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Abstract:
Ageing of particulate organic matter affects the composition and
properties of atmospheric aerosol particles. Driven by temperature and
humidity, the organic fraction can vary its physical state between
liquid and amorphous solid, or rarely even crystalline. These
transitions can influence the reaction kinetics due to limitations of
mass transport in such (semi-) solid states, which in turn may influence
the chemical ageing of particles containing such compounds. We have used
coated wall flow tube experiments to investigate the reaction kinetics
of the ozonolysis of shikimic acid, which serves as a proxy for
oxygenated, water-soluble organic matter and can form a glass at room
temperature. Particular attention was paid to how the presence of water
influences the reaction, since it acts a plasticiser and thereby induces
changes in the physical state. We analysed the results by means of a
traditional resistor model, which assumes steady-state conditions. The
ozonolysis rate of shikimic acid is strongly increased in the presence
of water, a fact we attribute to the increased transport of O-3 and
shikimic acid through the condensed phase at lower viscosities. The
analysis using the resistor model suggests that the system undergoes
both surface and bulk reaction. The second-order rate coefficient of the
bulk reaction is 3.7 (+1.5/-3.2) x 10(3) L mol(-1) s(-1). At low
humidity and long timescales, the resistor model fails to describe the
measurements appropriately. The persistent O-3 uptake at very low
humidity suggests contribution of a self-reaction of O-3 on the surface.
