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Abstract. This work presents the results from a flow reactor study on theformation of carboxylic acids from limonene oxidation in the presence ofozone 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 resultsrevealed that limonene oxidation produced large amounts of carboxylic acidswhich are important contributors to secondary organic aerosol (SOA)formation. The highest 10 acids contributed 56 %–91 % to the totalgas-phase signal, and the dominant gas-phase species in most experiments were C8H12O4 , C9H14O4 , C7H10O4 and C10H16O3 . The particle-phase composition was generally morecomplex than the gas-phase composition, and the highest 10 acids contributed47 %–92 % to the total signal. The dominant species in the particle phasewere C8H12O5 , C9H14O5 , C9H12O5 and C10H16O4 . The measured concentration of dimers bearing atleast one carboxylic acid function in the particle phase was very low,indicating that acidic dimers play a minor role in SOA formation viaozone ( O3 )/hydroxyl (OH) oxidation of limonene. Based on the various experimentalconditions, the acidic compositions for all experiments were modelled usingdescriptions from the Master Chemical Mechanism (MCM). The experimentand model provided a yield of large ( C7 – C10 ) carboxylic acid of the order of 10 % (2 %–23 % and 10 %–15 %, respectively). Significant concentrations of 11acids, from a total of 16 acids, included in the MCM were measured with theCIMS. However, the model predictions were, in some cases, inconsistent withthe measurement results, especially regarding the OH dependence.Reaction mechanisms are suggested to fill-in the knowledge gaps. Using theadditional mechanisms proposed in this work, nearly 75 % of the observedgas-phase signal in our lowest concentration experiment (8.4 ppb converted, ca. 23 % acid yield) carried out under humid conditions can be understood.

Carboxylic acids from limonene oxidation by ozone and hydroxyl radicals: insights into mechanisms derived using a FIGAERO-CIMS