Secondary organic aerosol formation from plant emissions: Overview of the Jülich Plant Chamber experiments

A considerable fraction of the organic aerosol component is of secondary origin, meaning it is formed through oxidation of volatile organic compounds (VOCs). Plant emissions, e.g. monoterpenes and sesquiterpenes, are a major source of VOCs in the troposphere (Guenther et al., 1995). VOC emissions commonly depend on light and temperature (Guenther et al., 1995). So far most laboratory investigations on the potential to form secondary organic aerosols (SOA) from plant emissions focused on single VOCs such as α-pinene which is probably the most investigated monoterpene (e.g. Saathoff et al, 2003). In this study we investigated the formation of SOA from the mixture of VOCs emitted by spruce, pine and birch trees by oxidation with ozone and OH. The experiments were performed in the Julich plant chamber in order to provide well defined conditions for plants. A fraction of the air carrying plant emissions was transferred from the plant chamber to a reaction chamber. The chamber was operated as a continuously stirred tank reactor. SOA formation from the VOCs introduced into the reaction chamber was initiated by UV-photolysis of ozone. VOC measurements were conducted simultaneously with a Proton-Transfer-Reaction Mass Spectrometer (PTRMS, IONICON, Lindiger et al. 1998) to determine the emission kinetics, and an online-GC-MS system for compound identification. VOC mixing ratios were in the lower ppbv to pptv range. Identification by GC-MS was based on mass spectra and retention times of pure chemicals (Fluka / Aldrich, purity > 93 %). The total number of particles formed was measured with a TSI Model 3025A Ultrafine CPC. The size distribution of the aerosols in the chamber was measured by a TSI SMPS 3936. The resulting SOA was analysed with an Aerodyne aerosol mass spectrometer (Q-AMS). We derived nucleation rates of 0.04 to 260 cm s from the uCPC data during these experiments which is high compared to ambient observations which range from 0.01 – 10 cm s (Kulmala, Vehkamaki et al. 2004). Growth rates of 3 to 29 nm h were calculated from the SMPS size distributions. This compares reasonably well with growth rates observed in mid latitudes with a range of 1-20 nm h (Kulmala, Vehkamaki et al. 2004). The maximum SOA volume produced during VOC oxidation was used as the quantity determining the SOA formation potential. We compared these results to those obtained using α-pinene as single VOC. Spruce, pine and birch were used as model plants representing the boreal forest. Changing temperature in the plant chamber led to changes of VOC emissions and furthermore, to changes of the maximum SOA volumes in the reaction chamber. Plots of maximum SOA volumes versus the total amount of carbon fed into the reaction chamber led to approximately linear relationships. The intercepts of these plots were seen as threshold for SOA formation. It was observed that this threshold was lower for the mixture of VOCs emitted from spruce, pine, and birch than for α-pinene as single compound. We therefore conclude that the threshold for SOA formation from real plant mixtures may be much lower than the threshold obtained from laboratory experiments that were focussed on single VOCs.