Predicting the night-time NO3 radical reactivity in the troposphere

Atmospheric Environment Vol. 24A, No. 1, pp. 73 78, 1990.

0004 6981/90 $3.00+0.00 Pergamon Press plc

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P R E D I C T I N G THE N I G H T - T I M E N O 3 R A D I C A L REACTIVITY IN THE T R O P O S P H E R E ALEKSANDAR SABLJIC* a n d HANS GIJSTEN Institut ffir Meteorologie und Klimaforschung, Kernforschungszentrum Karlsruhe/Universit~it Karlsruhe, 7500 Karlsruhe, F.R.G (First received 3 May 1989 and received for publication 7 August 1989) Abstract - A statistically significant linear correlation between the reaction rate constant, kNo~,for the N O 3

free radical reaction with 69 organic compounds in the gas phase at 298 K and the corresponding vertical ionization energies, E~.,., allows an a priori prediction to be made of hitherto not measured compounds. With these reaction rate constants and a mean concentration of N O 3 during the night, the upper limit of the tropospheric half-life of organic compounds and their persistence in the troposphere can be estimated. From the fairly good linear correlation between kNo3 and kon it can be deduced that in the gas phase both free radicals react in a very analogous manner with organic compounds. Key word index: Abiotic degradability, environmental chemicals, N O 3 radical reactivity, organic compounds, reaction rate constant, tropospheric lifetime, vertical ionization energy.

I. I N T R O D U C T I O N

Since the nitrate (NO3) free radical has been observed by spectroscopy in the lower troposphere during night-time hours (Platt et al., 1984), the gas phase reaction kinetics of this strongly oxidizing species with a variety of organic compounds has received much attention in recent years (Atkinson et al., 1988). Due to its strong absorption at wavelengths above 600 nm, N O 3 is readily photodecomposed in the daylight. During the night, however, the N O 3 free radical reactions can be an important process of tropospheric removal of trace gases and organic air pollutants. Thus, these reactions supplement the daytime removal process initiated by reactions with the OH radicals (Gtisten, 1986; Finlayson-Pitts and Pitts, 1986). In order to be able to assess the tropospheric lifetime of organic compounds reliable estimates must be made of the different radical reaction rate constants (Atkinson, 1987), particularly in view of the extremely large number of organic compounds of anthropogenic origin. Furthermore, the very low volatility of many organic compounds of industrial origin makes it impossible to measure their reaction rate constants in the laboratory. Finally, the a priori prediction of the tropospheric lifetime and other environmental quantities are of importance to the development of safer new organic chemicals (Sabljid, 1987, 1988; Proti6 and Sablji/:, 1989). We have shown that a statistically significant correlation between the reaction rate constants of the reactions of the O H radical with organic compounds in the gas phase and the corresponding ionization *Permanent address: Institute Ruder Bogkovi~:, P.O.B. 1016, 41001 Zagreb, Croatia, Yugoslavia. 73

energies allows a rapid estimate to be made of the upper limit of the tropospheric halfqife of organic compounds during daytime (Gfisten et al., 1984; Gfisten and Klasinc, 1986). We will explain in this paper that the same procedure for koH is also applicable as a rapid estimate of kNoa in the gas phase at night-time.

2. M E T H O D O F C A L C U L A T I O N A N D E X P E R I M E N T A L DATA

The photoelectron spectra (PE) of terpenes and 2,3-dimethylbutane listed in Table 1 were recorded on a Vacuum Generators UV-G 3 spectrometer using helium I excitation. If necessary, the inlet system was heated and the PE spectra were calibrated by addition of Ar, Xe or CH3I to the sample gas flow. The vertical ionization energies (E~.v) of the first band system were determined with an accuracy of +0.01 eV. A regression analysis was carried out on an Apple Macintosh SE personal computer using the statistical analysis system (SYSTAT, version 3.2). To test the quality of generated regression equations the following statistical parameters were used: the correlation coefficient (r), the standard error of the estimate (s), a test of the null hypothesis (F-test), and the amount of explained variance (E V).

