High-Resolution Mass Spectrometry (HRMS)

HRMS was used to analyze SOA chemical composition. SOA was extracted from the filters by shaking the filters in acetonitrile for 5 min. The solvent volume was chosen to achieve a concentration of SOA in acetonitrile of 400 μg mL^–1 assuming 100% extraction efficiency. Then an equal amount of water was added so that the SOA concentration would be 200 μg mL^–1 for HRMS analysis.

The instrument has been described previously in Chin et al.⁴³ A 10 μL aliquot of sample was injected into a Phenomenex Luna Omega Polar C18, 150 × 2.1 mm² column, with 1.6 μm particles and 100 Å pores for ultrahigh performance liquid chromatography (UPLC) separation and photodiode array (PDA) detection. The PDA detector was followed by a Thermo Q-Exactive Plus mass spectrometer with a heated electrospray ionization inlet and a resolving power of 1.4 × 10⁵ at m/z 400, which was operated in both positive (spray voltage +3.5 kV) and negative (spray voltage −2.5 kV) ion modes.⁴³ The UPLC solvents were water acidified to pH 3 with 0.1 wt % formic acid (solvent A) and acetonitrile acidified with 0.1 wt % formic acid (solvent B). The gradient was 95% solvent A and 5% solvent B for 3 min, followed by a linear ramp to 95% solvent B and 5% solvent A from for 11 min, a hold at 95% solvent B for 2 min, and finally a linear ramp back to 95% solvent A and 5% solvent B for 4 min in preparation for the next run.

FreeStyle 1.6 from Thermo Scientific was used to generate a raw time-integrated mass spectrum by integrating over the full total ion chromatogram (1–18 min). Peaks and their relative intensities were then extracted from the time-integrated mass spectrum using Decon2LS (https://omics.pnl.gov/software/decontools-decon2ls), and peaks representing ¹³C compounds were removed. Peaks from the solvent and SOA samples were then aligned with a tolerance of 0.0005 m/z, and peaks with equal or greater intensity in the solvent than in the samples were also removed. The resultant mass spectra were assigned within a tolerance of 0.0005 m/z to a formula of [C_cH_hO_xN_0–3 + Na]⁺ and [C_cH_hO_xN_0–3 + H]⁺ for positive ion mode and [C_cH_hO_xN_0–3 – H]⁻ for negative ion mode. The internal calibration of the m/z axis in both ion modes was verified using the assigned peaks, and the calibration was adjusted where necessary. This internal calibration improved the m/z accuracy, leading to a few additional assignments for peaks that could not be assigned within 0.0005 m/z in the uncalibrated mass spectra. Neutral formulas were calculated from the assigned ions by taking into account the ionization mode, and the two modes were clustered together. The mass spectra and elemental ratios presented here refer to the combined peak abundance in the positive and negative ion mode data referenced to formulas of the un-ionized SOA compounds.