Abstract
Abstract. Highly oxidized multifunctional compounds (HOMs) have been demonstrated to be important for atmospheric secondary organic aerosols (SOA) and new-particle formation (NPF), yet it remains unclear which the main atmospheric HOM formation pathways are. In this study, a nitrate-ion-based chemical ionization atmospheric-pressure-interface time-of-flight mass spectrometer (CI-APi-TOF) was deployed to measure HOMs in the boreal forest in Hyytiälä, southern Finland. Positive matrix factorization (PMF) was applied to separate the detected HOM species into several factors, relating these “factors” to plausible formation pathways. PMF was performed with a revised error estimation derived from laboratory data, which agrees well with an estimate based on ambient data. Three factors explained the majority (> 95 %) of the data variation, but the optimal solution found six factors, including two nighttime factors, three daytime factors, and a transport factor. One nighttime factor is almost identical to laboratory spectra generated from monoterpene ozonolysis, while the second likely represents monoterpene oxidation initiated by NO3. The exact chemical processes forming the different daytime factors remain unclear, but they all have clearly distinct diurnal profiles, very likely related to monoterpene oxidation with a strong influence from NO, presumably through its effect on peroxy radical (RO2) 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 confirming current knowledge and inspiring future research directions and provide new perspectives on using factorization methods to understand short-lived atmospheric species.
Highlights
Large amounts of volatile organic compounds (VOCs) are emitted into the atmosphere from both biogenic and anthropogenic sources (Atkinson and Arey, 2003)
Though the molar yield of Highly oxidized multifunctional compounds (HOMs) is only a few percent depending on the VOC structure and oxidant, a global model suggested HOMs play a crucial role in secondary organic aerosol (SOA) burden and cloud condensation nuclei concentrations (Jokinen et al, 2015)
Positive matrix factorization (PMF) analysis was initially done for two factors, and followed with a step-wise addition of one factor until the additional factor could no longer be interpreted based on the unique mass spectral feature or comparison of its time trend with auxiliary data
Summary
Large amounts of volatile organic compounds (VOCs) are emitted into the atmosphere from both biogenic and anthropogenic sources (Atkinson and Arey, 2003). Positive matrix factorization (PMF) (Paatero and Tapper, 1994) allows for time-resolved mass spectra to be expressed as a linear combination of a finite number of factors, assuming that the factor profiles are constant and unique (Ulbrich et al, 2009). Since this method does not require a priori information about the factors, it is an ideal technique for extracting information from ambient measurements where the detailed chemistry, sources, and atmospheric processes are complex. This work provides new perspectives on using PMF to understand the variation of short-lived species, e.g., HOMs
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