Abstract
Secondary organic aerosol (SOA) is a key uncertainty in quantifying the impact of humans on Earth's climate. SOA is a complex mixture of oxidized organic species, and a fundamental hurdle in determining its composition is the lack of authentic standards for comparison and quantification. Organic synthesis can be used to produce pure standards, but is limited to compounds for which there is a degree of confidence in the proposed structure and can be expensive and time-consuming. In this study, a flow reactor was developed to form SOA in sufficient quantities to be collected and pure compounds subsequently isolated from the mixture using semipreparative high performance liquid chromatography. The purity and yield of each isolated compound were obtained using proton nuclear magnetic resonance ((1)H NMR), whereas molecular formulas were confirmed by high resolution Fourier transform ion cyclotron mass spectrometry (FTICR-MS). The effectiveness of the methodology has been evaluated here by using α-pinene as the precursor because it is the monoterpene with the most well characterized SOA chemistry. Eleven individual α-pinene SOA compounds were produced from α-pinene oxidation experiments and used for quantitative analysis of SOA formed during chamber experiments carried out close to ambient conditions. These compounds represented 25% of the total SOA mass, a significant improvement in mass balance compared to previous studies. This relatively simple approach may be extended to produce other SOA components not available commercially to improve quantification of aerosol sources.
Highlights
Each isolated compound was subsequently recovered in 500 μL of D2O and one-fifth of this solution analyzed by 1H NMR spectroscopy, while the rest was used as stock solution for both identification and quantification
The aqueous extract of α-pinene Secondary organic aerosol (SOA) generated by the flow reactor was first analyzed using LC-UV/vis-MS using the analytical column
In order to allow larger injection volumes, the internal diameter of the column was increased to 10 mm. This larger column allows more mass to be loaded onto the column and the possibility of semipreparative chromatography, where fractions of the eluent are collected
Summary
1H NMR confirmed the purity and structure of the fractions and an internal standard was added to allow the mass of the compound to be calculated. Schematic of the Apparatus Used to Produce αPinene SOA in the Microflow Reactor by Introducing αPinene through a Heated Inlet and Using a Hg Pen-Ray Lamp to Generate Ozone from Air. Aldrich, Gillingham, UK) was injected into the gas stream using a syringe driver fitted with a 1 mL gastight syringe (Hamilton Bonaduz AG, Bonaduz, Switzerland) at a constant rate of 20 μL h−1 into the round-bottom flask. The reactor was run continuously for 3 days, consuming a total of 1.5 mL of αpinene After this time period, the filter paper was removed and the α-pinene SOA extracted into 4 mL of water (Fischer Optima LC-MS grade) and concentrated to 1 mL using a vacuum solvent evaporator. Full NMR experiments details can be found in the Supporting Information
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