Development of an in situ dual-channel thermal desorption gas chromatography instrument for consistent quantification of volatile, intermediate-volatility and semivolatile organic compounds

<p>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, <span class="inline-formula"><i>C</i>^*</span>, of 10<span class="inline-formula">⁻¹</span>–10<span class="inline-formula">³</span> <span class="inline-formula">µg m⁻³</span>) comprise a large fraction of organic aerosol, while intermediate-volatility organic compounds (IVOCs) (<span class="inline-formula"><i>C</i>^*</span> of 10<span class="inline-formula">³</span>–10<span class="inline-formula">⁶</span> <span class="inline-formula">µg m⁻³</span>) and volatile organic compounds (VOCs) (<span class="inline-formula"><i>C</i>^*</span> <span class="inline-formula">≥</span> 10<span class="inline-formula">⁶</span> <span class="inline-formula">µg m⁻³</span>) are gas-phase precursors to SOA and ozone.</p> <p>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 C<span class="inline-formula">₅</span>–C<span class="inline-formula">₁₆</span> alkane-equivalent-volatility VOCs and IVOCs on one channel and C<span class="inline-formula">₁₄</span>–C<span class="inline-formula">₃₂</span> 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.</p> <p>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.</p>