Search

In the atmosphere, reactive forms of mercury species can be produced by oxidation of the dominant gaseous elemental mercury (GEM). The oxidation of GEM is an important driver for deposition, but oxidation pathways currently are poorly constrained and likely differ among regions. In this study, continuous measurements of atmospheric nitrate radical (NO3) concentrations and mercury speciation (i.e., elemental and reactive, oxidized forms) were performed during a six week period in the urban air shed of Jerusalem, Israel during summer 2012, to investigate the potential nighttime contribution of nitrate radicals to oxidized mercury formation. Average nighttime concentrations of reactive gaseous mercury (RGM) were almost equivalent to daytime levels (25 pg m(-3) and 27 pg m(-3) respectively), in contrast to early morning and evening RGM levels which dropped to low levels (9 and 13 pg m(-3)). During daytime, the presence of RGM was increased when solar radiation exceeded 200 W m(-2), suggesting a photochemical process for daytime RGM formation. Ozone concentrations were largely unrelated to daytime RGM. Nighttime RGM concentrations were relatively high (with a maximum of 97 pg m(-3)) compared to nighttime levels in other urban regions. A strong correlation was observed between nighttime RGM concentrations and nitrate radical concentration (R(2) averaging 0.47), while correlations to other variables were weak (e.g., RH; R(2) = 0.35) or absent (e.g., ozone, wind speed and direction, pollution tracers such as CO or SO2). Detailed analyses suggest that advection processes or tropospheric influences were unlikely to explain the strong nighttime correlations between NO3 and RGM, although these processes may contribute to these relationships. Our observations suggest that NO3 radicals may play a role in RGM formation, possibly due to a direct chemical involvement in GEM oxidation. Since physical data, however, suggest that NO3 unlikely initiates GEM oxidation, NO3 may play a secondary role in GEM oxidation through the addition to an unstable Hg(I) radical species.

Deposition of atmospheric mercury is of global concern, primarily due to health effects associated with efficient bioaccumulation of mercury in marine food webs. Although oxidation of gaseous elementary mercury (GEM), the major fraction of atmospheric mercury, is a critical stage in regulating atmospheric mercury deposition efficiency, this oxidation is currently not well-characterized, limiting modeling-based assessments of mercury in the environment. Based on a previous study, we hypothesized that the oxidation of GEM is predominantly controlled by multistep bromine- and chlorine-induced oxidation (MBCO) in the remote marine boundary layer (RMBL), and by photochemical smog oxidants, primarily ozone (O3) and hydroxyl radical (OH), in the polluted continental boundary layer (PCBL). To test this hypothesis, we used the following analyses: (i) application of a newly developed criterion to evaluate the gaseous oxidized mercury (GOM)–O3 association based on previous studies in the RMBL and PCBL; (ii) measurement-based box simulations of GEM oxidation in the RMBL and at a PCBL site; and (iii) measurement-based analysis of photochemical oxidation vs. other processes which potentially influence GOM. Our model simulations indicated that the MBCO mechanism can reproduce GOM levels in the RMBL, but not in the PCBL. Our data analysis suggested the important role of photochemical smog oxidants in GEM oxidation in the PCBL, potentially masked by the effect of relative humidity and entrainment of free tropospheric air.

Accurate characterization of gaseous elemental mercury (GEM) chemical oxidation pathways and their kinetics is critically important for assessing the transfer of atmospheric mercury to bioaquatic systems. Recent comprehensive field measurements have suggested that the nitrate radical (NO3) plays a role in efficient nighttime oxidation of GEM, and that the role of the hydroxyl radical (OH) as a GEM oxidant has been underestimated. We used the CAABA/MECCA chemical box model and additional kinetic calculations to analyze these measurement results, in order to investigate the nighttime and daytime oxidation of GEM. We assumed a second-order reaction for the NO3 induced nighttime oxidation of GEM. Our analysis demonstrated that nighttime oxidation of GEM has to be included in the model to account for the measured variations in nighttime reactive gaseous mercury (RGM) concentration. A lower limit and best-fit rate constant for GEM nighttime oxidation are provided. To the best of our knowledge, this is the first time that a rate for nighttime oxidation of GEM has been determined based on field measurements. Our analysis further indicates that OH has a much more important role in GEM oxidation than commonly considered. A lower-limit rate constant for the OH–RGM reaction is provided.

