Abstract
Abstract. Aerosols are a source of great uncertainty in radiative forcing predictions and have poorly understood health impacts. Most aerosol mass is formed in the atmosphere from reactive gas-phase organic precursors, forming secondary organic aerosol (SOA). Semivolatile organic compounds (SVOCs) (effective saturation concentration, C*, of 10−1–103 µg m−3) comprise a large fraction of organic aerosol, while intermediate-volatility organic compounds (IVOCs) (C* of 103–106 µg m−3) and volatile organic compounds (VOCs) (C* ≥ 106 µg m−3) are gas-phase precursors to SOA and ozone. The Comprehensive Thermal Desorption Aerosol Gas Chromatograph (cTAG) is the first single instrument simultaneously quantitative for a broad range of compound-specific VOCs, IVOCs and SVOCs. cTAG is a two-channel instrument which measures concentrations of C5–C16 alkane-equivalent-volatility VOCs and IVOCs on one channel and C14–C32 SVOCs on the other coupled to a single high-resolution time-of-flight mass spectrometer, achieving consistent quantification across 15 orders of magnitude of vapor pressure. cTAG obtains concentrations hourly and gas–particle partitioning for SVOCs every other hour, enabling observation of the evolution of these species through oxidation and partitioning into the particle phase. Online derivatization for the SVOC channel enables detection of more polar and oxidized species. In this work we present design details and data evaluating key parameters of instrument performance such as I/VOC collector design optimization, linearity and reproducibility of calibration curves obtained using a custom liquid evaporation system for I/VOCs and the effect of an ozone removal filter on instrument performance. Example timelines of precursors with secondary products are shown, and analysis of a subset of compounds detectable by cTAG demonstrates some of the analytical possibilities with this instrument.
Highlights
In recent years, understanding of organic aerosol (OA) sources has changed substantially
Another widespread method is chemical ionization mass spectrometry (CIMS), e.g., the proton transfer reaction mass spectrometer (Ionicon Analytik) and related technologies, which compared to gas chromatograph (GC)-based methods offers far greater temporal resolution but less specificity as isomers cannot be separated, and detection is limited to compounds for which the ionization reaction with the chosen reagent is energetically favorable
The Comprehensive Thermal Desorption Aerosol Gas Chromatograph is a novel instrument capable of measuring nonpolar and some polar organic compounds as well as some more oxidized semivolatile organics from C5 through C32 alkane-equivalent volatility on two separate channels connected to a single HRToFMS. This set of quantifiable compounds encompasses many key Volatile organic compounds (VOCs) pollutants, reactive intermediates, and secondary products, all captured at hourly time resolution
Summary
In recent years, understanding of organic aerosol (OA) sources has changed substantially. Chung et al (2003) were able to speciate 55 %–85 % of total VOCs at urban sites in the Los Angeles Basin, with the lower end of the range corresponding to greater photochemical processing and more of the VOC mass present in oxidized species Another widespread method is chemical ionization mass spectrometry (CIMS), e.g., the proton transfer reaction mass spectrometer (Ionicon Analytik) and related technologies, which compared to GC-based methods offers far greater temporal resolution but less specificity as isomers cannot be separated, and detection is limited to compounds for which the ionization reaction with the chosen reagent is energetically favorable. The thermal desorption aerosol gas chromatograph (TAG) family of instruments, consisting of a reusable filter-based collection cell or impactor cell coupled to a GCMS, maximizes chemical speciation of gas and particle SVOCs, including separation of isomers, at hourly time resolution. Nonpolar and some polar VOCs and IVOCs as well as nonpolar and derivatization-amenable polar SVOCs are quantitatively collected, including many primary and secondary organics that lend insight into important sources and oxidation processes in atmospheric chemistry
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