OZONE LEVELS IN CENTRAL ISRAEL
Mordechai Peleg and Menachem Luria,
Environmental Sciences Division, School of Applied Sciences and Technology, the Hebrew University of Jerusalem,
Jerusalem 91904, Israel
The Israeli Meteorological Service,
P.O. Box 25, Bet Dagan 52250, Israel
Max-Planck-Institute for Chemistry, Department of Air Chemistry,
Postfach 3060, D-6500, Mainz, Germany
Department of Chemistry, University of California,
Irvine, California 92717, USA
Printed in the Israel Journal of Chemistry, Vol. 34, 1994, pp.375- 386. Reprinted with permission of Laser Pages Publishing (1992) Ltd.
Measurements performed during the early summer months of 1988-1991 at a rural site in central Israel, some 50 km east of the highly urbanized coastal region, have shown that during the afternoon hours the area was often under the influence of ozone mixing ratios above the Israel ambient standard (117 ppbv) and occasionally even above 150 ppbv. Analysis of air mass back trajectories has shown that only those air masses passing over the Tel Aviv metropolitan area cause elevated ozone mixing ratios at the rural site. This highly urbanized region emits large amounts of precursors which are entrapped in the air parcels entering Israel under the predominantly westerly wind flows. As these air masses travel inland, sufficient time is available (3-5 h) to allow the photochemical reactions to generate ozone before reaching the rural site. The above hypothesis is further supported by the fact that parallel to the increase of ozone at the rural site, elevated carbon monoxide (up to 0.8 ppmv) and other trace gases were also observed. A significant correlation (R2>0.8) was found to exist between the ozone mixing ratio and the NOx concentrations in photochemically aged air masses. In several cases an excess of up to 12 ozone molecules was formed for each NOx molecule present.
Elevated ozone levels were first observed at a rural site south of Jerusalem during an investigation of the transport of air pollutants inland from Israel’s coastal area towards the mountainous regions.(1) Similarly, the ozone levels measured in Jerusalem itself over a number of years have shown substantially greater values than those observed in other major cities throughout Israel.(2) Since the Jerusalem area lacks heavy industry and has only a relatively limited number of motor vehicles (less than 100,000), it is evident that the precursors producing O3, or the O3 itself, comes from the sources outside the area. The possibility of O3 being transported from the coastal urban area to Jerusalem was first suggested by Steinberger(3), who proposed that at least some of the ozone measured in Jerusalem results from transport from the Tel Aviv metropolitan area. Further investigations at an inland rural site(4,5) confirmed the frequent occurrence of ozone levels above the Israeli ambient air quality standard of 230 ug/m3 at STP (117 ppbv).
Of late it has become increasingly clear that elevated ozone mixing ratios greater than the US National Ambient Air Quality Standard of 120 ppbv are not limited to urban centers but also may occur in many rural areas.(6-8) Since ozone is a secondary pollutant, it has been estimated that peak concentrations will occur some 3 to 5 h after emission of the necessary precursors to the atmosphere.(9) Recently Trainer et al.(10) have shown that for six rural eastern US locations a significant correlation exists between the ozone mixing ratio and the NOy (sum of all nitrogen oxide compounds excluding N2O) concentrations. The above study also indicated that an average of 8.5 ozone molecules were formed for each NOx (NO + NO2) molecule consumed. Olszyna et al.(11) reported a similar correlation for a rural site in Tennessee and found that the excess ozone production is about 12 molecules of O3 for each NOx present.
In Israel, since most industrial and urban activity occurs in the coastal region, it is to be expected that high ozone mixing ratios would occur some 30 to 90 kilometers downwind as the pollutant envelope hits the inland mountainous regions, as shown in our previous studies.4,5 The present study reports results of four successive (1988-1991) research campaigns performed during early summer at a rural inland mountain site in Israel which is representative of inland sites in the country.
During the years 1988-1991 an air monitoring station was operated during May and June at a rural site situated at Etzion (31 39’N, 35 08’E), some 25 km south of Jerusalem and 50 km east of the Mediterranean coast. The site is located in a completely rural area situated on a mountain ridge 900 m above sea level and overlooks the densely industrialized and highly populated coastal strip. Continuous measurements were performed for SO2, NO/NOy, O3, and CO using standard instrumentation (Thermo Environmental Instruments Inc., TEII models 43, 42, 49, and 41, respectively), as well as complete meteorological information. The gas analyzers were calibrated against certified standards every 3 days. The accuracy of the monitors was +1 ppbv for SO2, NOy, and O3 monitors and +0.1 ppmv for CO. The major pollution sources in the central area are the metropolitan Tel Aviv region with a population of more than 2.5 million and approximately 800,000 vehicles, the 500 MW Tel Aviv and 1200 MW Ashdod oil power plants, and a 4-million-ton annual through-put oil refinery situated next to the Ashdod power plant. Air quality data were also available from other sites in the area such as Jerusalem (1988), Ashdod (1989), Ashkelon (1990, 1991), and Kiryat Gat (1990, 1991), which are operated by local authorities.