Atmospheric mercury depletion events (AMDEs) outside the polar region – driven by high levels of gaseous Br and BrO (i.e., BrOx) – were observed recently in the warm Dead Sea boundary layer. The efficient oxidation of gaseous elemental mercury (GEM) under temperate conditions by BrOx was unexpected considering that the thermal back dissociation reaction of HgBr is about 2.5 orders of magnitude higher under Dead Sea temperatures compared to polar temperatures, and hence was expected to significantly slow down GEM oxidation under warm temperatures. The goal of this modelling study was to improve understanding of the interaction of reactive bromine and mercury during Dead Sea AMDEs using numerical simulations based on a comprehensive measurement campaign in summer 2009. Our analysis is focused on daytime AMDE when chemical processes dominate concentration changes. Best agreements between simulations and observations were achieved using rate constants for kHg+Br and kHg+BrO of 2.7 × 10−13 cm3 molecule−1 s−1 and 1.5 × 10−13 cm3 molecule−1 s−1, respectively. Our model also predicted that a rate constant kHg+BrO of 5.0 × 10−14 cm3 molecule−1 s−1 may be considered as a minimum, which is higher than most reported values. These rate constants suggest that BrO could be a more efficient oxidant than Br in the troposphere as long as [Br]/[BrO] ratios are smaller than ~0.2 to 0.5. Under Dead Sea conditions, these kinetics demonstrate a high efficiency and central role of BrOx for AMDEs, with relative contributions to GEM depletion of more than ~90%. Unexpectedly, BrO was found to be the dominant oxidant with relative contributions above 80%. The strong contribution of BrO could explain why the efficiency of GEM oxidation at the Dead Sea does not critically depend on Br and, therefore, is comparable to that in cold polar regions. In order to confirm the suggested kinetics, additional studies, particularly for temperature-dependence of rate constants, are required.

As part of an ongoing USAID MERC research program investigating the transport of pollutants in the Eastern Mediterranean region, a joint coordinated campaign involving scientists from Jordan and Israel took place for a three-week period in June 2009. The study was aimed at examining previous modeling results that indicated the presence of elevated ozone levels occurring in Northern Jordan, due to emissions originating in coastal Northern Israel that were transported distances of more than 100 km. In the present investigation, ozone and other pollutants were monitored at five sites in Israel and two in Jordan. The sites were located along the prevailing wind direction that tends to move air-masses eastward from the Mediterranean coast, over the Israel Valley toward the Jordan Valley, and then into Northern Jordan. Analysis of the measurements showed that air masses arriving in Jordan passed over the Israeli metropolitan coastal region, entrapping large amounts of pollutant precursors before transporting them towards Jordan under the predominately westerly wind flows causing peak maximum ozone levels of around 100 ppbv. The present findings confirmed the previous modeling results that indicated higher NO x background levels for the inland stations, peak O 3 values at later hours depending on wind speed and distance from the Mediterranean coast and maximum O 3 levels over northern Jordan.

In the polar atmosphere, non-reactive gaseous elemental mercury is converted to a highly reactive form of mercury by halogens such as bromine. Measurements over the Dead Sea suggest that bromine also triggers reactive mercury formation over the mid-latitude ocean.

The present paper characterizes the background concentration of SO 2 , NO, NO 2 , NO x ∗ , O 3 , nitrate and sulfate aerosols throughout a three season campaign at a remote isolated site in southern Israel. Upper and lower thresholds were established for background levels based on two sets of criteria. The average background levels were 0.8, 2 , NO, NO x , O 3 , sulfate and nitrate respectively (ppbv for gaseous species and nm m −3 for the aerosols). Background concentrations of the primary pollutants were lower by a factor of 10–20 as compared to other sites in Israel, clearly showing that the remote background site was not influenced by local anthropogenic pollution. The nitrate and sulfate levels measured at the remote site were similar to those observed at other sites throughout Israel indicating that external sources are the major contributors to these pollution levels in Israel. The limited difference between the aerosol levels measured at a remote site as compared to polluted sites demonstrates that there is only a minor local contribution to nitrate and sulfate levels in Israel. The vast majority of the air masses that were measured showed high ratios of photochemical N-age and S-age, reflecting transport times of 2–3 days. Taken together with air trajectories, the transport time indicates that the majority of the air pollution reaching Israel is a result of long-range transport (LRT) originated in eastern and southern Europe. The present background levels provide an important database in attempts to address important policy questions regarding local air quality and the determination of the contribution of LRT to local air pollution levels. Further, regional meteorology appears to be a key influence on the concentrations in this and other background sites.

The Regional Atmospheric Modeling System (RAMS), Comprehensive Air Quality Model with Extensions (CAMx), and a new emissions inventory were used to study O3 formation downwind from the Israeli coast during summer 1997. While RAMS reproduced the observed diurnal variation of surface wind speeds and temperatures reasonably well, directions were somewhat less correlated. CAMx generally also reproduced the timing, location, and magnitude of measured surface and 300-m O3 max over Jerusalem, but also showed an even larger concurrent max over western Jordan, an area without observations. Observed near-zero nighttime surface O3 values, however, were over predicted because the CAMx grid was too large to accurately account for local emissions and titration effects. CAMx also generally accurately predicted another observed 300 m local max near the Dead Sea, but could not account for a unique O3 depletion mechanism from bromine monoxide released from its water surface. Factor-separation analysis showed that transportation emissions produced 66% and 29% of the Jerusalem and Jordan 300-m simulated maxima, respectively, while power plant emissions produced about 35% and 62%, respectively.

