Long-Term Measurements of NO3 Radical at a Semiarid Urban Site: 1. Extreme Concentration Events and Their Oxidation Capacity

Nitrate radical (NO3), 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, NO3 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 NO3 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 NO2 levels above 10 ppbv, which prevented a deficiency in NO3 precursors; Negligible NO levels during the night; and a substantial decrease in the loss processes, which led to a lower degradation frequency and allowed NO3 lifetimes to build up to a maximum mean of 25 min. The results indicate that the major sink pathway for NO3 was direct homogeneous gas phase reactions with VOC, and a smaller indirect pathway via hydrolysis of N2O5. The Jerusalem measurements were used to estimate the oxidation potential of extreme NO3 levels at an urban location. The 24 h average potential of NO3, OH, and O-3 to oxidize hydrocarbons was evaluated for 30 separate VOCs. NO3 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 NO3 radical as an important atmospheric oxidant in Jerusalem.