**4. Conclusions**

In this work, voltammetric methods have been used for the first time to study the presence of reduced sulfur species (RSS) in the water-soluble fraction of the fine marine aerosols collected during spring and early autumn in the area of Rogoznica Lake, a unique anoxic and sulfidic seawater environment at the Adriatic. Two types of non-volatile RSS peaks were detected and attributed to the presence of mercapto-type (RS–Hg peak at around −0.6 V and positive, calibrated with 3-MPA) and sulfide/S 0 -like compounds (HgS peak more negative than −0.6 V, calibrated with sulfide). In spring samples only, the presence of the mercapto-type compounds was evident and proved by the addition of the standard 3-MPA, ranging from 2.60 to 15.40 ng m−<sup>3</sup> , while both, the mercapto-type (0.48–2.23 ng m−<sup>3</sup> ) and sulfide and/or S<sup>0</sup> -like compounds (0.02–0.26 ng m−<sup>3</sup> ) were detected in early autumn samples collected during anoxic holomictic event [44] in the nearby marine lake water column.

Intensity of the RS–Hg peaks in ambient samples varied through the studied seasons with more expressed and defined peaks recorded in the spring, which possibly indicates their association with biological activity. It is worth to note that these mercapto-type RSS, show the same seasonal trend as biogenically derived sulfate and methanesulfonic acid (MSA) recorded in the same aerosol samples [59], which as the oxidation product of dimethyl sulfide, is widely used as a proxy for marine biogenic sources [32,46,59]. MSA was shown not to be electrochemically active at the given experimental conditions.

In this preliminary work, the 3-MPA is used as a representative of LMW reduced organosulfur compounds that can be usually found in natural aqueous environment, as detected by chromatographic methods [60–62]. Thus, we do not intend to declare exactly the presence of 3-MPA in the studied samples, but the presence of similar RSS organic compounds should be considered with regard to the electrochemical response. This statement is supported by preliminary source apportionment results indicating sulfur-containing organic aerosols as an important WSOC fraction in the same aerosol samples [48]. In addition, higher values of WSOC (0.81–3.45 µg m−<sup>3</sup> ) were recorded in the spring samples when higher amounts of non-volatile mercapto-type RSS (2.60–15.40 ng m−<sup>3</sup> ) were also detected.

The adopted protocol by selecting appropriate experimental conditions (accumulation at deposition potentials E*<sup>d</sup>* = −0.2 V and/or −0.4 V vs. Ag/AgCl and the acidification and purging step followed by adjustment of the pH to the original after purging) confirms that electrochemistry is a fast, direct, and non-destructive methodology for the analyses of different seawater and freshwater samples [2,6,9–11,40,41,45] and can be successfully applied for the single-scan characterization of the electrochemically active sulfur compounds in ambient aerosol samples. Further work is planned to test other atmospheric RSS-relevant substances and their possible contribution to the detected RSS peaks with possible optimization of the experimental conditions given here.

**Author Contributions:** Methodology, including sampling and filter sample preparation and measurements I.C. and S.F.; resources, funding: I.C.; measurements, analysis: A.C.K.; supervision: I.C. and S.F.; writing—original draft, and revised version, A.C., and I.C.

**Funding:** This research was funded by Hrvatska Zaklada za Znanost, grant numbers IP-11-2013-1205 SPHERE, IP-01-2018-1717 MARRES.

**Acknowledgments:** This work is supported by the Croatian Science Foundation projects IP-11-2013-1205 SPHERE and IP-01-2018-1717 MARRES. Authors thank Ana Krofliˇc and Irena Grgi´c from the Department of Analytical Chemistry, National Institute of Chemistry, Ljubljana, Slovenia for sulfate analysis"D. Omanovi´c for help in using ECDSOFT software during analysis of the RSS peaks, and the anonymous reviewers for their valuable comments that improved the manuscript. Marina Frapa Rogoznica is highly acknowledged for support during aerosol sampling.

**Conflicts of Interest:** The authors declare that they have no conflict of interest.

### **Abbreviations**

methyl 3-mercaptopropionate (3-MPA).
