Chlorine-initiated oxidation of <i>n</i>-alkanes under high-NO<sub><i>x</i></sub> conditions: insights into secondary organic aerosol composition and volatility using a FIGAERO–CIMS

Abstract

Abstract. Chlorine-initiated oxidation of n-alkanes (C8−12) under high-nitrogen oxide conditions was investigated. Observed secondary organic aerosol yields (0.16 to 1.65) are higher than those for OH-initiated oxidation of C8−12 alkanes (0.04 to 0.35). A high-resolution time-of-flight chemical ionization mass spectrometer coupled to a Filter Inlet for Gases and AEROsols (FIGAERO–CIMS) was used to characterize the gas- and particle-phase molecular composition. Chlorinated organics were observed, which likely originated from chlorine addition to the double bond present on the heterogeneously produced dihydrofurans. A two-dimensional thermogram representation was developed to visualize the composition and relative volatility of organic aerosol components using unit-mass resolution data. Evidence of oligomer formation and thermal decomposition was observed. Aerosol yield and oligomer formation were suppressed under humid conditions (35 % to 67 % RH) relative to dry conditions (under 5 % RH). The temperature at peak desorption signal, Tmax, a proxy for aerosol volatility, was shown to change with aerosol filter loading, which should be constrained when evaluating aerosol volatilities using the FIGAERO–CIMS. Results suggest that long-chain anthropogenic alkanes could contribute significantly to ambient aerosol loading over their atmospheric lifetime.

Seasonal characteristics of organic aerosol chemical composition and volatility in Stuttgart, Germany

“…3). Recent studies show that Tmax of a compound can be influenced by isomers (Thompson et al, 2017), thermal fragmentation of larger molecules during the heating of the filter (Lopez-Hilfiker et al, 2015), variations in filter mass loading (Huang et al, 2018;Wang and Ruiz, 2018), and/or differences in particles' viscosity (Huang et al, 2018). Since deposited organic mass loadings on the filter samples were similar for summer and winter, we can exclude a mass loading effect on the Tmax results presented here.…”

“…The pattern of the summer 2D thermogram in Stuttgart, particularly the "zigzag"-like behavior of e.g. m/z 280-380 Th (see Figure S12a), is comparable to that from alkane-Cl SOA at high RH (67 %) and high NOx conditions observed by Wang and Ruiz (2018), and was explained by increased hydroxyl functionalization over ketone functionalization. The winter 2D thermogram in Stuttgart also has a "zigzag"-like pattern but less pronounced and at higher Tmax (see Fig.…”

“…But for the larger compounds, such as dimers and trimers, contributions of thermal decomposition products become negligible (i.e. thermograms are unimodal; Huang et al, 2018;Wang and Ruiz, 2018). LVOC and ELVOC, which include compounds with larger molecular weight, exhibit higher mass contributions in winter (LVOC: 26.4 ± 3.1 %; ELVOC: 6.9 ± 1.9 %) than in summer (LVOC: 21.7 ± 2.5 %; ELVOC: 4.1 ± 1.1 %; see Fig.…”

“…framework developed recently to investigate the OOA thermal desorption behavior over the entire m/z and Tmax range (Wang and Ruiz, 2018). Each thermogram of each individual compound in the 2D space was normalized to its maximum signal.…”

A robust clustering algorithm for analysis of composition-dependent organic aerosol thermal desorption measurements

“…In the field of atmospheric chemistry, clustering methods have been applied to a variety of data types. Examples include the following: back trajectories of trace gases (Cape et al, 2000) or particles (Abdalmogith and Harrison, 2005;Pinero-Garcia et al, 2015), helping to elucidate the origin and transport of pollutants; particle size distributions, providing information on aerosol emission and formation (Beddows et al, 2009;Wegner et al, 2012); and organic functional groups comprising individual particles, allowing for the classification of the types of organic carbon (Takahama et al, 2007).…”