The Effect of Nitrogen Admixture in Carbon Dioxide on Formation of Ozone in the DC Corona Discharges

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1 WDS'1 Proceedings of Contributed Papers, Part II, 118 1, 1. ISBN MATFYZPRESS The Effect of Nitrogen Admixture in Carbon Dioxide on Formation of Ozone in the DC Corona Discharges Z. Lichvanová, J. Országh, Š. Matejčík Department of Experimental Physics, Comenius University, Mlynská dolina F, Bratislava, Slovakia. N. J. Mason Department of Physics and Astronomy, The Open University, Milton Keynes, MK7 6AA, Buckinghamshire, United Kingdom. Abstract. The aim of the experiment was simulation of processes present in the atmosphere of Mars. It consists mostly of (95%), nitrogen, argon, oxygen, etc. We focused on nitrogen and added different concentration of nitrogen (1%, 3%) to the because argon and nitrogen are the second most abundant gases in the Martian atmosphere. Positive and negative corona discharges were used to initiate plasmachemical processes. The UV spectrometry was used to analyse discharge influence on the gas mixture. We were interested in the influence of nitrogen on formation of ozone as it is considered to be an important biomarker. Introduction Mars has always been one of the most interesting planets in our Solar system for people especially because of the conditions on its surface. As they are relatively similar to the Earth conditions the question of sustaining life has emerged. Scientists have undertaken a lot of missions to gain basic information about the atmosphere, its composition and about active chemical and physical processes. The Martian atmosphere contains approximately 95.3% of,.7% of nitrogen, 1.6% of argon. The temperature fluctuates from C to 14 C during the year. The Martian day lasts 4.7 hours and atmospheric pressure is in the range of 5 1 mbar. This atmosphere does not contain ozone layer therefore UV radiation hits the surface of the planet directly and initiates different elementary processes near the surface. For initialization of these processes in laboratory conditions slow electrons can substitute the UV radiation. These are present in the drift region of corona discharge. Corona discharge was used for atmosphere mimics many times e.g. [1] Many scientists dealt with corona discharges, dissociation of and generation of ozone in Martian atmosphere. Niles et al. [] dealt with corona discharge generated in flowing in 197. They published research about reactions of ionospheric importance in corona. He used air with admixtures, NO, NO, N O. The initial NO density greatly affected the negative and observable positive ion densities. Evans and Inculet [3] published a paper focused on experimental measurements of positive and negative corona discharge. Discharge was created in cylindrical system of electrodes. They registered light emitted from initial discharge to spark. They presented resulting graphs, which provided reasonable estimate of the radius of the corona layer for corona currents from inception to sparkover. Brambilla et al. [4]were examining charge in unipolar corona. They were generating corona in cylindrical system of electrodes and determined charge density, electric current and losses in corona. Entire experiment was focused on processes in Earth s ionosphere. The electric field strength at the conductor surface increased with voltage keeping quite close to the geometrical field E g. The remarkable agreement was obtained with results of other researches. Cenian and Chernukho et al. [5] dealt with modelling of plasma-chemical processes in lasers during generating of corona discharge. They have proved that change of to CO depends on the ratio E/N. Higashi [6] and Weiss [7] have proved that concentration of in mixture of gases N - and pure is decreasing in DC corona discharge. Based on that authors suggested moving part of gas back to combustion chamber of diesel engine because of reduction of excessive emission of. Maezono and Chang [8] were studying the reduction of concentration of from combustion gases by DC corona torches. The corona torch consisted of two small diameter hollow electrodes and 118

