Key words:

An important role for nitrogen oxides in the chemistry of the lower atmosphere, both from the point of view of urban atmospheres (Leighton, 1961) and natural trace gas budgets, (Junge 1963, Levy 1972) has been recognised for some time now. The term 'NO' in air chemistry has usually been synonymous with the commonly known oxides of itrogen, NO and NO2, but recent fashion in terminology refers to total 'odd nitrogen' species which includes, in addition to NO and NO2, the higher oxides of nitrogen, N203, N2O4, NO3, N2O5 and also the oxyacids of nitrogen, HONO (nitrous) End HONO2 (nitric). A significant role is also How believed to be played by peroxynitric acid, 02N02.

In any model of the chemical transformations in urban air, the chemistry of the organic erivatives of the oxyacids, alkyl nitrites, alkyl itrates and the peroxynitrates must be considered. specially important are the peroxyacylnitrates PAN'S) which observational data show to be one f the most important compounds in photochemical rog.

An important atmospheric nitrogen oxide, not ormally included under the terminology 'NO', nitrous oxide N20. Present knowledge does not oint to a role for this oxide in the tropospheric aseous nitrogen cycle. However, observational ta suggests that there is a sizeable unidentified ink for N20 in the troposphere. It may be ppropriate, therefore, to consider any chemical inetic data which might relate to this problem.

Finally, reduced nitrogen compounds, NH, and s derivatives, should be mentioned, since the oblem of coupling of the NH3 and NO cycles has Een raised from time to time (Robinson and bbins, 1971). NH3 undeniably plays an important

role in the aerosol and precipitation chemistry of nitrates. Relating more to the current chemical kinetic data assessment, is the problem of oxidation of NH3 to NO (or NO2).

2. Importance of NOx in Atmospheric Chemistry

Nitrogen oxides and related species are important atmospheric pollutants in their own right, e.g. the toxicity of NO2 and the corrosive nature of NO2 and nitric acid toward many types of materials. However for the atmospheric chemist it is the interaction of NO with other chemical species in the atmosphere and the resulting influence on the basic trace-gas cycles and the formation of secondary pollutants, which is of interest. It is in the solution of problems arising from these interactions where chemical kinetics can play a role. These problems are primarily related, both in the natural and the polluted troposphere, to the photochemical oxidation of hydrocarbons.

Thus the involvement of NO2 in the tropospheric ozone budget, has a direct bearing on the average concentration of OH in the lower atmosphere and consequently on the atmospheric residence times of a variety of trace-gases.

A model of atmospheric NO chemistry is therefore necessary to (a) formulate a realistic model of photochemical smog on which to base control strategy and (b) to provide an understanding of the global tropospheric trace-gas cycles. The objective of such a model is to predict the total budget of NO, from source to sink, and the distribution of NO among the various chemical species in time and in space. This distribution depends on the nature and strength of the sources, the chemical interactions within the atmosphere and the role of the various sink mechanisms.

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The main source of NO is believed to be emission of NO from the ground, either from manmade sources, mainly combustion, or from soil processes. An additional source of NO is fixed atmospheric N2 from lightning. There is current argument about the magnitude, both relative and absolute, of these sources.

Once in the atmosphere, chemical oxidation of NO to NO2 occurs rapidly, primarily through the reaction

03 + NO = NO2 + 02.

In daylight NO is reformed by photolysis of NO2, but is also oxidised by photochemically generated radicals, i.e.

Removal of NO2 is primarily via formation of nitric acid, with alternative pathways via organic nitrates and pernitrates. Formation of peroxynitric acid and PANs is reversible but this can act as a sink if PANs are removed, for example, by absorption at the ground. Additional removal of NO2 can occur by reaction with 03 to give NO 3. NO3 is rapidly photodissociated (in daylight) but also reacts with NO (to reform NO2) or with NO2 to give N205. The latter reaction is reversible and so NO2, NO3, N205 and 03 can exist in equilibrium. N205 can also be converted to HNO3 by heterogeneous reaction with water.

The nitric acid, N20s and organic nitrates can all be removed from the atmosphere by absorption at the ground (dry deposition) or by incorporation into the precipitation elements - aerosol particles, cloud and fog droplets, which eventually leads to rain-out.

4. Status of Chemical Kinetic Data

Accurate chemical kinetic data is clearly required for the primary chemical processes involved in the transformation of NO to nitric acid. Also of interest is data relating to all possible minor interactions which would influence the basic atmospheric NO, cycles, or which produce unusual secondary pollutants in urban air. Due to the recent stimulus in the field of atmospheric kinetics, data for some of these processes is now rather well known. For some processes more and better data is badly needed, and these become self-evident during any detailed discussion of the data base. In the following paragraphs the status of the data base is very briefly indicated for some specific areas of NO chemistry mentioned above. The topics covered should not be considered exhaustive, but rather a minimum set necessary for modelling the basic NO cycle.

In order to calculate photodissociation rates for a species in the atmosphere, a knowledge of the absorption cross-section a as a function of wavelength and the quantum yield(s) of the photodissociation pathway(s), 1, is required. This is then combined with suitably averaged data for the photon-flux in the atmosphere to obtain the

ue to the chemical complexity of these systems, here is some uncertainty in the kinetic parameters. 0 may also undergo H-abstraction reactions with rganic molecules and some kinetic parameters for hese reactions have been reported. The effective ate of the reaction of N205 with water which is obably heterogeneous, is rather uncertain at this ine and could be of importance in the overall NO udget in the lower atmosphere. A quantitative reatment of the rate of heterogeneous removal of seous species on aerosol particles and cloud and g droplets, which is acceptable to many modelers, Es yet to be formulated.

X

The coupling of the NO and HO cycles is one f the most important aspects of atmospheric free

radical chemistry. The reactions of hydroxyl (HO) radicals with NO species has been widely studied in response to problems of aeronomy, and a reasonably good data base is available here. The important reactions are:

HO + NO (+M) = HONO (+M) HO+ NO2 (+M) = HONO2 (+M) HO+ HNO3 = H2O + NO 3

HO + HONO = H2O + NO2

Note that the M dependent reactions are in the transition region between third-order and secondorder kinetics at the pressures encountered in the troposphere. If the actual measurements of the rate constants as a function of pressure for M = Air are not available, the rate constants in the transition region can be estimated from a knowledge of the third order low pressure rate constants, kill, and the high pressure secondorder rate constant k. These two rate constants KIII and k therefore comprise a minimum data set for this type of reaction. The temperature dependence of these association reactions is also important since in the low pressure regime they usually exhibit a significant negative temperature coefficient. This can be important in modelling NO circulation in the global troposphere.

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The possible reactions of NO and NO2 with organic radicals are numerous. However reactions with organic peroxyradicals appear to be the most significant for atmospheric chemistry, and these are exemplified by the reactions of NO and NO2 with peroxyacetyl radicals:

CH3C(0)00+ NO CH3 + CO2 + NO2

CH3C(0)00 NO2CH3COO2 NO2 (PAN)

These reactions govern the formation of peroxyacetylnitrate in urban air and show clearly the competition between the chain carrying reaction