AN EVALUATION OF HOW NO 2, NO, O 3 AND RELATIVE HUMIDITY INTERACTS IN THE OXIDATION OF TERPENES

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1 AN EVALUATION OF HOW NO 2, NO, O 3 AND RELATIVE HUMIDITY INTERACTS IN THE OXIDATION OF TERPENES L Pommer 1,2,*, J Fick 1,2, B Andersson 1,2 and C Nilsson 3 1 Department of Chemistry, Environmental Chemistry, Umeå University, Umeå, Sweden 2 Centre for Musculoskeletal Research, National Institute for Working Life, Umeå, Sweden. 3 Unit for Biomass Technology and Chemistry, Swedish University of Agricultural Sciences, Umeå, Sweden. ABSTRACT 3 -carene and α-pinene were injected into a 6-12 meter long simulated ventilation tube (teflon tube i.d. 25 mm) together with the two oxidants ozone and nitrogen dioxide. The levels of the oxidants, reaction time and relative air humidity were varied according to an experimental design. The amount of terpene reacted in the tube varied between % and % for 3 -carene and α-pinene, respectively. Reaction time and ozone level were the parameters that influenced the amount of terpene reacted the most. The influence of relative air humidity was negative. Several interaction terms were identified (e.g. NO 2 -RH, NO-reaction time, O 3 - reaction time), which demonstrate the complexity of the gas-phase chemistry. Theoretical models, simulating the chemistry in the teflon tube, were calculated and compared to experimental data. These models underestimated the amount of terpene reacted with 11.8% for both of the terpenes. INDEX TERMS: Ozone, Nitrogen dioxide, Nitric oxide, Humidity effect INTRODUCTION Numerous organic compounds are emitted into the troposphere from plants, and common emitted compounds are terpenes and isoprene (Fuentes et al., 2000). The levels of terpenes in the atmosphere vary between 0,01-8 ppb depending on time of the day and location (Janson, 1992). The chemical structure of the terpenes was established by Wallach in 1887, by proposing the so-called "isoprene rule", by which the terpenes are considered to be fusions of two or more isoprene (C 5 H 8 ) unities. The different monoterpenes are a mixture of acyclic, cyclic and bicyclic structure. The general formula of monoterpenes are (C 5 H 8 ) 2 and it consist one or two double bondings in the structure which react readily with O 3, OH and NO 3 forming different oxygenated compounds (Grosjean et al., 1992, Hakola et al., 1994, Hoffmann et al., 1997, Wängberg et al., 1997). The reactions occurring when mixing terpene and NO x are slow, but when adding ozone the terpene is degraded to a higher extent than when NO x or ozone were added separately. The aim of the current study was to use experimental design as a tool to evaluate the gasphase reactions of terpenes with a mixture of ozone and nitrogen dioxide at different relative air humidity. The second part of the study was to compare experimental results with the results of a theoretical model where the most important gas-phase reactions were included. * Contact author linda.pommer@chem.umu.se 518

2 METHOD The chemicals used were α-pinene (Aldrich, 98%), (+)- 3 -carene (Aldrich, 90%), NO 2 in N 2 (AGA Gas AB, Sweden), NO in N 2 (AGA Gas AB, Sweden) and sodium sulphite (Acros, anhydrous 98% +). The pure terpenes were injected with a micro-syringe injector (CMA/100, CMA/Microdialysis) in a purified air stream at 20-22ºCwith a flow rate of l min -1 and a RH of 15-42%, in order to generate a gas-phase concentration of the terpenes of ppb ( µg m -3 ). The test atmosphere was led into a 6 or 12 m long Teflon chamber (i.d. 25 mm) where it was possible to simultaneously collect samples 4, 44, 105 or 213 s after mixing/evaporating the chemicals (Fick et al, 2002). The test atmosphere was sampled ( l, 10 min) with the Tenax TA adsorbent (60-80 mesh, 200 mg) packed in stainless steel tubes (Perkin Elmer). The amount of α-pinene and 3 -carene sampled were ng. During sampling, Na 2 SO 3 scrubbers were used to remove O 3 and NO 2 in front of all Tenax TA tubes, in order to minimise artefacts during sampling (Pommer et al., 2002). Experimental design To study the system described above, experimental design was an invaluable