Abstract
Abstract. Eighteen free amino acids (FAAs) were quantified in cloud water sampled at the Puy de Dôme station (PUY – France) during 13 cloud events. This quantification has been performed without concentration or derivatization, using liquid chromatography hyphened to mass spectrometry (LC-MS) and the standard addition method to correct for matrix effects. Total concentrations of FAAs (TCAAs) vary from 1.2 to 7.7 µM, Ser (serine) being the most abundant AA (23.7 % on average) but with elevated standard deviation, followed by glycine (Gly) (20.5 %), alanine (Ala) (11.9 %), asparagine (Asn) (8.7 %), and leucine/isoleucine (Leu/I) (6.4 %). The distribution of AAs among the cloud events reveals high variability. TCAA constitutes between 0.5 and 4.4 % of the dissolved organic carbon measured in the cloud samples. AA quantification in cloud water is scarce, but the results agree with the few studies that investigated AAs in this aqueous medium. The environmental variability is assessed through a statistical analysis. This work shows that AAs are correlated with the time spent by the air masses within the boundary layer, especially over the sea surface before reaching the PUY. The cloud microphysical properties' fluctuation does not explain the AA variability in our samples, confirming previous studies at the PUY. We finally assessed the sources and the atmospheric processes that potentially explain the prevailing presence of certain AAs in the cloud samples. The initial relative distribution of AAs in biological matrices (proteins extracted from bacterial cells or mammalian cells, for example) could explain the dominance of Ala, Gly, and Leu/I. AA composition of aquatic organisms (i.e., diatom species) could also explain the high concentrations of Ser in our samples. The analysis of the AA hygroscopicity also indicates a higher contribution of AAs (80 % on average) that are hydrophilic or neutral, revealing the fact that other AAs (hydrophobic) are less favorably incorporated into cloud droplets. Finally, the atmospheric aging of AAs has been evaluated by calculating atmospheric lifetimes considering their potential transformation in the cloud medium by biotic or abiotic (mainly oxidation) processes. The most concentrated AAs encountered in our samples present the longest atmospheric lifetimes, and the less dominant ones are clearly efficiently transformed in the atmosphere, potentially explaining their low concentrations. However, this cannot fully explain the relative contribution of several AAs in the cloud samples. This reveals the high complexity of the bio-physico-chemical processes occurring in the multiphase atmospheric environment.
Highlights
Free or combined amino acids (AAs) that make up proteins and cell walls in living organisms are ubiquitous chemical compounds found in various environments
The analytical method used in this study allows assaying of AAs directly in cloud samples
This study reports the quantification of amino acids in cloud waters sampled at the Puy de Dôme station using a new approach based on a direct in situ analysis of the sample
Summary
Free or combined amino acids (AAs) that make up proteins and cell walls in living organisms are ubiquitous chemical compounds found in various environments In the atmosphere, they are commonly detected in the condensed phases due to their low vapor pressures. Many efforts have been made in the past to assess their sources, their role in the atmospheric chemical and physical processes, and their fate (Cape et al, 2011) Despite those investigations, their exact role in the atmosphere is still poorly understood. AAs are part of the proteinaceous fractions of aerosol particles that significantly contribute to the organic carbon and organic nitrogen fraction of aerosol particles Their presence in aerosol particles can modify their chemical properties such as acidity/basicity and buffering ability (Cape et al, 2011; Zhang and Anastasio, 2003b). Atmospheric AAs can contribute to the nutrient cycling at a global scale as well as the global carbon and nitrogen cycles
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