Reactive oxidation products promote secondary organic aerosol formation from green leaf volatiles

Reactive oxidation products promote secondary organic aerosol formation from green leaf volatiles J. F. Hamilton, A. C. Lewis, T. J. Carey, J. C. Wenger, E. Borrás i Garcia, and A. Muñoz Department of Chemistry, University of York, Heslington, York, UK, YO10 5DD, UK Department of Chemistry and Environmental Research Institute, University College Cork, Cork, Ireland Fundación Centro de Estudios Ambientales del Mediterráneo (CEAM), EUPHORE laboratories, C/Charles Darwin, 14 – Parque Tecnológico, Paterna, Valencia, Spain Received: 24 November 2008 – Accepted: 15 January 2009 – Published: 5 February 2009 Correspondence to: J. F. Hamilton (jfh2@york.ac.uk) Published by Copernicus Publications on behalf of the European Geosciences Union.


Experimental Conditions and Results
Filter samples were extracted into high purity water, filtered and reduced to 1 ml using a vacuum solvent evaporator. Liquid chromatography-ion trap mass spectrometry was carried out using HCT Plus ion trap mass spectrometer (Bruker Daltonics GmbH, Bremen, Germany) equipped with an Eclipse ODS-C18 column with 5 µm particle size (Agilent, 4.6 mm×150 mm). Experiments to determine the yield of SOA produced from the hydroxyl radical initiated oxidation of cis-3-hexen-1-ol, cis-3-hexenyl acetate and isoprene were performed using nitrous acid (HONO, 100 ppbv) as the OH precursor and a VOC mixing ratio of 500 ppbv.
The EUPHORE chamber is a hemisphere (200,000 L) made of FEP foil surrounded by a retractable steel housing. Ozonolysis of the GLVs was performed in the absence of radical scavenger and seed aerosol at a relative humidity ca. 6%. The decay of the hydrocarbons was monitored by FTIR spectroscopy and gas chromatography and the formation and evolution of SOA was measured using a scanning mobility particle sizer. Samples of SOA were collected onto quartz filter papers when the particle concentration in the chamber had reached a maximum, approximately 4 h after the start of the reaction. The filters were taken at 81.2 L min −1 (at 25C) and stored in a freezer at −4C before analysis.
This work was supported by the EU 6FP Project EUROCHAMP and the European Science Foundation programme INTROP A series of simulation chamber experiments was performed to compare the yields of SOA generated from the atmospheric oxidation of isoprene, cis-3-hexen-1-ol and cis-3-hexenyl acetate under identical conditions. The following aerosol mass yields were obtained; cis-3hexen-1-ol (3.1%), cis-3-hexenyl acetate (0.9%) and isoprene (1.2%). Assuming that these SOA yields can be scaled according to the value typically used for isoprene (3%, Henze and Seinfeld, 2006), the relative yields of SOA produced from cis-3-hexen-1-ol and cis-3-hexenyl acetate in the atmosphere are 7.75% and 2.25% respectively. This leads to an estimated global SOA source of 1-5 TgC yr −1 from the OH-initiated oxidation of these two GLVs, up to a third of that from isoprene. The SOA yields from ozonolysis (at initial concentrations of 1600 ppbv) were 9.5% for cis-3hexen-1-ol and 8.6% for cis-3-hexenyl acetate, indicating that ozonolysis is likely to be an additional SOA production route for these GLVs.
A recent estimate of global SOA production based on VOC fluxes indicated that there may be significant missing SOA precursors that are currently unknown (Goldstein and Galbally, 2007). The results obtained in this study indicate that GLVs may be an important part of this unidentified global source of SOA, which have been overlooked as a consequence of their volatile first generation oxidation products. Here only 2 compounds have been considered, but there are a number of other GLV species emitted into the atmosphere (e.g. trans-3hexenal, cis-2-hexenal, cis-2-hexenol) indicating that the SOA potential of green leaf volatiles may be considerably higher than estimated here. Clearly, considerable further work on emission fluxes, simulation chamber studies, field measurements and modelling is required to fully evaluate the importance of GLVs to the SOA budget. Future scenarios may also be considered, where higher emissions of GLVs could arise from plant stress induced by extreme temperatures or high ozone concentrations, or from the use of plants for biofuels (GLV emissions are major emission from oil seed rape).

GLVs as a Global
Source of SOA

Oligomer Formation Oligomers identified by LC-ESI-MS/MS
The mass spectrum of cis-3hexenylacetate SOA contained small molecules (e.g., 3-acetoxypropanal, [M+Li] + = 123 Da) and oligomers with ester and ether linkages, produced from chemical reactions in the particle-phase. However, oligomer growth beyond dimers and trimers appeared to be blocked by the low reactivity of the acetate group.
The ability to form oligomers is therefore strongly linked to the reactivity of the oxidation products in the particle phase.