tool to get reliable results with as few experiments as possible (Box et al., 1978). All parameters were varied simultaneously, except the terpene concentration, which was kept constant. When using experimental design it was possible to extract information about the effects of the different parameters. The interaction between the different parameters could be estimated as well. The experimental design was evaluated using MODDE 5.0 from Umetrics AB, Sweden. The relationship between the parameters is evaluated with multiple linear regression (MLR), which gives a model relating the changes in the different parameters to the changes of the response. The model will also indicate which parameters are important, if they have positive or negative influence on the response, and the influence on the response of a combination of parameters. The models were evaluated using R 2 and Q 2. R 2 described the variation of the data included in the model. The predictive capacity of a model was described by Q 2 (cross validation). The results from the experimental design set of experiments were interpreted using the coefficient plot and the contour plot. The coefficient plot showed which parameters and interactions between parameters that influenced the response the most, both in positive and negative direction with a 90% confidence interval. The contour plot show the amount of terpene reacted at different settings of the parameters. Theoretical modelling Using a mathematical tool gave us the opportunity to compare experimental data with theoretical data involving rate constants and reaction pathways. The mathematical calculations was made using FACSIMILE, a data program that calculates levels of chemicals through coupled input data (differential equations, initial levels, and rate constants) (Malleson et al., 1990). The theoretic atmosphere modelled was similar to the atmosphere that was used during the experiments except for RH that was not taken in consideration by the reaction pathways used in the theoretical model. RESULTS AND DISCUSSION Evaluation of the experimental designs The results of the experiments in the reaction chambers were that % of the injected 3 -carene and % of injected α-pinene reacted during the time in the Teflon tube. The 519

3 different amount reacted in the experiments depends on the different initial levels of O 3, NO 2, NO, RH and reaction time (Table 1). Table 1. Variation and the choice of parameters in the experimental design. Parameters Low level (-) Centre level (0) High level (+) O 3 (ppb) NO 2 (ppb) NO (ppb) RH (%) [ppm] 15 [3600] 28 [6800] 42 [10000] Reaction time (s) The R 2 and Q 2 values for the developed model were 0.86 and 0.75 for 3 -carene, and 0.88 and 0.72 for α-pinene. The effect plots, Figure 1-2, showed the influence of the parameters varied in the experiments. Nine terms were presented for 3 -carene and 8 for α-pinene in the effect plots. For a term to be significant it must be larger than the standard deviation (90% confidence level). There were two types of terms, single terms and interaction terms. The single terms showed the change of terpene reacted when the parameter was changed from low to high level. The interaction terms showed the additive effect when both parameters were changed from low to high level. The O 3 level and reaction time are two significant parameters that influence the amount of terpene reacted positively when changed from low to high level. The influence on the amount 3 -carene reacted was less than for α-pinene both concerning the term ozone and reaction time. Raised RH influenced both the amount of 3 -carene and α-pinene reacted in a negative direction. The reason for this could be decreased importance of possible surface reactions or other humidity effects. Raised levels of NO increased the amount of terpene reacted. % tid O3 RH O3*tid NO2 NO*tid Figure 1. Effect plot for 3 -carene. The different columns/terms show the effect of the amount of 3 -carene reacted when the parameter/s was changed from centre to high level. (confidence level 90%) NO2*RH NO NO*RH 520

