Oxidation night-time

Though photochemistry does not take place at night, it is important to note, within the context of tropospheric oxidation chemistry, the potential for oxidation chemistry to continue at night. This chemistry does not lead to the production of ozone, in fact the opposite, but has importance owing to the potential for the production of secondary pollutants. In the troposphere, the main night-time oxidant is thought to be the nitrate radical formed by the relatively slow oxidation of NO2 by O3, viz. 

Case Study III — Evidence for the role of NO j in night-time oxidation chemistry from experimental based studies Significant NO3 concentrations have been detected over a wide range of atmospheric conditions, indicating a potential role for NO3 over large regions of the atmosphere. The atmospheric lifetime of NO3 can be estimated using the steady-state approximation (cf. PSS) to be... 

DMS concentrations in the remote marine troposphere vary in the range of 0.03-32 nmol m . Not surprisingly and as with the seawater concentrations, considerable temporal and geographic disparities occur. Furthermore, atmospheric DMS concentrations exhibit diurnal variations, with a night time maximum and an afternoon minimum consistent with a photochemical sink. Whereas oxidation involves HO free radicals... 

Reaction with NO3 radicals represents another important gas phase loss pathway for a-pinene during the night time. Under elevated NO c conditions, reaction with NO3 can also be significant during the daytime and contribute substantially to the monoterpene decay. However, only a few studies have been performed on NOs-initiated oxidation of pinenes [181-184]. To our knowledge, no theoretical studies have been performed to investigate the mechanistic behaviors of these reactions. 

The issue of night-time radical chemistry initiated by NO3 was not prominent at the start of LACTOZ. Although O3 is not generated in this process, NO3 chemistry is now considered to be of importance for the modification of the precursors for ozone formation, for example at high latitudes in wintertime, when anthropogenic pollutants can accumulate. Observations of night-time oxidation chemistry, including NO3 and RO2 radicals in the boundary layer, are also now feasible. LACTOZ data will enable the interpretation of these experiments. 

Reactions of NO3 with HO2 and simple RO2 radicals (CH3O2, C2H5O2 and CH3C(0)02) have been investigated within LACTOZ (Table 3), as possible chain propagation steps in the mechanism suggested for the night-time oxidation of VOCs (see above). 

Photo-oxidation studies of the nitrates and dinitrates investigated revealed that NO2 and PAN are important products (Becker). Thus, the photolysis of keto-nitrates and dinitrates will result in the release of NO2 from these species. As a consequence, organic nitrates formed in the reactions of NO3 with alkenes during night-time will act only as temporary reservoirs for NOx. 

Both laboratory and field measurement studies of RO2 and NO3 radicals were part of this LACTOZ study. The discovery of a rapid reaction between NO3 and RO2 and its involvement in a potentially important night-time oxidation mechanism for VOC are discussed. Investigation of the photo-excitation of mixtures of NO2, NO3 and O3 around 662 nm showed that reaction of N03(A E ) with N2 is not likely to be a source of N2O and provided an upper limit for rate coefficient for the reaction of NO3 with O3. N2O is produced by 185 nm photolysis of NO, NO2 and even air, as well as heterogeneously from NO2, NO3 and N2O5 mixtures. Finally the development of a tropospheric ambient RO2 detector, the analysis of RO2 measurements and their impact on our understanding of the chemistry of the daytime and night-time planetary boundary layer are discussed. 

T. Behmann, M. WeiBenmayer, J.P. Burrows Peroxy Radicals in night-time oxidation chemistry, in G. Angeletti, G. Restelli (eds), Proc. Sixth European Symp. on Physico-Chemical Behaviour of Atmospheric Pollutants, ECSC-EC-EAEC, Brussels and Luxembourg 1994, pp. 259-265, (ISBN 92-826-7922-5). 

The aims of the research were to provide kinetic and mechanistic data for NO3 and OH reactions of potential atmospheric relevance. In particular, a large body of the studies aimed at improving the knowledge of the NO3 initiated oxidation of VOCs during night-time in order to assess the contribution of the night-time processes to the overall atmospheric oxidation of VOCs and photo-oxidant formation. 

Mechanism for the NO3 initiated oxidation of VOCs in the atmosphere We suggested the following chain mechanism for the night-time oxidation of VOCs [1] ... 

U. Platt, G Le Bras, G. Poulet, J.P. Burrows, G.K. Moortgat, Nature 348 (1990) 147. G. Le Bras, C. Golz, U. Platt Production of peroxy radicals in the DMS oxidation during night-time, in G. Restelli, G. Angeletti (eds), Dimethylsulphide, Oceans, Atmosphere and Climate, Kluwer Academic Publ., Dordrecht 1993, p. 251. 

Ozone plays a major role in the degradation of unsaturated VOCs in the troposphere, especially during night-time. The rate constants of the ozonolysis of a variety of alkenes have been reported [1]. However, in most instances the fate of the primary products of the ozonolysis is unknown, although the secondary reaction products are of crucial importance for the overall understanding of the alkene/ozone chemistry. The classical Criegee mechanism of the ozonolysis reaction involves the primary ozonide (POZ, 1,2,3-trioxolane), which cleaves to the Criegee intermediate (carbonyl O oxide) and a carbonyl compound [2, 3]. The secondary ozonide (SOZ, 1,2,4-trioxolane) is formed from these components in a [l,3]-dipolar cycloaddition reaction. 

Laboratory study of aerosol formation from the night-time oxidation of DMS, in P.M. Borrell, P. Borrell, T. Cvitas, W. Seiler (eds), Proc. EUROTRAC Symp. 92, SPB Academic Publ., The Hague 1993, p. 416. 

Production of peroxy radicals in the DMS oxidation during night-time,... 

Peroxy radicals in night-time oxidation chemistry,... 

The interaction between nitrogen oxides and the naturally emitted isoprene (and structurally related compounds) in the night-time troposphere. 

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