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

Abstract

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.

Observationally constrained modelling of atmospheric oxidation capacity and photochemical reactivity in Shanghai, China

“…by the photolysis of CINO2 may also contribute to AOC (Bannan et al, 2015) , but unfortunately it has not been quantitatively characterized in this study. In general, OH dominated AOC during daytime and NO3 is the main oxidant at night, which is 240 consistent with previous studies (Asaf et al, 2009;Elshorbany et al, 2009).…”

Global modeling of the nitrate radical (NO3) for present and pre-industrial scenarios

“…The comparison of globally modeled NO 3 with in situ measurements is difficult because of the short lifetime of NO 3 and the location of the measurement sites. Most short-term NO 3 measurements were performed over highly polluted cities, and night-time values of more than 800 pptv have been reported by Asaf et al (2009). It is not reasonable to expect a background CTM to capture urban levels of NO 3 because the observed NO 3 values may not represent the average state of the region due to the unusual meteorology and the coarse emission grids in STOCHEM make it difficult to evaluate model NO 3 levels in the sites near strong source regions.…”

Review response

“…A large number of NO 3 measurements have been made using the active long-path DOAS technique (Platt et al, 1980(Platt et al, , 1981(Platt et al, , 1984Pitts et al, 1984;Heintz et al, 1996;Allan et al, 2000;Martinez et al, 2000;Geyer et al, 2001aGeyer et al, , b, 2003Gölz et al, 2001;Stutz et al, 2002Stutz et al, , 2004Stutz et al, , 2010Asaf et al, 2009;McLaren et al, 2010;Crowley et al, 2011;Sobanski et al, 2016). Here, a searchlight-type light source is used to transmit a beam of light across a kilometer-long light path in the open atmosphere to a receiving telescope-spectrometer combination.…”