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The climatology of summertime O3 and SO2 (1977–1981)
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Vd. 20. No. t2. pp. 2423-2433.1984.
Britain.
THE CLIMATOLOGY OF SUMMERTIME
0s AND SOz
~1977-1981)
FRED M. VUKOVICH ResearchTriangle Institute, Resarch Triangle Park, NC 27709. U.S.A.
and JACKFISHMAN AtmosphericSciencesDivision, NASA LangleyResearchCenter, Hampton,Virginia23665,U.S.A. (Firstreceived 11 March 1985 and infinaI~onn 5 Moy 1986) AbstracGI’he mean monthly ~~~ of the diurnal maximum 0s and of the SO, in the -tarn twot~~~oft~U.S.~~~~~for thes~cr (JuIy~d Au~t)of 1977-1981.Hit 0s concentrations varied from 60 to POppbv andcoveredan area of about 2 to 5 x IO*km*; and that for SO1Med from values greater than 10 ppbv to vatucs about 25 ppbv and covered an area of 0.3-1.3 x 106km3. The geographical location8of the centers of high O3 concentrations were related to the path of the anticyclones. The centers of high SO1concentration werea&ted by the path of anticyclones but to a lesserextent. The SOr distribution was controlled to a aignit%antextent by the location of major SO2 sources. The data suggested that highpressure systems that become stationary, weaken and dissipate in the eastern two-thirds of the U.S. have a profound effect on the O3 and SO2 ~~t~bution. Key word
index:
Ozone, climatology, anticyclone paths.
1. INTRODUCnON
A large number of case studies have been presented in the literature documenting widespread air pollution episodes that occur in the summer months and in the rural boundary layer in the eastern portions of the U.S., and which are associated with slow-moving, migratory high-pressure systems (Stasuik and Co&y, 1974;Bunt2 et at., 19?4;Decker et of., 197%Wolff et of., 1977,1979, 1980;Vukovich et a!., 1977;Ludwig et al., 1977;Husar et al., 1977; Samson, 1980;Wight et ai., 1978;Lyons et al., 1978;Mueller et al., 1980;Wolff and Lioy, 1980; and many others). Tbese studies have shown that an essentiallyclean air mass that moves out of Canada into the midwestem and E United States where large numbers of anthro~~nic sources exist becomes a polluted air mass. Pollutants were mixed into the air mass,transported eastward,and exposed to sunlight in essentiallyclear-sky, high-pressuresystems. Eventually, widespread haze and high concentrations of secondary pollutants are found in the boundary layer associated with the high-p~u~ systems. Tbe extent of the a&z&d areas has not been thoroughly documented. Since it has heen shown that these episodes are related to migratory high-pressure systems,it is reasonable to hypothesize that the extent of the area affected sbould be related to the path of the Antoine across the U.S. In this paper, the extent of air polIution episodes in the U.S. is documented using 5 years of data (1977-1981).The study focuses on the region east of fOO”W longitude and on the summer months of July and August. The relationship between
the area all&ted and the path of migratory anticyclones is explored. 2. DATAPROCJBSlNG The principal source of air pollution data for this analysis was the SAROAD data file which is maintained by the United States Env~onmen~~ Protection Agency (EPA). The file contains air pollution observations made by local air pollution agencies across the United States, and includes a broad spectrum of air pollution observations, including hourly observations of sulfur dioxide (S02) and ozone (0,). Analysesof the monthly average of the SO2 and of the diurnal maximum 0~ concentration were developed for July and August in the period 1977-1981, The diurnal maximum 0s concentration was used because it is an excellent indicator of Oj in an event condition (Vukovich et ol., 19772,and it is representative of the boundary layer concentration (Decker et al., 1976) because the boundary layer is generally well mixed when the diurnal maximum 0, concentration occurs. Analyseswere constructed for the eastern two. thirds of the United States, i.e. from the 100” W longitude line eastward to the E coast. The SAROAD data were carefully screened so that only those stations that were not directly ~n~rninat~ by an upwind source were used. Stations were selectedon the basis of their proximity to major u&an or industrial sources and on local climatology. By filtering the stations in tbis manner (whichis a marns of filteringda@ we have
2423
FRED M. VUKOVICHand JACK FISHMAN
2424
developed a data set that represents the large-scale background concentrations unencumbered by localscale variations (e.g. urban plumes). It is the analysis of the large-scale 0, and SO* distribution which we wish to examine. Rural stations least one station
were selected as often as possible. was selected
At
in States having small areas (e.g. Rhode Island). In most other States, two stations were selected, when it was possible. In large States, such as Texas and Florida, as many as three stations were selected. No stations could be found in S Dakota. However, Kelly et al. (1982) measured concentrations of SO1 and O1 in S Dakota in the summer of 1978. The average concentration of 03 at their site in S Dakota was the order of 40 ppbv; and for SO2, 5 ppbv. The authors contended that these values were natural background concentrations. In certain wind regimes, some of the selected stations were contaminated by urban and/or industrial sources. In those cases, substitutions were made. Figure 1 gives the position of the stations most often used in the analysis. About 70 y0 of the stations were rural stations. The remainder were, for the most part, upwind stations in urban regions. Isolated centers of high and low 0, and SO2 concentrations were determined only if at least two stations supported the analysis. Because the observed concentrations of SOZ were near the limits of detectability of the instrument and because of the potential problems with calibration of the variety of instrumentation used by local air pollution agencies across the U.S., it was decided not to analyze the SOZ concentrations unless the concentrations were equal to or exceeded 10 ppbv. Mean surface pressure analyses and average tracks
Fig. 1. Location of station used in the analyses of surface concentration of air polhants.
of the anticyclones were determined for the period using climatological data (National Summary) and daily weather maps prepared by the National Oceanic and Atmospheric Administration (NOAA). The analysis area was the same as that for the air pollution distribution. Data from all synoptic weather stations were used to develop the mean pressure analysis. However, data were screened and only those data were used that more appropriately describe the synopticscale variations, and were not influenced by local-scale events. lsobars were drawn at 2-mb increments. The tracks of the anticyclones are published in the climatological data up to 1980. In 1981, NOAA no longer published the climatological data, and the tracks were determined using the daily weather maps. Only the average path of major migratory anticyclones are depicted. 3. ANALYSIS RESULTS
In the period of 1977-1981, theeastern two-thirds of the U.S. was dominated by high pressure in July and August (Figs 2A, 4A, 6A. 8A and lOA). The highpressure systems were centered, for the most part, in the SE part of the U.S., and these systems were associated with the Bermuda high-pressure system. Only subtle changes in this pattern occurred from month to month and from year to year. Those changes were, for the most part, associated with the frequency and path of migratory high-pressure systems that traversed from W to E across the U.S. In July 1977 (Fig. 2B), high O3 characterized the central portions of the U.S. The highest O3 was found over Virginia, W Virginia and N Carolina, and the concentration in that region exceeded 90 ppbv. In August 1977, high OJ characterized the region from Oklahoma eastward to Virginia. The center of high OS was located over Oklahoma and Arkansas, and the concentration in that center exceeded 70 ppbv. High concentrations of SO1 were found across the central portions of the U.S. with an extension of this zone into the S part of the U.S. (Fig. 2C). In July, the highest SO2 concentrations were found over Virginia and N Carolina with the values exceeding 20 ppbv. A secondary center is noted over Iowa and Missouri where the concert trations exceeded 15 ppbv. In August, the highest concentrations of SO2 ( > 20 ppbv) were centered, for the most part, over W Virginia and Virginia. A secondary center was noted over Kansas and Oklahoma where the concentrations exceeded 10 ppbv. In July 1977 (Fig. 3), four major migratory highpressure systems affected the area. Two of these systems moved across the N portions of the U.S. The other two systems moved across the central portions of the U.S. In August, five major migratory high-pressure systems moved across the U.S. Four of these moved across the N part of the U.S. One moved across the central portions of the U.S., became stationary over Indiana, weakened, and eventually decayed.