International audience; The Dead Sea is an excellent natural laboratory for the investigation of Reactive Bromine Species (RBS) chemistry, due to the high RBS levels observed in this area, combined with anthropogenic air pollutants up to several ppb. The present study investigated the basic chemical mechanism of RBS at the Dead Sea using a numerical one-dimensional chemical model. Simulations were based on data obtained from comprehensive measurements performed at sites along the Dead Sea. The simulations showed that the high BrO levels measured frequently at the Dead Sea could only partially be attributed to the highly concentrated Br? present in the Dead Sea water. Furthermore, the RBS activity at the Dead Sea cannot solely be explained by a pure gas phase mechanism. This paper presents a chemical mechanism which can account for the observed chemical activity at the Dead Sea, with the addition of only two heterogeneous processes: the "Bromine Explosion" mechanism and the heterogeneous decomposition of BrONO2. Ozone frequently dropped below a threshold value of ~1 to 2 ppbv at the Dead Sea evaporation ponds, and in such cases, O3 became a limiting factor for the production of BrOx (BrO+Br). The entrainment of O3 fluxes into the evaporation ponds was found to be essential for the continuation of RBS activity, and to be the main reason for the jagged diurnal pattern of BrO observed in the Dead Sea area, and for the positive correlation observed between BrO and O3 at low O3 concentrations. The present study has shown that the heterogeneous decomposition of BrONO2 has a great potential to affect the RBS activity in areas influenced by anthropogenic emissions, mainly due to the positive correlation between the rate of this process and the levels of NO2. Further investigation of the influence of the decomposition of BrONO2 may be especially important in understanding the RBS activity at mid-latitudes.

High ozone levels are regularly measured during summer months over the inland and mountainous regions of Israel. Studies analyzing the back trajectories of air masses responsible for the high ozone levels showed that the precursors originated from the densely populated Israeli coastline. In order to better understand the contribution of those emission sources to ozone production, it is essential to have an accurate emission inventory that can be inputted into a photochemical model. The present paper describes the methods used in preparing an emission inventory for Israel based on information available and published until 1998. The source and accuracy of the data available are described. The calculations performed and the assumptions taken in order to obtain data not directly available are clarified. The sources reported in the inventory were the major polluters (power plants, oil refineries, and cement industries); industry; transportation; and biogenic sources. The pollutants studied were SO 2 , NO,, CO, saturated and unsaturated hydrocarbons, ethylene, isoprene, toluene, xylene, formaldehyde, and aldehydes. The inventory showed that transportation is responsible for almost the entire CO and 30% of the volatile organic compounds emitted, although transportation itself accounts for only a fifth of total fuel consumption. About 75% of the NO x emitted can be attributed to industrial sources and the remaining 25% to transportation. Model simulations using the emission inventory were performed and compared to data available from a monitoring station situated 30 km east of Tel Aviv. The results showed good agreement, validating the accuracy of the emission inventory. The present emission inventory provides an important database as input to photochemical models used in forecasting ozone levels over Israel.

A previous study performed at the Dead Sea during the summer (J. Geophys. Res. 106 (2001) 10375) has shown frequent ozone depletion episodes, when the mixing ratio of ozone was reduced to less than 5 ppb. These episodes were accompanied by the presence of high BrO levels (up to a level above 150 ppt). In order to establish the temporal and spatial extent of ozone depletion and subsequently indirect evidence of BrO formation, continuous ozone measurements were performed at three sites along the Dead Sea for a 14-month period starting from February 2001. The results indicated that extensive ozone depletion effects occur along the entire region of the Dead Sea basin during all periods of the year. The depletion effects were more pronounced in the southern areas (mid and south sites) than the north site. Since ozone depletion occurs with higher frequency in the south closer to the evaporation ponds and saltpans, it appears to be related to a higher sea-salt aerosol concentration in that region. Ozone depletion effects were also observed during a short-term study performed at a site situated approximately 1 km west and 400 m above the Dead Sea. Further, the ozone depletion effects appear to be more pronounced during summer than winter times. Additionally, these effects are prevalent during all periods of the year. Comparison of the Dead Sea data with ozone levels measured simultaneously at Beer Sheva, a site some 60 km west and outside of the Dead Sea Valley, demonstrates the extent of the ozone depletion. Further, while the Beer Sheva site showed the typical diurnal ozone cycle with ozone peaking at midday, the Dead Sea sites show a reverse situation and had decreased midday levels during the summer period. The present study indicates that ozone depletion events, and hence BrO production, occur all along the entire Dead Sea Valley and extend to a significant distance vertically and horizontally away from the water level.

