ∆O3 in this manner implicitly assumes that the O3 concentration measured at the station upwind of Atlanta is representative of the regional background O3 mixing ratio and that the highest O3 concentration measured in the metropolitan area is representative of the sum of this background plus the O3 photochemically produced in the urban plume. The daily value for ∆O3, inferred from Figure 3A as the vertical difference between the highest and lowest O3 concentrations, is plotted in Figure 4A. Inspection of this figure reveals that ∆O3 varied considerably over the period from near 20 ppbv (e.g., July 28 and August 6) to over 100 ppbv (e.g., July 31). Inspection of Figure 3, on the other hand, indicates that each of the five sites accounted for at least one of the daily maxima during the period. During the 7/19-8/11/87 period a high pressure system dominated the area. The surface meteorological analysis for August 1, 1987, at 1200Z (0700 EST), illustrated in Figure 5, was representative of the region throughout this period. Synoptic scale pressure gradients were generally quite weak. For example, note in Figure 5 that even at 2-hPa intervals only one isobar crosses Georgia. Moderately unstable condi-
FIGURE 3. Daily maximum O3 concentrations recorded at each of the Atlanta monitoring sites for the period from July 19 to August 11, 1987. (A) The 1-h averaged O3 maxima, with the red line indicating the current the 1-h/120-ppbv NAAQS. (B) The 8-h averaged O3 maxima, with the red line indicating the proposed new 8-h/80-ppbv NAAQS. tions favorable to the development of isolated, moderateto-strong thunderstorms persisted throughout this episode. In fact, it is likely that the extremely low values of ∆O3 on July 28 and August 6 were due to pollution-inhibiting effects of precipitation in the area on these days. (On August 6, precipitation was reported at Atlanta Hartsfield Airport between 1300 and 1400 EST, while the maximum O3 was observed at Dekalb at 1200 EST. On 28 July, on the other hand, no rain was recorded at the airport, but the cloud conditions reported at the airport before noon indicate the possibility that showers occurred in the local area during the morning, although neither thunder nor precipitation were reported at the airport.) In this regard it is also interesting to note that, while O3 concentrations did not exceed the 1-h or 8-h standards on August 6, a mild exceedence of the proposed 8-h/80-ppbv standard occurred at the Conyers and Sweetwater sites on July 28. This suggests that isolated cloud cover and shower activity, by inhibiting photochemistry for periods of a few hs, may have had a greater inhibiting effect on 1-h daily O3 maxima than on 8-h averaged daily O3 maxima on this date. Following the O3 productivity minimum on July 28, ∆O3 values increased each day until July 31, when the maximum O3 exceedence of 201 ppbv was observed with a ∆O3 value of 103 ppbv. It is interesting to note the shifting location of the O3 maximum during this 3-day period. On the 29th, the maximum was located to the southeast at Dekalb; on the 30th, it moved to the northwest at Dallas; and on 31st, it
FIGURE 4. ∆(O3) values in Atlanta for (A) June 19-August 9, 1987, and (B) September 5-9, 1990. Note the standard deviation of mean in ∆(O3) was 19 ppbv for the 1987 period and 9 ppbv for the 1990 period. returned to Dekalb. This pattern suggests a shifting wind flow pattern. In fact, a plot of hourly wind direction along with maximum O3 mixing ratios for the 3-day period shown in Figure 6 confirms this supposition. On July 29th, the dominant wind direction is out of the northwest, and the maximum O3 concentration was observed to the southeast of the city center. During the late-night hours of July 29 and early-morning hours of July 30, however, the wind shifted to southeasterly, and the resulting O3 maximum was observed to the northwest of the city center. On the next night, the wind shifted again to the northwest, and the maximum O3 on July 31 was observed to the southeast. This shifting pattern of winds and O3 maxima suggests an explanation for the increasing values of both maximum O3 concentration and ∆O3 that were recorded during this 3-day period: namely, a recirculation of Atlanta’s plume back through the city and a resulting “recooking” of its pollution on consecutive days. An indication of the strength of this recirculation can be obtained from a vector sum of the hourly winds observed at Atlanta’s Hartsfield International Airport for the 72-h period. This sum indicates a net displacement of only 60 mi from the city center toward the southeast. (To place this distance in perspective, note that the distance between the Dallas and Conyers sites shown in Figure 1 is also approximately 60 mi, while the distance between the intersections of I-75 and I-285 on the northwest and southeast sides of Atlanta is approximately 20 mi.) On August 1, the ∆O3 value dropped somewhat but remained significantly elevated (see Figure 4A) suggesting the continued influence of this recirculation process. This inference is supported by the fact that the 96-h total windvector displacement from July 29 through Aug 1 is only 72 mi to the east-southeast. From August 2 through the end of the
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FIGURE 5. Surface pressure analysis for August 1, 1987 (0700 EST). Station data include temperature and dew point (°F), wind barbs (knots), and sea level pressure (hP - 1000) × 10. Isobars every 2 hP are analyzed. The extremely weak pressure gradient is typical of the July-August 1987 episode. Note also the lack of organization in the winds between cities. (Meteorological data from the National Climatic Data Center.)