The climatologyof summertime0, and SO1(1977-1981)
Fig.2. (A) Meanmonthlysurfacepressure(mb) analysis for July (upper) and August (lower) 1977 in the eastern part of the U.S. (B) Mean monthly distribution of the diurnal maximum O3 concentration (ppb)for July (upper)and August (tower)1977in the easternpart of the U.S.(C) MeanmonthlySO2concentration @pb) for July super) and August (lower) f977 in the easternpart of the U.S.
Fig. 3, Characteristic paths of the anticyclones at the surface for Jufy (solid line) and August (dashed line) 1977 for the eastern part of the U.S.
July and August 1978 (Fig. 4B) were characterized with high O3 in the central portions of the U.S. from Arkansas northeastward into Indiana. The highest O3 was found over N Illinois and Indiana and central Ohio, having a concentration that exceeded 80 ppbv. The distribution of SO:!was very similar to that for the summer of 1977(Fig. 4C).High concentrations of SO2 were located in the central portion of the U.S. with an extension into the S States. In July, the highest SOz ( > 20 ppbv) was centered over parts of Virginia, W Virginia,Ohio and Kentucky. A secondary center was found over Missouri and Iowa where the concentrations exceeded 10 ppbv. In August, it appeared that the secondary center found over Missouri and Iowa in July joined with the primary zone of high SO2 producing a zone of high Sot that extended from Missouri eastward to Virginia. The highest concentrations of SO1 ( > 20 ppbv) were found over parts of Virginia, W Virginia, Kentucky, Ohio and Indiana. July 1978 (Fig. 5) was characterized by five major migratory high-pressure systems. Four moved across the N part of the US. One of the five systems moved out of the NW into Indiana where it became stationary, weakened, and eventually dissipated. In
2426
FRED M. VUKOVICH and JACK FISHMAN
Fig. 4. (A) Mean monthly surface pressure (mb)analysis for July (upper) and August (lower) 1978in the eastern part of the U.S. (B) Mean monthly distribution of the diurnal maximum 0, concentration (ppb) for July (upper) and August (lower) 1978 in the eastern part of the U.S. (C) Mean monthly SO1 concentration (ppb) for July (upper) and August (lower) 1978 in the eastern part of the U.S.
Fig. 5. Characteristic paths of antieyelones at the surface for July (solid line) and August (dashed line) 1978 for the eastern part of the U.S.
August, three major systems affected the area. All of these systems moved across the N portions of the U.S.; however, when two of these reached the NE of the U.S., they then moved SW across the S States into the Gulf of Mexico. In the summer of 1979, high 0, characterized the N part of the U.S. along the line that stretched from Illinois E into New England (Fig. 6B). In July, the highest O3 was located over N Illinois, Indiana and Ohio, having a concentration that exceeded 70 ppbv; and that over the New England area, having a concentration also exceeding 70 ppbv. A secondary high O3 center was found over Georgia and S Carolina having a concentration that exccedcd 5Oppbv. In Augus!, the highest O3 was found in the center that was located over Indiana, having a concentration that exceeded 70 ppbv. There were two secondary centers. The firsr was again located over Georgia and S Carolina, and appeared to have intensified since July insomuch as the concentration of OJ in that center exceeded 60 ppbv. The other secondary cenler was located over Oklahoma and Arkansas, and its concentration also exceeded 60 ppbv. The distributions of SO2 in July and August were essentially the same (Fig. 6C). The zone of high SO2
The climatology of summertime0, and SQ (1977-1981)
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Pressure
(A)
Fig. 6. (A) Mean monthly surfacepressure(mb)an&&s for July (upper) and August (Iowe~j 1979 in the eastern part of the US (B) Mean monthly distribution of the diurnal maximum0~ concentration(apb)for July [upper) and August @wet) 1979 in the eastern pan of the U.S. (C) Mean monthly SO1concentration@pb)for July (upper) and August (lower) I979 in the easternpartof the U.S.
was found in the cast central portions of the U.S. The area covered by the high SO2 was small in 1979 compared to the previous years, The high SO2 concentration was, for the most part, centered over Virginia and W Virginia In July, the concentration exceeded 13 ppbv; and in August, 20 ppbv. In July 1979, three major systems afTected the region of interest (Fig. 7), and all of these systems moved across the N portions of the U.S. In August, six major systems moved across tht E two-thirds of the U.S. Four affected the N of the U.S. Two moved across the south central portions of the U.S. High 03 was found across the S States from Oklahoma E to N Carolina and along the E seaboard in the summer of 1980 (Fig. 8B). In July, O3 was high tbr~u~~ut this region xi&sing ~n~ntmtions in excessof80 ppbv. In August, the highest 0, was found over N Carolina and the E seaboard, and the concentration in that region exceeded 70 ppbv. The distribution of high SO1 (Fig. 8C) stretched from Alabama NE to New York. In August 1977 and July 1974 r+somewhat sin&w pattern exist&, but was not as distinct as the 1980 pattern. In July, there were two wnt+rs of highest SO1 concentrations. The N center was found over W Virginia and Pennsylvania,
Fis f- CJ==dtii paths of anticycl~nc~ at the surfacefor July (s&l line) and August (dashedline) 1979for the easternpart of the U.S.
FRED M. VUKOVICHand JACK FISHMAN Pressure
Fig. 8. (A) Mean monthly surke pressure (mb) analysis for July [upper)and August (lower) 1980 in theastern part of the U.S. (B) Mean monthly distribution of the diurnal maximum 0, concentration (ppb) for July (upper) and August (lower) 1930 in the eastern part of the U.S. (C) Mean monthly SO2concentration (ppb) for July (upper)and August (lower) 1980in
the eastern part of the U.S.
and the S center over parts of Alabama, Georgia, S Carolina, N Carolina, Tennessee and Kentucky. The concentrations in both centers exceeded 15 ppbv. In August, only the N center was present and it was displaced N over Pennsylvania and New York. The concentration in that center exceeded 15 ppbv. In both July and August, a secondary center persisted over Oklahoma and Kansas where the concentrations exceeded 10 ppbv. July 1980 (Fig. 9) was characterized by seven major systems. Four moved across the N part of the U.S. Two moved from the NW across the mid-west, ESE to N Carolina where they became stationary, weakened, and dissipated. One system moved out of the Gulf of Mexico into the S States and became stationary over Georgia where it weakened and dissipated. In August, four major systems affected the region of interest. Two ofthc systems moved acrossthe N part of the U.S. Two moved E across the centre of the U.S. until they reached the E part of the U.S. where they then moved SW into Tennessee. In both cases, these systems became stationary over Tennessee, weakened, and dissipated.
Fig. 9. Mean path of anticyclones at the surface for July (solid line) and August (dashed line) 1980for the eastern part of the U.S.
