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.

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.

Few studies have characterized the regional scale (300-500 km) variability of the mutagenicity of respirable airborne particles (PM2.5). We previously collected 24-h PM2.5 samples for 1 year from background, suburban, and urban sites in Massachusetts (MA) and rural and urban sites in upstate New York (NY) (n = 53-60 samples per site). Bimonthly composites of these samples were mutagenic to human cells. The present report describes our effort to identify chemical classes responsible for the mutagenicity of the samples, to quantify spatial differences in mutagenicity, and to compare the mutagenicity of samples composited in different ways. Organic extracts and HPLC fractions (two nonpolar, one semipolar, and one polar) of annual composites were tested for mutagenicity in the h1A1v2 cells, a line of human B-lymphoblastoid cells that express cytochrome P450 CYP1A1 cDNA. The mutagenic potency (induced mutant fraction per microg organic carbon) of the semipolarfractions was the highest at all five sites, accounting for 35-82% of total mutagenic potency of the samples, vs the nonpolar (4-38%) and polar (14-32%) fractions. These results are consistent with previous studies. While unfractionated extracts exhibited no spatial variations, the mutagenicity of semipolar fractions at the NY sites was approximately 2-fold higher than at the MA sites. This suggests there may be significant regional differences in the sources and/ or transport and transformation of mutagenic compounds in PM2.5. In addition, mutagenic potency was sensitive to whether samples were fractionated and how they were composited: unfractionated annual composite samples at the NY sites were significantly less mutagenic than their semipolar fractions and the annual average of bimonthly composites; spatial differences in the mutagenic potency of bimonthly composites and the semipolar fractions were not apparent in the annual composites.