Chamber investigation of the formation and transformation of secondary organic aerosol in mixtures of biogenic and anthropogenic volatile organic compounds

Abstract

Abstract. A comprehensive investigation of the photochemical secondary organic aerosol (SOA) formation and transformation in mixtures of anthropogenic (o-cresol) and biogenic (α-pinene and isoprene) volatile organic compound (VOC) precursors in the presence of NOx and inorganic seed particles was conducted. Initial iso-reactivity was used to enable direct comparison across systems, adjusting the initial reactivity of the systems towards the assumed dominant oxidant (OH). Comparing experiments conducted in single precursor systems at various initial reactivity levels (referenced to a nominal base case VOC reactivity) and their binary and ternary mixtures, we show that the molecular interactions from the mixing of the precursors can be investigated and discuss limitations in their interpretation. The observed average SOA yields in descending order were found for the α-pinene (32 ± 7 %), α-pinene/o-cresol (28 ± 9 %), α-pinene at ½ initial reactivity (21 ± 5 %), α-pinene/isoprene (16 ± 1 %), α-pinene at ⅓ initial reactivity (15 ± 4 %), o-cresol (13 ± 3 %), α-pinene/o-cresol/isoprene (11 ± 4%), o-cresol at ½ initial reactivity (11 ± 3 %), o-cresol/isoprene (6 ± 2 %) and isoprene systems (0 ± 0 %). We find a clear suppression of the SOA yield from α-pinene when it is mixed with isoprene, whilst the addition of isoprene to o-cresol may enhance the mixture’s SOA formation potential, however, the difference was too small to be unequivocal. The α-pinene/o-cresol system yield appeared to be increased compared to that calculated based on the additivity, whilst in the α-pinene/o-cresol/isoprene system the measured and predicted yield were comparable. However, in mixtures where more than one precursor contributes to the SOA mass it is unclear whether changes in the SOA formation potential are attributable to physical or chemical interactions, since the reference basis for the comparison is complex. Online and offline chemical composition and SOA particle volatility, water uptake and “phase” behaviour measurements that were used to interpret the SOA formation and behaviour are introduced and detailed elsewhere.

The influence of the addition of isoprene on the volatility of particles formed from the photo-oxidation of anthropogenic–biogenic mixtures

“…This resulted in appreciable amounts of VOC(s) remaining at the end of each experiment (see Fig. 2 in Voliotis et al, 2022). In turn, this might suggest a continuous formation of earlier-generation products during our experiments that might contributed to the relatively high volatility observed.…”

“…Three types of experiments were conducted: (a) single-precursor experiments, in which the photo-oxidation of each precursor was studied individually, (b) binary experiments, in which the photo-oxidation of mixtures of α-pinene/isoprene and o-cresol/isoprene was studied, and (c) ternary experiments, in which all three precursors, α-pinene, o-cresol, and isoprene, were oxidized simultaneously. Following Voliotis et al (2021Voliotis et al ( , 2022, all experiments were designed to start with initial iso-reactivity; the injected VOC concentrations were selected based on the ratios of the inverse of their IUPAC-recommended rate constants at 298 K towards the OH (see Voliotis et al, 2022). This approach ensures that at the beginning of each experiment each VOC in the mixture will have equal chances of reacting with the dominant oxidant, and thereby they will have equal contributions to the first generation of products.…”

“…A summary of the initial experimental conditions is provided in Table 1. Each experiment was performed following procedures described in detail in Voliotis et al (2021Voliotis et al ( , 2022. Briefly, the "chamber stabilization" phase (∼ 1 h) was initialized when the chamber was filled with clean air in the absence of any of the reactants, followed by the "dark unreactive" phase (∼ 1 h) where all the reactants (VOC, NO 2 , and seeds) were added in the dark.…”

“…Steady-state actinometry experiments showed a photolysis rate of NO 2 (J NO 2 ) at 0.135 ± 0.024 min −1 , while the average OH concentrations for all experiments conducted here were estimated at around 8 ± 3 × 10 5 molec. cm −3 (Voliotis et al, 2022). VOCs are produced by evaporating the corresponding liquids (α-pinene, isoprene, and o-cresol; Sigma-Aldrich, GC grade ≥ 99.99 % purity) in a gently heated glass bulb and are transferred to the MAC using an electronic capture device-grade nitrogen stream (ECD N 2 ; N4.8 purity ≥ 99.998 %).…”

Combined application of online FIGAERO-CIMS and offline LC-Orbitrap mass spectrometry (MS) to characterize the chemical composition of secondary organic aerosol (SOA) in smog chamber studies

“…The selectivity and instrumental sensitivity make it impossible to use one instrument to capture all oxidation products; thus, the combination of instruments in order to better explore the evolution of SOA chemical composition is strongly recommended. Additionally, future work will report SOA ox-idation products and their chemical properties from more complex systems utilizing the analytical approaches from the two instruments, such as the chamber photooxidation of anthropogenic VOCs or biogenic and anthropogenic mixtures of VOCs (Voliotis et al, 2022). As this study only assessed a single α-pinene photooxidation system, more work is needed to evaluate how those analytical methods from the two instruments perform with other systems.…”