The ckatofogy of summertimeO3 and SO2 (1977-1981)
O1 characterixed the N part of the U.S. in the summer of 1981 (Fig. 10B); the region of high 03 extended into the S States. In July, the OJ concentration exceeded 60 ppbv. In August,the center of high 0, was over E Pennsylvania and New Jersey, where the concentrations exceeded 7Oppbv. The distributions of SO2 (Pig. 1OC)in July and August were similar to those found in July and August of 1979.The zone of high SO2 covered a very small area in the E-central portion of the U.S. The center of high SO2 was,for the most part, found over Kentucky and Ohio. The co~~tion in those centers exceeded 15 ppbv. In July 1981 (Fig. 1l), five major migratory highpressure systems affected the region of interest. All moved across the N portions of the U.S.; however, when two systems reached N Ohio, they then moved directly S into the Gulf of Mexico. In August, five major systemsaffected the region of interest. Again,all moved across the N of the U.S. However, when four of these reached the NE of the U.S.,they moved directly S along the coast, moving out to sea off S Carolina. High
2429
4. DISCUSSION OF RESULTS
The data indicate that the climatoIogical distribution of O5 (a secondary pollutant) is significantly more variable with respect to location of centers than the climatological SO2 (a primary pollutant) distribution from year to year in the summertime. Centers of high O3 co~nt~tion were seldom found in the same locations. However, the centers of high SO2 concentration were usually found in the region from Illinois eastward to the coast and in the latitude belt 35” N to 40” N (Ohio River Vailey).According to Clark (1980), this region is characterized by high-density SO2 sources. This concentration pattern is also consistent with the observed distribution of sulfate concentrations in rainwater found during the same period of time (Hidy et af., 1984). The strong relationship between the observed sulfate distribution in rainwater, the distribution of SO2 sources, and the subsequent SO2 dist~bution is not surprising since conversion of SO2 to sulfate is on the order of one day.
tA)
Fig. 10. (A) MeanmonthlysurfPoeprcssurc(mbfainalysisfor July(u~)a~ Aunt (bwcr) 1981in t~~tern~~oft~ U.S. (B) Mean monthlydistributionof thediurnalmaximum0, umocntration(ppb)for July (upper)and August(lower) 1981in thecustcrnFIrI of the U.S.(C) km monthlySO* conccntrntion (pph)for July(uppr)nnd Aupr~sl (lower)19131 in the castcrnpart of the U.S.
2430
FREDM. VUKOVICH and JACKFISHMAN 100
90
80
70
60
Fig. Il. Mean path of anticyclones at the surface for July (solid line) and August (dashed line) 1981 for the eastern part of the U.S. Secondary centers of SOz were found over Iowa and Missouri and over Oklahoma and Kansas. Clark showed that there are major sources of SO1 over these areas also. The area affected by high OJ concentrations ( > 40ppbv) varied from 2 x lo6 to 5 x IO6 km*; whereas, the area affected by high SOZ concentrations ( > 10ppbv) varied from 0.3 x IO6 to 1.3 x IO6 km*. The high OS concentrations affect an area about 5-10 times greater than that for SOZ. The surface pressure distribution over the 5 years did not vary substantially from year to year in the summer. The major high-pressure center was always found in the SE portions of the U.S. and was associated with the Bermuda high-pressure system. When a large number of migratory high-pressure systems moved across the N of the U.S., an apparent NW penetration of the Bermuda high-pressure system may, at times, be found in the monthly average surface pressure analysis; e.g. as in the July 1981 case (Fig. 1OA) when a secondary,high-pressure center was found over Ohio which was the result of five major migratory highpressure systems that moved across that region in that month. Previous investigators (Ripperton et (I/., 1977; Vukovich cr ul., 1977; Ludwig et ul., 1977; Wolffn ul., 1977;Stasuik and Coffey, 1974; Decker et ul., 1976, etc.) have shown, on a day-to-day basis,a strongcorrelation between high 0, and migratory high-pressure systems in the summertime. High OJ was found at the rear of the high-pressure system. As the high-pressure system moved E, the area of high 0, on the side of falling pressure generally persisted. Over the period the highpressure system moved across the U.S., it swept out a time-dependent zone of high 03.
Since SOz is a primary pollutant and high concentrations of SO2 are usually found near major source areas, spatial variability, on a day-to-day basis, similar to 03 should not occur. However, variations in the magnitude of the SO2 concentrations in major source regions have been previously noted to be associated with the passage of a migratory high-pressure system (Hidy et ol., 1978; Vukovich, 1979; WolfT et nl., 1982; Mueller and Hidy, 1983). Though the above factors have been noted, there is not any apparent correlation between monthly average pressure distributions and monthly average OJ or SO2 distribution in the summertime. This is no doubt a result of the fact that the pressure analyses are dominated by the quasi-stiionary Bermuda highpressure system rather than the migratory highpressure systems. Since a strong correlation has been previously found between migratory high-pressure systems and high O3 concentrations, and, to a lesser extent, with high SO2 concentration, it was theorized that, if there was a persistent path for the migratory high-pressure systems, then the climatological distribution of the O3 concentration and, possibly, the SO2 concentration should be correlated with, or influenced by, the path and frequency of migratory high-pressure systems. In the summers of 1979 and 1981, most of the migratory high-pressure systems moved across the N part of the eastern two-thirds of the U.S. and regions of high OJ were found in the N portions of the U.S. stretching from N Illinois E into New England. However, in both years high OJ was also found in the S part of the U.S. and migratory high-pressure systems were observed to move into the S States. The SO2 distributions in July and August have high centers in the regions where there is a high density of SOz sources (i.e. E-central U.S.) as expected. In the summers of 1977,1978 and 1980, most of the migratory high-pressure systems moved across the central and S parts of the E two-thirds of the U.S. In many cases, the high-pressure systems became stationary in these areas, weakened, and eventually dissipated. These characteristics for the high-pressure systems had profound effects on the O3 distribution. Centers of high OJ shifted into the S parts of the U.S. Though the major center of SO2 remained in the E-central U.S., secondary centers of high SO2 appeared in the S parts of the U.S. in regions where, according to Clark, significant sources of SO1 exist. The effect of the movement of these migratory highpressure systems into thecentral and S portions of the U.S. was most profoundly depicted in 1980. In the summer, five migratory high-prcssurc systems moved into the S portions of the U.S. where they became stationary, weakened, and eventually dissipated. The ccntcr of high O., stretched across the S portions of~hc E two-thirds of the U.S. Though a major center of high concentration remained over the high-density source region for SO2 in the E, a southern center of equal magnitude developed over the high source regions of
The climatology of summertime O3 and SO1 (1977-198 I)
Georgia and Alabama, and high SOz. stretched from the S States NE into New England. Daily analyses of the diurnal maximum 0, concentrations and of the daily average SO1 concentrations were developed for the summer 1980. These analyses are too voluminous to present as a part of this paper. Some of these analyses were presented in Vukovich et al. (1985,1986). It is important to point out that in the summer of 1980, the daily analyses showed persistent centers of high O3 in the S of the U.S. associated with the migratory high-pressure systems that moved into that region. Comparisons of the daily analyses of OJ with the monthly average analyses (July and August) indicated that the monthly averages reflected the persistence of the high OS centers in the S. Similarly, the daily analyses of SO1 showed that secondary centers of high SO1 consistently appeared over major source areas in the S U.S. Therefore, secondary centers of high SO2 resulted in the analyses of the average concentrations for July and August. A number of daily analyses were also performed in the summer of 1979 and 1981. The daily analyses in these cases also supported the notion that the monthly average analyses reflected the results of the persistence of centers of high 0, or of high SOI in a region that was consistently affected by the passage of migratory highpressure systems. The climatological reservoir of both SO2 and OJ in the boundary layer of the eastern U.S. is summarized in Table 1. To calculate these burdens, a boundary layer depth of 1SOOm was assumed, consistent with the observations of Vukovich et al. (1985) and Galloway ef ol. (1984). For 03, a constant molecular density was assumed throughout the entire boundary layer. The summary by Georgii (1978) suggests that SO, concentrations fall off to approximately one-half of their surface values at the top of the continental boundary layer. Galloway et al. (1984) describe a mean SO2 profile over the eastern U.S. which falls off more sharply with altitude (to 20-30 y0 of the surface values), but which actually has a slightly higher average concentration in the lower boundary than at the surface. Having calculated a reservoir of 0.69 x 103’ molecules for 1977 (using the vertical distribution of Georgii, 1978) and assuming that the emissions in the region of interest or 26 x 10” g (S) a-’ (U.S. EPA, 1976Xwe compute an SOI resident time of nearly 13 h; using the approximation of Galloway et al. (1984), the resident time is LV11 h. The model described by Fishman and Carney (1984) with the boundary conditions stated in Fishman et 01. (1985) computes a diurnally-averaged OH concentration of 2.3