3. R E S U L T S A N D D I S C U S S I O N

The survey of the literature yielded 74 reaction rate constants for the reactions of the NO3 radical in the gas phase with organic compounds. About 90% of the published NOa reaction rate constants, kNo3 (in cm 3 molecule-I s - t ) , were measured and compiled by Atkinson et al. (1988). The data base for our statistical evaluation, kNo3, the reaction rate constant of the OH radical, koH (in cm a molecule- 1 s - 1), and the corresponding vertical ionization energies (in eV) of 74 organic compounds are summarized in Table 1. Other

74

ALEKSANDARSABLJICand HANS Gt~STEN Table 1. The reaction rate constants - l o g kNo~ (in cm a molecule-1 s - t ) and - l o g koH (in cm 3 molecule - 1 s- 1) of NO 3 and OH radicals with organic compounds in the gas phase at 298 K and their first vertical ionization energies (in eV) Compound

- l o g kr~o3

E~.v

- log koR

Literature

Aliphatics Ethene Propene 1-Butene

trans-2-Butene 2-Methyl-2-butene 2,3-Dimethyl-2-butene 2-Methylpropene

cis-2-Butene Cyclopentene Cyclohexene Cycloheptene 1,3-Butadiene 2-Methyl- 1,3-butadiene 2,3-Dimethyl- 1,3-butadiene 1,3-Cyclohexadiene

1,3-Cycloheptadiene 1,3,5-Cycloheptatriene Bicyclo[2.2.1]-2-heptene Bicyclo[2.2.2]-2-octene Bicyclo[2.2.1]-2,5-heptadiene 1,4-Cyclohexadiene ct-Pinene* fl-Pinene* A3-Carene * d-Limonene* Myrcene* cis- and trans-Ocimene 7-Terpinene* ~t-Terpinene* ~t-Phellandrene* Propyne Vinyl chloride Allyl chloride 1,1-Dichloroethene

cis.l,2-Dichloroethene trans-l,2-Dichloroethene Trichloroethene Pyrrole Furan Thiophene n-Butane n-Pentane n-Hexane n-Heptane n-Octane n-Nonane 2-Methylpropane 2,3-Dimethylbutane* Cyclohexane Formaldehyde Acetaldehyde Acrolein Crotonaldehyde Dimethyl sulfide Dimethyl disulfide Methanthiol Ethanthiol Tetrahydrofuran

15.670 14.027 13.910 12.412 11.030 10.245 12.504 12.460 12.336 12.279 12.319 13.009 12.228 11.980 10.914 11.190 11.928 12.611 12.842 11.996 12.276 11.237 11.627 10.996 10.883 10.979 10.654 10.533 9.742 10.071 15.740 15.352 15.253 14.893 15.836 15.955 15.533 10.338 11.842 13.408 16.187 16.097 15.979 15.866 15.742 15.622 16.013 15.391 15.873 15.140 14.520 14.936 14.293 12.006 12.131 12.004 11.921 14.312

10.51 9.66 9.63 9.12 8.68 8.27 9.24 9.12 9.10 8.94 9.04 9.20 8.85 8.62 8.25 8.40 8.55 8.97 9.05 8.69 8.82 8.31 8.68 8.59 8.62 8.61 -8.23 7.67 8.01 10.37 10.18 10.38 10.00 9.80 9.80 9.66 8.20 8.90 8.87 11.20 10.90 10.72 10.45 10.33 10.23 11.00 10.60 10.30 10.88 10.21 10.13 9.75 8.59 8.96 9.44 9.29 9.65

11.069 10~580 10.503 10.196 10.061 9.959 10.289 10.251 10,174 10.171 10.130 10.175 9.996 9.914 9.788 9.857 10.025 10.309 10.391 9.921 10.004 10.274 10.107 10.060 9.772 9.678 9.573 9.754 9.444 9.509 11.218 11.180 10.770 11.092 11.623 11.745 11.627 9.914 10.393 11.023 11.596 11.391 11.253 11.143 11.060 11.000 11.625 11.208 11.132 11.046 10.790 10.708 10.444 11.201 -10.480 10.333 10.824

16.190 15.427 15.603 15.347 14.733

8.78 8.57 8.57 8.45 8.42

11.208 10.833 10.611 10.818 10.500

Benzene derivatives Toluene o-Xylene m-Xylene p-Xylene 1,2,3-Trimet hylbenzene

:~ :~ :~ :~

§ §

II

Night-time NO 3 radical reactivity prediction

75

Table 1 (Contd.) Compound 1,2,4-Trimethylbenzene 1,3,5-Trimethylbenzene Tetralin Methoxybenzene 1,4-Benzodioxan 2,3-Dihydrobenzofuran Benzaldehyde Phenol o-Cresol m-Cresol p-Cresol