Atmospheric measurements were performed during a 1 month period in early summer of 1997 at the Dead Sea in Israel in an attempt to identify bromine monoxide BrO, and evaluate its effect on ozone chemistry. The differential optical absorption spectroscopy (DOAS) technique was utilized to identify and measure BrO present in the air masses. Concurrent to the DOAS measurements, continuous monitoring of SO2, NO/NOx, O3, and CO was performed. Filter samples for aerosol analysis and whole air canister samples for bromocarbon analysis were also collected. The present paper reports the complete comprehensive data set of the measurements at the Dead Sea site and is a continuation to our preliminary communication [Heberstreit et al., 1999]. The more complete data now available enable a more detailed examination of the sources and mechanisms of the reactive halogen species and the presentation of new conclusions. The results showed a diurnal repeating cycle of O3 and BrO variations, correlated with solar radiation and wind direction. During the elevated BrO events, where bromine oxide rose to daily maximum values often exceeding 100 ppt, a clear negative correlation with O3 was observed. During these episodes, the O3 regularly decreased from noontime levels of 50–80 ppb or higher down to 10–30 ppb and occasionally to levels below the detection limit of 2 ppb. The enhanced BrO levels were associated with southerly winds that are typical for the location during midday hours. This suggests that a possible source for the reactive bromine species is the interaction of atmospheric oxidants with bromide at the surface of the large salt pans located at the southern end of the Dead Sea. Research flights flown over the area showed that ozone destruction to levels well below the background values were observed over large areas of the Dead Sea Valley.

Samples of Jerusalem limestones were exposed to high levels of common air pollutants (SO2, NO) in the presence and absence of hydrocarbon, water vapor and ultra-violet light. After exposure, the outer layers were shaved off and analyzed for sulfate and nitrate. The results revealed that even after one day of exposure significant concentrations of CaSO4 and Ca(NO3)2 could be detected in the external 40 μm layers. Sulfate formation was found to relate very strongly to relative humidity with nearly undetectable production at humidities below 10%. Nitrate formation was found to relate to UV light and to a limited extent to the presence of hydrocarbon but was unchanged at different humidities. Surface samples were taken from different sites of the old city wall and were also analyzed for the same substances. The data showed that the concentrations of especially CaSO4, and to a limited extend Ca(NO3)2, measured at various points along the city walls were higher than the expected values extrapolated from laboratory simulations. The elevated pollution content may be explained in part due to the deposition of transported ions, especially sulfate, onto the stone surface. It was concluded that although air pollution may not cause any structural damage in the foreseeable future it definitely deteriorates the fine details of the ancient monument.

The present paper reports results of a study that attempted to elucidate the factors causing relatively high levels of particulate sulfate that have frequently been observed over central Israel. Aircraft research flights were performed some 70 km west of and parallel to the Israeli coastline during September 1993 and June 1994. Comparison between the two measurement periods revealed a distinctive difference between the two different sampled air masses. While both air masses were nearly homogeneous throughout the measurement period and along the 180 km flight path, the air mass sampled in September 1993 was much cleaner than the air mass sampled during June 1994. The concentrations of the air pollutants measured during the 1993 campaign averaged 0.7 ± 0.4 parts per billion by volume (ppbv) SO 2 , 1.0 ± 0.6 ppbv NO y , 39 ± 7 ppbv O 3 and 38 ± 7 nmol/m 3 particulate sulfate, whereas in the second period the levels averaged 3.0 ± 1.0, 3.9 ± 1.8, 48 ± 9, and 108 ± 63, respectively. These results suggest that the two air masses traveled different paths before reaching the eastern Mediterranean region. Further examination of the air mass sources and transport were performed using the Regional Atmospheric Modeling System for meteorological simulations and the Hybrid Particle and Concentration Transport Package for dispersion modeling. The model simulation showed that during the 1993 measurement period, the pollution sources in southern Europe and the Balkans did not effect the eastern coasts of the Mediterranean, while the synoptic conditions and simulation results for the June 1994 period indicated that the winds over the eastern Mediterranean tended to be northwesterly and thus forcing the polluted air masses toward the coast of Israel.

Research flights have been performed over the Greater Athens Area (GAA) and southwards over the Island of Aegina and east of Peloponnisos in order to investigate the evolution of an ozone episode over GAA and the transportation of the urban pollution plume southwards from the Athens region. During the 3 day period of 6 July to 8 July 1994, the GAA was under the influence of an ozone episode with ground-level noontime concentrations of more than 120 ppbv. Upper-air ozone concentrations measured during the flights were as high as 135 ppbv. The interaction of the weak synoptic conditions over the area along with the development of a mesoscale thermal circulation created poor dispersion conditions during the period of interest and resulted in elevated ozone levels. The primary pollutants emitted in the GAA during the night and early morning hours, were funneled out to the Saronic Gulf and southwards along the southwestern Aegean Sea, near the coast of east Peloponnisos. Under the influence of strong sunlight these primary pollutants continued to undergo photochemical reaction giving rise to elevated ozone levels tens of kilometers downwind of the pollution emission sources. Further evidence of the photochemically-aged air masses was the high correlation (R2 = 0.8) observed between NOy, and ozone. The ozone production efficiency in these transported air masses reached a value of close to six.