2431
x 10~ crnm3 for thesummertimecontinental boundary characterized in this study. With the recommended rate constant for OH + SO2 (DeMore et al., 1985) the computed summertime lifetime of SO2 against gas-phase destruction by OH is nearly 18 h. Since other processes such as heterogeneous removal (Barrie and Georgii, 1976) and dry deposition (Sehmel, 1980) also are significant removal mechanisms for SO1, we conclude that our calculated reservoir over the eastern U.S. is consistent with our current understanding of the sources and removal mechanisms for this trace gas. The integrated amount of 03 in 1977.1978 and 1980 is remarkably constant, and the highest (1977 and 1980) and lowest (1979) burdens differ by less than 20%. On the other hand, the largest SOZ reservoir (1977)is more than 70 % greater than the 1981 (lowest) value, and it appears that there is a downward trend during this time in the SOz burdens found over the eastern U.S. This finding is consistent with the fact that emissions of SO2 during this period have been reduced by 20-25 % (Clark, 1980; Hidy er al., 1984). Perhaps the most significant difference between the climatological OJ and SO2 reservoirs is the fact that the spatial behavior of OJ is not directly tied to the distribution of the precursor emissions of the species most responsible for Oa formation (i.e. nonmethane hydrocarbons (NMHC) and nitrogen oxides, NO,). The distribution of NMHC emission is closely related to population density and is the highest in the ‘northeast corridor’ which extends from Washington, D.C., through Boston, MA. Though this region is noted for high OJ, of the 10 months analyzed,centers of high 0, values are found over this general area only in July 1979 and August 1981. This region, however, is a region of relatively high OJ in most of the analyses. The distribution of NO, emissions, on the other hand, more closely resembles the emission pattern of SOz, which is primarily centered around the upper Ohio Valley. Centers of high 03 concentrations were found in this region only in July and August 1978 and July and August 1979. Thus, the reservoir of boundary layer 03 is not directly affected by a major source region and is dependent on factors other than the emissions of the precursors that lead to the photochemical production of 03. One of the most important of these factors appears to be the path and frequency of major migratory anticyclones. The climatological distribution of 01 is profoundly affected if the highpressure systems become stationary, weaken, and dissipate in a certain geographical regions; i.e. centers of high 03 appear in that geographical region in the monthly averages. layer
Table 1. Boundary-layer reservoir over the E U.S. of’0, and SO1 (10” molecules)
5. CONCLUSIONS 0, reservoir so, reunwir
1977
1978
1979
1980
1981
6.4 0.69
6.3 0.68
5.5 0.50
6.4 0.58
5.6 0.41
The
primary purpose of this research was to examine
the hypothesis that the climatological distribution of 03 and SO1 in the E two-thirds of the U.S. should be
FRED M.
2432
VUKOVICH and JACKFISHMAN
correlated with, or influenced by, the path and frequency of migratory high&&sure _ systems. Previous studies have shown that, on a day-to-day basis, there is a strong correlation between centers of hinh - 03- and to a lesser extent, centers of hiah _ SO,- and migratory high-pressure systems in the summertime. The results of this study have shown that if there is a persistent path for major migratory high-pressure systems, thegeographical location ofpersistent regions of high 0, correlates well with the geographical location of those paths. Because SOz is a primary pollutant, the climatological distribution of SOz is primarily governed by the distribution of sources. However, there was evidence that the climatological distribution of SO1 is also affected by the path of migratory anticyclones. In any case, the climatological distribution of both OJ and SOz was profoundly affected if a number of migratory anticyclones became stationarv. weakened. and dissioated in a certain geographical region. 1; those I&S, major climatological centers of high O3 appeared in that region and the effect of sources of SOz in that region on the SOI distribution intensified.
Acknowledgmrnr -A portion of this research was supported by the National Aeronauticsand Space Administration under Contract number NAS I - 17014.
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Chem.3,29%320. Galloway J. N, Whelpdale D. M. and Wolff G. T. (1984) The flux of Sand N eastward from North America. Atmospheric Environment 18, 2595-2608.
Georgii H. W. (1978) Large scale spatial and temporal di&ibution of sulfur co6pounds. Atmospheric E&ironmenr 12,681-690.
Hidy G. M., Hansen D. A., Henry R. C., Ganesan K, and Collins J. 119841Trends in historical acid wecursor emissions and their &borne and precipitation products. J. Air Pollut. Control Ass. 34, 332-j54. _ _ Hidy G. M., Mueller P. K. and Tong E. Y. (1978) Spatial and temporal distributions of airborne sulfate in parts of the U.S. Atmospheric Environment 12, 735-752. Husar R..B.. Patterson D. E., Paley C. C. and Gillani N. V. (1977) Ozone in hazy air masses. Proc. Int. Co@ on Photochemical Oxidanr Pollution and its Control, EPA600/3-77-OOlA, ResearchTriangle Park, N.C., pp. 275-283. Kelly N. A., Wolff G. T. and Ferman M. A. (1982) Background pollutant measurements in air massesaffecting the eastern half of the United States--l. Air masses arriving from the Northwest. Atmospheric Environment 16,
1077-1088. Ludwig F. L., Johnson W. B., Ruff R. E. and Singh H. B. (1977) Important factors affecting rural ozone concentration. Proc. Int. Con$ on Phorochemical Oxidant Pollution and Its Control, EPA-600/3-77-OOIA, Research Triangle Park, N.C., pp. 425-439. Lyons W. A., Dooley J. C., Jr. and Whitby K. T. (1978) Satellite detection of long range pollution transport and sulfate aerosol hazes. Atmospheric Environment 12, 621-631. Mueller P. K. and Hidy G. M. (1983) The sulfate regional experiment: report of findings, Vol. 2, EPRI EA-1901. Electric Power Research Inst., Palo Alto, CA. Mueller P. K.. Hidy G. M., Warren K., Lavery T. F. and Baskett R. L. (1980) The occurrence of atmospheric aerosols in the Northeastern United States. Ann. N. Y. Acad. Sci. 388, 463+82. Ripperton L. A., Worth J. J. B., Vukovich F. M.and Decker C. E. (1977) ResearchTriangk Institute studies of high ozone concentration in non-urban areas. Proc. fnt. ConJ on Phororhemical Oxidant Pollurion and Its Conrrol, &PA600/3-77-OOIA. Research Triannle Park. NC.. DO.413-425. Samson P. J. (i980) Trajectory analysis of ‘s;mmertime sulfate concentrations in the Northeastern U.S. J. appl. Mer. 19, 1382-1394. Sehmel G. A. (1980) Particle and gas dry deposition: a review. Atmospheric Environment 14,983-1011. Stasuik W. N. and Coffey D. E. (1974) Rural and urban,ozone relationship in New York State. J. Air Pollu~ Conrrol Ass. 24, 564-568. U.S. Environmental Protection Agency (1976) 1973 National Emissions Report National emissions data system of the aerometric and emissions reporting system, U.S. EPA Report No. EPA-450/2-76-007, Research Triangle Park, N.C. Vukovich F. M. (1979) A note on air quality in high pressure systems.Atmospheric Environmenr 13, 255-265. Vukovich F. M., Bach W. D., Crissman B. W. and King W. J. (1977) On the relationship between high ozone in the rural boundary layer and high pressure systems. Atmospheric Eneironmenr 11, 967-983. Vukovich F. M.. Fishman J. and Browell E. V. (1985) The reservoir of ozone in the boundary layer of the eastern United States and its potential imp& on the global tropospheric ozone budget. J. geophys. Res. 90,5687-5698. Vukovich F. M., Fishman J. and Browell E. V. (1986) Factors that influence vertical transport in large scale air pollution episodes. Submitted to J. geophys. Res. Wight G. D., Wolff G. T., Lioy P. J., Meyers R. E. and Cederwall R. T. (1978) Formation and transport of ozone in the Northeast Quadrant of the United States in air
quality meteorology and atmospheric ozone, pp. 445457. Amer. Sot. Testing Materials, Philadelphia, PA. wola G. T., Ferman M. A. and Monson P. R. (1979) The distribution of Beryllium-7 within high-pressuresystemsin
The climatology of summertime 0, and SO* (1977-1981) the eastern United States. Geophys.Res. La. 6,637-639. WoItTG. T., Kelly N. A. and Ferman M. A. (1982)Source regions of summertime ozone and haze episodes in the eastern U.S. Wu&r Air Soil Pollut. 1$65-81. Wolff G. T. and Lioy P. 1. (1980)Development of an ozone river associated with synoptk seak episodes in the eastern U.S. Ettvir. Sci. Technol. 14, 1257-1260. WolB 0. T., Lioy P. J. and Wight G. D. (1980)Transport of ozone associated with an air mass. J. En&. Sci. H&h. AS, 193-199.
