An aircraft gas chromatograph–mass spectrometer System for Organic Fast Identification Analysis (SOFIA): design, performance and a case study of Asian monsoon pollution outflow

摘要

Volatile organic compounds (VOCs) are important for global airquality and oxidation processes in the troposphere. In addition toground-based measurements, the chemical evolution of such species duringtransport can be studied by performing in situ airborne measurements.Generally, aircraft instrumentation needs to be sensitive, robust and sampleat higher frequency than ground-based systems while their construction mustcomply with rigorous mechanical and electrical safety standards. Here, wepresent a new System for Organic Fast Identification Analysis (SOFIA), whichis a custom-built fast gas chromatography–mass spectrometry (GC-MS) systemwith a time resolution of 2–3 min and the ability to quantify atmosphericmixing ratios of halocarbons (e.g. chloromethanes), hydrocarbons (e.gisoprene), oxygenated VOCs (acetone, propanal, butanone) and aromatics (e.g. benzene,toluene) from sub-ppt to ppb levels. The relatively high timeresolution is the result of a novel cryogenic pre-concentration unit whichrapidly cools (∼ 6 °C s) the sample enrichment traps to−140 °C, and a new chromatographic oven designed for rapid cooling rates(∼ 30 °C s) and subsequent thermal stabilization. SOFIAwas installed in the High Altitude and Long Range Research Aircraft (HALO)for the Oxidation Mechanism Observations (OMO) campaign in August 2015, aimedat investigating the Asian monsoon pollution outflow in the tropical uppertroposphere. In addition to a comprehensive instrument characterization wepresent an example monsoon plume crossing flight as a case study todemonstrate the instrument capability. Hydrocarbon, halocarbon and oxygenatedVOC data from SOFIA are compared with mixing ratios of carbon monoxide (CO)and methane (CH), used to define the pollution plume. By using excess(ExMR) and normalized excess mixing ratios (NEMRs) the pollution could beattributed to two air masses of distinctly different origin, identified byback-trajectory analysis. This work endorses the use of SOFIA for aircraftoperation and demonstrates the value of relatively high-frequency,multicomponent measurements in atmospheric chemistry research.