The reaction between unsaturated VOCs and NO3 free radicals
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The reaction between unsaturated VOCs and NO3 free radicals

Posted By admes liliy     Apr 17    


Organic nitrates play an important role as sinks or temporary reservoirs of nitrogen oxides and in the generation of atmospheric ozone (Perring et al., 2013, 2010; Ito et al., 2007). They are formed by the degradation of volatile organic compounds (VOCs) in the presence of NOx through two main processes:
Reaction of peroxy radicals produced by VOC oxidation with NO. The main pathway is usually the reaction (R1a) leading to NO2 formation. Reaction (R1b) is a minor pathway, but it becomes progressively more important as the carbon chain length of the peroxyl radical increases (Atkinson and Arey, 2003; Finlayson-Pitts and Pitts Jr., 2000).

The reaction between unsaturated VOCs and NO3 free radicals mainly generates nitroalkyl free radicals through the addition reaction of nitrate on the double bond, and then evolves into organic nitrates.
Among organic nitrates, a variety of multifunctional species such as hydroxynitrates, carbonylnitrates, and dinitrates are formed. Formed species have been shown to contribute significantly to the nitrogen budget in both rural and urban areas (Perring et al., 2013). Beaver et al. (2012) observed that carbonyl nitrate, a second-generation nitrate formed from isoprene, was a significant fraction of total organic nitrate observed in the Sierra Nevada in summer. These observations are supported by several studies investigating isoprene photooxidation in simulated chambers (Paulot et al., 2009; Müller et al., 2014). These multifunctional organic nitrates are also semi-volatile/non-volatile and highly soluble species; thus, they are able to partition into atmospheric condensed states (droplets and aerosols). Extensive field observations of the chemical composition of atmospheric particles have shown that organic nitrates constitute a significant fraction (up to 75% by mass) of total organic aerosols (OA), suggesting that these species are an important component of total OA (Ng et al., 2017 ).

Several modeling studies have also confirmed that multifunctional organic nitrates, especially isoprene nitrate, play a key role in the transport of reactive nitrogen and thus the formation of ozone and other secondary pollutants on a regional and global scale (Horowitz et al. , 2007; Mao et al., 2013; Squire et al., 2015). In particular, Mao et al. (2013) based simulations on ICARTT (International Consortium for Atmospheric Transport and Translational Research) data on aircraft movement across the eastern United States in 2004. They show that organic nitrates, consisting mainly of secondary organic nitrates, including a large fraction of carbonyl nitrates, provide a significant pathway for NOx export from the US boundary layer, even surpassing that of peroxyacyl nitrate (PAN). However, these modeling studies also point to the need for additional experimental data to better describe sinks in the gas and condensed phases of multifunctional organonitrates in the model.

Recent experimental studies have shown that hydrolysis in the aerosol phase can be a very efficient sink for atmospheric organic nitrates (Bean and Hildebrand Ruiz, 2016; Rindelaub et al., 2015). These studies also show that the rate of these reactions depends strongly on the chemical structure of the organic nitrate, and more work is needed to better understand these processes. In the gas phase, photolysis and reaction with OH radicals are expected to dominate the fate of organic nitrates (Roberts, 1990; Turberg et al., 1990). In a previous study, we measured three carbonyl nitrates (α-nitrooxyacetone, 3-nitrooxy-2-butanone, and 3-methyl-3-nitrooxy-2-butanone) The photolysis frequency and OH oxidation rate constant, and we found that photolysis is the main sink for these compounds (Suarez-Bertoa et al., 2012). By comparison with the absorption cross section provided by Barnes et al. (1993), Müller et al. (2014) suggested the following: (i) α-nitrooxyketones have an enhanced absorption cross section due to the interaction between the –C=O and –ONO2 chromophores; (ii) the photolysis quantum yield is close to unity, O-NO2 dissociation may be the main channel. They also showed that this enhancement is greater at higher wavelengths, with very little absorption by the nitrate chromophore. Therefore, they concluded that the uptake by the carbonyl group was enhanced due to the adjacent nitrate group.