2433
Wolff
G.T., Lioy P. J., Wight G. 13.and Meyers R. E (1977) An investigation of Iong-range transport of ozone across the midwestem and eastern United States. Pruc. Ink Con/. on Photochemical Oxidmt Pollution and Its Control, EPA~~77~1~ ResearzhTriande Park, N.C.,pp. 30-319. WolfT G. T.. Liov P. J.. Wiaht G. D.. Mevers R. E. and Cedenvall~ R. T. (197j) A~I investi&od of long-range transport of ozone across the mid-western and eastern U.S. Atmospheric En~iron~nr 11, 797-802.
&a&omen1
Ptinted L omt
Vd. 20. No. t2. pp. 2423-2433.1984.
Britain.
THE CLIMATOLOGY OF SUMMERTIME
0s AND SOz
~1977-1981)
FRED M. VUKOVICH ResearchTriangle Institute, Resarch Triangle Park, NC 27709. U.S.A.
and JACKFISHMAN AtmosphericSciencesDivision, NASA LangleyResearchCenter, Hampton,Virginia23665,U.S.A. (Firstreceived 11 March 1985 and infinaI~onn 5 Moy 1986) AbstracGI’he mean monthly ~~~ of the diurnal maximum 0s and of the SO, in the -tarn twot~~~oft~U.S.~~~~~for thes~cr (JuIy~d Au~t)of 1977-1981.Hit 0s concentrations varied from 60 to POppbv andcoveredan area of about 2 to 5 x IO*km*; and that for SO1Med from values greater than 10 ppbv to vatucs about 25 ppbv and covered an area of 0.3-1.3 x 106km3. The geographical location8of the centers of high O3 concentrations were related to the path of the anticyclones. The centers of high SO1concentration werea&ted by the path of anticyclones but to a lesserextent. The SOr distribution was controlled to a aignit%antextent by the location of major SO2 sources. The data suggested that highpressure systems that become stationary, weaken and dissipate in the eastern two-thirds of the U.S. have a profound effect on the O3 and SO2 ~~t~bution. Key word
index:
Ozone, climatology, anticyclone paths.
1. INTRODUCnON
A large number of case studies have been presented in the literature documenting widespread air pollution episodes that occur in the summer months and in the rural boundary layer in the eastern portions of the U.S., and which are associated with slow-moving, migratory high-pressure systems (Stasuik and Co&y, 1974;Bunt2 et at., 19?4;Decker et of., 197%Wolff et of., 1977,1979, 1980;Vukovich et a!., 1977;Ludwig et al., 1977;Husar et al., 1977; Samson, 1980;Wight et ai., 1978;Lyons et al., 1978;Mueller et al., 1980;Wolff and Lioy, 1980; and many others). Tbese studies have shown that an essentiallyclean air mass that moves out of Canada into the midwestem and E United States where large numbers of anthro~~nic sources exist becomes a polluted air mass. Pollutants were mixed into the air mass,transported eastward,and exposed to sunlight in essentiallyclear-sky, high-pressuresystems. Eventually, widespread haze and high concentrations of secondary pollutants are found in the boundary layer associated with the high-p~u~ systems. Tbe extent of the a&z&d areas has not been thoroughly documented. Since it has heen shown that these episodes are related to migratory high-pressure systems,it is reasonable to hypothesize that the extent of the area affected sbould be related to the path of the Antoine across the U.S. In this paper, the extent of air polIution episodes in the U.S. is documented using 5 years of data (1977-1981).The study focuses on the region east of fOO”W longitude and on the summer months of July and August. The relationship between
the area all&ted and the path of migratory anticyclones is explored. 2. DATAPROCJBSlNG The principal source of air pollution data for this analysis was the SAROAD data file which is maintained by the United States Env~onmen~~ Protection Agency (EPA). The file contains air pollution observations made by local air pollution agencies across the United States, and includes a broad spectrum of air pollution observations, including hourly observations of sulfur dioxide (S02) and ozone (0,). Analysesof the monthly average of the SO2 and of the diurnal maximum 0~ concentration were developed for July and August in the period 1977-1981, The diurnal maximum 0s concentration was used because it is an excellent indicator of Oj in an event condition (Vukovich et ol., 19772,and it is representative of the boundary layer concentration (Decker et al., 1976) because the boundary layer is generally well mixed when the diurnal maximum 0, concentration occurs. Analyseswere constructed for the eastern two. thirds of the United States, i.e. from the 100” W longitude line eastward to the E coast. The SAROAD data were carefully screened so that only those stations that were not directly ~n~rninat~ by an upwind source were used. Stations were selectedon the basis of their proximity to major u&an or industrial sources and on local climatology. By filtering the stations in tbis manner (whichis a marns of filteringda@ we have
2423
FRED M. VUKOVICHand JACK FISHMAN
2424
developed a data set that represents the large-scale background concentrations unencumbered by localscale variations (e.g. urban plumes). It is the analysis of the large-scale 0, and SO* distribution which we wish to examine. Rural stations least one station
were selected as often as possible. was selected
At
in States having small areas (e.g. Rhode Island). In most other States, two stations were selected, when it was possible. In large States, such as Texas and Florida, as many as three stations were selected. No stations could be found in S Dakota. However, Kelly et al. (1982) measured concentrations of SO1 and O1 in S Dakota in the summer of 1978. The average concentration of 03 at their site in S Dakota was the order of 40 ppbv; and for SO2, 5 ppbv. The authors contended that these values were natural background concentrations. In certain wind regimes, some of the selected stations were contaminated by urban and/or industrial sources. In those cases, substitutions were made. Figure 1 gives the position of the stations most often used in the analysis. About 70 y0 of the stations were rural stations. The remainder were, for the most part, upwind stations in urban regions. Isolated centers of high and low 0, and SO2 concentrations were determined only if at least two stations supported the analysis. Because the observed concentrations of SOZ were near the limits of detectability of the instrument and because of the potential problems with calibration of the variety of instrumentation used by local air pollution agencies across the U.S., it was decided not to analyze the SOZ concentrations unless the concentrations were equal to or exceeded 10 ppbv. Mean surface pressure analyses and average tracks
Fig. 1. Location of station used in the analyses of surface concentration of air polhants.
