ACROSS_2022_RambForest_LISA_PTRMS_VOCs_Belowcanopy_10min_20220617 - 20220723

Type of PTR-MS: The LISA PTR-ToF-MS, manufactured by Kore Technology Ltd., was used for monitoring ground-level concentrations of Volatile Organic Compounds (VOCs) at the Rambouillet forest site from June 17th, 2022, to July 23rd, 2022, during ACROSS field campaign. The instrument operated in the PEGASUS mobile lab in H3O+ mode with a drift tube pressure of 1.30 mbar and a temperature of 40°C, along with a drift tube voltage of 397V, resulting in an E/N ratio of 134.2 Td. Air samples were drawn at a flow rate of approximately 300 mL min-1 through a 3-meter-long Silcosteel® coated stainless steel tube (2.1 mm inner diameter) ®. The PTR-MS conducted full mass scan mode analysis (10 – 270 amu).

Software Data Processing: Data analysis was performed using Grams/AI version 9.1 software.
Measured species: 23 protonated VOCs and their isomers were analyzed by the PTR-ToF-MS, including Methanol (m33), Acetonitrile (m42), Acetaldehyde (m45), Acetone (m59), Acetic acid (m61), DMS (m63), Isoprene (m69), MVK + MACR (m71), Methylglyoxal (m73), Butan-2-one (MEK) (m73_2), Benzene (m79), Propionic acid + Hydroxyacetone (m75), Toluene (m93), C8-aromatics (m107), C9-aromatics (m121), Monoterpenes (m121), and Monoterpenes oxidation products (m139, m151, and m153). For the complete list of measured VOCs, refer to the table summarizing the gas tracer measurements conducted by PTR-MS during the ACROSS campaign.

Data Averaging: Raw data were collected every 1 minute, followed by additional averaging over 10 minutes, resulting in an average of 10 spectra. Invalid data are indicated as “-9999”.
Blank measurements were conducted for 8 min every 1 hour. Blank signals were subtracted from measured signals. Purified air was generated from ambient air by passing the ambient air through an HC charcoal cartridge.

Regular calibration was performed approximately every 3 days. 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). The cylinder contained various species nominally at 1 ppmv ±5% for each standard, including Acetaldehyde, Methanol, Ethanol, Isoprene, Acetone, Dimethyl sulfide, Acetonitrile, and others. Methanol, Acetonitrile, Acetaldehyde, Acetone, Isoprene, Butan-2-one (MEK), Benzene, Toluene, m-Xylenes, 1,2,4-Trimethylbenzene, and 3-Carene were directly used for calibration. Sensitivity determined from calibration, corrected for water dependency, was used to derive the volume mixing ratios of targeted VOCs. Water dependencies of various compounds were determined by performing calibration at various relative humidity before and after the campaign using the same NPL standard cylinder. Non-calibrated compounds were retrieved from reactor conditions, rate constants, fragmentations, and ion transmissions determined using the same NPL standard cylinder.

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.
Technical issues: The measurement periods for June 20th at 23:20, June 22nd at 12:16, June 23rd at 11:26, June 27th at 15:51, and July 02nd are not reported in the database due to technical issues.

Limit of Detection (LOD): The LOD was calculated based on a signal-to-noise ratio of 3 over a period of 10 minutes. Estimated average LODs in pptv ranged as follows: 5.21 for m33, 7.20 for m42, 7.91 for m45, 20.15 for m59, 5.10 for m61, 12.33 for m63, 2.76 for m69, 3.09 for m71, 2.75 for m73, 4.28 for m73_2, 3.67 for m75, 3.06 for m79, 3.73 for m87, 3.18 for m89, 3.39 for m93, 3.73 for m101, 3.16 for m107, 7.09 for m113, 3.06 for m121, 3.40 for m137, 6.84 for m139, 6.73 for m151, and 6.84 for m153 (see tables for LOD values of the targeted species).