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Highly oxidized multifunctional compounds (HOMs) have been demonstrated tobe important for atmospheric secondary organic aerosols (SOA) and new-particle formation (NPF), yet it remains unclear which the main atmosphericHOM formation pathways are. In this study, a nitrate-ion-based chemicalionization atmospheric-pressure-interface time-of-flight mass spectrometer(CI-APi-TOF) was deployed to measure HOMs in the boreal forest inHyytiälä, southern Finland. Positive matrix factorization (PMF) wasapplied to separate the detected HOM species into several factors, relatingthese “factors” to plausible formation pathways. PMF was performed with arevised error estimation derived from laboratory data, which agrees wellwith an estimate based on ambient data. Three factors explained the majority(> 95 %) of the data variation, but the optimal solution foundsix factors, including two nighttime factors, three daytime factors, and atransport factor. One nighttime factor is almost identical to laboratoryspectra generated from monoterpene ozonolysis, while the second likelyrepresents monoterpene oxidation initiated by NO<sub>3</sub>. The exact chemicalprocesses forming the different daytime factors remain unclear, but they allhave clearly distinct diurnal profiles, very likely related to monoterpeneoxidation with a strong influence from NO, presumably through its effect onperoxy radical (RO<sub>2</sub>) chemistry. Apart from these five “local”factors, the sixth factor is interpreted as a transport related factor.These findings improve our understanding of HOM production by confirmingcurrent knowledge and inspiring future research directions and provide newperspectives on using factorization methods to understand short-livedatmospheric species

Source characterization of highly oxidized multifunctional compounds in a boreal forest environment using positive matrix factorization