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.