Advancing our understanding of dimethyl sulfide oxidation products by field observations of formaldehyde and photolysis frequencies

海洋生物排放的二甲基硫（DMS）是大气中硫的最大自然来源，也是主要的气溶胶前体（Carslaw等，2010）。随着人为SO2来源的减少，了解DMS的命运对于理解新的硫酸盐气溶胶颗粒的形成、现有气溶胶向云凝结核（CCN）的增长以及遥远海洋环境中气溶胶的负担/组成至关重要。最近发现的在DMS氧化过程中形成的新分子氢过氧甲基硫代甲酸酯（HOOCH2SCHO； HPMTF）突出表明，我们对DMS化学的主要途径和产物的了解存在不确定性（Veres et al., 2020）。为了解决这些关键的不确定性，已经开发了一个大型野外和建模项目（限制海洋硫循环在地球系统中的作用（CARES））。作为CARES的一部分，一套全面的测量将在一艘研究船上和在爱尔兰西海岸飞行的FAAM飞机上进行。与本研究特别相关的是，将测量HPMTF中存在的光化通量观察以及官能团（醛和过氧化氢）的吸收截面，从而确定HPMTF的光解速率，并推导出光解产物。HCHO的观测结果，再加上HCHO光解频率和一组VOCs、CH4、CO、NOx的观测结果（从中可以得出(a)加权于所有OH反应的有效HCHO产率），将提供OH浓度的代表（Wolfe et al., 2019），并使通过OH氧化的HPMTF损失以及通过OH氧化的DMS氧化速率得以确定。通过这些有针对性的实地观测和随后的模拟研究，将更好地了解DMS在海洋硫循环中的作用及其对气候系统的影响。

Dimethyl sulfide (DMS) from marine biogenic emissions is the largest natural source of sulphur in the atmosphere and a major aerosol precursor (Carslaw et al., 2010). With anthropogenic sources of SO2 decreasing, knowledge of the fate of DMS is critical for understanding the formation of new sulphate aerosol particles, growth of existing aerosols to cloud condensation nuclei (CCN), and the burden/composition of aerosols in the remote marine environment. The recent discovery of a new molecule, hydroperoxy methylthioformate (HOOCH2SCHO; HPMTF) which is formed during the oxidation of DMS has highlighted that there are uncertainties in our knowledge of the main pathways and products of DMS chemistry (Veres et al., 2020). To address these key uncertainties, a large field and modelling project (Constraining the role of the marine sulfur cycle in the Earth System (CARES)) has been developed. As part of CARES, a comprehensive suite of measurements will be made on board a research ship and from the FAAM aircraft flying off the west coast of Ireland. Of particular relevance to this studentship, actinic flux observations, coupled with absorption cross sections for the functional groups (aldehydic and hydroperoxide) present in HPMTF will be measured and will enable photolysis rates for HPMTF to be determined and the photolytic products to be derived. HCHO observations, coupled with HCHO photolysis frequencies and observations of a suite of VOCs, CH4, CO, NOx (from which (a) the effective HCHO yield weighted over all OH reactions, can be derived) will provide a proxy for the OH concentration (Wolfe et al., 2019) and enable the loss of HPMTF via OH oxidation to be determined as well as the DMS oxidation rate via OH. Through these targeted field observations and subsequent modelling studies, an improved understanding of the role DMS plays in the marine sulfur cycle and the impact on the climate system will be achieved.