OZONE CHEMILUMINESCENT DETECTION OF OLEFINS:

HYDROCARBON EMISSIONS POTENTIAL APPLICATIONS FOR REAL-TIME MEASUREMENTS OF NATURAL

Nancy A. MarIey*, Jeffrey S. Gaffhey and Mary M. Cunningham

Environmental Research Division Argonne National Laboratory, Argonne, Illinois, 60439

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Extended abstract of oral presentation (Paper J3.11) to be presented at the 78th Meteorological Society Annual Meeting, Phoenix, Arizona, January 11-16, 1998.

October 1997

This research was carried out at Argonne National Laboratory for the US. Department of Energy under contract No. W-3 1-109-ENG-38 as part of the Office of Biological and Environmental Research Atmospheric Chemistry Program.

The submitted manuscript has been created by the University of Chicago as Operator of Argonne National Laboratory ("Argonne") under Contract No. W-31-109-ENG-38 with the U.S. Department of Energy. The U.S. Government retains for itself, and others act- ing on its behalf, a paid-up, nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, dis- tribute copies to the public, and perform pub- licly and display publicly, by or on behalf of the Government.

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OZONE CHEMILUMINESCENT DETECTION OF OLEFINS:

HYDROCARBON EMISSIONS POTENTIAL APPLICATIONS FOR REAL-TIME MEASUREMENTS OF NATURAL

Nancy A. Marley*, Jeffrey S. Gaffney, and Mary M. Cunningham Argonne National Laboratory

1. INTRODUCTION

Natural reactive hydrocarbons which are emitted from both hardwood and softwood forests can be important in the formation of atmospheric oxidants and aerosols when they interact with the energy-related emissions of oxides of nitrogen and sulfur. Isoprene, the best studied of these natural olefins, is predominately produced by deciduous (hardwood) trees. Isoprene is a hemiterpene (C-5), that is quite reactive with OH radical and can lead to the formation of formaldehyde, methacrolein, methyl vinyl ketone, and other photochemically active products. The emission of isoprene from plants is known to be directly associated with the activity of plant stomata; studies to date indicate that isoprene emissions show a strong diurnal variation that is dependent on temperature, relative humidity, and light intensity (Baldocchi, ef a/., 1995). The ultimate fate of isoprene in the atmosphere is controlled by the competitive processes of chemical oxidation and deposition to plant and soil surfaces (Gao, ef a/., 1993).

Monoterpenes (C-IO), such as a-pinene, p- pinene, and d-limonene are the most predominant natural olefins released from pine (softwood) forests. The emission rates of these heavier hydrocarbons do not show the strong correlation with stomatal activity associated with isoprene emissions. The basal emission rate appears to be primarily a function of the monoterpene pool size within the plant and its vapor pressure (Monson, et a/., 1995). Therefore, variations of the emission rate for a single plant species are controlled by temperature effects of the monoterpene vapor pressure or by plant wounding which triggers increases in monoterpene synthesis (increase in pool size). The monoterpenes are very reactive with ozone and nitrate radical, as well as with OH, and may play an important role in the nighttime chemistry of the lower troposphere.

The atmospheric concentrations of isoprene and the monoterpenes, as predicted by the rates of emission, reactio, and deposition, are controlled by a

* Corresponding author address: Nancy A. Marley, Environmental Research Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439. e-mail: marfey@anl.gov.

complex series of biological, physical, and chemical processes. A great deal of uncertainty still exists in both the emission rates and atmospheric fate of these biogenic hydrocarbons, partly because of the lack of accurate, rapid response techniques for measurement of these reactive hydrocarbons (Cavis, et a/., 1994).

Traditionally, two approaches have been used to determine the concentrations of reactive hydrocarbons in the atmosphere. The first is the nonmethane hydrocarbon analyzer, which is based on flame ionization detection (FID) after separation of methane on a short precolumn. This method is simple and rapid but yields only the amount of total carbon (excluding methane) in the air sample. The second method relies on detailed gas chromatography to determine the constituents in a hydrocarbon mixture after the whole- air sample is trapped in a stainless-steel sampling vessel or separating the hydrocarbons onto adsorption tubes. This method yields detailed information of the hydrocarbon mixture but is time consuming and does not have sufficient time resolution for use in field flux measurements or laboratory kinetic studies.