An air quality observatory was operated at a rural site on the Mediterranean coast of Israel near the ancient city of Caesarea between May 1993 and October 1995. The objective of the study was to monitor transport of air pollutants from remote sources that arrive at the Israeli coast. Normally, under onshore westerly winds, which come from the Mediterranean sea, the levels of NO, NOy and SO2 at the site dropped to below 0.5 ppbv, CO to below 150 ppbv, and the O3 levels ranged between 30 and 60 ppbv. During the last week of October 1993, an unusual series of pollution episodes occurred, with elevated values for all the pollutants being recorded during onshore flows. The SO2 concentration reached 30 ppbv, NO and NOy more than 100 ppbv and O3 levels rose above 200 ppbv. On 26 October the O3 levels were the highest observed with an abnormally high concentration of 230 ppbv being recorded. The episode that occurred on that day was investigated in detail in order to understand the cause for this high ozone value. During the above day, night and early morning, easterly winds swept the air parcels containing the locally emitted pollution westwards over the sea. The intense photochemical activity that occurred while the air mass was over the sea for a relatively extensive time combined with the low mixing height on that day and the fact that the returning air mass reached the measuring site at peak ozone formation time, give rise to record ozone levels. The elevated concentrations were measured when the polluted air parcels returned to the coast during the afternoon onshore flows. An excellent linear correlation between O3 and NOy was obtained for the time period between 1400 and 1700. The linear fit between O3 and NOy indicates that approximately 11 O3 molecules were formed for each NOy molecule present. Similar ratios have been reported in other studies dealing with aged air masses under NOy limited conditions.

Particulate sulfate has been measured intermittently at various ground sites in Israel and from an instrumented aircraft for a 10-year period between 1984 and 1993. The measurements were accompanied by concurrent monitoring of primary and secondary air pollutants and meteorological parameters. In three studies, Etzion 1984–1986, Jerusalem 1987–1988, and Jerusalem 1990–1991, measurements were taken for at least a year. The other studies were performed during summer months when higher levels of secondary pollutants, including particulate sulfate, were observed. In most of the studies samples were taken for four sequential 6-hour time segments. In one study, at Caesarea 1993, sulfate samples were taken only when wind flows were perpendicular to the coastline. The airborne measurements were performed along three north-south paths inside the planetary boundary layer, over the Mediterranean coast, over the Judea-Samaria mountains and over the Jordan Valley. Each flight path consisted of 30 to 45 minutes of continuous sampling of SO2 and one integrated sample of particulate sulfate. In all of our studies the concentration of particulate sulfate observed was relatively high compared with other world locations. The highest values, occasionally exceeding 500 nmole m−3, were found during the summer. Wintertime levels were in the range of 50–100 nmole m−3. The annual average, calculated for the three long studies, is 100 ± 15 nmole m−3, which is twice as high as predicted for the region by a global model and as high as reported for some of the more polluted regions in the US. Several indicators suggested that the origin of the sulfate in the region is not from local sources but the result of long range transport. The indicators include the lack of correlation between particulate sulfate and primary pollutants, the high sulfate to total sulfur values, the origin of the airmass back trajectories and the fact that similar levels were observed during concurrent periods at different sites. Throughout the study, higher concentration of particulate sulfate was found during the afternoon hours, especially during the summer and at the inland locations. The contribution to the afternoon elevated values could not be associated with long range transport and results probably from major sulfur emission sources located along the Israeli Mediterranean coast.

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.

An instrumented aircraft was used to locate the centerline of plumes emitted from two large power plants situated along the Israeli Mediterranean coast line. The results of the measurements that were carried out at several downwind distances and at various altitudes were compared with model calculations. The models included the classical Briggs formulae, the Turner method and Petersen-Dowd modifications. The latter two take into account the effect of plume penetration and reflection of inversion layers and surface roughness. The comparison showed moderate correlation between the model and the observations for the lower stacks (80 m), and even weaker correlation for the taller stacks (250 m). In general, all three models underestimate plume rise for stable atmospheric conditions. For unstable and neutral conditions, the Briggs model overestimates the plume rise, while the other two modified models underestimate plume rise.

This study is the first to present long-term measurements of the nitrate radical in an urban location. Extensive nitrate radical measurements were conducted together with ancillary parameters during a continuous two year campaign (2005-2007) in the semiarid location of Jerusalem. The average nighttime NO3 concentration was 27.3+/-43.5 ppt, the highest ever reported, with a seasonal average peak during summer (33.3+/-55.8 pptv) with maximum levels exceeding 800 pptv. Significant diurnal changes in NO3 concentrations were observed, caused by an unusual nighttime increase in ozone concentrations. The NO3 loss processes exhibited strong seasonal variability. Homogeneous gas-phase losses were the main removal processes during summer and spring. The heterogeneous losses of N2O5, averaged over the entire campaign, contributed to less than half of the direct losses even though they dominated the winter seasons and part of the autumn months. Statistical regression analysis showed that NO3 was inversely correlated with relative humidity and positively correlated with temperature and to a lesser extent with NO2 and O3, indicating that the heterogeneous removal processes were also important. The diurnal behavior of NO3 was examined using a one-dimensional chemical transport model. The simulations showed that NO3 trends and concentrations were influenced mainly by changes in ozone and nitrogen oxide levels and that the very high levels of NO3 can be explained by the entrainment of fresh ozone from the upper atmospheric levels. After sunset and in the early morning, the homogeneous processes are the major loss pathways, while the heterogeneous N2O5 removal pathway dominates the intermediate times.