of the anticyclones were determined for the period using climatological data (National Summary) and daily weather maps prepared by the National Oceanic and Atmospheric Administration (NOAA). The analysis area was the same as that for the air pollution distribution. Data from all synoptic weather stations were used to develop the mean pressure analysis. However, data were screened and only those data were used that more appropriately describe the synopticscale variations, and were not influenced by local-scale events. lsobars were drawn at 2-mb increments. The tracks of the anticyclones are published in the climatological data up to 1980. In 1981, NOAA no longer published the climatological data, and the tracks were determined using the daily weather maps. Only the average path of major migratory anticyclones are depicted. 3. ANALYSIS RESULTS
In the period of 1977-1981, theeastern two-thirds of the U.S. was dominated by high pressure in July and August (Figs 2A, 4A, 6A. 8A and lOA). The highpressure systems were centered, for the most part, in the SE part of the U.S., and these systems were associated with the Bermuda high-pressure system. Only subtle changes in this pattern occurred from month to month and from year to year. Those changes were, for the most part, associated with the frequency and path of migratory high-pressure systems that traversed from W to E across the U.S. In July 1977 (Fig. 2B), high O3 characterized the central portions of the U.S. The highest O3 was found over Virginia, W Virginia and N Carolina, and the concentration in that region exceeded 90 ppbv. In August 1977, high OJ characterized the region from Oklahoma eastward to Virginia. The center of high OS was located over Oklahoma and Arkansas, and the concentration in that center exceeded 70 ppbv. High concentrations of SO1 were found across the central portions of the U.S. with an extension of this zone into the S part of the U.S. (Fig. 2C). In July, the highest SO2 concentrations were found over Virginia and N Carolina with the values exceeding 20 ppbv. A secondary center is noted over Iowa and Missouri where the concert trations exceeded 15 ppbv. In August, the highest concentrations of SO2 ( > 20 ppbv) were centered, for the most part, over W Virginia and Virginia. A secondary center was noted over Kansas and Oklahoma where the concentrations exceeded 10 ppbv. In July 1977 (Fig. 3), four major migratory highpressure systems affected the area. Two of these systems moved across the N portions of the U.S. The other two systems moved across the central portions of the U.S. In August, five major migratory high-pressure systems moved across the U.S. Four of these moved across the N part of the U.S. One moved across the central portions of the U.S., became stationary over Indiana, weakened, and eventually decayed.
The climatologyof summertime0, and SO1(1977-1981)
Fig.2. (A) Meanmonthlysurfacepressure(mb) analysis for July (upper) and August (lower) 1977 in the eastern part of the U.S. (B) Mean monthly distribution of the diurnal maximum O3 concentration (ppb)for July (upper)and August (tower)1977in the easternpart of the U.S.(C) MeanmonthlySO2concentration @pb) for July super) and August (lower) f977 in the easternpart of the U.S.
Fig. 3, Characteristic paths of the anticyclones at the surface for Jufy (solid line) and August (dashed line) 1977 for the eastern part of the U.S.
July and August 1978 (Fig. 4B) were characterized with high O3 in the central portions of the U.S. from Arkansas northeastward into Indiana. The highest O3 was found over N Illinois and Indiana and central Ohio, having a concentration that exceeded 80 ppbv. The distribution of SO:!was very similar to that for the summer of 1977(Fig. 4C).High concentrations of SO2 were located in the central portion of the U.S. with an extension into the S States. In July, the highest SOz ( > 20 ppbv) was centered over parts of Virginia, W Virginia,Ohio and Kentucky. A secondary center was found over Missouri and Iowa where the concentrations exceeded 10 ppbv. In August, it appeared that the secondary center found over Missouri and Iowa in July joined with the primary zone of high SO2 producing a zone of high Sot that extended from Missouri eastward to Virginia. The highest concentrations of SO1 ( > 20 ppbv) were found over parts of Virginia, W Virginia, Kentucky, Ohio and Indiana. July 1978 (Fig. 5) was characterized by five major migratory high-pressure systems. Four moved across the N part of the US. One of the five systems moved out of the NW into Indiana where it became stationary, weakened, and eventually dissipated. In
2426
FRED M. VUKOVICH and JACK FISHMAN
Fig. 4. (A) Mean monthly surface pressure (mb)analysis for July (upper) and August (lower) 1978in the eastern part of the U.S. (B) Mean monthly distribution of the diurnal maximum 0, concentration (ppb) for July (upper) and August (lower) 1978 in the eastern part of the U.S. (C) Mean monthly SO1 concentration (ppb) for July (upper) and August (lower) 1978 in the eastern part of the U.S.
Fig. 5. Characteristic paths of antieyelones at the surface for July (solid line) and August (dashed line) 1978 for the eastern part of the U.S.
August, three major systems affected the area. All of these systems moved across the N portions of the U.S.; however, when two of these reached the NE of the U.S., they then moved SW across the S States into the Gulf of Mexico. In the summer of 1979, high 0, characterized the N part of the U.S. along the line that stretched from Illinois E into New England (Fig. 6B). In July, the highest O3 was located over N Illinois, Indiana and Ohio, having a concentration that exceeded 70 ppbv; and that over the New England area, having a concentration also exceeding 70 ppbv. A secondary high O3 center was found over Georgia and S Carolina having a concentration that exccedcd 5Oppbv. In Augus!, the highest O3 was found in the center that was located over Indiana, having a concentration that exceeded 70 ppbv. There were two secondary centers. The firsr was again located over Georgia and S Carolina, and appeared to have intensified since July insomuch as the concentration of OJ in that center exceeded 60 ppbv. The other secondary cenler was located over Oklahoma and Arkansas, and its concentration also exceeded 60 ppbv. The distributions of SO2 in July and August were essentially the same (Fig. 6C). The zone of high SO2
The climatology of summertime0, and SQ (1977-1981)
2427
Pressure
(A)
Fig. 6. (A) Mean monthly surfacepressure(mb)an&&s for July (upper) and August (Iowe~j 1979 in the eastern part of the US (B) Mean monthly distribution of the diurnal maximum0~ concentration(apb)for July [upper) and August @wet) 1979 in the eastern pan of the U.S. (C) Mean monthly SO1concentration@pb)for July (upper) and August (lower) I979 in the easternpartof the U.S.
was found in the cast central portions of the U.S. The area covered by the high SO2 was small in 1979 compared to the previous years, The high SO2 concentration was, for the most part, centered over Virginia and W Virginia In July, the concentration exceeded 13 ppbv; and in August, 20 ppbv. In July 1979, three major systems afTected the region of interest (Fig. 7), and all of these systems moved across the N portions of the U.S. In August, six major systems moved across tht E two-thirds of the U.S. Four affected the N of the U.S. Two moved across the south central portions of the U.S. High 03 was found across the S States from Oklahoma E to N Carolina and along the E seaboard in the summer of 1980 (Fig. 8B). In July, O3 was high tbr~u~~ut this region xi&sing ~n~ntmtions in excessof80 ppbv. In August, the highest 0, was found over N Carolina and the E seaboard, and the concentration in that region exceeded 70 ppbv. The distribution of high SO1 (Fig. 8C) stretched from Alabama NE to New York. In August 1977 and July 1974 r+somewhat sin&w pattern exist&, but was not as distinct as the 1980 pattern. In July, there were two wnt+rs of highest SO1 concentrations. The N center was found over W Virginia and Pennsylvania,
Fis f- CJ==dtii paths of anticycl~nc~ at the surfacefor July (s&l line) and August (dashedline) 1979for the easternpart of the U.S.
