Abstract
Abstract. This work presents the results from a flow reactor study on the formation of carboxylic acids from limonene oxidation in the presence of ozone under NOx-free conditions in the dark. A High-Resolution Time-of-Flight acetate Chemical Ionisation Mass Spectrometer (HR-ToF-CIMS) was used in combination with a Filter Inlet for Gases and AEROsols (FIGAERO) to measure the carboxylic acids in the gas and particle phases. The results revealed that limonene oxidation produced large amounts of carboxylic acids which are important contributors to secondary organic aerosol (SOA) formation. The highest 10 acids contributed 56 %–91 % to the total gas-phase signal, and the dominant gas-phase species in most experiments were C8H12O4, C9H14O4, C7H10O4 and C10H16O3. The particle-phase composition was generally more complex than the gas-phase composition, and the highest 10 acids contributed 47 %–92 % to the total signal. The dominant species in the particle phase were C8H12O5, C9H14O5, C9H12O5 and C10H16O4. The measured concentration of dimers bearing at least one carboxylic acid function in the particle phase was very low, indicating that acidic dimers play a minor role in SOA formation via ozone (O3)/hydroxyl (OH) oxidation of limonene. Based on the various experimental conditions, the acidic compositions for all experiments were modelled using descriptions from the Master Chemical Mechanism (MCM). The experiment and model provided a yield of large (C7–C10) carboxylic acid of the order of 10 % (2 %–23 % and 10 %–15 %, respectively). Significant concentrations of 11 acids, from a total of 16 acids, included in the MCM were measured with the CIMS. However, the model predictions were, in some cases, inconsistent with the measurement results, especially regarding the OH dependence. Reaction mechanisms are suggested to fill-in the knowledge gaps. Using the additional mechanisms proposed in this work, nearly 75 % of the observed gas-phase signal in our lowest concentration experiment (8.4 ppb converted, ca. 23 % acid yield) carried out under humid conditions can be understood.
Highlights
Atmospheric aerosol particles have an impact on climate and human health, and their respective effects depend on particle properties determined by the particle size and chemical composition
Explicit formation pathways for the C10H16O4 and C9H12O5 compounds implemented in the model and additional reaction pathways for C7H10O4, C8H12O4−5, C9H14O4−5 and C10H16O3 are proposed
Structures for C10H14O5 and C10H16O4 have been proposed in previous studies (Jaoui et al, 2006; Rossignol et al, 2012; Leungsakul et al, 2005a; Walser et al, 2008; Glasius et al, 2000), but the current mechanistic understanding is inadequate to explain the formation of compounds with the proposed structures
Summary
Atmospheric aerosol particles have an impact on climate and human health, and their respective effects depend on particle properties determined by the particle size and chemical composition. Identifying the chemical pathways of condensable products is essential for predicting SOA formation (Hallquist et al, 2009; Ziemann and Atkinson, 2012; Ehn et al, 2014; Shrivastava et al, 2017; McFiggans et al, 2019). This identification is inherently difficult as such products often reside in both the gas and particulate phases and continuous partitioning occurs between these two phases. The main constituent of the essential oil from citrus plants, is a widely used chemical in personal care and household-related consumer products (owing to its pleasant smell); elevated indoor concentrations of limonene can be expected
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