Simultaneously with the conventional air quality monitoring, a differential optical absorption spectrometer (DOAS) was operated at Etzion and oriented to face a light source approximately 11 km due west. The DOAS instrument, developed by Platt and Perner,(12) is capable of measuring compounds such as O3, HCHO, HNO2, and NO3 in air parcels traveling between the source and collector. The pathway of the light beam was along the prevailing wind direction, west to east. The DOAS technique measures the average concentration within the entire air mass passing between the light source and the detector, and is capable of detecting sub-ppbv levels of the pollutants.
During the 1989 and 1991 studies, grab-samples (almost 100 each year, 5-10 per day) were collected into evacuated inert stainless- steel containers and analyzed for CO, CH4, saturated hydrocarbons up to C5, benzene, as well as halogenated hydrocarbons (such as CCl3F, CCl4, and C2Cl4) using standard gas chromatography techniques.
Selected results for the ozone values measured at the rural site show for the sampling periods during 1988-1991 show that, on certain days, the area can be influenced by ozone levels above 100 ppbv during the afternoon hours, while for other days, almost no increase in ozone is observed. Table 1 summarizes the measuring periods at the Etzion site together with the number of days during which the half-hourly average ozone mixing ratios exceeded 100 and 120 ppbv and also the maximum half-hour average level recorded for each year. Usually, the ozone reaches its peak value between 1400 and 1600 local summer time, LST (GMT + 3).
A comparison between ozone values measured by the DOAS technique and the conventional, continuous method for the 1991 study shows that the DOAS levels observed for the 11-km length air mass follow trends similar to the values measured locally at the light receiving system of the DOAS at Etzion. Usually the DOAS data are lower than the point measurements, although on a few occasions equivalent or higher values for the DOAS were observed. This difference may be due to the additional photochemical reactions that have taken place in the air mass during its 11-km travel from the light source to the monitoring site.
Table 1. Summary of ozone measurements at the Etzion site
No. of 1/2 h Maximum periods exceeding 1/2 h Year Measuring No. of average period days 120 ppbv 100 ppbv 1988 May 4-23 20 1 11 121 1989 May 4-Jun 7 25 4 13 153 1990 May 13-31 19 3 11 138 1991 May 12-Jun 9 29 2 6 133
Because the immediate region has almost no ozone precursor emission it is obvious that the rural area is strongly affected by outside emission sources and varying atmospheric conditions.
Atmospheric Conditions and Wind Direction
The Etzion region, as is typical for most of central Israel, is mainly under the influence of westerly wind flows for more than 80% of the time. During the afternoon hours, when the Mediterranean sea breeze reaches the site, this tendency is even more pronounced. For about 10 to 15 percent of the time the area can be influenced by eastern winds flowing in from the desert regions. In a previous study,4 and confirmed in the present investigation, it was observed that almost all elevated ozone levels
(above 60 ppbv) were accompanied by increased sulfur dioxide and nitrogen oxide values. This indicates a common source both for the primary pollutants and the precursors of secondary pollutants.
During May 1988, the area was under normal prevailing westerly wind conditions except for a distinct interruption during May 14-16 when the region was influenced by easterly winds. The wind flow change caused a drop from peak midday maximum ozone levels of above 100 ppbv which were present for four consecutive days, to values of less than 40 ppbv. During the first three days of the period between May 10 through May 24, at 1500, the Etzion area was under the normal west wind regime with air masses passing over the Tel Aviv metropolitan area. On May 13 a change of the wind direction occurred as the wind veered easterly, and for the three successive days (May 14-16) the area was influenced by air masses originating over the desert regions causing average daily temperatures of more than 30 C. From May 17 the region returned to its normal west wind patterns with the air masses coming from the Ashdod-Ashkelon area for the rest of the period, except for May 19 when the wind came from the Jerusalem area.
The midday elevated ozone mixing ratios observed during the first four days of the period (May 10-13) are due to incoming westerly air parcels that contain precursors emitted from the Tel Aviv area. Following the change in the wind flows (May 14), the ozone levels dropped to baseline levels with monotonous diurnal cycles with almost no midday peaks. Even when westerly flows resumed, ozone levels remained low with midday peaks of less than 60 ppbv due to the fact that from May 17 onward, the air masses originated over the southern, less populated, part of Israel and not the Tel Aviv region.
During the three days in May 1988 that the area was under the influence of the continuous easterly wind flows, the daily average ozone level slowly decreased to 50 ppbv and finally to almost 40 ppbv, as the air masses reaching the measuring site became stripped of any anthropogenic precursors.. A daily average ozone mixing ratio of 40 ppbv range can be viewed as being representative of the ozone content of easterly traveling air masses that originated over unpolluted desert regions.