Nitrate radical (NO(3)), an important nighttime tropospheric oxidant, was measured continuously for two years (July 2005 to September 2007) in Jerusalem, a semiarid urban site, by long-path differential optical absorption spectroscopy (LP-DOAS). From this period, 21 days with the highest concentrations of nitrate radical (above 220 pptv) were selected for analysis. Joint measurements with the University of Heidelberg's LP-DOAS showed good agreement (r = 0.94). For all daytime measurements, NO(3) remained below the detection limit (8.5 pptv). The highest value recorded was more than 800 pptv (July 27, 2007), twice the maximum level reported previously. For this subset of measurements, mean maximum values for the extreme events were 345 pptv (SD = 135 pptv). Concentrations rose above detection limits at sunset, peaked between midnight and early morning, and returned to zero at sunrise. These elevated concentrations of NO(3) were a consequence of several factors, including an increase in ozone concentrations parallel to a substantial decrease in relative humidity during the night; Mean nighttime NO(2) levels above 10 ppbv, which prevented a deficiency in NO(3) precursors; Negligible NO levels during the night; and a substantial decrease in the loss processes, which led to a lower degradation frequency and allowed NO(3) lifetimes to build up to a maximum mean of 25 min. The results indicate that the major sink pathway for NO(3) was direct homogeneous gas phase reactions with VOC, and a smaller indirect pathway via hydrolysis of N(2)O(5). The Jerusalem measurements were used to estimate the oxidation potential of extreme NO(3) levels at an urban location. The 24 h average potential of NO(3), OH, and O(3) to oxidize hydrocarbons was evaluated for 30 separate VOCs. NO(3) was found to be responsible for approximately 70% of the oxidation of total VOCs and nearly 75% of the olefinic VOCs; which was more than twice the VOC oxidation potential of the OH radical. These results establish the NO(3) radical as an important atmospheric oxidant in Jerusalem.

Episodes of elevated bromine oxide (BrO) concentration are known to occur at high latitudes in the Arctic boundary layer and to lead to catalytic destruction of ozone at those latitudes; these events have not been observed at lower latitudes. With the use of differential optical absorption spectroscopy (DOAS), locally high BrO concentrations were observed at mid-latitudes at the Dead Sea, Israel, during spring 1997. Mixing ratios peaked daily at around 80 parts per trillion around noon and were correlated with low boundary-layer ozone mixing ratios.

CO and NOx monitors in conjunction with a data acquisition system were installed inside a small van, together with an independent battery operated power supply, to study air pollution levels in the zones where Jerusalem commuters are exposed. The results revealed that along most sections of the test road, CO and NOx concentrations exceed the short term lower Israeli national air quality standards of 30 and 0.5 ppm, respectively. In addition, there were also a large number of violations of the higher standard for NOx (1.0 ppm). Traffic volume correlated reasonably well with pollution levels; however, the geometry of the road was found to be equally important. The highest pollution levels were recorded in the narrower segment of the test road near the market place, despite the fact that the traffic counts along these road segments were relatively low. This phenomenon is related to the trapping caused by the canyon effect resulting from the buildings bordering the road. No significant relationship with the general meteorological conditions was found, however, rain washout of NOx was observed during one sampling trip, while CO concentrations ramained almost unaffected.

The occurrence of mercury depletion events (MDE) in the Polar Regions during the spring periods has raised global concern due to the biomagnifications of the deposited mercury into the aquatic food chain. However, it now appears that MDE is not limited to the Polar Regions and can also occur at mid-latitudes. Diurnal cycles of mercury, ozone, and BrO behavior based on short-time resolution measurements are presented for the Dead Sea, Israel, for Summer 2006. The results show that mercury depletion events occur almost daily, accompanied always by the presence of BrO and concurrent ozone destruction. The intensity of the MDE corresponded to increasing BrO levels. Mercury depletions of more than 40% were observed when BrO levels rose above 60-70 ppt. Based on the present measurements and supported bytheoretical considerations, it appears that BrOx (BrO + Br) is the primary species responsible for the mercury depletion at the Dead Sea. The present study also suggests, especially at low ozone levels, that the Br atom may play a major role in conversion of the gaseous elemental mercury to the reactive species, HgBr2. The implications of the present study are that even at low BrO levels (10 ppt), mercury depletion may well occur at other mid-latitude sites and thus needs to be taken into consideration in the global mercury cycle.

