Characteristics of oxygenated volatile organic compounds in Zurich, Switzerland: Sources, composition, and implication for secondary aerosol formation.

Volatile organic compounds (VOCs) are of vital importance in the formation of secondary organic aerosol (SOA). Understanding SOA formation remains challenging, requiring further investigation of both oxygenated VOCs (OVOCs) and SOA composition with novel measurement techniques. In this work, we deployed a proton-transfer-reaction time-of-flight mass spectrometer (PTR-TOF-MS) to measure VOCs and their oxidation products in urban Zurich in summer 2016. The positive matrix factorization (PMF) source apportionment method identified five sources, including two primary sources (traffic and local), and three OVOC sources associated with different oxidation processes in the atmosphere. Together with the deployment of an extractive electrospray ionization time-of-flight mass spectrometer (EESI-TOF-MS), this enabled a detailed understanding of the SOA components which were dominated by biogenic SOA and distinguished by daytime and nighttime chemistry. The combination of the two instruments provided new insights in the understanding of atmospheric processes by comparison of molecular-level secondary components between gas phase and particle phase. In the gas phase, two OVOC factors (32.1% and 16.7%) showed strong influence from the oxidation of aromatic compounds, exhibiting low atomic H to C ratios, and were distinguished by daytime and night-time chemistry. The third OVOC factor (19.3%) was characterized by strong biogenic influence. Similar temporal variations were found for the gas and aerosol phase, indicating co-evolution of OVOCs and SOA in summer. Besides, comparisons of OVOC compounds and SOA composition exhibited similar H to C ratio distributions for both the gas phase and particle phase.