A similar effect was noted for the ozone levels observed at Etzion during the beginning of June 1991. From the air mass back trajectories, it is apparent that for the period June 1 through June 5 the air masses arriving at Etzion passed over the Tel Aviv area causing elevated ozone levels
(half hour maxima above 120 ppbv) during the afternoon hours at the rural site. For the next two days (June 6 and 7) the air masses originated from the Ashdod- Ashkelon region and no elevated ozone values (above 60 ppbv) were noted. For the following days the rural site was once more influenced by air masses from the Tel Aviv area which caused increased ozone levels of almost 100 ppbv.
Thus it is apparent that only air masses which cross over the densely populated Tel Aviv area entrap sufficient precursors (nitrogen oxides and hydrocarbons) so that during the 3 to 5 hours of inland travel of the polluted air parcel significant quantities of ozone can be photochemically produced.
Carbon Monoxide, Organic Compounds, and Ozone
To determine the exact source of the ozone precursors, a comparison was made with the levels of other pollutants in the air parcels touching Etzion. Almost every occurrence of elevated ozone mixing ratios was accompanied by increased carbon monoxide concentrations. While normal CO concentrations were at background levels of around 0.1 to 0.2 ppmv, values as high as 0.8 ppmv were observed coinciding with ozone peaks. Since about 75% of all the carbon monoxide emitted in Israel is from the greater metropolitan Tel Aviv area (350,000-400,000 tons per year),(13) it is apparent that air masses reaching the rural site have entrapped the ozone precursors during their passage over the coastal strip.
Further indications of the source of the ozone precursors can be shown by the presence of organic compounds such as isobutane, benzene and tetrachloroethylene in the air masses reaching Etzion. All three organic materials (especially C2Cl4 which is extensively used as a degreaser and in dry cleaning) are emitted as a result of anthropogenic activity typical of highly populated urban centers. The June 1991 measurements show that elevated ozone levels are accompanied by increased levels of the specific organic compounds. Conversely, when ozone mixing ratios are at background values, the concentrations of carbon monoxide, isobutane, benzene, and tetrachloroethylene drop to levels that are lower by one order of magnitude.
It is of interest to examine the levels of ozone measured at other sites in central Israel. The results for 1991 of the daily averages and half-hour maximum ozone mixing ratios for Etzion and also for Ashkelon and Kiryat Gat (two small towns with no major pollution sources) together with the corresponding DOAS values show a trend of increasing ozone levels with increasing inland distance. Generally speaking, the coastal site
(Ashkelon) has the lowest ozone levels, followed by those measured at Kiryat Gat (situated some 20 km inland) and Etzion which has the highest levels, supporting the fact that ozone is being produced during inland travel of the air parcels. A similar trend is observed for other years and for other sites (Ashdod and Jerusalem).
Comparison of the maximum half-hour ozone values shows that the DOAS values are often the lowest, even though they represent the ozone content of inland air masses. However, the DOAS daily averages are frequently above those for Ashkelon and Kiryat Gat, as would be expected due to the inland location of the measuring site. The low maximum DOAS values are not surprising, since on many of the days sampling was not performed during the period of maximum ozone levels.
Nitrogen Oxides and Ozone
The photochemical processes responsible for ozone formation are the oxidation of volatile organic compounds (VOC) in the presence of the nitrogen oxides (NOy). While the VOC are consumed in these processes, the NOy acts as a catalyst. The basic oxidation scheme includes the oxidation NO to NO2 by the HO2 radical
(1) NO + HO2 NO2 + HO
followed by the photolysis of NO2
(2) NO2 + hv NO + O
and the production of ozone
(3) O + O2 (+M) O3 (+M)
Each molecule of NO emitted can produce a number of ozone molecules before termination occurs. For example:
(4) NO2 + HO HNO3
The role of the volatile organic compounds in the process is to facilitate the rapid conversion of NO to NO2 through a sequence of free radical chain reactions.(14)
Since NO is the predominant reactive nitrogen oxide species emitted directly to the atmosphere, the titration of ozone in the reaction with NO will cause a negative correlation between ozone and NOy as long as NO is the major component of NOy. Only after photochemical reactions form ozone and oxidize the bulk of NO to NOy, can a positive correlation of ozone with NOy be expected. In order to examine the terminal relationship between the photochemically produced ozone and the nitrogen oxides, data must be examined only from photochemically aged air. Some of the air masses arriving at the Etzion rural site should fit the above criterion and allow investigation of the correlation between ozone and NOy.
