10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Fourteen Orders of Magnitude of Organic Volatility in One Instrument: The Comprehensive Thermal Desorption Aerosol Gas Chromatograph (cTAG)
REBECCA WERNIS, Nathan Kreisberg, Susanne Hering, Allen H. Goldstein, University of California, Berkeley
Abstract Number: 1438 Working Group: Instrumentation
Abstract Aerosols are a source of great uncertainty in radiative forcing predictions and have poorly understood impacts on human health. Most aerosol mass is formed in the atmosphere from reactive gas phase organic precursors, forming secondary organic aerosol (SOA). Semi-volatile organic compounds (SVOCs) (effective saturation concentration, C*, of 10-1 to 103 μg/m3) comprise a large fraction of organic aerosol (OA), while intermediate volatility organic compounds (IVOCs) (C* of 103 to 106 μg/m3) have been demonstrated to efficiently react to form SOA and to be abundant in the atmosphere. Volatile organic compounds (VOCs) (C*≥106 μg/m3) are also critical to measure precursors to SOA due to their even greater abundance and their reactivity, as well as their impacts on ozone formation.
Until now, no single instrument existed that is sensitive to compound-specific VOCs, IVOCs and SVOCs (C* 10-1 to 109 μg/m3). The Comprehensive Thermal Desorption Aerosol Gas Chromatograph (cTAG) can detect this full range of SOA-producing organic precursors and semi-volatile products. cTAG can obtain concentrations hourly and gas-particle partitioning for SVOCs bihourly, enabling observation of the chemical evolution of these species through oxidation and partitioning into the particle phase. Online derivatization for SVOCs measured by cTAG enables detection of polar and oxidized species in addition to non-polar ones, expanding speciated identification and quantification to a broader range of SOA compounds.
cTAG is a two channel instrument which measures concentrations of C5 – C16 alkane equivalent volatility VOCs and IVOCs on one channel and C14 – C30 IVOCs and SVOCs on the other channel coupled to a single High-Resolution Time-of-Flight Mass Spectrometer (HR-TOF-MS) detector, thus achieving consistent quantification across 14 orders of magnitude of vapor pressure. The HR-TOF-MS enables detection limits below 1 ppt over the entire range as well as improved compound identification by assigning chemical formulas to exact mass ion measurements from continuously measured full mass spectra. Dual miniature gas chromatographs allow for use of two chromatography columns each optimized for the volatility range of its respective channel. The VOC/IVOC collector consists of a layered bed of adsorbent materials while the SVOC collector is a passivated metal mesh filter. Collectors are thermally desorbed and their contents injected onto the corresponding chromatography column and analyzed in series. Online derivatization on the SVOC channel is achieved by exposing the sample to derivatization agent-saturated helium during desorption. On line calibration systems tailored to each channel allow accurate quantification and identification confirmation for hundreds of compounds.
In this work we present the operating principle and design details of cTAG as well as data from a winter and spring field deployment in Livermore, California, a city which had multiple exceedances of the ozone and PM2.5 U.S. National Ambient Air Quality Standards over the last few years, placing it among the cities in the San Francisco Bay Area with the poorest air quality. In winter, wood burning for heat combined with reduced boundary layer mixing leads to elevated PM2.5 levels. Regional contributions from the Bay Area and the San Joaquin Valley add to the local air pollution burden. Hourly concentrations of VOCs/IVOCs and SVOCs were obtained concurrent to routinely monitored VOCs and other criteria and regularly measured air pollutants at a collocated Bay Area Air Quality Management District monitoring site; the cTAG data are compared with these measurements as validation. Based on known chemical tracers and temporal correlations between measured compounds, we present the initial findings of VOC, IVOC and SVOC concentrations and partitioning to elucidate the relative contributions of pollution sources to PM2.5 in Livermore.