[1] A study has been performed in an attempt to better understand and characterize BrO formation in the Dead Sea region. The DOAS technique was employed to quantify BrO levels at three sites along the Dead Sea representing three different Br− and pH levels of the water. In order to characterize the spatial extent of BrO formation, measurements were performed at a site south of the Dead Sea itself and also at a site 400 m above the water level. Additionally, measurements were concurrently performed for ozone, nitrogen oxides and meteorological parameters at each of the campaign sites. The results showed that BrO was produced everywhere in the Dead Sea region up to maximum levels of around 200 ppt. The frequency and intensity of BrO production was more intense in the southern part of the Dead Sea corresponding to the increased acidity and bromide content of the water and increased solid salt depositions. Maximum BrO production during the winter was similar to that of BrO produced during summer. BrO was also observed at 4 km inland from the Dead Sea shores and at heights 400 m above the water. However, these levels were less than half the concentration measured at the water level. While it would appear that the Br− content and pH of the Dead Sea water are the two major factors responsible for the elevated BrO levels observed at the Dead Sea, the presence of NO2, up to levels of several parts per billion, might also play an important role in the BrO production. Additional factors influencing BrO production could be the extensive salt deposition at the Dead Sea as well as the high sulfate aerosol levels present in the region.

Both field measurements and model simulation studies have shown that Israel is the recipient of long range transported air pollutants that originated over various parts of Europe. The present paper presents results of aircraft measurements aimed at quantitizing the sulfur flux arriving at Israel's western coast from Europe and the Israeli pollution contribution to the air masses leaving its eastern borders towards Jordan. During the research flights, measurements of sulfur dioxide and sulfate particulates and meteorological data were recorded. Two different legs were performed for each research flight: one over the Mediterranean Sea, west of the coast and the second along the Jordan Valley. All flights were carried out at a height of approximately 300 m above ground level. A total of 14 research flights were performed covering the summer and autumn seasons. The results indicate that the influx of sulfur arriving at the Israeli coast from Europe varied in the range of 1-30 mg S/h, depending on the measuring season. The particulate sulfate level in the incoming LRT air masses was at least 50% of the total sulfur content. The contribution of the local pollutant sources to the outgoing easterly fluxes also varies strongly according to season. During the early and late summer, the Israeli sources contributed an average of 25 mg S/h to the total pollution flux as compared to only approximately 9 mg S/h during the autumn period. Synoptic analysis indicates that conditions during the summer in Israel favor the accumulation of pollution species above the Mediterranean basin from upwind European sources. This season features a shallow mixed layer and weak zonal flow leads to poor ventilation rates, inhibiting an efficient dispersion of these pollutants while being transported eastward. Under these conditions, in flux, local contribution and the total out-flux of these pollutants are elevated as opposed to during other seasons. During the fall, the eastern Mediterranean region is usually subjected to weak easterly winds, interrupted at times by strong westerly wind flows inducing higher ventilation rates. These meteorological conditions and the lack of major emitting sources eastwards of Israel result in lower sulfur budgets to and from Israel for this season. An estimate of the yearly flux showed that approximately 0.06 tg S arrived at the Israeli coast from the west. This is approximately 15% of the estimated pollution leaving Europe towards the eastern edge of the Mediterranean basin. The local contribution to the out-flux towards Jordan was calculated to be 0.13 tg S per year, almost all of the sulfur air pollutants emitted in Israel.

In several studies during the past, the urban plumes have been extensively considered. In these studies, the spatial and temporal scales of episodic conditions have been described and emphasis was given to the formation and evolution of air pollution episodes within city limits (or in an area covering a few tens of kilometers around the city) and for a time period of one to two days. Moreover, the weather phenomena exhibiting strong diurnal variations (e. g. sea/land-breezes, upslope/downslope and drainage flows, orographic effects, heat islands etc.) were emphasized. The influence of the regional scale phenomena in such cases was not considered on a systematic manner. Actually, the role of phenomena with wavelengths larger than a few tens of kilometers was considered as not important for the formation of a specific air quality over the city of consideration. During the last few years, the influence of regional scale forcing on the formation of specific air quality conditions was found to be important. Kallos et al. (1993) reported that the regional scale phenomena should contribute significantly in the formation of specific air quality conditions in the Greater Athens Area (GAA). Luria et al. (1996) showed that significant degradation of the air quality in some areas should be attributed to regional scale transport phenomena. While the physicochemical properties of various urban plumes have been described at the urban scale with the aid of organized experimental campaigns and/or mesoscale and photochemical modeling (e. g. Ziomas, 1996), not enough attention was paid to the properties of the urban plume as it is passing to areas relatively far from its origin. Consequently, the urban plume impact on remote locations has not been extensively studied. Such phenomena should be considered as very important in some cases, especially in areas with specific characteristics like the Mediterranean Region.