- log kNo3

El,,

- log kon

14.745 15.102 13.959 15.682 15.229 12.943 14.595 11.440 10.680 10.800 10.660

8.27 8.42 8.44 8.39 --9.57 8.61 8.32 8.40 8.22

10.416 10.218 -10.804 10.602 10.432 10.886 10.548 10.398 10.244 10.357

Literature

* New data for Ei, v. "1Benter and Schindler (1988). Atkinson (1987). §Atkinson et al. (1984b). IIDlugokencky and Howard (1988). ' Atkinson et al. (1984c),

kNo3 data than those of Atkinson et al. (1988) are indicated in Table 1. The kon data were taken from a compilation by Atkinson (1986). Most of the vertical ionization energies, Ei,v, which are necessary for the correlation with kNo~ have been measured and compiled by G/isten et al. (1984). New ionization energies measured in this work are marked with an asterisk in Table 1. The plot of - log kN% vs Ei. v (Fig. 1) shows clearly that the 72 data points form two non-overlapping, welldefined classes of compounds. The smaller group consists of nine benzene derivatives (with the exception of benzaldehyde, phenol and cresols) which lies on the steeper correlation line. It is located in the upper left corner of the correlation diagram (Fig. 1). The larger group (62 compounds), termed here 'aliphatics', consists of alkanes, alkenes, polyenes, terpenes, alkynes, chloroalkenes, aldehydes, ethers, thiols, thioethers, plus benzaldehyde, phenol and cresols. All these functionally diverse classes of compounds form very compact groups around the diagonal line of the correlation diagram (Fig. 1). For the sake of clarity, the two groups in Fig. 1. will be analyzed and discussed separately. Aliphatic compounds

The linear regression analysis for the larger group of compounds shows that the N O 3 radical rate constants ( - l o g kNo~) correlate strongly with their first vertical ionization energies. The regression equation and statistical parameters describing this quantitative model are: - log kNo~ = -- 7.02( + 0.87) + 2.16( + 0.09)Ei,v, (1) where N = 6 2 , r=0.948, s=0.468, F 1 6 ° = 5 3 7 , E V= 89.8%. The statistical parameters show that Equation 1 is statistically significant above the 95% level and it accounts for 90% of the variation in the log kNo3 data. (The 95% confidence intervals are shown in parenth-

eses in Equation 1.) In fact, the correlation coefficient (r) is approaching unity when the above listed classes are considered separately within a homologues series such as alkanes, etc. Thus, the small individual slopes of the different classes of organic compounds which we summarized under 'aliphatics' are averaged by the regression Equation 1. The residual analysis shows that only the group of chloroalkenes systematically deviates from the above model. A similar behaviour of bromo- and chloroalkenes was already described (Gfisten et al., 1984) in the correlation study of ionization energies with O H radical rate constants. It was recommended either to use the fluorine substitution for bromine and chlorine or to approximate those compounds by the 'parent' hydrocarbon molecule. We have tested both approximations for chloroalkenes and the result is presented in Equations 2 and 3, respectively. - log kNo3 = -- 6.28( _ 0.68) + 2.07( + 0.07)Ei.v, (2) where N = 6 2 , r=0.965, s=0.418, F 1 6 ° = 8 2 0 , EV=93.1%.

- l o g kNo~ = --6.55(+0.71) + 2.10(+0.08)Ei. v, (3) where N = 6 2 , r=0.963, s=0.420, F t 6 ° = 7 7 0 , E V = 92.6 %. -33OO - 330 15-

33 3.3

13-

.33

"~ ~r2 [ d ]

.03 11-

.003 -.0003

9

-

, 8

-

i

-

9

, 10

.

i 11

12

E i,v

Fig. 1. Correlation of - log kNo~ (in cm 3 molecule- 1s- l ) vs E~.v(in eV) for 71 organic compounds in the gas phase at room temperature.

76

ALEKSANDAR SABLJI(~ and HANS GI3STEN

It is immediately obvious that both approximations improve the original model and that the levels of improvement are very similar. However, due to the lack of ionization energies for organofluorine compounds, the 'parent' hydrocarbon approximation may be more readily applicable. A comparison of Equations 2 and 3 with Equation 1 shows that their slopes are approximately the same while their intercepts differ considerably. This result additionally supports the idea that chloroalkenes had an undue influence on the resulting model (Equation 1). Equations 2 and 3 enable the log kNo~ data to be predicted within one power often, the probability being about 90%. In view of the significant variability in measured NO 3 rate constants, the correlation with the ionization energies is surprisingly good. The experimental kNo~ values from different laboratories have been measured using both relative and absolute rate techniques, namely smog chambers or flow tube reactors. This fact implies a good quality of the correlation of kNo~ with the ionization energies. Benzene derivatives

