Aerosol Formation from the Reaction of Α-pinene and Ozone Using

A kinetic mechanism was used to describe the gas-phase reactions of α-pinene with ozone. This reaction scheme produces low vapor pressure reaction products that distribute between gas and particle phases. Partitioning was treated as an equilibrium between the rate of particle uptake and rate of particle loss of semivolatile terpene reaction products. Given estimated liquid vapor pressures and activation energies of desorption, it was possible to calculate gas-particle equilibrium constants and aerosol desorption rate constants at different temperatures. Gas and aerosol phase reactions were linked together in one chemical mechanism and a chemical kinetics solver was used to predict reactant and product concentrations over time. Aerosol formation from the model was then compared with aerosol production observed from outdoor chamber experiments. Approximately 10-40% of the reacted α-pinene carbon appeared in the aerosol phase. Models vs. experimental aerosol yields illustrate that reasonable predictions of secondary aerosol formation are possible from both dark ozone and light/NOx α-pinene systems. INTRODUCTION The atmospheric chemistry of non-methane biogenic hydrocarbons has received much attention because of their significant global emissions, high photochemical reactivity, and their high aerosol forming potential. Although the potential of aerosol formation from terpenes was noted as early as 1960 by Went1, the magnitude of the natural contribution by biogenics to the particulate burden in the atmosphere is still not well characterized. In this paper, we will describe the feasibility of a predictive technique for the formation of secondary aerosols from biogenic hydrocarbons. An advantage of this approach is that it has the ability to embrace the range of different atmospheric chemical and physical conditions that bring about secondary aerosol formation. EXPERIMENTAL SECTION Gas-particle samples for this study were generated in a large outdoor 190 m Teflon film chamber (Kamens et al.,1995, Fan et al., 1996). All experiments were carried out under darkness to exclude photochemical effects. Rural background air was used to charge the chamber without not any additional injections of oxides of nitrogen. Secondary aerosols were created by the reaction of α-pinene with O3 in the chamber. O3 from an electric discharge ozone generator was added to the chamber over the course of an hour with initial concentrations ranging, depending on the experiment, from 0.25 to 0.65 ppm. It was measured using a Bendex chemilumenesent O3 meter (model 8002, Roncerverte,WV) and calibrated via gas phase titration using a NIST traceable NO tank. O3 addition to the chamber was followed by volatilizing, depending on the experiment, 0.4 to 1 mL of liquid α-pinene into the chamber atmosphere. The gas-phase concentration of αpinene was monitored with an online gas chromatograph (Shimadzu Model 8A, column: 1.5 m, 3.2 mm stainless steel packed with Supelco 5% Bentanone 34) using a flame ionization detector, and calibrated with a known concentration of a mixture of toluene and propylene. Gas and particle phase α-pinene products were simultaneously collected with a sampling train that consisted of an upstream 5-channel annular denuder, followed by a 47mm Teflon glass fiber filter (type T60A20, Pallflex Products Corp., Putnam, CT, USA) and another denuder. In one of the experiments, a parallel sampling system consisting of a filter, followed bv a denuder was also used. The details of the sample workup procedure and quantitative analysis have been reported in previous manuscripts Kamens et al., 1995 and Fan et al., 1996). Carbonyl products of α-pinene-O3 aerosols were derivatized by O-(2,3,4,5,6-pentafluorobenzyl)hydroxyl-amine hydrochloride (PFBHA) as described by Yu et al.,1997, and carboxylic acids, using pentafluorobenzyl bromide (PFBBr) as a derivatizing agent as described by Chien et al., (1998). (±)-α-Pinene, O-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine hydrochloride (PFBHA), pentafluorobenzylbromide (PFBBr), decafluorobibenzyl (internal standard for derivatization), cispinonic acid, n-hexanoic acid, n-octanoic acid, hexane-1,6-dionic acid, and heptane-1,7-dionic acid were all purchased from Aldrich (Milwaukee, WI). In one of the experiments the particle size distribution and aerosol concentration for particles ranging from 0.018 to 1.0 μm were monitored by an Electrical Aerosol Analyzer (EAA) (Thermo Systems, Inc., Model 3030, Minneapolis, MN). Total aerosol number concentration was also measured by a Condensation Nuclei Counter (CNC, Model Rich 100, Environment One Corp., Schenectady, NY). A GAS-PARTICLE MECHANISM As a result of new aerosol compositional information (Jang and Kamens, 1999, Yu et al., 1998) we have developed an exploratory model for predicting aerosol yields from the reaction of α−pinene with ozone in the atmosphere. Reaction pathways (Scheme1 and 2) were constructed from experimentally measured products which include: pinonaldehyde, norpinonaldehyde, pinonic acid, norpinonic acid, pinic acid, 2,2dimethylcyclobutane-1,3-dicarboxylic acid, and hydroxy and aldehyde substituted pionaldehydes and hydroxy pinonic acids. To simplify the mechanism in this study, six generalized semivolatile products were defined: 1. “pinald” to represent pinonaldehyde and norpinonaldehyde, 2. “pinacid” to represent pinonic and norpinonic acids, 3. “diacid” for pinic acid and norpinic acid, (2,2-dimethylcyclobutane1,3dicarboxylic acid), 4. “oxy-pinald” for hydroxy and aldehyde substituted pinonaldehydes (called oxo-substituted), 5. “P3” for 2,2-dimethylcyclobutyl-3-acetylcarboxylicacid, (a pinic acid precursor), and 6. “oxy-pinacid” for hydroxy and aldehyde substituted pinonic acids. A last group, frag, was employed to account for volatile oxygenated products. O O O