FRED M. VUKOVICHand JACK FISHMAN Pressure
Fig. 8. (A) Mean monthly surke pressure (mb) analysis for July [upper)and August (lower) 1980 in theastern part of the U.S. (B) Mean monthly distribution of the diurnal maximum 0, concentration (ppb) for July (upper) and August (lower) 1930 in the eastern part of the U.S. (C) Mean monthly SO2concentration (ppb) for July (upper)and August (lower) 1980in
the eastern part of the U.S.
and the S center over parts of Alabama, Georgia, S Carolina, N Carolina, Tennessee and Kentucky. The concentrations in both centers exceeded 15 ppbv. In August, only the N center was present and it was displaced N over Pennsylvania and New York. The concentration in that center exceeded 15 ppbv. In both July and August, a secondary center persisted over Oklahoma and Kansas where the concentrations exceeded 10 ppbv. July 1980 (Fig. 9) was characterized by seven major systems. Four moved across the N part of the U.S. Two moved from the NW across the mid-west, ESE to N Carolina where they became stationary, weakened, and dissipated. One system moved out of the Gulf of Mexico into the S States and became stationary over Georgia where it weakened and dissipated. In August, four major systems affected the region of interest. Two ofthc systems moved acrossthe N part of the U.S. Two moved E across the centre of the U.S. until they reached the E part of the U.S. where they then moved SW into Tennessee. In both cases, these systems became stationary over Tennessee, weakened, and dissipated.
Fig. 9. Mean path of anticyclones at the surface for July (solid line) and August (dashed line) 1980for the eastern part of the U.S.
The ckatofogy of summertimeO3 and SO2 (1977-1981)
O1 characterixed the N part of the U.S. in the summer of 1981 (Fig. 10B); the region of high 03 extended into the S States. In July, the OJ concentration exceeded 60 ppbv. In August,the center of high 0, was over E Pennsylvania and New Jersey, where the concentrations exceeded 7Oppbv. The distributions of SO2 (Pig. 1OC)in July and August were similar to those found in July and August of 1979.The zone of high SO2 covered a very small area in the E-central portion of the U.S. The center of high SO2 was,for the most part, found over Kentucky and Ohio. The co~~tion in those centers exceeded 15 ppbv. In July 1981 (Fig. 1l), five major migratory highpressure systems affected the region of interest. All moved across the N portions of the U.S.; however, when two systems reached N Ohio, they then moved directly S into the Gulf of Mexico. In August, five major systemsaffected the region of interest. Again,all moved across the N of the U.S. However, when four of these reached the NE of the U.S.,they moved directly S along the coast, moving out to sea off S Carolina. High
2429
4. DISCUSSION OF RESULTS
The data indicate that the climatoIogical distribution of O5 (a secondary pollutant) is significantly more variable with respect to location of centers than the climatological SO2 (a primary pollutant) distribution from year to year in the summertime. Centers of high O3 co~nt~tion were seldom found in the same locations. However, the centers of high SO2 concentration were usually found in the region from Illinois eastward to the coast and in the latitude belt 35” N to 40” N (Ohio River Vailey).According to Clark (1980), this region is characterized by high-density SO2 sources. This concentration pattern is also consistent with the observed distribution of sulfate concentrations in rainwater found during the same period of time (Hidy et af., 1984). The strong relationship between the observed sulfate distribution in rainwater, the distribution of SO2 sources, and the subsequent SO2 dist~bution is not surprising since conversion of SO2 to sulfate is on the order of one day.
tA)
Fig. 10. (A) MeanmonthlysurfPoeprcssurc(mbfainalysisfor July(u~)a~ Aunt (bwcr) 1981in t~~tern~~oft~ U.S. (B) Mean monthlydistributionof thediurnalmaximum0, umocntration(ppb)for July (upper)and August(lower) 1981in thecustcrnFIrI of the U.S.(C) km monthlySO* conccntrntion (pph)for July(uppr)nnd Aupr~sl (lower)19131 in the castcrnpart of the U.S.
2430
FREDM. VUKOVICH and JACKFISHMAN 100
90
80
70
60
Fig. Il. Mean path of anticyclones at the surface for July (solid line) and August (dashed line) 1981 for the eastern part of the U.S. Secondary centers of SOz were found over Iowa and Missouri and over Oklahoma and Kansas. Clark showed that there are major sources of SO1 over these areas also. The area affected by high OJ concentrations ( > 40ppbv) varied from 2 x lo6 to 5 x IO6 km*; whereas, the area affected by high SOZ concentrations ( > 10ppbv) varied from 0.3 x IO6 to 1.3 x IO6 km*. The high OS concentrations affect an area about 5-10 times greater than that for SOZ. The surface pressure distribution over the 5 years did not vary substantially from year to year in the summer. The major high-pressure center was always found in the SE portions of the U.S. and was associated with the Bermuda high-pressure system. When a large number of migratory high-pressure systems moved across the N of the U.S., an apparent NW penetration of the Bermuda high-pressure system may, at times, be found in the monthly average surface pressure analysis; e.g. as in the July 1981 case (Fig. 1OA) when a secondary,high-pressure center was found over Ohio which was the result of five major migratory highpressure systems that moved across that region in that month. Previous investigators (Ripperton et (I/., 1977; Vukovich cr ul., 1977; Ludwig et ul., 1977; Wolffn ul., 1977;Stasuik and Coffey, 1974; Decker et ul., 1976, etc.) have shown, on a day-to-day basis,a strongcorrelation between high 0, and migratory high-pressure systems in the summertime. High OJ was found at the rear of the high-pressure system. As the high-pressure system moved E, the area of high 0, on the side of falling pressure generally persisted. Over the period the highpressure system moved across the U.S., it swept out a time-dependent zone of high 03.