The linear regression analysis for the benzene derivatives shows that the NO 3 radical rate constants ( - l o g kNo~) does not correlate well with their first vertical ionization energies (correlation coefficient 0.639). However, the analysis of residuals and visual inspection of the plot of - l o g kNo~ vs Ei.v made it clear that tetraline and methoxybenzene are extreme outliers. Thus, their elimination from the regression analysis made a very significant contribution to the benzene derivatives quantitative model which is described by Equation 4 and its statistical parameters: - l o g kNo3 = -10.02(-t-4.10)+ 2.98(+0.48)El.v,

(4) where

N = 7 , r=0.940, s=0.191, F1'7=38, EV=86.1%.

The statistical parameters show that Equation 4 is statistically significant at the 85% level and it accounts for 86% of the variation in the log kNo3 data. (The 95% confidence intervals are shown in parentheses.) This is probably the best we can get due to the limited number of measured rate constants for benzene derivatives. Test o f models

In the following section, the predictive ability of our models, Equations 3 and 4, will be tested on the set of test compounds listed in Table 2 as part of the validation process. Their rate constants have been determined only semiquantitatively (only as slower than); thus they could not be included in the regression analysis. Equations 3 and 4 furnished qualitatively correct predictions for the rate constants of eight aliphatic compounds and three benzene derivatives, respectively. With the exception of tetrachloroethene, all predicted kNo3 values are slower than observed

Table 2. Comparison of observed and predicted NO a radical rate constants - l o g kNo3 (in cm 3molecule-I s -1) for aliphatic compounds and benzene derivatives at room temperature Compound

- l o g kNo3

--log kNo3

Ei, v

(obs.)*

(predict.)

(eV)

>/16.459 >/16.208 > 16.292 > 14.921 i> 15.222t /> 15.046t ~>14.638t >/14.523++

17.414 15.543 16.952 14.723 16.508 15.815 15.290 14.534

11.40 10.51 11.18 10.13 10.98 10.65 10.40 10.04

>/16.495 /> 15.155 > 15.155

17.743 16.196 17.869

9.25 8.76 9.29

Aliphatics

Acetylene Tetrachloroethene Carbonyl sulfide Carbon disulfide Methanol Ethanol 2-Propanol Dimethylether Benzene derivatives

Benzene Ethylbenzene Benzyl chloride

*Data from Atkinson et al. (1988). tData from Wallington et al. (1987). ++Data from Wallington et al. (1986).

experimentally. In experiment, however, only upper limits of the rate constants were obtained experimentally (Atkinson et al., 1988) for these less reactive organic compounds with room temperature rate constants of < 10-15 cm 3 molecule- t s - 1. These results let us trust that the models above will be also accurate in the future estimations made for compounds whose reaction rate constants for the reaction with NO 3 radicals have not been measured yet or whose low volatility prevents the direct measurement of the NO3 reaction rate constants. Since both reaction rates of NOa and OH radicals in the reactions with organic compounds correlate highly with the ionization energies of selected chemicals, it was reasonable to expect also a high intercorrelation between these rate constants. Furthermore, a correlation plot of the OH and NO 3 radical constants for 28 alkenes was published (Atkinson, 1986) indicating that such correlation does occur. If a sufficiently high correlation exists, it can be used also to estimate the NO 3 radical rate constants from the measured koH data. OH radical rate constants have been measured about 4 times more frequently on organic chemicals than for NO 3. As expected, a fairly good linear correlation was found between kNo~ and koH for 72 aliphatic compounds and benzene derivatives (correlation coefficient 0.854). The residual analysis shows that dimethyl sulfide is an extreme outlier exerting an undue influence on the regression model and that phenol, cresols, and trimethyibenzenes deviate systematically from that model. Moreover, for benzene derivatives, with the exception of benzaldehyde, the OH (H atom abstraction from the substituent groups) and NO 3 (radical addition to the aromatic ring) radical reactions proceed by different reaction pathways. Eliminating dimethyl sulfide and benzene derivatives (except benzaldehyde) from regression

Night-time NO 3

radical reactivity prediction

by a simple calculation of kNo3 from a plot - l o g kso 3 vs Ei.v. By use of Equations 3, 4 and 5 together with the average tropospheric night-time NO3 concentration, the a priori estimation of a mean tropospheric half-life of organic compounds of environmental concern can be made (Gfisten et al., 1984; Atkinson et al., 1984a). By assuming a pseudo first-order behaviour of radical reactions in the troposphere, the tropospheric half-life, rl/2, of a chemical compound can be calculated:

15

e 13

1

.......