2 LICHVANOVÁ ET AL.: EFFECT OF N IN ON FORMATION OF O 3 IN DC CORONA gas flow entered the upstream cylindrical hollow electrode and existed at a downstream cylindrical electrode. The combustion gas (N : O : =.745:.15:.15) was used together with different mixtures of argon gas. The results showed that the gas reduction rate seeded with argon increased with increasing corona current until % of argon mixture and decreased with increasing argon mixture above %. Černak and Skalny [9] were investigating the dependence of discharge current on concentration of ozone at pressures ( ) kpa in negative corona discharge. The voltage on the electrodes was constant and the corona discharge was generated in short-time pulses (3s). Concentration of ozone was calculated from absorption of UV radiation. Chen and Davidson [1] focused on the production of ozone in positive corona discharge. They studied physical processes in corona and chemical aspects of ozone formation. The 8% of produced ozone was result of presence of excited molecules of nitrogen and oxygen. Concentration of ozone was increasing with increasing discharge current and diameter of wire and decreasing with rising temperature. Discharge in atmosphere of Mars was examined by Manning [11]. Paschen s curves were measured in different mixtures of gases, which are present in this atmosphere. Measured results were compared with literature concerning break-down voltage in, N and H. and Mars gas had comparable curves suggesting that the addition of small amounts of N and Ar did not considerably alter the discharge potential. In this research, we were interested in ozone generation in corona discharge fed by and mixtures of and N (1%, 3%). Experiment A scheme of the experimental apparatus is shown in Figure 1. The experiment was carried out at atmospheric pressure and ambient temperature. It was made in flowing regime. The mixture of gases was flowing through the discharge reactor which was located outside the UV spectrometer during the entire experiment. Then the gas flowed into the cell of UV spectrometer (Shimadzu). The gas flow rate was kept constant 1 cm 3 /min. The used mixtures were pure, 1% N, 3% N. The gases were flowing through the positive and negative corona. The discharge reactor contained cylindrical system of electrodes with 1cm of active length. The discharge current and the voltage on the electrodes were measured by voltmeter and ammeter and recorded using the computer. The UV spectrometer was used to determine the concentration of generated ozone. After each voltage change the system was let to relax for 3 minutes and then the Figure 1. Experimental apparatus (VN high voltage power supply, A ammeter, V voltmeter, K discharge reactor, UV UV spectrometer, P flow controllers,, N gases. 119

3 LICHVANOVÁ ET AL.: EFFECT OF N IN ON FORMATION OF O 3 IN DC CORONA transmittance was read by the UV spectrometer. The concentration of ozone was calculated according to the Lambert-Beer formula nσl T = e (1) where T is transmittance of UV light, n is concentration of ozone, σ is ozone absorption cross section and l is the length of the UV cell. The transmittance was measured near the maximum of the ozone absorption cross section at 55 nm. Results and Discussion The current-voltage characteristic for corona discharge is defined as I 8πε µ U ( U U ) = () l R ln R / r where ε is permittivity of vacuum, μ is mobility of ions, R is radius of outer electrode, r is radius of inner electrode, l is length of the inner electrode and U o is onset voltage. This is semi-empirical formula for coaxial system of electrodes. The current-voltage (CV) characteristics were measured in positive and negative corona discharge fed by pure and different mixtures of and N (1%, 3%). Electric current is recalculated on 1 cm. l- the length of the inner electrode, is 1 cm. Positive corona (CV characteristic) Discharge current increased with electrical voltage according to formula (). Addition of the nitrogen in small amount caused an increase of the discharge current. In discharge reactor, there were present chemical processes, which produced ions of nitrogen and oxygen. The mass of nitrogen ions is slightly lower than oxygen ions so their mobilities can be slightly higher. Apart from that the excited states of nitrogen can contribute to increasing the level of ionization if the gas mixture. These processes could be responsible for the increase of the discharge current after addition of nitrogen. [1] Negative corona (CV characteristic) Influence of the added nitrogen in the is not clear in comparison to the positive corona. The reason is that in the negative corona the discharge current is composed of electron and ion components. The electrons have very high mobility in comparison to ions and thus affect the measured discharge current significantly. On the other hand in positive corona the measured discharge current is composed only of ions. The values of discharge current used in current-voltage characteristics in both positive and negative corona were recalculated over the length of inner electrode. In Figures 4 and 5 the concentrations of ozone in dependence on Becker s parameter η are shown. The Becker s parameter η expresses an amount of energy handed over to the unit volume of the gas by the corona discharge. The Becker s parameter is defined by following formula U I η = (3) Q where U is electrical voltage, I is discharge current and Q is the gas flow rate. The concentration of ozone was calculated by formula (1) and transmittance of UV light was determined by UV spectrometer. Positive corona (concentration of ozone) The concentration of ozone in positive corona dramatically decreased after mixing of nitrogen to the pure. Even 1% of nitrogen affected the concentration of ozone significantly. Negative corona (concentration of ozone) Adding of nitrogen to the pure did not influence concentration of ozone in negative corona so significantly as in positive corona. Concentration of produced ozone was approximately hundred times higher than in positive corona. 1