4 % O3 tid RH NO O3*NO O3*tid Figure 2. Effect plot for α-pinene. The different columns/terms show the effect of the amount of α-pinene reacted when the parameter/s was changed from centre to high level. (confidence level 90%) NO*RH NO*tid Terms affecting the oxidation of 3 -carene The term NO 2 represented the amount of terpene reacted with NO 2 and the size of this term was large in comparison to the O 3 term if the size of the gas-phase rate constants were taken into consideration. The term showing the interaction between O 3 and reaction time could according to Fick et al. (2002) be interpreted as the effect of OH, C.I. or any other radical specie/s that could react with 3 -carene and that was/were formed during the initial reaction between O 3 and 3 -carene. The NO-reaction time interaction was affected of the level of RH. At low levels of RH the highest amount of 3 -carene reacted were at low levels of NO, and at high RH the setting of NO should be high to reach the highest amount of 3 -carene reacted. The negative interaction between NO 2 -RH showed that a change in the RH level had larger influence on the amount of 3 -carene reacted at high settings of NO 2 than on low settings. The strongest interaction was when the NO 2 level was 75 ppb and RH was 42%. The positive interaction between the NO level and RH showed the maximum amount of 3 -carene reacted at high levels of NO and low RH, similar to the NO 2 -RH interaction (Figure 2). a) b) Figure 2. Contour plot of the interaction between a) NO 2 -RH and b) NO-RH showing the amount of 3 -carene reacted at different settings of NO 2, NO and RH. The parameters not included were kept constant, O 3,(50 ppb) NO/NO 2 (50 ppb) and reaction time (105 s). 521

5 Terms affecting the oxidation of α-pinene The terms affecting the oxidation of α-pinene besides O 3, reaction time, RH and NO, were four interaction terms; O 3 -NO, O 3 -reaction time, NO-RH and NO-reaction time (Figure 1). The effect of the parameters O 3, reaction time and RH were almost the same as for 3 -carene, but the level of NO showed almost the double influence of α-pinene compared to 3 -carene. The interaction between O 3 -NO were found only for α-pinene and the highest amount of α- pinene reacted were reached at high settings of both parameters. The O 3 -reaction time term showed the same positive interaction as for 3 -carene. This interaction term contributed with 1.1% and 1.5% of α-pinene and 3 -carene reacted when changing the parameter from low to high level. The interaction of NO-RH was negative in the α-pinene design but positive in the 3 -carene design. This did not mean that the effect of the parameters were opposite to each other but just that that the interaction between the parameters were different (Figure 2, 3). In this study it was shown that the influence of RH was present at different levels. The influence of RH was negative 1.7 and 2.0 for 3 -carene and α-pinene, respectively, and this influence was obvious when studying the response data. The interpretation of the interaction terms NO 2 -RH and NO-RH was not that straightforward. Several studies have been done to examine the effect of water vapour in different systems. Both zero, positive and negative effect of adding water to the air mixture have been observed (Altshuller 1971, Wilson and Levy 1969, Neeb et al. 1997, Warscheid and Hoffmann, 2000, Akimoto 1980, Cox and Penkett, 1972, Tobias et al. 2000). Figure 3. Contour plot of the amount of α-pinene reacted. The settings of parameters not included in the figure were O 3 75 ppb and reaction time 44 s. Evaluation of the theoretical model The results of the theoretical model (initial levels of O 3 75 ppb, NO 2 75 ppb, NO 75 ppb), calculated by the help of a program FACSIMILE, was that 0.3% of 3 -carene and 0.6% of α- pinene had reacted after 45 s. After 215 s the amounts reacted were 1.1% for 3 -carene and 1.5% for α-pinene. Four reaction pathways including the terpenes reaction with O 3, NO 2, NO and NO 3 were evaluated further. The reaction of O 3 with the 3 -carene was the dominating pathway (molecules/s) at shorter reaction times. But after 215 s the reaction time the pathway including NO 3 and 3 -carene was as important as the ozone reaction