C27. Kinetics of the Gas-phase Reactions of the NO3 Radical with selected Monoterpenes over the Temperature range 296-430 K, using L.I.F. Dectection

 

E. Martinez, B. Cabañas, A. Aranda and P. Martin

Universidad de Castilla-La Mancha

Dpto Química-Física

Facultad de Ciencias

Campus Universitario

13071 Ciudad Real (Spain)

 

Introduction

Monoterpene hydrocarbons (C10H16) are important constituents of the organic compounds emitted into the atmosphere from vegetation1.On a regional and global scale the emissions of these biogenic compounds are comparable to, or exceed, the emission of non-methane organic compounds from antrhropogenic sources2,3.The oxidation of natural organic compounds has been recognized as an important atmospheric source of CO and other reactive intermediates such as HO2 and RO2 and as important precursors in photochemical oxidant (ozone) formation in urban and rural areas. From this it can be inferred that biogenic non-methane hydrocarbons (NMHC) emissions contribute significantly to regional-scale air pollution.1-3 .These compounds, as other unsaturated compounds, react in the atmosphere with OH radical, O3, NO3 radical and O(3P) atom with their relative importance depending on atmosphere conditions. During the day the most important removal processes for terpenes are the reactions with OH radical and with O3. At night the high atmospheric concentrations of NO3 radical and its fast reactions with different organics, have led to suggestion that reactions with NO3 may dominate the nightime chemistry of volatile biogenic organic compounds.4

 

Experimental

Reactions of NO3 radical with terpenes have been subject of several group studies, using principaly a relative technique 5-9 and only one temperature dependence( for a -pinene) has been published.

The aim of the present study was to investigate the temperature dependence of the reactions of NO3 with atmospherically important monoterpenes (a -pinene, b -pinene and camphene). At room temperature the rate coeficients are 5.78 x 10-12, 2.16 x 10-12 and 6.64 x 10-13 molecule cm-3 s-1 for a -pinene, b -pinene and camphene respectively. These values are in excellent agreement with the absolute rate constant measured by other authors.8,10 The study at different temperatures provides us the Arrhenius parameters and some information about the posible mechanism.

The experiments were carried out with an absolute kinetic technique, using the flow tube reactor with laser induced fluorescence(L.I.F.), to monitor NO3 concentration11 . Figure 1 shows a schematic diagram of the experimental system.

Nitrate radical was generated by reaction of nitric acid vapour with fluorine atoms. The NO3 was detected by monitoring the fluorescence emitted after exciting the (0-0) 2E´¬ 2A2´ transition of NO3, pumping with l = 662 nm radiation from a dye laser.

 

Figure 1. Schematic diagram of apparatus.

The experiments were carried out without a large excess of reactants over NO3. The ratios [terpene] /[NO3] were between 1 and 3, then the integrate rate equation applyed to these reaction conditions was:

 

ln [ (M-Xa)/(M(1-Xa))] = (Bo-Ao)kt (I)

 

where M = [monoterpene] t/[NO3] t, Bo = [monoterpene] o, Ao = [NO3] o and Xa = ([NO3] o-[NO3] t)/[NO3] o

The rate constants obtained at differents temperatures are listed in table 1.

Tropospheric half lives of these compounds have been estimated at night based on reactions with NO3 and during the day for the reactions with the OH radical. The results are summarized in table2.

 

Table 1. Summary of the measured rate constants and Arrhenius parameters.

Compound

T/K

k/ 10-12 cm3 molecule-1 s-1

Ea/kJ mol-1

A/ 1012 cm3 molecule-1 s-1
 

296

2.16 ± 0.17

    

b -pinene

336

3.84 ± 0.24

14.1 ± 1.1

758 ± 29
 

362

6.03 ± 0.51

    
 

391

8.73 ± 0.63

    
         
 

296

5.78 ± 0.40

    

a -pinene

336

4.81 ± 0.39

-4.7 ± 0.2

0.93 ± 0.07
 

391

3.87 ± 0.27

    
 

420

3.36 ± 0.12

    
         
 

296

0.66 ± 0.17

    

camphene

349

0.58± 0.25

-2.9± 0.3

0.21± 0.9
 

403

0.50± 0.40

    
 

433

0.46± 0.37

    
         
         

 

Table 2. Calculated half-lives

Compound kNO3

/10-12cm3 molecu1s-1

 kOH

/10-11cm3

molecu1s-1

 t OH,

day-time

minutes

 t NO3,

nigt-time

minutes
b -pinene 2.16 7.76a 14.9 5.3
a -pinene 5.78 6.00a 19.2 2
camphene 0.66 5.33b 21.6 17.4

[ NO3] = 109 molecule cm-3; [ OH] = 106 molecule cm-3.a From ref.10, b from ref. 8

 

References.

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(11)- E. Martinez, B. Cabañas, A. Aranda and R. P. Wayne, J. Chem. Soc. Faraday Trans., 1996, 92, 53.