Investigation of mineral-mediated photochemical processes and the development of an automated TD-GC-FID/MS system coupled to an atmospheric reaction chamber

Abstract

The atmospheric aqueous phase is an important reaction medium for the processing of both biogenic and anthropogenic organic compounds. Particularly, the formation of aqueous-phase secondary organic aerosol (aqSOA) and the influence on ambient air quality and regional climate are of increasing interest compared to data from comparable gas-phase processes contributing to SOA formation. A fundamental component for the investigation of atmospheric aqueous-phase reactions is the availability of a photochemical reactor. For this purpose, a multiposition photoreactor was designed and characterized with detailed comparisons of exchangeable sample adaptors regarding their stirring performance for the application in heterogeneous, aqueous-phase photochemistry. The developed photoreactor was applied for the investigation of a new, mineralmediated, photochemical formation mechanism of organosulfates (OS) in the aqueous phase using methacrolein, a major atmospheric oxidation product of isoprene, an abundant biogenic volatile organic compound, as precursor. Since organosulfates are known to make a significant contribution to particulate matter (PM) mass loadings and SOA, the OS formation was studied under several conditions, such as the illumination time, catalyst loading, sulfate concentration, counterion identity, and methacrolein concentration. SOA formation in the atmosphere also occurs from gas-phase precursors, the concentrations of which can be monitored with both optical spectroscopic techniques, such as differential optical absorption spectroscopy (DOAS) and via gas chromatographic (GC) separation and hyphenation techniques, such as flame ionization detection (FID) and mass spectrometry (MS). GC-MS is a powerful tool for the analysis and structural elucidation of gas-phase species. However, volatile organic compounds and related atmospheric oxidation products are usually detected at low concentration ranges, which require an analyte preconcentration and thermal desorption (TD) step prior to a gas chromatographic analysis. Consequently, a thermal desorption unit for GC-FID/MS coupling was designed, characterized and further developed to achieve an automated TD-GC-FID/MS system for a continuously operated analysis process of samples generated in smog chamber studies.