It has been well recognized that reactive nitrogen compounds, specifically the nitrate radical (NO3) and dinitrogen pentoxide (N2O5), play a key role in nighttime chemistry (Wayne et al., 1991; Brown and Stutz, 2012). NO3 is the most important oxidant in the nighttime and can be considered the nighttime analogue of the hydroxyl radical (OH) for certain volatile organic compounds (VOCs; Wayne et al., 1991; Benton et al., 2010). NO3 can initiate the removal of many kinds of anthropogenic and biogenic emissions after sunset. In NOx-rich plumes, NO3 is responsible for the vast majority of the oxidation of biogenic VOCs because of its rapid reactions with unsaturated hydrocarbons (Edwards et al., 2017). NO3 is predominantly formed by the reaction of NO2 with O3 (Reaction R1) and further reacts with NO2 to produce N2O5 (Reaction R2). N2O5 is rapidly decomposed back to NO3 (Reaction R3), NO3, and N2O5 are in dynamic equilibrium in the troposphere.
Photolysis of NO3 and the reaction of NO3 with NO are rapid, which leads to a daytime NO3 lifetime being shorter than 5 s with extremely low concentrations, whereas in low-NO air masses, the fate of NO3 is mainly controlled by the mixing ratios of various VOCs and N2O5 heterogeneous hydrolysis because the two terms are the dominating loss pathways of NO3 and N2O5. The VOC reaction is significant downwind of an urban area or a strongly urban-influenced forested area in summer. The NO3 oxidation of VOCs was responsible for more than 70 % of nocturnal NO3 loss in Houston (Stutz et al., 2010) and contributed approximately 50 % in a forest region in Germany (Geyer et al., 2001). The reactions of NO3 with several biogenic VOCs (BVOCs) produce considerable amounts of organic nitrates (ONs) with efficient yields, which act as important precursors of secondary organic aerosols (SOAs). The reaction of NO3 with isoprene has a SOA mass yield of 23.8 % (Ng et al., 2008). For the reaction with monoterpene, such as limonene, the SOA mass yield can reach 174 % at ambient temperatures (Boyd et al., 2017). The reactions of NO3 + BVOCs are critical to the studies of aerosols on regional and global scales (Fry et al., 2009; Rollins et al., 2009; Pye et al., 2010; Ng et al., 2017). For example, ON had extensive percentages of fine particulate nitrate (pNO3-) (34-44 %) in Europe (Kiendler-Scharr et al., 2016).
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The heterogeneous hydrolysis of N2O5 produces soluble nitrate (HNO3 or NO3-) and nitryl chloride (ClNO2) on chloride-containing aerosols (Reaction R4) (Finlayson-Pitts et al., 1989). This reaction is known to be an important intermediate in the NOx removal processes (Brown et al., 2006). The pseudo-first-order loss rate constant of N2O5 via heterogeneous uptake is given in Eq. (1) (Wahner et al., 1998).
Here c is the mean molecule speed of N2O5, Sa is the aerosol surface concentration, and γ (N2O5) is the N2O5 uptake coefficient. N2O5 heterogeneous hydrolysis is one of the major uncertainties of the NO3 budget since the N2O5 uptake coefficient can be highly variable and difficult to quantify (Brown and Stutz, 2012; Chang et al., 2011; Wang and Lu, 2016). Laboratory and field measurement studies have reported that the N2O5 uptake coefficient has large variability and ranges from < 0.001 to 0.1; the N2O5 uptake coefficient depends on relative humidity (RH), particle morphology, compositions (water content, nitrate, sulfate, and organic or mineral particles), and other factors (Wahner et al., 1998; Mentel et al., 1999; Hallquist et al., 2003; Thornton et al., 2003, 2005; Brown et al., 2006; Bertram and Thornton, 2009; Tang et al., 2012, 2014; Gaston et al., 2014; Gržinić et al., 2015; Tang et al., 2017). The coupled chemical mechanisms in ambient conditions are still not well understood. ClNO2 forms and accumulates with a negligible sink during the night and further photolyzes and liberates the chlorine radical (Cl) and NO2 after sunrise. Hundreds of parts per trillion by volume to parts per billion by volume of ClNO2 can lead to several parts per billion by volume of O3 enhancement and significant primary ROx production (Osthoff et al., 2008; Thornton et al., 2010; McLaren et al., 2010; Riedel et al., 2014; Sarwar et al., 2014; Tham et al., 2016).
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Large amounts of NOx have been emitted for the past several decades in China, but comprehensive field studies of the nighttime chemical processes of reactive nitrogen oxides remain sparse. Previous studies have found high mixing ratios of NO3 associated with high NO3 reactivity in the megacities in China, including Shanghai, the Pearl River Delta (PRD), and Beijing (Li et al., 2012; S. S. Wang et al., 2013; D. Wang et al., 2015). The N2O5 concentration was elevated in Beijing (H. C. Wang et al., 2017a, b) but was moderate in other parts of the North China Plain (NCP), such as Wangdu, Jinan, and Mount Tai (Tham et al., 2016; X. F. Wang et al., 2017; Z. Wang et al., 2017). Recently, the N2O5 uptake coefficients were determined to be very high, even up to 0.1 in the NCP, but the reason is still not well studied (H. C. Wang et al., 2017b; X. F. Wang et al., 2017; Z. Wang et al., 2017). Reactive N2O5 chemistry was also reported in Hong Kong and showed the highest field-observed N2O5 concentration to date (Wang et al., 2016; Brown et al., 2016). Observations and model simulations revealed that fast heterogeneous uptake of N2O5 is an important pathway of pNO3- formation in China (H. C. Wang et al., 2017b; Z. Wang et al., 2017; Su et al., 2017); the reaction also contributed significantly to removal (Z. Wang et al., 2017; Brown et al., 2016). Moreover, chlorine activation from N2O5 uptake had a significant effect on daytime photolysis chemistry in China (Xue et al., 2015; Li et al., 2016; Tham et al., 2016; T. Wang et al., 2016).
In this study, to quantify the contribution of NO3 and N2O5 chemistry to the atmospheric oxidation capacity and the NOx removal process in the outflow of urban Beijing, we report the measurement of N2O5, ClNO2, and related species in the surface layer of a suburban site in Beijing and determine the N2O5 heterogeneous uptake coefficients and ClNO2 yields. The nighttime NO3 oxidation of BVOCs and its impact on ON formation in a NOx-rich region were diagnosed. Finally, the nighttime NOx removal via NO3 and N2O5 chemistry was estimated and discussed.
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