On the atmospheric chemistry of NO2 - O3 systems : a laboratory study

In this dissertation a laboratory study dealing with the atmospheric chemistry of NO 2 -O 3 systems is described. Knowledge of this system is relevant for a better understanding of a number of air pollution problems, particularly that of acid deposition.In chapter 1 a short overview of atmospheric chemistry is given, in which the formation of oxidants, the SO 2 chemistry and the NO X chemistry are considered.It appears that, in absence of light, interactions between NO 2 and O 3 may lead to the formation of nitric acid. After oxidation of NO 2 by O 3 , the NO 3 radical is formed. Next, this radical can react in several ways. one of the possibilities is the reaction with NO 2 resulting in N 2 O 5 . The actual nitric acid formation is the N 2 O 5 hydrolysis. In theory, this process can substantially contribute to the nitric acid formation In the atmosphere. Some gaps in the present knowledge are the exact mechanism and kinetics of the N 2 O 5 hydrolysis and the NO 3 reactivity.Chapter 2 contains a review of the recent literature. Subsequently, laboratory, field and model studies are considered. Without doubt, it can be concluded that the non-photochemical nitrate formation by the NO 2 -O 3 system is recognized as an important atmospheric chemical process. The above-mentioned knowledge gaps are confirmed which, among other things. follows from measurements of the ambient NO 3 radical concentration profile. Regularly, it is suggested that heterogeneous processes of NO 3 or N 2 O 5 removal are involved. The parameters needed to quantify such processes are currently unavailable.After the general Introduction and the literature review, the experimental methods applied in the present investigation are described. The construction of the employed laboratory equipment is given. The basic principles of the techniques used for the generation and analysis of the reactants are listed.In the chapters 4 to 7 the results are described and discussed. Chapter 4 treates the results of a study on the NO 2 -O 3 chemistry at sub-ppm concentrations as well as the influence of temperature and relative humidity (R.H.). The experiments are performed using the standard techniques for measurement and calibration of NO 2 and O 3 .If we consider the stoichiometry of the reaction system at R.H. < 0.1%, It appears that it deviates from the theoretical value of two. Obviously a side-reaction, that regenerates NO 2 , consumes extra O 3 or both, is involved. Since the stoichiometry significantly differs in reaction vessels of different size (other variables constant), we have to deal with a wall reaction. Further analysis of the results combined with literature data leads to the interpretation that the low stoichiometry is caused by the heterogeneous NO 3 decay on the vessel wall with regeneration of NO 2 , With this reaction mechanism, kinetic parameters can be obtained from the massbalance of every component. The rate constant of the NO 2 -O 3 reaction appears to agree reasonably well with literature values.The influence of R.H. results in an increase of the stoichiometry caused by N 2 O 5 hydrolysis. The kinetics of the NO 2 -O 3 reaction remains unchanged. The kinetics of the N 2 O 5 hydrolysis can be described with Its pseudo-first-order rate constant. This rate constant Is not directly proportional to the R. H. , which is an indication that the N 2 O 5 hydrolysis -at least in part- occurs heterogeneously at the wall of the reaction vessel.In the atmosphere, aerosol particles are involved in these heterogeneous processes. Before studying the NO 2 -O 3 -aerosol chemistry, the dynamical behaviour of aerosol particles in the reaction vessels is considered. The results are given in chapter 5. The differences in the particle number concentration and in the particle size distribution of the feed and steady state aerosol have been measured. It appears that these differences are caused by coagulation and wall deposition processes.The influence of aerosol particles on the NO 2 -O 3 chemistry is the subject of chapter 6. 'Dry' aerosol (NaCl; R.H.=15%) and 'wet' aerosol (MgCl 2 ; R.H.=78%) are distinguished. In case of a 'dry' aerosol a small decrease in the stoichiometry is observed. This can be interpreted as a NO 3 decay on the aerosol surface. In case of 'wet' aerosol NO 3 decay as well as N 2 O 5 hydrolysis is important. This follows from the nearly constant stoichiometry and the nitrate formation in the aerosol.The kinetics of the NO 2 -O 3 reaction do not change in the presence of aerosol particles. Moreover, the rate constants of the heterogeneous reactions can be obtained. From these the so- called 'accommodation coefficient' can be deduced. This coefficient Indicates what part of the gas- aerosol collisions really leads to reaction and characterizes the rate of the heterogeneous reaction. The heterogeneous N 2 O 5 hydrolysis can also be understood as a bulk aqueous phase process. The product of Henry's law constant and the rate constant of the aqueous phase hydrolysis can be found. It appears that for N 2 O 5 this product is in fairly good agreement with the product for N 2 O 3 and N 2 O 4 .In chapter 7 the aqueous phase chemistry of NO 2 and O 3 is considered. It is investigated using a gas-liquid contact reactor and analysis of the nitrite and nitrate formation. The results of the Interaction between NO 2 and liquid water are in agreement with the current literature. The influence of O 3 is restricted to the oxidation of nitrite. From the results it can be deduced that aqueous phase NO 2 -O 3 reactions do not significantly contribute to atmospheric nitrate formation.Finally, a general evaluation is given in chapter 8. It can be concluded that this investigation confirms the present insights in the atmospheric chemistry of NO 2 and O 3 and that new insights in the reactivity of NO 3 and N 2 O 5 are obtained.