FIGURE 6. Hourly wind directions at the Atlanta airport (circles, scale on right) and hourly O3 concentrations (scale on left) at the Dallas (thin line) and Dekalb (thick line) sites from July 29 to August 1, 1987. period on August 11, a steady wind flow began to dominate the area, and the average 24-h wind-vector displacements is approximately 140 mi. Inspection of Figures 3 and 4 indicates a gradual return to ∆O3 values in the 50-60 ppbv range and a corresponding drop in the O3 maxima. Exceedences of the 1-h/120-ppbv and 8-h/80-ppbv standards generally persist, however, until August 7 and 8, respectively. The slow, 3-day rise in O3 and ∆O3 from 7/29 to 7/31 and the similarly slow decay in these parameters from 8/1 to 8/3 suggest that multiple-day events of this type can have a development phase while the wind direction is periodically reversing, causing a buildup in pollutant levels as well as a recovery phase when the wind direction is steady and flushes the pollutants from
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the area. In general, this recovery phase can be brief or rapid depending on meteorological conditions. The multiple-day episode from 7/23 to 7/25/87 has some similarities to and some differences from the 7/31 to 8/3 episode of that year discussed above. As in the late July and early August episode, we find a rapid rise in the 1-h and 8-h averaged O3 maxima as well as in the ∆O3 value during the initiation phase of the episode on 7/23 and a maintenance of these high values until 7/26 (see Figures 3 and 4A). However, in this case the location of the O3 maximum remains relatively close to the urban core (i.e., at the Sweetwater and MLK sites) for the entire episode. The reason for this can again be found through inspection of the prevailing wind patterns during this period. Unlike the pattern from July 28 to July 31, when the winds reversed direction during the nighttime hours, the winds during this period reversed during the O3-producing daylight hours. For example, Figure 7 illustrates the hourly O3 concentrations at the Sweetwater and MLK Blvd sites along with the wind direction recorded at Atlanta Hartsfield Airport on July 24. Note that the peak O3 concentration at Sweetwater occurred around noon and then dropped dramatically. At the MLK site, on the other hand, the maximum was not reached until 1500 h. Interestingly, meteorological data indicate that there was a shift in the winds from southeasterly to northwesterly direction between 1400 and 1500 h at the Atlanta airport. With measured wind speeds of about 5-10 knots, it is likely that the wind shift occurred at Sweetwater between 1200 and 1300, corresponding to the time of the O3 maximum at that site. Similar patterns occurred on July 23 and 25. As a result, the urban plume remained over the city on three consecutive days. If the wind vector sum for the 25th is added to the sum from the 23rd and 24th, we obtain a net displacement of 114
FIGURE 7. Hourly wind directions at the Atlanta airport (circles, scale on right) and hourly O3 concentrations (scale on left) at the Sweetwater (thick line) and MLK (thin line) sites for July 24, 1987. mi toward the southeast. This 3-day displacement is somewhat larger than that calculated for the July 29-August 1 case, and thus it is perhaps not surprising that the peak O3 concentrations and ∆O3 values measured were less dramatic. The Plume Recirculation Episode as a Special Subset of Multiple-Day Episodes. The analysis in the previous section suggests that reversing winds and the concomitant recooking of the pollutants in the Atlanta plume on successive days played a key role in the promulgation of the severe O3 episodes encountered in Atlanta during the months of July and August 1987. The key common characteristics of these events, hereafter referred to as plume recirculation episodes; include the following: (i) reversing wind directions which either move the urban plume back into the city center each afternoon on consecutive days or cause the maximum site to be on opposite sides of the city on consecutive days; (ii) a total wind-vector sum of less than about 125 mi from 0800 the previous day until 1800 on the exceedence day during the development phase of the event; (iii) significant increases in ∆O3 values during the development phase and slowly decreasing ∆O3 values during the recovery phase of the event; and (iv) a return to steady winds (usually from the northwest) and more normal values of ∆O3 during the recovery phase. It is also possible that increases in the regional background O3 concentration may add to the severity of these types of pollution episodes. However, the low variability in the minimum daily maxima illustrated in Figure 3A suggests that this effect can be quite small. It should be noted that this so-called plume recirculation episode actually represents a special case of a more general type of ozone exceedence commonly referred to as a multipleday episode (see, for example, ref 9). In a standard multipleday episode, ozone exceedences occur on successive days. However if the winds are steady during these days, the air mass within the city on each