The gas-phase ozonolysis of b-caryophyllene (C 15 H 24 ). Part I: an experimental study Richard Winterhalter,* a Frank Herrmann, a Basem Kanawati, a Thanh Lam Nguyen, b Jozef Peeters, b Luc Vereecken b and Geert K. Moortgat a Received 13th October 2008, Accepted 6th March 2009 First published as an Advance Article on the web 25th March 2009 DOI: 10.1039/b817824k The gas phase reaction of ozone with b-caryophyllene was investigated in a static glass reactor at 750 Torr and 296 K under various experimental conditions. The reactants and gas phase products were monitored by FTIR-spectroscopy and proton-transfer-reaction mass spectrometry (PTR-MS). Aerosol formation was monitored with a scanning mobility particle sizer (SMPS) and particulate products analysed by liquid chromatography/mass spectrometry (HPLC-MS). The different reactivity of the two double bonds in b-caryophyllene was probed by experiments with different ratios of reactants. An average rate coefficient at 295 K for the first-generation products was determined as 1.1  10 16 cm 3 molecule 1 s 1 . Using cyclohexane as scavenger, an OH-radical yield of (10.4  2.3)% was determined for the ozonolysis of the more reactive internal double bond, whereas the average OH-radical yield for the ozonolysis of the first-generation products was found to be (16.4  3.6)%. Measured gas phase products are CO, CO 2 and HCHO with average yields of (2.0  1.8)%, (3.8  2.8)% and (7.7  4.0)%, respectively for the more reactive internal double bond and (5.5  4.8)%, (8.2  2.8)% and (60  6)%, respectively from ozonolysis of the less reactive double bond of the first-generation products. The residual FTIR spectra indicate the formation of an internal secondary ozonide of b-caryophyllene. From experiments using HCOOH as a Criegee intermediate (CI) scavenger, it was concluded that at least 60% of the formed CI are collisionally stabilized. The aerosol yield in the ozonolysis of b-caryophyllene was estimated from the measured particle size distributions. In the absence of a CI scavenger the yield ranged between 5 and 24%, depending on the aerosol mass. The yield increases with addition of water vapour or with higher concentrations of formic acid. In the presence of HCHO, lower aerosol yields were observed. This suggests that HCOOH adds to a Criegee intermediate to form a low-volatility compound responsible for aerosol formation. The underlying reaction mechanisms are discussed and compared with the results from the accompanying theoretical paper. Introduction Biogenic volatile organic compounds (BVOC), whose emissions largely exceed anthropogenic emissions, 1 play an important role in the atmosphere. Although isoprene and monoterpenes are the most abundantly emitted biogenic compounds, the sesquiterpenes (C 15 H 24 ) are of special importance due to their high reactivity towards ozone and their large aerosol formation potential. 2 Helmig et al. 3 found that sesquiterpene emission from a variety of pine tree species can account for as much as 29% of the monoterpene emissions. Sesquiterpene emission rates remain highly uncertain because few quantitative emission rate measurements have been made and biological pathways for their formation are not well understood. Kanakidou et al. 4 estimated that the uncertainties in global BVOC emissions could be as high as a factor of 5 for sesquiterpenes and other terpenes, and a factor of 3 for isoprene. Recently, a sensitivity study for mono- and sesquiterpene emissions was performed for the United States taking into account the contribution of individual plant functional types, chemical speciation, light and temperature dependence of the emissions. 5 Sesquiterpenes are emitted from the flowers and foliage of a variety of coniferous and deciduous plants. 3,6,7 Among the most common sesquiterpenes, b-caryophyllene has been observed to be emitted by pine trees, 3 orange orchards 8 and a variety of agricultural plant species such as potato plants, leaves of tobacco, sunflower, maize and cotton. 9,10 Due to the rapid degradation of b-caryophyllene, and the low volatility of some of the degradation products, b-caryophyllene is assumed to have a high particle formation potential. 11 Results from reaction chamber experiments 12–14 and ambient observations have confirmed the important role of b-caryophyllene in the formation of secondary organic aerosol (SOA). Aerosol particles sampled in a coniferous forest in Greece were found to consist partly of photooxidation products of b-caryophyllene. 15 a Max Planck Institute for Chemistry, Atmospheric Chemistry Department, P.O. Box 3060, D-55020, Mainz, Germany. E-mail: winterha@mpch-mainz.mpg.de b Department of Chemistry, University of Leuven, Celestijnenlaan 200F, B-3001, Heverlee-Leuven, Belgium 4152 | Phys. Chem. Chem. Phys., 2009, 11, 4152–4172 This journal is  c the Owner Societies 2009 PAPER www.rsc.org/pccp | Physical Chemistry Chemical Physics Downloaded by University of Texas Libraries on 22 May 2012 Published on 25 March 2009 on http://pubs.rsc.org | doi:10.1039/B817824K View Online / Journal Homepage / Table of Contents for this issue