Advances in speciating highly oxygenated organic aerosol using in-situ thermal desorption semi-volatile aerosol gas chromatography (SV-TAG) with on-line derivatization
Author
Nathan Kreisberg — Aerosol Dynamics, Inc.
Category
Secondary Organic Aerosol
Description
Average fraction in the particle phase as a function of volatility for semi-volatile organic compounds including known biogenic VOC oxidation products as measured by the dual channel SVTAG instrument with on-line derivatization. Partitioning can be measured for highly oxygenated compounds and unknowns. Results show consistently higher levels in the particle phase compared to absorptive partitioning theory (green line). Approximate saturation concentration based on gas chromatograph (GC) retention index and authentic standards.
Speciation at the molecular level is necessary to more fully understand the sources, sinks, transport and transformation of organic matter in the atmosphere. The Thermal desorption Aerosol Gas chromatograph (TAG) is a field deployable instrument for in-situ, hourly speciation of organic aerosols. To measure the highly polar fraction of semi-volatile organics, which are important to secondary aerosol formation, a new version of the instrument has been developed to simultaneously collect both particle and semi-volatile vapor phases (Zhao et al., 2013; Isaacman et al., 2014) using a dual cell SV-TAG system with on-line derivatization of hydroxyl functional groups. Parallel samples collected on a reusable metal-fiber filter with and without removing the gas phase provide direct partitioning measurements of organic compounds. Hydroxyl groups are replaced with more stable trimethylsiloxyl groups using a standard silylation technique to provide quantitative recovery of highly oxygenated compounds by minimizing interactions with the chromatography column. Two recent field campaigns, in rural Alabama, US (2013) and in Amazonia, Brazil (2014), offer unique data sets for evaluating the field performance of this instrument. Using perdeuterated internal standards injected with every sample, the recovery of compounds ranging from pure non-polar hydrocarbons to highly oxygenated SOA can be evaluated for two very different environments to determine the role of aerosol composition, environmental factors and small changes in sampling protocols in instrument performance. Results show that these highly polar species can be measured with comparable precision to that obtained with off-line filter samples but now at hourly time resolution. While the extremely wet Amazonian environment presented unique challenges operationally, instrument performance is shown to be improved over that observed in Alabama, demonstrating that field performance of TAG instruments with derivatization can be expected to match prior capabilities for non-polar compounds. The addition of a gas phase channel for the measurement of SOA precursors in the range of C5 to C15 is a current area of development that will provide a more comprehensive look at SOA from a single instrument.
