Simulated tropospheric reactions

Presently very little is known about the atmospherie behaviour of nitrophenols and nitrocresols. The aim of the present study is to increase our knowledge with respeet to the fate of this compound class under simulated tropospheric conditions in the laboratory. Reeent smdies in our laboratory on the photolysis of nitrophenol and nitroeresols with aetinie lamps (Philips TL 05/40 W 320 < k< 480, Xmax = 360 nm) have indieated that these eompounds could be important secondary organic aerosol sources (Bejan et al., 2003). In order to assess the importance of this aerosol formation pathway for the atmosphere, kinetie information concerning other atmospherie loss processes, e.g. reactions with OH and NO3 radicals, is required. 

Details of the kinetics of the various reactions have been explored in detail using large-volume chambers that can be used to simulate the reactions in the troposphere, and have frequently used hydroxyl radicals formed by photolysis of methyl (or ethyl) nitrite, with the addition of NO to inhibit photolysis of N02. This would result in the formation of 0(3P) atoms, and subsequent reaction with 02 to produce ozone and hence NOs radicals from NOz. Nitrate radicals are produced by the thermal decomposition of N2Os, and in experiments with 03, a scavenger for hydroxyl radicals is added (Chapter 5, Section 5.1). Details of the different experimental procedures for the measurement of absolute and relative rates have been summarized, and attention drawn to the often considerable spread of values for experiments carried out at room temperature ( 298 K) (Atkinson, 1986). It should be emphasized that in the real troposphere, both the rates — and possibly the products — of transformation will be determined by seasonal differences both in temperature and the intensity of solar radiation. These are determined both by latitude and altitude. 

Reactions that simulate tropospheric conditions have been carried out in Teflon bags with volumes of 6 m3 fitted with sampling ports for introduction of reactants and substrates, and removal of samples for analysis. Substrates may be added in the gas phase or as aerosols that form a surface film. The reactants have been noted in Section 4.1.2 and are the hydroxyl and nitrate radicals, and ozone. These must be prepared before use by Reactions 5.1 to 5.3. 

The Rates and Mechanisms for VOC Photo-oxidation Reactions under Simulated Tropospheric Conditions... 

The temperature dependence of the OH-radical reaction of some aromatic compounds under simulated tropospheric conditions,... 

Semadeni, M., D.W. Stocker, and J.A. Kerr (1995), The temperature dependence of the OH radical reactions with some aromatic compounds under simulated tropospheric conditions, Int. J. Chem. Kinetics, 27, 287-304. 

Veillerot, M., P. Foster, R. Guillermo, and I.C. Galloo (1996), Gas-phase reaction of n-butyl acetate with the hydroxyl radical under simulated tropospheric conditions Relative rate constant and product study, Int. J. Chem. Kinet., 28, 235-243. 

The results of computer simulations can be used to estimate the degree of sensitivity required for measurement of the peroxy radicals in the troposphere. Madronich and Calvert (22) gave results of 5-day simulations for free tropospheric ( clean ) and Amazon boundary layer ( moderately polluted ) conditions (Figures 1 and 2, respectively). The solid and dotted lines show the simulations with and without reactions among the peroxy radicals... 

Tropospheric chemistry models have to take into account a significant number of chemical reactions required to simulate correctly tropospheric chemistry. In the global background marine troposphere, it seems reasonable to consider a simplified chemistry scheme based on O3/ NOx/ CH, and CO photochemical reactions. However, natural emissions of organic compounds from oceans (mainly alkenes and dimethyl sulphide-DMS) might significantly affect the marine boundary layer chemistry and in particular OH concentrations. Over continental areas both under clean and polluted conditions,... 

Note that careful evaluation and minimization of uncertainties and errors in CTMs is requested to enable the application of these CTMs to the study of observed changes in 03 as small as < 1.5 %/yr. However, actually 03 concentrations are simulated by the models within 20-50%. Chemical reaction rates are also uncertain, for instance in the 90 s determinations of the rates of CH4 and CH3CC13 reactions with OH suggested that these reactions are about 20% slower than believed. Similarly OH reaction with N02 which is an important sink for NOx in the troposphere is measured to be 10-30% lower than earlier estimates [23]. Thus, the past years a number of studies (mainly based on Monte Carlo simulations) focused on the identification and evaluation of the importance of various chemical reactions on oxidant levels to highlight topics crucial for error minimization. Temperature dependence of reaction rates can also introduce a 20-40% uncertainty in 03 and H20 computations in the upper troposphere. It has been also shown that 03 simulations are particularly sensitive to the photolysis rates of N02 and 03 and to PAN chemistry. 

The model tropopause is defined by a PV level of 3.5 pvu poleward of 20° latitude, and by a -2 K km 1 temperature lapse rate equatorward of 20° latitude. Consequently, in this study the troposphere is defined as the volume between the surface and the simulated tropopause. Because the model does not consider typical stratospheric chemical reactions explicitly, ozone concentrations are prescribed from 1-2 levels above the model tropopause up to the top of the model domain at 10 hPa. In both hemispheres we apply monthly and zonally averaged distributions from a 2D stratospheric chemistry model [31]. In the present version of the model, we use the simulated PV and the regression analysis of the MOZAIC data (Section 2) to prescribe ozone in the NH extratropical lower stratosphere, which improves the representation of ozone distributions influenced by synoptic scale disturbances [32, 33]. Furthermore, the present model contains updated reaction rates and photodissociation data [34]. 

Current research on the atmospheric cycling of sulfur compounds involves the experimental determination of reaction rates and pathways (see Plane review, this volume) and the field measurement of ambient concentrations of oceanic emissions and their oxidation products. Photochemical models of tropospheric chemistry can predict the lifetime of DMS and H2S in marine air however there is considerable uncertainty in both the concentrations and perhaps in the identity of the oxidants involved. The ability of such models to simulate observed variations in ambient concentrations of sulfur gases is thus a valuable test of our assumptions regarding the rates and mechanisms of sulfur cycling through the marine atmosphere. 

Aqueous phase tropospheric simulation chamber, On-line mass spectrometry, N-methylpyrrolidone, Kinetic rate constants, OH-oxidation, Reaction products. Chemical mechanisms... 

The reactivity of NMP towards OH radicals was studied in the aqueous phase, under tropospheric conditions. The kinetic results show that the OH oxidation of NMP is fast compared to that of other WSOC, and thus should induce modifications of the composition of water droplets, due to the reaction products formed. A new experimental technique was developed to study the aqueous phase OH oxidation of NMP. A mass spectrometer was coupled to an aqueous phase simulation chamber, thus providing an on-line analysis of the solution. The mass spectrometer was equipped with an electrospray ionisation (ESI) unit and a triple quadrupole, which allowed ESI-MS, ESI-MS, and ESI-MS-MS analysis. The results proved that this experimental technique is highly promising, as it allowed us to detect the formation of 66 reaction products, of which 24 were positively identified. Based on the results obtained, a chemical mechanism has been suggested for the OH oxidation of NMP in the aqueous phase. The developed equipment can be used to study other molecules and other reactions of atmospheric interest. 

A chemical mechanism is the set of chemical reactions and associated rate constants that describes the conversion of emitted species into products. From the point of view of tropospheric chemistry, the starting compounds are generally the oxides of nitrogen and sulfur and organic compounds, and ozone is a product species of major interest. Chemical mechanisms are a component of atmospheric models that simulate emissions, transport, dispersion, chemical reactions, and removal processes (Seinfeld, 1986, 1988). 

---