pathway. For α-pinene the ozone reaction pathway dominates during both shorter and longer reaction times, and the reaction of α-pinene with NO 3 and OH was of equal importance with the stress put on NO 3. Comparison of experimental designs and theoretical models The chemistry of the interaction between O 3, NO 2 and NO in a dark chamber is a well-studied area and the rate constants are numerously tested (Atkinson et al., 1997). The experimental measurements of O 3, NO 2 and NO at the end of the Teflon tube corresponded well with the estimated theoretical values. This demonstrated that the reaction kinetics of O 3, NO 2 and NO (the part of the theoretical model not including terpenes) was well functioning in this experimental setup. This agreement was not found when the amount of terpene reacted was compared between experimental and theoretical values. The theoretical calculations underestimated the reactivity 522

6 of the gas mixture in this experimental set-up. The difference between this experimental setup and the conditions used when determining gas-phase constants was that the influence of RH was excluded and the surface to air volume ratio was very large using this experimental set-up simulating a ventilation duct. CONCLUSION AND IMPLICATIONS In the work by Fick et al the amounts of α-pinene and 3 -carene reacted were 8.1% and 10.9%, respectively, which were 4.8% and 2.9% lower compared to the amounts reacted in this work. The same experimental set-up was used as in the current work with the difference that O 3 was the only oxidising agent added. The effect of adding NO 2 and NO to the system was an increase of the oxidising capacity of the air mixture. The oxidising capacity increased 59% in the α-pinene air mixture and 22% in the 3 -carene air mixture. The two terpenes α- pinene and 3 -carene was affected in different degrees by the oxidising mixture of O 3, NO 2, NO and radicals formed during reciprocal reactions or reaction of these species with the two terpenes. REFERENCES Atkinson, R. Baulch, D L. Cox, R A. Hampson Jr, R F. Kerr, J A. Rossi, M J. Troe, J. Evaluated kinetic and photochemical data for atmospheric chemistry: Supplement VI. Journal of physical and chemical reference data, Vol 26, pp Box, G.E.P., Hunter, W.G. and Hunter, J.S., In Statistics for experimenters. An introduction to design, data analysis, and model building. Bradley, R.A., Hunter, J.S., Kendall, D.G. and Watson, G.S. John Wiley & Sons, New York. Fick, J., Pommer, L., Andersson, B. and Nilsson, C Experimental design as a tool to study the gas-phase ozonolysis of monoterpenes. A feasible way to evaluate the effect of OH radicals and other species formed during chemical reactions. Submitted to Atmospheric Environment. Fuentes, J.D., Lerdau, M., Atkinson, R., Baldocchi, D., Bottenheim, J.W., Ciiccioli, P., Lamb, B., Geron, C., Gu, L., Guenther, A., Shrakey, T.D. and Stockwell, W Biogenic hydrocarbons in the atmospheric boundary layer: A review. Bulletin of the American Meterological Society, Vol 81, pp Grosjean, D., Williams, E.L. and Seinfeld, J.H Atmospheric oxidation of selected terpenes and related carbonyls: Gas-phase carbonyl products. Environmental Science & Technology, Vol 26, pp Hakola, H., Arey, J., Aschmann, S.M. and Atkinson, R Product formation from the gas-phase reactions of OH radicals and O 3 with a series of monoterpenes. Journal of Atmospheric Chemistry, Vol 18, pp Hoffmann, T., Odum, J.R., Bowman, F., Collins, D., Klockow, D., Flagan, R.C. and Seinfeld, J.H Formation of organic aerosol from the oxidation of biogenic hydrocarbons. J Atmospheric Chemistry, Vol 26, pp Janson, R Monoterpenes from the boreal coniferous forest. Their role in atmospheric chemistry. Ph. D. Thesis ISBN x Malleson, A.M., Kellet, H.M., Myhill, R.G. and Sweetenham, W.P., Harwell Lab, Oxfordshire. Pommer, L., Fick, J., Andersson, B. and Nilsson, C Development of a NO 2 scrubber for accurate sampling of ambient levels of terpenes. Atmospheric Environment, Vol 36, pp Wängberg, I., Barnes, I. and Becker, K.H Product and mechanistic study of the reaction of NO 3 radicals with alpha-pinene. Environmental Science & Technology, Vol 31, pp