[1] The detection of high ozone levels over large inland areas in Israel during the early, mid and late summer triggered an analysis of air mass back-trajectories. This, in turn, pointed to the transportation system in the metropolitan coastal Tel Aviv region as the possible origin of the ozone’s precursors. To link the daily dynamics of rush hour transportation emissions to inland air pollution, in general, and airborne ozone measurements, in particular, an interdisciplinary modeling system was established. The simulations of transportation-to-inland air pollution integrated transportation, emission factor, atmospheric, transport/diffusion, and photochemical models. The modeling results elucidated a spatial and temporal overlap between the ozone precursors and ozone production. The model simulations indicated east to southeasterly dispersion of the pollution cloud. The results agreed well with both spatial and temporal ozone levels as recorded by aircraft over central Israel, as well as with ground-based monitoring station observations. The impact of the Tel Aviv metropolitan area as well as the Gaza Strip, as pivotal coastal transportation sources for inland air pollution in general and ozone formation in particular, is discussed. The synoptic analysis identified the conditions prevailing when elevated air pollution, and especially high ozone levels, exists over central Israel. The analysis showed that this season features a shallow mixed layer and weak zonal flow, which leads to poor ventilation rates and inhibit efficient dispersion of this secondary pollutant. These poor ventilation rates result in the slow transport of ozone precursors, enabling their photochemical transformation under intense solar radiation during their travel from the coast inland. Under these conditions, model results showed that traffic emissions during the morning rush hour from the Tel Aviv metropolitan area contribute about 60% to the observed ozone concentrations. INDEX TERMS: 0345 Atmospheric Composition and Structure: Pollution—urban and regional (0305); KEYWORDS: air pollution, numerical atmospheric modeling, transportation model, emission factors, photochemical model, ozone, photochemical aged air mass

Ambient particulate sulfate measurements have been intermittently performed at a rural site in Israel over a period of more than two years. Concurrent measurements of ambient pollutants (SO2, NO−NOx, and O3), as well as meteorological data, were also carried out. The daily data included four particulate sulfate samples representing four successive 6 h accumulating periods. The measured concentrations of sulfate ions ranged from a low 2 μg m-3 observed during the winter season to a high of >50 μg m-3 obtained during the summer. Little correlation was obtained between the sulfate concentration and either O3 or SO2, although sulfate and O3 showed a similar diurnal and annual trend. Based on the data distribution and on a photochemical model, it was concluded that a large part of the particulate sulfate observed at the eastern coast of the Mediterranean Sea must be related to long-range transport from distant sources.

Continuous measurements of particle mass concentration, particle size distribution, and other environmental measurements were conducted at a monitoring station in Jerusalem, Israel, during the 1978/79 winter season in order to characterize the processes which occur in suspended dust particles in the metropolitan area. Most of the man-made aerosol mass is initially emitted to the air in the form of small particles (submicron), but, as a result of atmospheric processes, larger particles are evolved. The rate constant which is characterized in this coagulation process (2.7 x 10 /sup -8/ cm/sup 3//S) was estimated from the field measurements. This rate constant indicated that urban particles coagulate on almost every collision and that the speed of the process is the main mechanism responsible for the sharp decrease in particle number density shortly after midnight. The influence of climatic factors (duststorms and precipitation) on suspended dust particles in Jerusalem air is also discussed.

A survey of the contribution of city buses to the total emission of gaseous and particulate pollutants was conducted in the city of Jerusalem, Israel. The results reveal that buses are one of the main sources for nitrogen oxides, and, to a lesser extent, particulate matter. Contribution of buses to carbon monoxide and hydrocarbon levels was insignificant as compared to gasoline powered private cars. Possible vehicular restrictions in downtown districts are examined.

Air quality data obtained at the Jerusalem municipal air monitoring station, during the years 1979–1983, was analyzed to determine seasonal and long-term trends in air quality. The results of this study provide further support to previous findings indicating that ambient air quality levels in Jerusalem are influenced not only by local sources but also by transport of air pollution from Israel's coastal area. The annual and monthly averages of most pollutants indicated that in 1981 record high levels of pollutants were measured. Since total suspended paniculate (TSP) level also peaked in 1981, and since TSP is mostly associated with natural sources, it was concluded that air pollutant levels in the city are influenced more by multiannual change in dispersion conditions than by the combination of all local anthropogenic sources.

A comparison of air quality data obtained at a downtown roof top location, a surburban area, a main road curbside and adjacent to a major highway was conducted. The results of this study revealed that a reasonably good correlation between daily average concentrations at these locations can be established. For primary pollutants it was found that within the immediate release zone their average concentration is 3.5 to 4.0 times higher than at a close location but at roof top level. Total suspended particulate (TSP) concentration was found to increase by about 80%, and O3 is reduced by about 25%. No significant difference was obtained between daily averages obtained at the city center location and the suburb site 5 km away.