Sugars in clouds: Measurements and modelling investigation of their aqueous photodegradation

Abstract

Currently, studies show that only 10–30% at best of the organic matter dissolved in cloud water is characterized and sugars are among the compounds still unexplored. Numerous studies on the atmospheric particulate matter (PM) demonstrate their ubiquity, and their incorporation into cloud phase is therefore expected in light of their solubility.

In this work, a large array of sugars are measured at low concentration levels with an innovative LC-MSMS technique in 28 cloud water samples collected at the puy de Dôme and their environmental variability is analyzed. The total sugar concentration reaches 4.0 μmol L⁻¹, with an average value of 0.98 ± 0.94 μmol L⁻¹, with no clear seasonal trend. This represents on average 1.2% of dissolved organic carbon but reaches up to 3.7% for one sample. The most abundant sugars is levoglucosan (average 247.2 nmol L⁻¹) followed by glucose (168.2 nmol L⁻¹), mannitol (136.5 nmol L⁻¹) and 2-methyl-tetrols (130.6 nmol L⁻¹). Reactivity in the aqueous phase is simulated with the cloud chemistry model CLEPS to gain insights in the half-life time of four selected compounds in different meteorological and chemical scenarios. Results show a fast degradation of sugars in the aqueous phase, especially during summer, that could question the use of these tracers for long range transport in the free troposphere.