The Aarhus Chamber Campaign on Highly Oxygenated Organic Molecules and Aerosols (ACCHA): particle formation, organic acids, and dimer esters from α-pinene ozonolysis at different temperatures
Authors: Kristensen Kasper , Jensen Louise N. , Quéléver Lauriane L. J. , Christiansen Sigurd , Rosati Bernadette , Elm Jonas , Teiwes Ricky , Pedersen Henrik B. , Glasius Marianne , Ehn Mikael , Bilde Merete 
Published: November 2020

Abstract. Little is known about the effects of subzero temperatures on the formation
of secondary organic aerosol (SOA) from α-pinene. In the current
work, ozone-initiated oxidation of α-pinene at initial
concentrations of 10 and 50 ppb, respectively, is performed at
temperatures of 20, 0, and −15 ∘C in
the Aarhus University Research on Aerosol (AURA) smog chamber during the Aarhus Chamber Campaign on Highly Oxygenated
Organic Molecules and Aerosols (ACCHA). Herein, we show how temperature influences the
formation and chemical composition of α-pinene-derived SOA with a
specific focus on the formation of organic acids and dimer esters. With
respect to particle formation, the results show significant increase in
particle-formation rates, particle number concentrations, and particle mass
concentrations at low temperatures. In particular, the number concentrations
of sub-10 nm particles were significantly increased at the lower 0 and −15 ∘C temperatures. Temperature also affects
the chemical composition of formed SOA. Here, detailed offline chemical
analyses show that organic acids contribute from 15 % to 30 % by mass,
with highest contributions observed at the lowest temperatures, indicative
of enhanced condensation of these semivolatile species. In comparison, a
total of 30 identified dimer esters were seen to contribute between 4 % and 11 % to the total SOA mass. No significant differences in the chemical
composition (i.e. organic acids and dimer esters) of the α-pinene-derived SOA particles are observed between experiments performed at
10 and 50 ppb initial α-pinene concentrations, thus suggesting a
higher influence of reaction temperature compared to that of α-pinene loading on the SOA chemical composition. Interestingly, the effect
of temperature on the formation of dimer esters differs between the
individual species. The formation of less oxidized dimer esters – with
oxygen-to-carbon ratio (O:C)<0.4 – is shown to increase at low
temperatures, while the formation of the more oxidized species (O:C>0.4) is suppressed, consequently resulting in
temperature-modulated composition of the α-pinene-derived SOA.
Temperature ramping experiments exposing α-pinene-derived SOA to
changing temperatures (heating and cooling) reveal that the chemical
composition of the SOA with respect to dimer esters is governed almost
solely by the temperature at which oxidization started and is insusceptible to
subsequent changes in temperature. Similarly, the resulting SOA mass
concentrations were found to be more influenced by the initial α-pinene oxidation temperatures, thus suggesting that the formation
conditions to a large extent govern the type of SOA formed, rather than the
conditions to which the SOA is later exposed. For the first time, we discuss the relation between the identified dimer
ester and the highly oxygenated organic molecules (HOMs) measured by
chemical ionization–atmospheric pressure interface–time-of-flight mass spectrometer (CI-APi-ToF) during the ACCHA experiments. We propose
that, although very different in chemical structures and O:C ratios, many
dimer esters and HOMs may be linked through similar RO2 reaction
pathways and that dimer esters and HOMs merely represent two different
fates of the RO2 radicals.