4 LICHVANOVÁ ET AL.: EFFECT OF N IN ON FORMATION OF O 3 IN DC CORONA.4.3 I [ma/cm]. Positive corona.1 1% N 3% N U [kv] Figure. Current- voltage characteristic of positive corona for different gas mixtures..5 I [ma/cm]. Negative corona.15 1% N 3% N U [kv] Figure 3. Current-voltage characteristics of negative corona for different gas mixtures. Ozone concentration [ppm] Positive corona 1% N 3% N η [J/cm 3 ] Figure 4. Concentration of ozone depending on Becker s parameter in positive corona. Since the nitrogen is electropositive gas it creates only positive ions. Because of this fact the processes in positive and negative corona are slightly different. In positive corona discharge only the neutral and positively charged particles (positive ions) are present in drift region. On the other hand in the negative discharge only the neutral and negatively charged species (electrons and negative ions) are present in the drift region. In both polarities of the discharge there are ions of both polarities in the ionization region. So in the negative corona there are no nitrogen ions in the drift region and in the positive corona the nitrogen ions are present in the drift region. The ionization regions are similar in both cases. Thus the presence of nitrogen ions seems to be responsible for the significant ozone diminishing effect in positive corona after adding nitrogen. We suggest that this effect of ozone diminishing can be cause by reactions of nitrogen ions with oxygen atoms and molecules such as 11

5 LICHVANOVÁ ET AL.: EFFECT OF N IN ON FORMATION OF O 3 IN DC CORONA Ozone concentration [ppm] Negative corona 1% N 3% N η [J/cm 3 ] Figure 5. Concentration of ozone depending on Becker s parameter in negative corona. N O NO O [13] (4) k 4 = cm 3 s 1 N O NO O [13] (5) k 5 = cm 3 s 1 N O NO N [13] (6) k 6 = cm 3 s The reaction rates of these reactions are high and show that they are very significant in the chemistry of the discharge. Such reactions are impossible in the negative corona drift region as there are no nitrogen ions present. Conclusion We were partly simulating the atmosphere of Mars using discharge reactor with positive and negative corona discharge and flowing mixture of gas (pure, 1% N, 3% N ). We were observing the influence of adding nitrogen as an abundant gas in this atmosphere to the carbon dioxide. The results of experiment show that the processes in positive and negative corona discharge are different. The admixture of 1% and 3% of nitrogen caused significant decrease of concentration of ozone in in positive corona discharge. In negative corona discharge the influence of admixture of nitrogen was weak almost negligible. We assume that this difference was caused by the role of nitrogen ions in the drift region of the positive corona discharge. In the future, when there is more facts about character of processes caused by UV radiation, we will compare our results with these results. References [1] Vušković, L., Ash, R. L., Shi, Z., Popović, S. a Dinh, T.,in SAE Paper, 97499, [] Niles, F.E., in The Journal of Chemical Physics, Vol 5, pp. 48, 197. [3] Evans, R.W., Inculet, I.I.,in IEEE Trans. On Industry Appl. IA 14, 53, [4] Brambilla, R. et al, Gas Discharges,in Physical Approach to Unipolar Corona Space Charge in Book of Contributed Papers Oxford, pp. 191, [5] Cenian, A., Chernukho, A., Borodin, V., Slivinsky, G.,in Contrib. Plasma Phys, Vol. 34, pp. 1 5, [6] Higashi, M., Sugaya, M., Ueki,K., Fujii, K., Plasma processing of exhaust gas from a diesel engine vehicle, in Proc. Int. Conf. Plasma Chem, Vol., pp. 366, [7] Weiss, H.R., Plasma Induced Dissociation of Carbon Dioxide, in Proc. Int. Conf. Plasma Chem., Vol. pp , [8] Maezono, I., Chang, J.S.,in IEEE Trans.On Industry Appl., Vol 6, No 4 pp , 199. [9] Cernak, M., Skalny, J., Veis, S., Dindosova, D.,in Acta Phys. Slov., Vol 9, No.,1979. [1] Chen, J., Davidson, J.H., in Plasma Chemistry and Plasma Processing, Vol., No. 4, pp. 495,. [11] Manning, H.L.K., Kate, I.L, Battel, S.J., Mahaffy, P.R.,in Advances in Space Research 46, pp , 1. [1] Samson, J.A.R., Weissler, G.L., Mobilities of Oxygen and Nitrogen ions, in Phys.Rev. 137, [13] Hokazono, H., Obara, M.,Theoretical operational life study of the closed-cycle transversaly excited atmosferic CO, in J. Appl. Phys. Vol 69, 685,