Since SOz is a primary pollutant and high concentrations of SO2 are usually found near major source areas, spatial variability, on a day-to-day basis, similar to 03 should not occur. However, variations in the magnitude of the SO2 concentrations in major source regions have been previously noted to be associated with the passage of a migratory high-pressure system (Hidy et ol., 1978; Vukovich, 1979; WolfT et nl., 1982; Mueller and Hidy, 1983). Though the above factors have been noted, there is not any apparent correlation between monthly average pressure distributions and monthly average OJ or SO2 distribution in the summertime. This is no doubt a result of the fact that the pressure analyses are dominated by the quasi-stiionary Bermuda highpressure system rather than the migratory highpressure systems. Since a strong correlation has been previously found between migratory high-pressure systems and high O3 concentrations, and, to a lesser extent, with high SO2 concentration, it was theorized that, if there was a persistent path for the migratory high-pressure systems, then the climatological distribution of the O3 concentration and, possibly, the SO2 concentration should be correlated with, or influenced by, the path and frequency of migratory high-pressure systems. In the summers of 1979 and 1981, most of the migratory high-pressure systems moved across the N part of the eastern two-thirds of the U.S. and regions of high OJ were found in the N portions of the U.S. stretching from N Illinois E into New England. However, in both years high OJ was also found in the S part of the U.S. and migratory high-pressure systems were observed to move into the S States. The SO2 distributions in July and August have high centers in the regions where there is a high density of SOz sources (i.e. E-central U.S.) as expected. In the summers of 1977,1978 and 1980, most of the migratory high-pressure systems moved across the central and S parts of the E two-thirds of the U.S. In many cases, the high-pressure systems became stationary in these areas, weakened, and eventually dissipated. These characteristics for the high-pressure systems had profound effects on the O3 distribution. Centers of high OJ shifted into the S parts of the U.S. Though the major center of SO2 remained in the E-central U.S., secondary centers of high SO2 appeared in the S parts of the U.S. in regions where, according to Clark, significant sources of SO1 exist. The effect of the movement of these migratory highpressure systems into thecentral and S portions of the U.S. was most profoundly depicted in 1980. In the summer, five migratory high-prcssurc systems moved into the S portions of the U.S. where they became stationary, weakened, and eventually dissipated. The ccntcr of high O., stretched across the S portions of~hc E two-thirds of the U.S. Though a major center of high concentration remained over the high-density source region for SO2 in the E, a southern center of equal magnitude developed over the high source regions of
The climatology of summertime O3 and SO1 (1977-198 I)
Georgia and Alabama, and high SOz. stretched from the S States NE into New England. Daily analyses of the diurnal maximum 0, concentrations and of the daily average SO1 concentrations were developed for the summer 1980. These analyses are too voluminous to present as a part of this paper. Some of these analyses were presented in Vukovich et al. (1985,1986). It is important to point out that in the summer of 1980, the daily analyses showed persistent centers of high O3 in the S of the U.S. associated with the migratory high-pressure systems that moved into that region. Comparisons of the daily analyses of OJ with the monthly average analyses (July and August) indicated that the monthly averages reflected the persistence of the high OS centers in the S. Similarly, the daily analyses of SO1 showed that secondary centers of high SO1 consistently appeared over major source areas in the S U.S. Therefore, secondary centers of high SO2 resulted in the analyses of the average concentrations for July and August. A number of daily analyses were also performed in the summer of 1979 and 1981. The daily analyses in these cases also supported the notion that the monthly average analyses reflected the results of the persistence of centers of high 0, or of high SOI in a region that was consistently affected by the passage of migratory highpressure systems. The climatological reservoir of both SO2 and OJ in the boundary layer of the eastern U.S. is summarized in Table 1. To calculate these burdens, a boundary layer depth of 1SOOm was assumed, consistent with the observations of Vukovich et al. (1985) and Galloway ef ol. (1984). For 03, a constant molecular density was assumed throughout the entire boundary layer. The summary by Georgii (1978) suggests that SO, concentrations fall off to approximately one-half of their surface values at the top of the continental boundary layer. Galloway et al. (1984) describe a mean SO2 profile over the eastern U.S. which falls off more sharply with altitude (to 20-30 y0 of the surface values), but which actually has a slightly higher average concentration in the lower boundary than at the surface. Having calculated a reservoir of 0.69 x 103’ molecules for 1977 (using the vertical distribution of Georgii, 1978) and assuming that the emissions in the region of interest or 26 x 10” g (S) a-’ (U.S. EPA, 1976Xwe compute an SOI resident time of nearly 13 h; using the approximation of Galloway et al. (1984), the resident time is LV11 h. The model described by Fishman and Carney (1984) with the boundary conditions stated in Fishman et 01. (1985) computes a diurnally-averaged OH concentration of 2.3
2431
x 10~ crnm3 for thesummertimecontinental boundary characterized in this study. With the recommended rate constant for OH + SO2 (DeMore et al., 1985) the computed summertime lifetime of SO2 against gas-phase destruction by OH is nearly 18 h. Since other processes such as heterogeneous removal (Barrie and Georgii, 1976) and dry deposition (Sehmel, 1980) also are significant removal mechanisms for SO1, we conclude that our calculated reservoir over the eastern U.S. is consistent with our current understanding of the sources and removal mechanisms for this trace gas. The integrated amount of 03 in 1977.1978 and 1980 is remarkably constant, and the highest (1977 and 1980) and lowest (1979) burdens differ by less than 20%. On the other hand, the largest SOZ reservoir (1977)is more than 70 % greater than the 1981 (lowest) value, and it appears that there is a downward trend during this time in the SOz burdens found over the eastern U.S. This finding is consistent with the fact that emissions of SO2 during this period have been reduced by 20-25 % (Clark, 1980; Hidy er al., 1984). Perhaps the most significant difference between the climatological OJ and SO2 reservoirs is the fact that the spatial behavior of OJ is not directly tied to the distribution of the precursor emissions of the species most responsible for Oa formation (i.e. nonmethane hydrocarbons (NMHC) and nitrogen oxides, NO,). The distribution of NMHC emission is closely related to population density and is the highest in the ‘northeast corridor’ which extends from Washington, D.C., through Boston, MA. Though this region is noted for high OJ, of the 10 months analyzed,centers of high 0, values are found over this general area only in July 1979 and August 1981. This region, however, is a region of relatively high OJ in most of the analyses. The distribution of NO, emissions, on the other hand, more closely resembles the emission pattern of SOz, which is primarily centered around the upper Ohio Valley. Centers of high 03 concentrations were found in this region only in July and August 1978 and July and August 1979. Thus, the reservoir of boundary layer 03 is not directly affected by a major source region and is dependent on factors other than the emissions of the precursors that lead to the photochemical production of 03. One of the most important of these factors appears to be the path and frequency of major migratory anticyclones. The climatological distribution of 01 is profoundly affected if the highpressure systems become stationary, weaken, and dissipate in a certain geographical regions; i.e. centers of high 03 appear in that geographical region in the monthly averages. layer
Table 1. Boundary-layer reservoir over the E U.S. of’0, and SO1 (10” molecules)
5. CONCLUSIONS 0, reservoir so, reunwir
1977
1978
1979
1980
1981
6.4 0.69
6.3 0.68
5.5 0.50
6.4 0.58
5.6 0.41
The
primary purpose of this research was to examine
the hypothesis that the climatological distribution of 03 and SO1 in the E two-thirds of the U.S. should be
FRED M.
2432
VUKOVICH and JACKFISHMAN
correlated with, or influenced by, the path and frequency of migratory high&&sure _ systems. Previous studies have shown that, on a day-to-day basis, there is a strong correlation between centers of hinh - 03- and to a lesser extent, centers of hiah _ SO,- and migratory high-pressure systems in the summertime. The results of this study have shown that if there is a persistent path for major migratory high-pressure systems, thegeographical location ofpersistent regions of high 0, correlates well with the geographical location of those paths. Because SOz is a primary pollutant, the climatological distribution of SOz is primarily governed by the distribution of sources. However, there was evidence that the climatological distribution of SO1 is also affected by the path of migratory anticyclones. In any case, the climatological distribution of both OJ and SOz was profoundly affected if a number of migratory anticyclones became stationarv. weakened. and dissioated in a certain geographical region. 1; those I&S, major climatological centers of high O3 appeared in that region and the effect of sources of SOz in that region on the SOI distribution intensified.
Acknowledgmrnr -A portion of this research was supported by the National Aeronauticsand Space Administration under Contract number NAS I - 17014.
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