of these successive days is freshly imported from outside the urban area. Thus, the ozone exceedence on each of the days of a standard multiple-day episode is the result of only one day’s urban photochemistry working on top of a lower regional background, as opposed to the successive cooking of the urban plume on a plume recirculation episode. The quintessential example of this more standard type of multiple-day episode occurred in Atlanta during the September 5-8, 1990, period. Maximum 1-h O3 concentrations at the five Atlanta monitoring sites for this episodes are illustrated in Figure 8, and daily values for ∆O3 are plotted in Figure 4B. We find a dramatic contrast between these plots and the similar plots prepared for the two 1987 episodes in Figures 3 and 4A. It turns out that the September 1990 period was characterized by fairly steady westerly to north-
FIGURE 8. Daily maximum 1-h averaged O3 concentrations recorded at each of the Atlanta monitoring sites for the period September 5-9, 1990. westerly winds. As result, the location of the O3 maximum remained at the Conyers site throughout the episode. Moreover, because plume recirculation of the Atlanta plume did not occur, ∆O3 and peak O3 concentrations remained relatively modest throughout the episode. In this light, we see that, in terms of the peak O3 and the amount of O3 generated, a standard multiple-day episode (as opposed to a plume recirculation episode) is actually quite similar to a single-day episode. To determine if the occurrence of plume recirculation episodes is characteristic of Atlanta’s non-attainment climatology or an anomaly of the 1987 ozone season, an objective classification of O3 air pollution events from 1987 to 1993 was carried out. In this scheme, all O3 violations of the 1-h/120ppbv standard from 1987 to 1994 were placed into one of three categories: (i) plume recirculation episodes development phase; (ii) plume recirculation recovery phase; and (iii) normal episodes. A day was designated as a plume recirculation episode development phase if the ∆O3 increased by at least 20 ppbv over the previous day and one of the following sets of criteria was met: (A) The location of the Atlanta maximum switched from one side of the city to the other (e.g., at Sweetwater one day and at Conyers the next) and the total wind vector was less than or equal to 126 (statute) mi. (We used the vector sum of hourly winds at the airport from 0800 EST on the previous day until 1800 EST of the day being evaluated, i.e., a 34-h total). (B) The location of the Atlanta maximum remained at the downtown site (MLK Blvd) for two or more days in a row. (C) The location of the Atlanta maximum was at the downtown site (MLK Blvd) on one day and at one of the close-in sites (Sweetwater or Dekalb) the next day or at one of the close-in sites the first day and at the downtown site the following day, and the total wind vector was less than or equal to 126 (statute) mi. (D) The total wind vector was less than or equal to 62 (statute) mi. Note that by including criteria A, B, and C, we have attempted to capture both types of plume recirculation episodes encountered during July and August 1987: those where the urban plume moved out of and back into the city on consecutive days and those where the urban plume remained trapped in the urban core on consecutive days. A plume recirculation episode recovery phase day was selected if the previous day was a plume recirculation episode development phase, the current day was otherwise a normal day, and the O3 maximum dropped from the previous day. All other days, including the standard multiple-day episodes, were deemed to be normal days.
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FIGURE 9. Daily maximum O3 concentrations recorded at each of the Atlanta monitoring sites for the period June 20-26, 1988. The results of the objective classification scheme are shown in Table 1. On the basis of our classification scheme, we estimate that 24% of the exceedence days during the period were plume recirculation episodes, either in the development or recovery phase. Moreover, the majority of the exceedences with the highest O3 maxima (1-h and 8-h) were associated with these plume recirculation episodes. For example, note in Table 1 that of the 10 most severe O3 exceedences, six were classified by our objective scheme as plume recirculation days (including Atlanta’s top three exceedences), two could not be classified because of missing data, and only two (ranked fifth and seventh) were classified as normal. In fact, closer inspection of the O3 data during the two severe events that were classified as normal (i.e., 6/25/88 and 7/24/87) suggests the presence of plume recirculation effects as well. For example, consider the O3 exceedence on 6/25/ 88. Figure 9 illustrates the maximum O3 concentrations at Atlanta’s five monitoring sites on the days leading up and following 6/25/88 as well as the day itself. The shifting location of the O3 maximum from Dekalb to MLK from 6/23 to 6/25 as well as the rapid increase in ∆(O3) over this period are more reminiscent of the patterns found in Figure 3 (i.e., examples of