Environ. Sci. Technol. 1997, 31, 3166-3172
Atmospheric Fate of Carbonyl Oxidation Products Originating from r-Pinene and ∆3-Carene: Determination of Rate of Reaction with OH and NO3 Radicals, UV Absorption Cross Sections, and Vapor Pressures M A T T I A S H A L L Q U I S T , * ,† I N G V A R W A¨ N G B E R G , ‡ A N D EVERT LJUNGSTRO ¨ M† Department of Inorganic Chemistry, University of Go¨teborg, S-412 96 Go¨teborg, Sweden, and Physikalische Chemie/ Fachbereich 9, Bergische Universita¨t-Gesamthochschule Wuppertal, Gaussstrasse 20, D-42097 Wuppertal, Germany
Large yields of dicarbonyl compounds have been found in the atmospheric oxidation of R-pinene and ∆3-carene. These terpenes are emitted in large quantities by biogenic sources, and it is important to know the fate of their reaction products. In this investigation, ultraviolet and infrared absorption cross sections, vapor pressures, and rate coefficients for hydroxyl and nitrate radical reactions have been determined for the main product in each case, i.e., 3-acetyl-2,2-dimethylcyclobutaneacetaldehyde (pinonaldehyde) and 2,2-dimethyl-3-(2-oxopropyl)cyclopropaneacetaldehyde (caronaldehyde). Photolysis lifetimes at noon on July 1 at 50° N using a photolysis quantum yield of 1 are 3.3 h for pinonaldehyde and 5.8 h for caronaldehyde. The infrared absorption cross sections obtained (base e) were (8.08 ( 0.32) × 10-19 and (1.04 ( 0.05) × 10-18 cm2 molecule-1 for pinonaldehyde and caronaldehyde at 1725.4 and 1741.5 cm-1, respectively. Vapour pressures determined by Knudsen effusion measurements are 5.1 and 3.0 Pa at 298 K. The rate coefficients obtained using the relative rate technique for reaction with OH radicals were (8.72 ( 1.14) × 10-11 and (1.21 ( 0.36) × 10-10 molecules cm-3 s-1 for pinonaldehyde and caronaldehyde and for the corresponding NO3 reactions were (2.35 ( 0.37) × 10-14 and (2.71 ( 0.15) × 10-14 molecules cm-3 s-1. The vapor pressures are too high for homogenous nucleation or direct condensation to take place in the atmosphere. The dominant gas-phase removal processes will be reaction with OH radicals and possibly also photodissociation.
Introduction Emission of monoterpenes is a major source of organic compounds in the troposphere. Monoterpenes constitute about 11% of the total global amount of naturally emitted, volatile non-methane organic compounds (1). Locally, the emission of monoterpenes may even dominate the volatile * To whom correspondence should be addressed. E-mail:
[email protected]; fax: + 46 31 7722853. † University of Go ¨ teborg. ‡ Bergische Universita ¨ t-Gesamthochschule Wuppertal.
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organic load into the atmosphere. Kinetic investigations have shown that major sinks for monoterpenes in the atmosphere are reactions with hydroxyl radicals, nitrate radicals, and ozone (2). Some of the products from the oxidation of monoterpenes are believed to contribute to aerosols of natural origin found in the atmosphere (3). Aerosol formation in the atmosphere is an important issue since particles may act as cloud condensation nuclei, may affect visibility, and could locally be troublesome from a health aspect. Aerosol particles may also catalyze heterogeneous reactions and remove potential ozone-forming compounds from the gas phase (4). Product information, e.g., from smog chamber experiments is available for several monoterpene-oxidant systems (3, 5-10). The mass balances reported are in most cases far from closed. The mass deficiency is frequently explained by the formation of aerosols in the reaction mixture, removing material from the gas phase. Part of this aerosol formation could be caused by the high concentration of reactants that is often used in such reaction systems. The major gas-phase products found in monoterpene-oxidant systems are various types of carbonyl species, originating from the oxidative attack on a double bond. Two of the dominant monoterpenes emitted from coniferous forest are R-pinene and ∆3-carene (11). Studies of the oxidation of these two terpenes under atmospheric conditions in static reactors show large yields of 3-acetyl-2,2-dimethyl-cyclobutaneacetaldehyde (known as pinonaldehyde) and 2,2-dimethyl-3-(2-oxopropyl)cyclopropaneacetaldehyde (by analogy, here known as caronaldehyde) (3, 5-10). Both products and starting materials are shown in Figure 1. These 10-carbon atom dicarbonyl compounds are expected to have a much lower vapor pressure than the corresponding monoterpene. They may therefore be suspected of contributing to natural particle formation in the atmospheres and pinonaldehyde has been identified in ambient aerosols (3). Several gas-phase oxidation routes are available if pinonaldehyde or caronaldehyde are to be removed from the troposphere. The ketone and aldehyde groups make them prone to photodissociation. Other possible, competitive channels are reaction with hydroxyl radicals and, during nighttime, reaction with nitrate radicals. The aim of this work was to determine the relative importance of different removal processes of pinonaldehyde and caronaldehyde. The work includes determination of OH and NO3 radical reaction rate coefficients, ultraviolet (UV) absorption cross sections important for photodissociation, and vapor pressure data needed for assessment of the aerosol formation capability of the two dicarbonyls.
Experimental Section Vapor Pressure Measurements. The vapor pressure measurements were made by using a Knudsen effusion apparatus. The substance, the vapor pressure of which is to be determined, is placed in an aluminium container equipped with a copper foil lid, sealed by an o-ring, and held down by a nut. The lid is supplied with a circular effusion orifice. Two nominal orifice diameters were used in this investigation, 0.3 and 0.9 mm, in foils of 0.05 and 0.1 mm thickness, respectively. The complete cell is placed in a copper block inside a vacuum chamber that is pumped by a 120 L s-1 diffusion pump. The pressure in the vacuum chamber is estimated by using a Penning gauge. The copper block is cooled by a two-stage Peltier element unit, set at a constant cooling power suitable for the measurement to be made. The temperature of the block is held constant to within (0.05 K by controlled heating. Pt-100 thermometers are used, both for the temperature controller and for the actual temperature measurement of
S0013-936X(97)00151-X CCC: $14.00
1997 American Chemical Society
FIGURE 1. Structural relation between r-pinene and pinonaldehyde and between ∆3-carene and caronaldehyde. the container. Measurements for the dicarbonyls were made in the temperature range from 255 to 276 K, where the vapor pressures fell between 0.01 and 0.5 Pa. (1 Pa ) 0.01 mbar). This means that the pressure in the vacuum chamber was always at least a factor of 100 lower than in the effusion cell and that the mean free path in the cell was always considerably longer than the orifice diameter but not necessarily greater than the cell dimensions. Experiment times between 2 h and 2 day were used, resulting in weight losses between 0.2 and 5 mg. Experiment times were long as compared with the times for the transients at the beginning and end of an experiment. The vapor pressure may be calculated from the weight loss and the nominal orifice dimension or via a calibration procedure for the orifice. IR Absorption Calibration. Accurate absorbanceconcentration calibrations of pinonaldehyde and caronaldehyde in the infrared (IR) were necessary for the concentration measurements in the determination of UV absorption cross sections. Measurements of IR absorption cross sections were made using the 0.480 m3 borosilica glass reactor described below, in the UV absorption measurements section. Due to the comparatively low vapor pressures of the dicarbonyls, these compounds are difficult to transfer in a quantitative manner to the gas phase, and they are lost by adsorption to the reactor walls. A special procedure was therefore developed to handle these problems. The measurements of the absorption cross sections were performed at low pressure,
