4.3 Kinetic experimental results and lifetime calculations for nitramines and nitrosamines
4.3.3 Nitramines + Cl
The chlorine atom is one of the powerful oxiding agents playing a big role in removing pollutants from the atmosphere particularly at the Marine Boundary layer (MBL) where it is found in significant amount. Thus, its rate of reaction with nitramines was investigated to evaluate its contribution on removing this group of compounds from the atmosphere.
Recent measurements of Thornton et al., (2010), suggests that Chlorine chemistry may be more important in continental air.
The photo-stability of the nitramines in the Oslo chamber without the presence of Cl2 was investigated in separate experiments, and that no significant photolysis was registered A series of kinetic experiments were carried out employing the relative rate method to determine the rate coefficients of Cl atom reactions with four nitramines. The obtained relative rates and the derived absolute rates for all experiments are compiled in Table 4.3.3.1.
4.3.3.1 CH3NHNO2 + Cl
Three experiments were carried out using CH3OH as a reference compound from which krel = 0.234 ± 0.004 (2V) was derived (Figure 4.3.3.1.1). Also 3 experiments were done with CH3OCHO as reference compound from which krel = 4.07 ± 0.31 (2V) was derived (Figure 4.3.3.1.2).
Figure 4.3.3.1.1. Relative rate plot showing decays of CH3NHNO2 and CH3OH in the presence of Cl atoms plotted as ln{[CH3NHNO2]0/[CH3NHNO2]t} vs.
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Figure 4.3.3.1.2. Relative rate plot showing decays of CH3NHNO2 and CH3OCHO in the presence of Cl atoms plotted as ln{[CH3NHNO2]0/[CH3NHNO2]t} vs.
ln{[CH3OCHO]0/[CH3OCHO]t}.
The statistical errors in the relative rates are smaller than the uncertainty factors in the rate coefficients of the reference compounds. The latter therefore dominate the uncertainty in the absolute rate coefficients. The present results suggest a weighted average for the rate coefficient for the CH3NHNO2 + Cl reaction to be kCH3NHNO2+Cl = (9.3 r 1.1) u 10-12 molecule cm-3 s-1 at 298 K.
4.3.3.2 (CH3)2NNO2 + Cl
Three experiments were carried out with CH3OH as reference compound from which krel
= 0.907 ± 0.010 (2V) was derived (Figure 4.3.3.2.1), and 3 experiments with CH3OCH3
as reference compound from which krel = 0.402 ± 0.011 (2V) was derived (Figure 4.3.3.2.2). Because these two experiments suggest that the absolute rate constant for the (CH3)2NNO2+Cl reaction is almost the same as that of (CH3CH2)2NNO2+Cl (see later), a third series of experiments was carried out employing ethylacetate (CH3CH2OC(O)CH3) as reference compound. The results from these experiments gave krel = 4.35 ± 0.03 (2V) (Figure 4.3.3.2.3). The statistical errors in the relative rates are much smaller than the uncertainty factors in the rate coefficients of the reference compounds. The latter therefore dominate the uncertainty in the absolute rate coefficients. The present results suggest a weighted average for the rate coefficient for the (CH3)2NNO2 + Cl reaction to be k(CH3)2NNO2+Cl = (6.5 r 0.7) u 10-11 molecule cm-3 s-1 at 298 K.
0.00 0.05 0.10 0.15 0.20 0.25
0.0
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ln{[CH3OH]0/[CH3OH]t}
Figure 4.3.3.2.1. Relative rate plot showing decays of (CH3)2NNO2 and CH3OH in the presence of Cl atoms plotted as ln{[(CH3)2NNO2]0/[(CH3)2NNO2]t} vs.
ln{[CH3OH]0/[CH3OH]t}.
ln{[CH3OCH3]0/[CH3OCH3]t}
Figure 4.3.3.2.2. Relative rate plot showing decays of (CH3)2NNO2 and CH3OCH3 in the presence of Cl atoms plotted as ln{[(CH3)2NNO2]0/[(CH3)2NNO2]t} vs.
ln{[CH3OCH3]0/[CH3OCH3]t}.
0.0 0.2 0.4 0.6 0.8 1.0
0.0 0.2 0.4 0.6 0.8 1.0
ln{[Me2NNO2]0/[Me2NNO2]t} krel = 0.907 0.010 42 data points 3 experiments
0.0 0.5 1.0 1.5 2.0 2.5 3.0
0.0 0.2 0.4 0.6 0.8 1.0 1.2
ln{[Me2NNO2]0/[Me2NNO2]t} krel = 0.402 0.011 24 data points 3 experiments
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0.0 0.2 0.4 0.6 0.8 1.0
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
ln{[Me2NNO2]0/[Me2NNO2]t}
ln{[CH3CH2OC(O)CH3]0/[CH3CH2OC(O)CH3]t} krel = 4.35 0.03
38 data points 3 experiments
Figure 4.3.3.2.3. Relative rate plot showing decays of (CH3)2NNO2 and CH3CH2OC(O)CH3 in the presence of Cl atoms plotted as ln{[(CH3)2NNO2]0/[(CH3)2NNO2]t} vs. ln{[CH3CH2OC(O)CH3]0/ [CH3CH2OC(O)CH3]t}.
