Trace Organic Gas Analyzer Time-of-Flight mass spectrometer (TOGA-TOF) system for airborne observations of formaldehyde

Abstract. Formaldehyde (HCHO) is a ubiquitous atmospheric constituent, originating from primary emissions (natural and anthropogenic) and secondary production via the oxidation of volatile organic compounds (VOCs). In addition to being a regulated pollutant, HCHO is a key species used as a tracer of recent photochemical activity due to its short atmospheric lifetime and its role as a source of HO_x radicals. Given its diverse sources and high spatial variability, HCHO is challenging to represent accurately in chemical transport models, often resulting in significant discrepancies with observations. Airborne in-situ measurements of HCHO, especially when combined with VOC precursor data, offer valuable insights into its atmospheric distributions for evaluating models. Here, we present HCHO observations from the NSF NCAR Trace Organic Gas Analyzer with Time-of-Flight mass spectrometer (TOGA-TOF), deployed during the 2019 Fire Influence on Regional to Global Environments and Air Quality (FIREX-AQ) campaign. While most HCHO instruments target at most a few selected species for measurement, the TOGA-TOF employs a rapid gas chromatography-mass spectrometry (GC/MS) technique and provides discrete VOC measurements—including >100 C₁–C₁₀ species—at a time resolution of less than 2 minutes. We compare TOGA-TOF HCHO data to measurements from three 1-Hz instruments aboard the NASA DC-8: the Compact Atmospheric Multi-species Spectrometer (CAMS), the In Situ Airborne Formaldehyde (ISAF) instrument, and a proton-transfer-reaction time-of-flight mass spectrometer (PTR-ToF-MS). The wide dynamic range of observed HCHO concentrations (from < 100 ppt to ~100 ppb) during FIREX-AQ enabled a robust intercomparison. TOGA-TOF HCHO agreed well with CAMS (slope = 1.1), with similar agreement with the PTR-ToF-MS, while larger discrepancies were observed with ISAF (slope = 1.5), likely due to differences in calibrations. Normalized excess mixing ratios (NEMRs) of HCHO relative to CO in wildfire plumes exhibited consistent trends with plume age across instruments. These findings highlight the TOGA-TOF’s capability for highly sensitive and accurate airborne HCHO measurements.