A chemiluminescence analyzer has been constructed that takes advantage of the temperature dependence of the ozone-hydrocarbon reaction. When operated at a temperature of 170OC, the analyzer functions as a total nonmethane hydrocarbon analyzer with sensitivities 10-1 000 times better than a conventional FID (Marley, N.A. and J.S. Gaffney, 1997). However, with operation at varying temperatures, the chemiluminescent signal reflects the differences in rates of reaction of the hydrocarbons with ozone. Preliminary studies at room temperature indicated that the relative rates of reaction of isoprene, a-pinene, p-pinene, and limonene with ozone correlated with the observed chemiluminescence signal (Gaffney et a/., 1985).

When hydrocarbons are grouped in classes of similar structure, their rates of reaction with electrophillic atmospheric oxidants (e.g. OH, 03, NO3) can be correlated with each other (Gaffney and Levine, 1979). By varying the temperature of the reaction chamber, the chemiluminescence analyzer can be tuned to more reactive classes of hydrocarbons. Therefore, the chemiluminescence analyzer has the ability to determine atmospheric hydrocarbon concentrations as a function of class and will also provide a measure of the atmospheric reactivity of the hydrocarbons.

We have examined the temperature dependence of ozone chemiluminescent reactions as a means of monitoring volatile organic compounds. Reported here are recent studies on the use of ozone chemiluminescent detection for real-time monitoring of olefins (including isoprene and the monoterpenes) and on the application of fast gas chromatography coupled to such a detection system for rapid differentiation of these natural olefins. Potential applications of this instrumentation are in the areas of dry deposition and in tracking of air parcels associated with natural forest ecosystems.

2. EXPERIMENTAL APPROACH

An ozone chemiluminescent reaction chamber was constructed on the basisof a design currently used for NOx chemiluminescent detection by Monitor Labs. Details are available elsewhere (Marley and Gafiey, 1997). The reaction cell was heated by using a 0.25-in. diameter cartridge heater (Chromalox Type CIR) and a temperature controller unit (Omega Series CN8500) with a type-# thermocouple that monitored the reaction temperature. Ozone was generated by using an electrical corona-discharge-type generator (Matheson Gas Products Model GEN-021) with bottled oxygen. This generator produces ozone in the oxygen stream at a maximum of 6 gmlhr of. The total ozone/oxygen flow rate into the reaction chamber was 70 mumin.

Light detection was achieved by using a large- area, head-on photomultiplier tube (Hamamatsu R1332) and a variable high-voltage supply (Hamamatsu Model C3350). The tube was cooled to -30% by a peltier heat pump assembly (Amherst scientific Corp. Model 7602. The chemiluminescent signal was optically chopped and detected by using a lock-in amplifier (Oriel, Merlin).

3. RESULTS AND DISCUSSION

The temperature at which a chemiluminescent signal is observed from the reaction of a hydrocarbon with ozone is a measure of the reactivity of the hydrocarbon. At low temperatures only the very reactive hydrocarbons will give an obsewable signal. As the temperature of the reaction chamber is increased, less reactive hydrocarbons begin to show chemiluminescence (Figure 1). At 1 OOOC the aromatics begin to react, and at 125OC the unreactive alkanes give a chemiluminescent signal.

Because olefins are very reactive, the chemiluminescent reaction with ozone proceeds at room temperature and shows little increase with temperature. The reaction of ethylene with ozone has been used commercially to monitor ozone concentrations in the troposphere, but it could just as easily be used to monitor ethylene concentrations.

5 -

0 50 100 150 200

TEMPERATURE

Figure 1. Temperature dependence (OC) for the ozone chemiluminescent reaction of alkanes e), alkenes (o), aromatics (e), and aldehydes (0). Signal response is given in integrated counts X 106,

350

300

250

200

150

I

100

50

0

I I

I I

4 I

0 50 100 150 200 TEMPERATURE

Figure 2. Temperature dependence (OC) for the ozone chemiluminescent reaction of the biogenic olefins, isoprene (.), limonene (o), and a-pinene e). Signal response is given in integrated counts X 106.

Because many other olefins, including isoprene and the monoterpenes, also react with ozone at room temperature, the reaction could be used to monitor reactive alkenes in the atmosphere.

