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

Abstract

Abstract. Volatile organic compounds (VOCs) are important for global air quality and oxidation processes in the troposphere. In addition to ground-based measurements, the chemical evolution of such species during transport can be studied by performing in situ airborne measurements. Generally, aircraft instrumentation needs to be sensitive, robust and sample at higher frequency than ground-based systems while their construction must comply with rigorous mechanical and electrical safety standards. Here, we present a new System for Organic Fast Identification Analysis (SOFIA), which is a custom-built fast gas chromatography-mass spectrometry (GC-MS) system with a time resolution of 2-3 min and the ability to quantify atmospheric mixing ratios of halocarbons (e.g. chloromethanes), hydrocarbons (e.g isoprene), oxygenated VOCs (acetone, propanal, butanone) and aromatics (e.g. benzene, toluene) from sub-ppt to ppb levels. The relatively high time resolution is the result of a novel cryogenic pre-concentration unit which rapidly cools ( ∼ 6 • C s −1 ) the sample enrichment traps to −140 • C, and a new chromatographic oven designed for rapid cooling rates (∼ 30 • C s −1 ) and subsequent thermal stabilization. SOFIA was installed in the High Altitude and Long Range Research Aircraft (HALO) for the Oxidation Mechanism Observations (OMO) campaign in August 2015, aimed at investigating the Asian monsoon pollution outflow in the tropical upper troposphere. In addition to a comprehensive instrument characterization we present an example monsoon plume crossing flight as a case study to demonstrate the instrument capability. Hydrocarbon, halocarbon and oxygenated VOC data from SOFIA are compared with mixing ratios of carbon monoxide (CO) and methane (CH 4 ), used to define the pollution plume. By using excess (ExMR) and normalized excess mixing ratios (NEMRs) the pollution could be attributed to two air masses of distinctly different origin, identified by back-trajectory analysis. This work endorses the use of SOFIA for aircraft operation and demonstrates the value of relatively high-frequency, multicomponent measurements in atmospheric chemistry research.

Upper tropospheric CH<sub>4</sub> and CO affected by the South Asian summer monsoon during the Oxidation Mechanism Observations mission

“…Bangladesh, which are more polluted, convection transports the pollutants to the upper troposphere. At the same time, the strength of the convection changes within the monsoon due to differences in thermal heating between the relatively cold ocean and the hot land (Dethof et al, 1999).…”

“…For example, the in-service airborne projects CARIBIC (Civil Aircraft for the Regular Investigation of the atmosphere Based on an Instrument Container; e.g. Schuck et al, 2012;Rauthe-Schöch et al, 2016) and IAGOS-MOZAIC (IAGOS -In-service Aircraft for a Global Observing System) and MOZAIC (Measurements of OZone by Airbus In-service aircraft); Dethof et al, 1999;Barret et al, 2016) reported trace gas measurements in the Asian monsoon region. In addition, aircraft campaigns investigated the Asian monsoon during the MINOS campaign (Lelieveld et al, 2002;Scheeren et al, 2003), the Earth System Model Validation (ESMVal) campaign (Gottschaldt et al, 2017), and the Transport and Composition in the Upper Troposphere and Lowermost Stratosphere (TACTS) campaign (Vogel et al, 2014).…”

Stability of halocarbons in air samples stored in stainless- steel canisters

“…Although canister air sampling typically generates data at a much lower spatial coverage, the postflight analysis allows a wider range of substances to be measured. Thus, air sample collection is a well-established part of the scientific payload of both campaign-type studies such as the HIPPO (Wofsy, 2011) and ATTREX (Navarro et al, 2015;Jensen et al, 2017) projects and long-term projects such as CONTRAIL (Machida et al, 2008) and IAGOS-CARIBIC (Brenninkmeijer et al, 2007;Petzold et al, 2015).…”

“…The Civil Aircraft for the Regular Investigation of the atmosphere Based on an Instrument Container (CARIBIC) project (http://www.caribic-atmospheric.com/, last access date: 27 June 2019) investigates atmospheric composition from aboard a Lufthansa passenger aircraft equipped with a sophisticated air inlet system (Brenninkmeijer et al, 2007). CARIBIC is a long-term scientific project, employing a comprehensive set of instruments for performing simultaneous measurements of trace gases and aerosol particles inside a 1.5 t airfreight container during regular long-distance flights of the aircraft.…”

“…Measurement flights take place over 2-4 consecutive days 6-12 times per year. The instrument package consists of remote-sensing instruments, fast in situ instruments, and the collection of particulate matter and air samples (Brenninkmeijer et al, 2007;Petzold et al, 2015).…”