Chemical characterization of α-pinene secondary organic aerosol constituents using gas chromatography, liquid chromatography, and paper spray-based mass spectrometry techniques.

Rationale: Despite ample research into the atmospheric oxidation of α-pinene, an important precursor to biogenic secondary organic aerosol formation, the identification of its reaction products, specifically organic nitrates, which impact atmospheric NOx concentrations, is still incomplete. This negatively impacts our understanding of α-pinene oxidation chemistry and its relation to air quality. Methods: Photochemical chamber experiments were conducted in conjunction with mass spectrometric techniques, including gas chromatography/mass spectrometry (GC/MS), high-performance liquid chromatography/time-of-flight (HPLC/TOF), and paper spray ionization MS, to investigate products from the OH radical initiated oxidation of α-pinene under high NOx conditions. Results: Over 30 compounds were tentatively identified, including those newly detected from photochemical chamber studies of α-pinene oxidation, pinocamphenol, fencholenic aldehyde, and α-pinene-derived nitrate isomers. α-Pinene-derived hydroxynitrate isomers were successfully detected using chromatographic methods, demonstrating, for the first time, the identification of individual first-generation organic nitrate products derived from α-pinene. The application of paper spray ionization to particle-phase compounds collected on filters represents a novel method for the direct analysis of filter samples at ambient pressure and temperature. Conclusions: The use of HPLC/TOF and paper spray ionization methods to identify previously unobserved α-pinene-derived products helps lower the uncertainty in α-pinene oxidation chemistry and provides new platforms that can be used to identify and quantify important atmospheric compounds that relate to air quality in a complex sample matrix, such as ambient aerosol particles. Additionally, the use of paper spray ionization for direct filter analysis is a fast, relatively inexpensive sample preparation technique that can be used to reduce sample manipulation from solvent-induced reactions.