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Abstract. 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 03 and BrO variations, correlated with solar radiation and wind direction. During the elevated BrO events, where bromine oxide rose to daily maximum values oRen exceeding 100 ppt, a clear negative correlation with 03 was observed. During these episodes, the 03 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.

Abstract. 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 BrONO 2 . Ozone frequently dropped below a threshold value of ∼1 to 2 ppbv at the Dead Sea evaporation ponds, and in such cases, O 3 became a limiting factor for the production of BrO x (BrO+Br). The entrainment of O 3 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 O 3 at low O 3 concentrations. The present study has shown that the heterogeneous decomposition of BrONO 2 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 NO 2 . Further investigation of the influence of the decomposition of BrONO 2 may be especially important in understanding the RBS activity at mid-latitudes.

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.