**Aqueous-Phase Production of Secondary Organic Aerosols from Oxidation of Dibenzothiophene (DBT)**

#### **Yu Liu <sup>1</sup> , Junchen Lu <sup>1</sup> , Yanfang Chen <sup>1</sup> , Yue Liu <sup>1</sup> , Zhaolian Ye <sup>2</sup> and Xinlei Ge 1,\***


Received: 13 January 2020; Accepted: 29 January 2020; Published: 30 January 2020

**Abstract:** Intermediate-volatility organic compounds (IVOCs) have been recognized as an important contributor to the secondary organic aerosol (SOA) formation via gas-phase reactions. However, it is unclear whether or not IVOCs-SOA can be produced in the aqueous phase. This work investigated aqueous oxidation of one model compound of IVOCs, dibenzothiophene (DBT). Results show that DBT can be degraded by both hydroxyl radical and the triplet excited states of organic light chromophores ( <sup>3</sup>C\*). Aqueous dark oxidation of DBT was also possible. SOA yields of 32% and 15% were found for hydroxyl radical (OH)-mediated photo-oxidation and dark oxidation, respectively. A continuous and significant increase of oxidation degree of SOA was observed during OH photo-oxidation, but not during the dark oxidation. Factor analyses revealed that there was a persistent production of highly oxygenated compounds from the less oxygenated species. OH-initiated photochemical reactions can also produce species with a relatively large light-absorbing ability, while such photo-enhancement due to direct light irradiation and <sup>3</sup>C\*-initiated oxidation could occur, but is much less important. In the future, studies on the second-order rate constants, molecular characterization of the oxidation products from this and other IVOCs precursors are needed to better understand the role of this reaction pathway in SOA budget, air quality and climate change.

**Keywords:** dibenzothiophene; intermediate-volatility organic compounds; secondary organic aerosol; aqueous phase; brown carbon
