ACROSS_LISA_RambForest_Orbitrap_L2

Molecular scale chemical composition of the organic aerosol was studied using high resolution mass spectrometry (HRMS). Filters of 46 mm were extracted following Kourtchev et al. (2014). Non-target mass spectrometric analysis was performed using LTQ Orbitrap mass spectrometer (Thermo Scientific™, Bremen, Germany) equipped with a chip-based nanoESI source (TriVersa NanoMate, Advion) operating in negative ionization mode. The source parameters were: gas pressure of 0.30 psi, spray voltage -1.4 kV, and an injection volume of 7 L. NanoESI source was used to achieve higher ionization efficiency for a variety of analytes (Kourtchev et al., 2014). All spectra were recorded in triplicates with one-minute data acquisition time, with a nominal resolution of 100,000, and in two scan ranges (50-500 m/z and 150-1000 m/z). HRMS data treatment was conducted following Zielinski et al. (2018) and it included the following atomic constraints: 14N (0-5), 16O (0-50), 12C (1-100), 1H (1-200), 32S (0-2), 34S (0-1), and 13C (0-1). Formula lists and peak intensities were further processed including noise removal, blank subtraction, and additional atomic constraints: elemental ratios of 0.3 ≤H/C≤ 2.5, O/C≤ 2, N/C≤ 1.3, S/C≤ 0.2, 12C/13C≤ 0.011 and 32S/34S≤ 0.045, double bond equivalent (DBE) values, nitrogen rule, and isotopic filtering (Zielinski et al., 2018). The smallest absolute mass error was used for multiple assignments after mass shift correction, and only formulae found in all triplicates were considered. 14 samples from 3 periods: July 3 to 4, July 11 to 13, and July 17 to 19 were using HRMS, with the compounds identification and the relative intensity associated to the measurements. From the formulae identified, the O/C, H/C, DBE (double bond equivalent), Xc (aromaticity equivalent) are also presented. For samples were data is not available, -9999 values are presented. Information regarding the sampling time of each sample is presented in the collection time files.