•

'o 9

•

,

,

10

11

12

7712 = In 2/(k x" IX]),

-Iogk OH

Fig. 2. Plot

of

- l o g k ~ o , (in

77

cm3mole -

cule i S - l ) vs -logko, (in cm3molecule 1S-~) for aliphatic compounds at room temperature. analysis resulted in an improved correlation and also in a mechanistically and statistically more reasonable and meaningful quantitative model, described by Equation 5 and its statistical parameters: - log kNo3= -- 18.86( + 1.56)+ 3.05( -t-0.15)( -- log koH),

(5) where N = 5 7 , r=0.941, s=0.535, FI 55=426, E V= 88.4%. It is fair to conclude that Equation 5 can be used with confidence to calculate NO 3 radical reaction rates for aliphatic compounds from their measured OH radical reaction rates and to cross-check the predictions made by use of ionization energies. The correlation of koH vs kNo~ of Equation 5 is depicted in Fig. 2. It is also of interest to point out that a poor correlation (correlation coefficient around 0.5) is found between kNo, and ko. data for benzene derivatives. It remains to explain why benzaldehyde, phenol and the cresols behave like 'aliphatics' in Fig. 1 where they are well located on the regression line for 'aliphatics'. Their reaction rates are markedly faster, by factors of 104-105 , than those of the other benzene derivatives in Table 1. Generally, little is known about the reaction mechanism of NO 3 radicals with organics. Carter et al. (1981) raised strong arguments for a rapid hydrogen atom abstraction reaction by NO 3 from the weakly phenolic O - H bond. The same reaction mechanism was discussed for the NO 3 reaction with acetaldehyde (Morris and Niki, 1974). It is interesting to note that in the plot - l o g ko~ vs E~.~benzaldehyde is also located on the 'aliphatic' regression line (G~sten et al., 1984) in accordance with the established aldehyde hydrogen atom abstraction mechanism. Thus, it seems that the H atom abstractions from the weak O - H bond of phenol and cresols, and from the H - C O bond of benzaldehyde are responsible for their fast reaction rates with N O 3 radicals and their behaviour as 'aliphatics'. Prediction of the tropospheric residence times

The purpose of this paper was not to replace sound reaction rate measurements of kNo~ in the laboratory

(6)

where [X] is the concentration of the free radical active in the troposphere such a s N O 3 , OH, O(3P) etc. (Finlayson-Pitts and Pitts, 1986). From Fig. 1, the half-life of the organic compounds of Table 1 can be directly obtained for a 'clean' troposphere with 10 ppt ( ~ 2.4 × l0 s molecules cm- 3 ) of N O 3 radicals during night-time hours. The residence times so obtained of organic molecules at night can then be intercompared by their daylight removal in the reaction with OH radicals (Gfisten et al., 1984) in the 'clean' troposphere. With an average tropospheric night-time NO 3 concentration of ~ 2 × 10 9 molecules cm -3 (i.e. ~-100ppt) and an OH concentration during daylight hours of 2× l 0 6 moleculescm -3 for a moderately polluted urban atmosphere (Atkinson et al., 1984a), a NO 3 rate constant of 2.33x10 -14 cm 3 molecules-I s-~ at night equals the halflife for a degradation of a given organic compound during the day. Thus, with the environmental concentrations indicated above for OH and NO3, all organic compounds in the lower left part of Fig. 2 are degraded faster by NO 3 than by OH radicals while the organic compounds in the upper right part of the correlation in Fig. 2 are degraded faster by OH than by NO 3 radicals. This estimation method constitutes a safe upper limit for judgement of the persistence of an organic molecule in the troposphere. If there are other loss processes in the troposphere such as direct photolysis, reaction with ozone, or deposition to soil, the half-life predicted from Equation 6 will be too long. Acknowledyement We thank Dr Branka Kova~ from the Institute Ruder Bo~kovi6in Zagreb for measuring the ionization energies of the terpenes. A. S. gratefully acknowledges the financial support by the EEC, Brussels, given as S&T grant. REFERENCES

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78

ALEKSANDARSABLJI(~and HANS GOSTEN

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