Table 2 shows the correlation between ozone and NOy during the years studied for air masses arriving at Etzion during the afternoon period
(1200-1800). The data taken were restricted to results when the NO was less than 0.5 ppbv, in an attempt to limit the data examined to photochemically aged air masses. Table 2 summarizes all the data: the slope indicates the number of ozone excess molecules per NOy present, and the intercept value the baseline ozone mixing ratio in the absence of nitrogen oxides.
Table 2. Summary of the correlation between O3 and NOy
Year No. of R2 Intercept Slope Observations 1988 129 0.73 39.5 6.0 1989 232 0.33 40.2 2.6 1990 150 0.90 33.0 9.8 1991 657 0.47 38.9 3.3
The results in the above table indicate that as the correlation factor (R2) between NOy and O3 increases, a corresponding increase in the slope of the linear fit is also observed. The highest average slope was observed during the 1990 campaign with a slope of nearly ten, which agrees well with the reports of Olszyna et al.11 and Imhoff et al.(15) for rural southeastern United States. Examination of individual days showed that the value of the slope varied from almost zero to as high as 12 (data not shown). For example, on June 5, 1991 when high ozone levels were recorded and the air masses came from the Tel Aviv region, the R2 value was 0.85 and the slope more than 9. However, on the following day when the air masses originated over the Ashdod-Ashkelon region the correlation was very poor and the slope almost zero.
It is evident that air masses passing over the Tel Aviv metropolitan region entrap sufficient VOC to exhaust all the ozone producing capacity of the NOy present. However, while the air masses from the Ashdod-Ashkelon region are also NOy-rich due to the power plant emissions, they are lean in VOC due to the relatively small number of motor vehicles in the area and thus have a low ozone-generating capacity.
The present study indicates that inland areas of Israel can be affected by elevated ozone concentrations which are produced inside air masses that have entrapped pollutants during their passage over the Tel Aviv metropolitan region. In photochemically aged air parcels a maximum excess of up to twelve ozone molecules is observed for each NOy molecule present, in good agreement with previous studies. For all air masses traveling westerly and passing over the Tel Aviv region, slope values in the range between 4 and 12 were always obtained at the rural site. The lower values indicate that the air parcel has not been fully photochemically developed. These values agree well with other studies performed at rural sites in the US10,11 and inside an aged urban plume.15
For other air masses arriving at the rural site the correlation is mild or even weak with decreasing slopes. Apparently the presence of NOy even with low levels of NO (less than 0.5 ppbv) is not sufficient for enhanced ozone production.
Weak correlation between NOy and ozone was observed occasionally even for NO less than 0.5 ppbv. These cases were typical for air masses traveling over the southern coastal region where high emission of NO is not accompanied by sufficient VOC levels.
(1) Luria, M.; Almog, H.; Peleg, M. Atmos. Environ. 1984-18:2215.
(2) Luria, M.; David, T.; Peleg, M. Atmos. Environ. 1985, 19:715.
(3) Steinberger, E. H. In Atmospheric Pollution; Benarie E.M., Ed.; Elsevier, Amsterdam, 1980, p. 165.
(4) Lifshitz, B.; Peleg, M.; Luria, M.J. Atmos. Chem. 1988, 7:19.
(5) Peleg, M.; Perner, D.; Lahav, D.; Luria M. In Environmental Quality and Ecosystem Stability; Luria, M.; Steinberger, Y.; Spanier, E., Eds.; ISSEQS Pubs.: Jerusalem, Israel, 1989; Vol. 4-A, p. 57.
(6) Logan, J.A. J. Geophys. Res. 1989, 94: 8511.
(7) Meagher, J.F.; Lee, N.T.; Valente, R.J.; Parkhurst, W.J. Atmos. Environ. 1987, 21:605.
(8) Luria, M.; Boatman, J.F.; Wellman, D.L.; Gunter, R.L.; Watkins, B.A.; Wilkinson, S.W.; Van Valin, C.C. Atmosp. Environ. 1992, 26:3265.
(9) Seinfeld, J.H. Science 1989, 243: 745.
(10) Trainer, M. et al. J. Geophys. Res. 1993, 98:2917.
(11) Olszyna, K.J.; Bailey, E.M.; Simonaitis, R.; Meagher, J. J. Geophys. Res. 1994, 99: 14,557.
(12) Platt, U.; Perner,D. In Optical and Laser Remote Sensing; Killinger, D.K.; Mooradian, A., Eds. Springer Ser. Optical Sci., Vol 39, 1983, p. 106.
(13) Ministry of the Environment. The Environment in Israel. Israel, Yearly Report No. 17-18, 1992 (in Hebrew).
(14) Finlayson-Pitts, B.J.; Pitts, J.N. Jr. Atmospheric Chemistry; John Wiley, New York, 1986.
(15) Imhoff, R.E.; Valente, R..; Meagher, J.F.; Luria, M. Atmos. Environ. 1995, in press.