AEROCLO-sA PEGASUS PTR-MS volatile organic compounds

The concentration of volatile organic compounds was measured by a proton transfer reaction time-of-flight mass spectrometer (PTR-ToF-MS) Kore Inc™ (second generation) operated in the PEGASUS mobile lab. Air was taken into the instrument through a 1.5 m long Silcosteel® coated stainless steel tube (2.1 mm inner diameter), at a constant flow rate of about 3 cm3/s . Instrumental settings were: glow discharge region P=2.00 mbar, proton transfer reaction tube P=1.69 mbar, T=60 °C and E=390 V, leading to an E/N of 112 Td (1 Td=10−17 V cm2). List of gas standard mixtures used for calibration: (i) built-in GCU gas standard containing formaldehyde, methanol, acetonitrile, acetaldehyde, ethanol, acrolein, acetone, isoprene, crotonaldehyde, 2-butanone, benzene, toluene, o-xylene, chlorobenzene, α-pinene, 1,2-dichlorobenzene, and 1,2,4-trichlorobenzene at a nominal concentration of 1 ppm; (ii) NPL primary reference gas mixture containing ethane, ethene, propane, propene, 2-methylpropane, butane, ethyne, trans-but-2-ene, but-1-ene, cis-but-2-ene, 2-methylbutane, pentane, 1,3-butadiene, trans-pent-2-ene, pent-1-ene, 2-methylpentane, hexane, isoprene, heptane, benzene, 2,2,4-trimethylpentane, octane, toluene, ethylbenzene, m-xylene, p-xylene, o-xylene, 1,3,5-trimetrylbenzene, 1,2,4-trimethylbenzene, and 1,2,3-trimethylbenzene at a nominal concentration of 4 ppb; (iii) standard mixture made in-house containing dimethyl sulphide (DMS, 1.39 ppm), dimethyl sulfoxide (DMSO, 1.34 ppm), diethyl sulphide (DES, 2.03 ppm), dimethyl disulphide (DMDS, 1.7 ppm) and dimethyl sulphate (DMSO4, 1.79 ppm).