Abstract
Abstract. The interest in organic nitrogen and particularly in quantifying and studying the fate of amino acids (AAs) has been growing in the atmospheric-science community. However very little is known about biotic and abiotic transformation mechanisms of amino acids in clouds. In this work, we measured the biotransformation rates of 18 amino acids with four bacterial strains (Pseudomonas graminis PDD-13b-3, Rhodococcus enclensis PDD-23b-28, Sphingomonas sp. PDD-32b-11, and Pseudomonas syringae PDD-32b-74) isolated from cloud water and representative of this environment. At the same time, we also determined the abiotic (chemical, OH radical) transformation rates within the same solutions mimicking the composition of cloud water. We used a new approach by UPLC–HRMS (ultra-performance liquid chromatography–high-resolution mass spectrometry) to quantify free AAs directly in the artificial-cloud-water medium without concentration and derivatization. The experimentally derived transformation rates were used to compare their relative importance under atmospheric conditions with loss rates based on kinetic data of amino acid oxidation in the aqueous phase. This analysis shows that previous estimates overestimated the abiotic degradation rates and thus underestimated the lifetime of amino acids in the atmosphere, as they only considered loss processes but did not take into account the potential transformation of amino acids into each other.
Highlights
The organic matter (OM) content of the cloud water phase is very complex; it has been described using Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) (Bianco et al, 2018; Zhao et al, 2013)
Organic nitrogen compounds contribute a significant fraction to the total nitrogen in cloud water (18 %) (Hill et al, 2007) and in aerosol particles (7 %–10 % in urban areas) (Xu et al, 2017) and even exceed other nitrogen contributions in marine aerosol (Miyazaki et al, 2011)
Incubations were performed in an artificial-cloud water medium containing inorganic ions, carboxylic acids, and amino acids within the same range of concentrations as those measured in clouds that were impacted by marine air masses collected at the Puy de Dôme station (Table S1, pH = 6.0) (Bianco et al, 2016a; Deguillaume et al, 2014)
Summary
The organic matter (OM) content of the cloud water phase is very complex; it has been described using Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) (Bianco et al, 2018; Zhao et al, 2013). Organic nitrogen compounds contribute a significant fraction to the total nitrogen in cloud water (18 %) (Hill et al, 2007) and in aerosol particles (7 %–10 % in urban areas) (Xu et al, 2017) and even exceed other nitrogen contributions in marine aerosol (Miyazaki et al, 2011) Among these organic nitrogen molecules, amino acids (AAs) have been recently analyzed and quantified in cloud droplets collected at the Puy de Dôme station and on the Cabo Verde islands (Triesch et al, 2021). Free-AA concentrations range from 2.4 ± 2.0 to 74.3 ± 43.8 μg C L−1 at the rural site of the Puy de Dôme station (Bianco et al, 2016a) and from 17 to 757 μg C L−1 at the marine site of Cabo Verde (Triesch et al, 2021) These AAs are of biological origin and are the building blocks of peptides ( called “combined AA”) and proteins. Primary and secondary atmospheric sources of AAs are discussed in previ-
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