4.3.3.3 CH3CH2NHNO2 + Cl
Methylformate (CH3OCHO) was used as reference compound in an initial experiment from which krel = 10.24 ± 0.4 (1V) was derived, Figure 4.3.3.3.1. When rates differ by an order of magnitude the relative rate method becomes sensitive to systematic errors and hence CH3OCHO was not used in additional experiments. CH3OH was used as reference compound in 4 experiments from which krel = 0.3027 ± 0.0024 (1V) was derived, Figure 4.3.3.3.2. Another set of 3 experiments with ethylacetate (CH3CH2OC(O)CH3) as reference compound was done from which krel = 1.233 ± 0.007 (1V) was derived, Figure 4.3.3.3.3.
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Figure 4.3.3.3.1. Relative rate plot showing decays of CH3CH2NHNO2 and CH3OCHO in the presence of Cl atoms plotted as ln{[(CH3CH2NHNO2]0/[(CH3CH2NHNO2]t} vs.
ln{[CH3OCHO]0/[CH3OCHO]t}.
Figure 4.3.3.3.2. Relative rate plot showing decays of CH3CH2NHNO2 and CH3OH in the presence of Cl atoms plotted as ln{[(CH3CH2NHNO2]0/[(CH3CH2NHNO2]t} vs.
ln{[CH3OH]0/[CH3OH]t}.
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Figure 4.3.3.3.3. Relative rate plot showing decays of CH3CH2NHNO2 and CH3CH2OC(O)CH3 in the presence of Cl atoms plotted as ln{[(CH3CH2NHNO2]0/[(CH3CH2NHNO2]t} vs. ln{[CH3CH2OC(O)CH3]0/ [CH3CH2OC(O)CH3]t}.
The statistical errors in the relative rates are much smaller than the uncertainty factors in the rate coefficients of the reference compounds. The latter therefore dominate the uncertainty in the absolute rate coefficients. The present results suggest a weighted average for the rate coefficient for the CH3CH2NHNO2 + Cl reaction to be kCH3CH2NHNO2+Cl = (1.88 r 0.22) u 10-11 molecule cm-3 s-1 at 298 K.
4.3.3.4 (CH3CH2)2NNO2 + Cl
Four experiments were carried out with CH3OH as reference compound from which krel = 0.892 ± 0.007 (1V) was derived (Figure 4.3.3.4.1), and 3 experiments with CH3CH2OC(O)CH3 as reference compound from which krel = 3.56 ± 0.05 (1V) was derived (Figure 4.3.3.4.2). The statistical errors in the relative rates are much smaller than the uncertainty factors in the rate coefficients of the reference compounds. The latter therefore dominate the uncertainty in the absolute rate coefficients. The present results suggest a weighted average for the rate coefficient for the (CH3CH2)2NNO2 + Cl reaction to be k(CH3CH2)2NNO2+Cl = (5.5 r 0.7) u 10-11 molecule cm-3 s-1 at 298 K.
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Figure 4.3.3.4.1. Relative rate plot showing decays of (CH3CH2)2NNO2 and CH3OH in the presence of Cl atoms plotted as ln{[(CH3CH2)2NNO2]0/[(CH3CH2)2NNO2]t} vs.
ln{[CH3OH]0/[CH3OH]t}.
Figure 4.3.3.4.2. Relative rate plot showing decays of (CH3CH2)2NNO2 and CH3CH2OC(O)CH3 in the presence of Cl atoms plotted as ln{[(CH3CH2)2NNO2]0/[(CH3CH2)2NNO2]t} vs.
ln{[CH3CH2OC(O)CH3]0/[CH3CH2OC(O)CH3]t}.
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Table 4.3.3.1. Experimental relative rate coefficients and derived absolute rate coefficients (/cm3 molecule-1 s-1) for the reactions of nitramines with Cl atoms. (IUPAC, 2011). Reference compound dimethylether (CH3OCH3), kCl+CH3OCH3 = 1.8 ×10-10 cm3 molecule-1 s-1 (Extracted best value from previous studies). cReference compound ethylacetate (CH3C(O)OCH2CH3), kOH+CH3C(O)OCH2CH3 =1.7×10-11 cm3 molecule-1s-1 (extracted best value from previous studies).
Table 4.3.3.2. Lifetimes for the nitramines with respect to their reaction with cl atom in the atmosphere
The lifetimes are estimated based on a global average concentration of Cl atom at the Marine Boundary Layer of 104 cm-3 (Finlayson and Pitts, 2000, Pszenny, et al., 1993, Singh et al., 1996a).
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From Table 4.3.3.2 above we see that, the atmospheric lifetime of nitramines with respect to reaction with Cl atoms will be around half a month and above. Hence it can be concluded that Cl atom chemistry will not constitute an important loss process for nitramines in the atmosphere.