plume recirculation episodes) than those illustrated in Figure 8 (i.e., an example of a normal episode). However, this day was not categorized as a plume recirculation episode by our objective criteria because the 34-h wind displacement exceeded our threshold of 126 mi. The next highest exceedence classified as normal occurred on 7/24/87. Inspection of Figure 3 indicates that maximum O3 concentration on this day and the previous day was located at Sweetwater. While this aspect of the episode meets one of our criteria for a plume recirculation episode, the exceedence was not categorized as one by our objective criteria because the wind displacement again exceeded the threshold. These two examples suggest that the objective criteria we have used to identify plume recirculation episodes may be overly conservative and that the influence of plume recirculation phenomena on Atlanta’s most severe exceedences may actually be underestimated in our analysis. Our analysis thus suggests that a special subset of multipleday episodes, referred to here as plume recirculation episodes, are responsible for Atlanta’s most severe O3 pollution events. That ozone pollution episodes are most likely to occur during stagnant conditions associated with anticyclones has been well established (see, for example, refs 3 and 10-14). However, we find that these anticyclonic conditions are a necessary but not sufficient condition for Atlanta’s most severe episodes. For these severe episodes to occur, wind reversals
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occurring on the subsynoptic scale appear to be needed. There are several possible explanations for these wind reversals; e.g., differential heating under varying cloud cover patterns, urban heat island effects, or outflow boundaries from isolated showers. Determining the mechanisms responsible for wind reversals in specific pollution events is beyond the scope of this paper. Nevertheless, the data show that these wind reversals do occur and that they coincide with Atlanta’s most severe O3 pollution events. In the Georgia Department of Natural Resources’s most recent SIP for Atlanta, the three historical exceedence days chosen for simulation using the Urban Airshed Model (as per U.S. EPA guidelines) all turned out to be plume recirculation episodes according to our classification scheme. It is interesting to note, in this light, that Georgia was not able to demonstrate attainment using the appropriate model simulations for these episodes (6). Moreover, because the most extreme 8-h episodes also tend to be associated with the plume recirculation episodes, the challenge of dealing with these events will have to be confronted regardless of whether or not a new 8-h standard is promulgated.
Acknowledgments This research was supported in part by the U.S. EPA under the Southern Oxidant Study Cooperative Agreement CR 824 849 and by the Georgia Department of Natural Resources under Grant AGR DTD 950401.
Literature Cited (1) Air Quality Criteria for Ozone and Related Photochemical Oxidants U.S. Environmental Protection Agency, Office of Research and Development; U.S. Government Printing Office: Washington, DC, 1995; EPA/600/AP-93/004a-c. (2) Wolff, G. T. J. Air Waste Manage. Assoc. 1996, 46, 807. (3) Wolff, G. T. EM, 1996, 2, 27-32. (4) U.S.-EPA. National Ambient Air Quality Standards for Ozone: Proposed Decision. 40 CFR Part 50, 1996. (5) Berglund, R. L.; Dittenhoefer, A. C.; Ellis, H. M.; Watts, B. J.; Hansen, J. L. Evaluation of the stringency of alternative forms of a national ambient air quality standard for ozone. In Scientific and Technical Issues Facing Post 1987 Ozone Control Strategies; AWMA: 1988; pp 343-369. (6) Guideline for Regulatory Application of the Urban Airshed Model; U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards, Research Triangle Park, NC; U.S. Government Printing Office: Washington, DC, 1991; EPA-450/ 4-91-013. (7) Georgia Department of Natural Resources. The 1994 State Implementation Plan for the Atlanta Ozone Nonattainment Area. Environmental Protection Division, Air Protection Branch; Atlanta, GA, 1994. (8) Lindsay, R. W.; Richardson, J. L.; Chameides, W. L. Air Pollut. Control Assoc. J. 1989, 39, 40-43. (9) Tesche, T. W.; McNally, D. E. J. Appl. Meteorol. 1991, 30, 745763. (10) Use of Meteorological Data in Air Quality Trend Analysis; U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards: Research Triangle Park, NC, 1978; EPA-450/378-024. (11) Cox, W. M.; Chu, S.-H. Atmos. Environ. 1993, 27, 425-434. (12) Cox, W. M.; Chu, S.-H. Atmos. Environ. 1995, 27, 2615-2625. (13) Vukovich, F. M. Regional-scale boundary layer ozone variations in the Eastern United States and the association with meteorological variations. Atmos. Environ. 1995, 29, 2259-2273. (14) Vukovich, F. M. Boundary layer ozone variations in the eastern United States and their association with meteorological variations: long term variations. J. Geophys. Res. 1994, 99, 16,83916,850.
Received for review December 30, 1996. Revised manuscript received April 3, 1997. Accepted June 2, 1997.X ES961068A X
Abstract published in Advance ACS Abstracts, August 1, 1997.
