Characterisation of tracers for aging of a-pinene secondary organic aerosol using liquid chromatography/negative ion electrospray ionisation mass spectrometry

Secondary organic aerosol (SOA) from the oxidation of a-pinene is a very complex and dynamic mixture containing products with a different chemical nature and physicochemical properties that are dependent on chemical evolution or aging processes. In this study, we focus on the chemical characterisation of major products that are formed upon a-pinene ozonolysis SOA and subsequent aging throughOH-initiated reactions in the absence of NOx, which include known as well as unknown tracers. The mass spectrometric data obtained for selected unknown compounds that show an increased relative abundance upon aging are interpreted in detail and tentative structures for them are proposed taking into account their formation through photooxidation of a-pinene. Known tracers for a-pinene SOA aging that were identified include norpinic acid, 10-hydroxypinonic acid, diaterpenylic acid acetate, and diesters formed by esterification of pinic acid with terpenylic acid or 10-hydroxypinonic acid. Novel tracers for a-pinene SOA aging that were tentatively identified include dinorpinic acid and 8-hydroxypinonic acid. In addition, reaction mechanisms are proposed to explain the formation of the observed a-pinene SOA tracers. Received 1 December 2011, accepted 23 February 2012, published online 3 May 2012 Introduction It is becoming increasingly recognised that secondary organic aerosol (SOA) from the oxidation of a-pinene is a very complex and dynamic mixture containing products with a different chemical nature and physicochemical properties that are dependent on chemical evolution or aging processes. These processes comprise oxidation reactions of semivolatile precursors in the gas phase that result in more polar oxygenated products with an increased O/C ratio, which are less volatile and more hydrophilic, and thus may enhance the capability of SOA particles to serve as cloud condensation nuclei. On the other hand, oxidation reactions of semivolatile precursors in the gas phase may also result in fragmentation to smaller molecules. In addition, aging processes occur in the particle phase where heterogeneous reactions take place; examples of the latter reactions are esterification with sulfuric acid of hydroxyor epoxy-containing SOA products, esterification of pinic acid with hydroxy-containing terpenoic acids and OH-initiated oxidation reactions. Limited information is available on the oxidation processes that form the molecular basis of a-pinene SOA aging processes. A useful marker compound for aged a-pinene SOA is the C8-tricarboxylic acid 3-methyl-1,2,3-butanetricarboxylic acid (MBTCA), which is a higher-generation photooxidation product of a-pinene generated in the presence of NOx and is formed through further reaction of pinonic acid, a first-generation a-pinene SOA tracer. MBTCA has been reported in several field studies to occur at significant concentrations, supporting its atmospheric relevance (for a review, see Hallquist et al.; see recent field studies). CSIRO PUBLISHING Environ. Chem. 2012, 9, 236–246 http://dx.doi.org/10.1071/EN11148 Journal compilation CSIRO 2012 Open Access www.publish.csiro.au/journals/env 236 Research Paper SPECIAL ISSUE