ACROSS_LISA_SAFIRE-ATR42_PTRMS-VOCs_20220617-20220705

Type of PTR-MS: The LISA PTR-ToF-MS, manufactured by Kore Technology Ltd., was specially customized for integration into the SAFIRE ATR-42 to meet the security requirements of airborne measurements. It operated in H3O+ mode with a drift tube pressure of 1.329 mbar and a temperature of 60°C, resulting in an E/N ratio of 132.7 Td. Air samples were drawn at a flow rate of approximately 300 mL min-1 through a 1.5-meter-long silcosteel tubing (1/8” OD) heated to 60°C connected to a pressurized manifold maintained at 1018 hPa. The PTR-MS conducted full mass scan mode analysis (10 – 200 amu).

Software Data Processing: Data analysis was performed using Grams/AI version 9.1 software.

Measured species: Acetaldehyde, Acetone, Isoprene, Butane-2-one (MEK), Benzene, Toluene, C8-aromatics, C9-aromatics, and sum of monoterpenes (see Table summarizing the gas tracer measurements conducted by PTR-MS during the ACROSS campaign).

Data Averaging: Raw data were collected every 30 seconds for the majority of flights, with the exception of flights 24 and 25 (where data were collected every 1 minute), followed by additional averaging over 90-second intervals (or 3-minutes for flights 24 and 25), resulting in an average of three spectra. Invalid data are indicated as “-9999”. The concentrations of compounds measured for each flight are provided along with the trajectory coordinates of the SAFIRE-ATR42 aircraft.

Blank measurements were conducted once on the ground before the flight, or twice before and after the flight. Blank signals were subtracted from measured signals. Zero air was generated for approximately 45 minutes from ambient air using a Gas Calibration Unit (GCU, IONICON Analytic GmbH) equipped with a catalytic VOC scrubber made of Pt/Pd and heated to 350°C in a catalytic oven. Blank changes for organic species were assumed to be minimal or unchanged during flight.

Regular calibration was performed before and after the ACROSS campaign, and at least once before each flight. VOC standards were injected through the dilution system inside the GCU to achieve different volume mixing ratios in the range of 5 – 20 ppb using a certified calibration mixture from a cylinder provided by NPL (National Physical Laboratory). Methanol, Acetonitrile, Acetaldehyde, Acetone, Isoprene, Butane-2-one (MEK), Benzene, Toluene, m-Xylenes, 1,2,4-Trimethylbenzene, and 3-Carene were used for calibration. Sensitivity determined from calibration, corrected for water dependency, was used to retrieve the volume mixing ratios of targeted VOCs.

Technical Issues: Between flights, the PTR-MS was powerless, resulting in the inability to maintain high vacuum levels, leading to relatively high and variable background levels. Some compounds were difficult to measure due to potential contributions from interfering species or zero ion signals, such as Methanol, Acetic acid, Acetaldehyde, as well as MVK, MACR, and MEK in several flights. Pilot communication manifested as a significant increase in background noise, leading to notable offsets in the count rates of targeted VOCs for some spectra. Consequently, affected data were eliminated from the dataset. These interferences may introduce uncertainties into the volume mixing ratios of targeted compounds, which must be considered as an upper limit for the observed species.

Uncertainties: Uncertainties (1σ) for each averaged data point were calculated. The accuracy associated with calibration was 5% (1σ) for gases present in the calibration standard.

Limit of Detection (LOD): The LOD was calculated based on a signal-to-noise ratio of 2 over a period of 90 seconds (or 3 min for flights 24 and 25). Estimated LODs in pptv ranged as follows: 331 – 679 for acetaldehyde, 157 – 325 for acetone, 14 – 120 for isoprene, 70–323 for MVK + MACR, 142–308 for MEK, 30 – 122 for benzene, 25 – 52 for toluene, 18 – 55 for C8-aromatics, 23 – 57 for C9-aromatics, and 46 – 183 for monoterpenes (see table for LOD values of the targeted species).