Fourteen Orders of Magnitude of Organic Volatility in One Instrument: The Comprehensive Thermal Desorption Aerosol Gas Chromatograph (cTAG)

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 - 10³ μg/m³) comprise a large fraction of organic aerosol, while intermediate volatility organic compounds (IVOCs) (C* of 10³ - 10⁶ μg/m³) are abundant and have been demonstrated to efficiently react to form SOA. VOCs (C* ≥ 10⁶ μg/m³) are also critical to measure precursors to SOA due to their reactivity, even greater abundance, and impacts on ozone formation.

The Comprehensive Thermal Desorption Aerosol Gas Chromatograph (cTAG) is the first single instrument sensitive to 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 - C₃₀ IVOCs and SVOCs on the other channel coupled to a single High-Resolution Time-of-Flight Mass Spectrometer, achieving consistent quantification across 14 orders of magnitude of vapor pressure. cTAG obtains concentrations hourly and gas-particle partitioning for SVOCs bihourly, enabling observation of the evolution of these species through oxidation and partitioning into the particle phase. Online derivatization for SVOCs enables detection of polar and oxidized species. Online calibration enables accurate quantification for hundreds of compounds.

In this work we present the design details of cTAG as well as data from a spring 2018 field deployment in Livermore, CA, which had multiple exceedances of the ozone and PM_2.5 U.S. National Ambient Air Quality Standards over the last few years. 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 concentration and partitioning findings to elucidate the relative contributions of pollution sources to PM_2.5 in Livermore.