Improving the characterization of dissolved organic carbon in cloud water: Amino acids and their impact on the oxidant capacity

Abstract

Improving our understanding of cloud chemistry depends on achieving better chemical characterization (90% of the organic carbon [OC] fraction remains uncharacterized) and, consequently, assessing the reactivity of this complex system. In this manuscript, we report for the first time the concentrations of 16 amino acids (AAs) in 25 cloud water samples. The concentrations of individual AAs ranged from a few nM up to ~2.0 μM, and the average contribution of AAs corresponded to 9.1% (4.4 to 21.6%) of the dissolved OC (DOC) concentration. Considering their occurrence and concentrations, AAs were expected to represent an important hydroxyl radical (HO^•) sink in aqueous cloud samples. In this work, we estimated that approximately 17% (from 7 to 36%) of the hydroxyl radical-scavenging ability of the DOC could be attributed to the presence of AAs, whereas comparing the AAs suggested that an average of 51% (from 22 to 80%) of their reactivity with HO^• could account for the presence of tryptophan. These results clearly demonstrate that the occurrence and reactivity of AAs must be considered to better estimate the chemical composition and oxidant capacity of the cloud aqueous phase.

Figure 1. Distribution of each AA in the cloud samples.

The bottom and top lines of the box correspond to the 25^th and 75^th percentiles, respectively. The middle line represents the median. The ends of the whiskers are the 10^th and 90^th percentiles, and the filled circle is an outlier. The y-right scale shows the total AA concentration.

Figure 2. Pie plot showing the distributions of AAs, aldehydes and the most relevant short-chain CAs (formate, acetate, oxalate, malonate and succinate) relative to the total DOC in cloud water samples.

The percentages were calculated based on the average concentrations of individual AAs, aldehydes, CAs and DOC in mg C L⁻¹. The second plot shows the distribution of a single AA relative to the total distribution.

Figure 3. Ratio of the scavenging rate constants of HO^• (expressed in s⁻¹) with AAs and short-chain CAs (C_AA/CA) without (filled circles) and with (empty circles) formate (calculated using equation 1).

The error bars were determined by considering the uncertainties associated with AA quantification in each sample.

Figure 4. HO^• scavenging rates (k′) of AAs, TRP and DOC.

These values were determined based on the average values of , and using the concentrations and second-order rate constants reported in Tables S1 and S3. The value was taken from Arakaki et al..

Figure 5. Example HPLC chromatograms obtained from samples 2 and 9.