The temperature dependence of the natural alkenes is shown in Figure 2. The behavior of isoprene is similar to that of the simple alkenes shown in Figure 1. The reaction proceeds at room temperature and shows little increase with increasing temperature. This is typical of an addition reaction mechanism. The reaction of ozone with the monoterpenes (limonene and apinene) proceed at room temperature similar to the simpler alkenes. However, the reaction with ozone also gives an increasing signal at elevated temperatures. This is an indication that the reaction proceeds by addition at room

_. , -

temperature and also by abstraction at temperatures above 125OC similar to that shown by the alkanes in Figure 1.

At 170OC all hydrocarbons react to give a chemiluminescent signal in the reaction chamber and the detector behaves as a total non-methane hydrocarbon detector. The detection sensitivities of the ozone chemiluminescence detector operated at 170OC have been compared with those of a FID under similar conditions (Marley and Gaffney, 1997). Table 1 includes a partial summary of these results. All compounds studied showed some detection sensitivity enhancement over the conventional FID. Detection sensitivities for the biogenic hydrocarbons were 50 - 1250 times better for chemiluminescence detection than for the FID with the terpenes showing the best improvement.

Table 1. Detection sensitivities for the ozone chemiluminescence detector and sensitivity enhancement factors compared to the FID.

Compound Sensitivity Enhancement (countslpg)

Alkanes 3.8 x 105

Isoprene 3.4 x 106 Limonene 3.2 x 107 a-Pinene 4.0 x 107

Alkenes 1.5X 106

Aromatics 3.0 X 106 Aldehydes & Ketones 2.6 X 106 Alcohols 1.9x 106 _-_____________I_---__1___1________1____---------.

12 10 56

1250 1100

87

112 119

4. CONCLUSIONS

Because the biogenic olefins, isoprene, limonene and pinene react with ozone at room temperature to give a chemiluminescent signal, while other classes of atmospheric hydrocarbons only react at higher temperatures, the ozone chemiluminescence detector can be used as a fast-response natural hydrocarbon detector when operated at room temperature. To achieve sensitivities up to 1000 times better than a conventional FID, the detector can also be operated at 17OOC with the addition of a short capillary column to separate the olefins of interest. This approach will be useful for both field flux measurements and laboratory kinetic studies.

5. ACKNOWLEDGMENT

The authors wish to gratefully acknowledge the support of the United States Department of Energy, Office of Energy Research, Office of Biological and Environmental Research, Atmospheric Chemistry

Program under contract W-31-109-Eng-38. This work was performed at Argonne National Laboratory.

6. REFERENCES

Baldoucchi, D., A. Guenther, P. Harley, L. Klinger, P. Zimmerman, B. Lamb, and H. Westberg, 1995: The fluxes and air chemistry of isoprene above a deciduous hardwood forest. Phil. Trans. R. SOC. Llond. A 350,279-296.

Davis, K.J., D.H. Lenschow, and P.R. Zimmerman, 1994: Biogenic nonmethane hydrocarbon emissions estimated from tethered balloon observations. J. Geophys. Res. 99,25,587-25,598.

Gaffney, J.S. and S.Z. Levine, 1979: Predicting gas phase organic molecule reaction rates using linear free energy correlation’s.: I. O(3P) and OH addition and abstraction reactions. Int. J. Chem. Kinef.. IX, 1 197-1 209.

Gaffney, J.S., R.L. Tanner, T.J. Kelley, and D.J. Spandau, 1985: Use of ozone chemiluminescent reactions with olefins and reduced sulfur compounds in analytical and environmental chemistry. Paper presented at the 189th American Chemistry Society National Meeting, Environmental Division Award Symposium, Miami Beach, Florida, 28 April-3 May.

Gao, W., M.L. Weseley, and P.V. Doskey, 1993: Numerical modeling of the turbulent diffusion and chemistry of Nox, 03, isoprene. and other reactive trace gasses in and above a forest canopy. J. Geophys. Res. 98, 18,339-18,353.

Marley, N.A. and J.S. Gaffney, 1997: A comparison of flame ionization detection and ozone chemiluminescence for the detection of atmospheric hydrocarbons. Atmos. Environm. in press.

Monson, R.K., M.T. Lerdau, T.D. Sharkey, D.S. Schimel, and R.Fall, 1995: Biological aspects of constructing volatile organic compound emission inventories. Afmos. Environm. 29,2989-3002.