Contribution of dimethyl sulfide to new particle formation in the Arctic atmosphere

DoctorAtmospheric new particle formation and growth significantly influence climate by supplying new seeds for cloud condensation. Thus, observations of chemical processes in the atmosphere are crucial to understanding and predicting climate change. There is still a lack of knowledge on whether and how marine biota emissions affect aerosol-cloud-climate interactions in the Arctic. Among many naturally produced gases, dimethyl sulfide (DMS) plays an important role in atmospheric sulfur budgets and radiative balance, having potentially climate-cooling effects. Especially during the bloom and post-bloom period when the anthropogenic influence is limited and biological activity is high, the marine biological DMS is of great importance for cloud formation. The harsh environment of the Arctic makes it difficult to perform in situ measurements of atmospheric DMS and related aerosols. But more measurements in the Arctic need to be successfully accomplished because the Arctic is the best place for evaluating the impact of DMS and its oxidation products (e.g. H2SO4 and MSA) on particle formation and growth, further radiative forcing. For this dissertation study, I conducted research that developing an analytical system capable of continuous measurement of atmospheric DMS and proving the robustness of the system (Chapter 2). And I collected aerosol samples (PM 2.5) to quantify the concentration of oxidation products of DMS in the Arctic atmosphere and elucidate the factors controlling the seasonal- and interannual variability of them (Chapter 3). By applying stable S isotope measurement technique, source apportionment of sulfate aerosols and the relationship between biogenic sulfate and fine aerosol particles were investigated (Chapter 4). First, I developed an analytical system capable of continuous measurement of atmospheric DMS at pptv levels. The system uses customized devices for detector calibration and for DMS trapping and desorption that are controlled using a data acquisition system designed to maximize the efficiency of DMS analysis in a highly sensitive pulsed flame photometric detector housed in a gas chromatograph. The fully integrated system, which can sample approximately 6 L of air during a 1-hr sampling, was used to measure the atmospheric DMS mixing ratio over the Atlantic sector of the Arctic Ocean over for more than several years, with minimal routine maintenance and interruptions. During the field campaigns, the measured atmospheric DMS mixing ratio varied over a considerable range, from < 1.5 pptv to maximum levels of a few hundred pptv. The operational period covering the pre-bloom to post-bloom periods showed that the system is suitable for uninterrupted measurement of atmospheric DMS mixing ratios in extreme environments. Moreover, the data obtained using the system showed it to be useful in identifying ocean DMS source regions and changes in source strength. Seasonal to interannual variations in the concentrations of sulfur aerosols (< 2.5 micron in diameter; non sea-salt sulfate: NSS-SO42-; anthropogenic sulfate: Anth-SO42-; biogenic sulfate: Bio-SO42-; methanesulfonic acid: MSA) in the Arctic atmosphere were investigated using measurements of the chemical composition of aerosols collected at Ny-Ålesund, Svalbard from 2015 to 2019. In all measurement years the concentration of NSS-SO42- was highest during the pre-bloom period and rapidly decreased towards summer. This was because more than 50 % of the NSS-SO42- measured during this period was Anth-SO42-, which was transported to the Arctic in Arctic haze. Unexpected increases in the concentration of Bio-SO42- aerosols (an oxidation product of DMS) were occasionally found during the pre-bloom period. These probably originated in regions to the south (the North Atlantic Ocean and the Norwegian Sea), rather than in ocean areas in the proximity of Ny-Ålesund. The concentration of MSA during the pre-bloom period remained low, primarily because of the greater loss of MSA relative to Bio-SO42-, and the suppression of condensation of gaseous MSA onto particles already present in air masses being transported northwards from distant ocean source regions. In addition, the low light intensity during the pre-bloom period resulted in a low concentration of photochemically activated oxidant species including OH radicals and BrO. The concentration of MSA peaked in May or June, and was positively correlated with phytoplankton biomass in the Greenland and Barents seas around Svalbard. As a result, the mean ratio of MSA to the DMS-derived aerosols was low (0.09 ± 0.07) in the pre-bloom period, but high (0.32 ± 0.15) in the bloom and post-bloom periods. There was large interannual variability in the ratio of MSA to Bio-SO42- (i.e., 0.24 ± 0.11 in 2017, 0.40 ± 0.14 in 2018, and 0.36 ± 0.14 in 2019) during the bloom and post-bloom periods. This was probably associated with changes in the chemical properties of existing particles, biological activities surrounding the observation site, and air mass transport patterns. The results indicate that MSA is not a conservative tracer for predicting DMS-derived particles, and the contribution of MSA to the growth of newly formed particles may be much larger during the bloom and post-bloom periods than during the pre-bloom period. The connection between marine biogenic DMS and the formation of aerosol particles in the Arctic atmosphere was evaluated by analyzing atmospheric DMS mixing ratios, aerosol particle size distributions and aerosol chemical composition data that were concurrently collected at Ny-Ålesund, Svalbard from 2015 to 2019. Measurements of aerosol sulfur compounds showed distinct patterns during periods of Arctic haze (April), phytoplankton blooms (May), and post-bloom periods (June–August). Specifically, during the phytoplankton bloom period the contribution of DMS-derived SO42– to the total SO42– increased by 7-fold compared with that during the proceeding Arctic haze period, accounting for up to 70% of fine SO42– particles (< 2.5 µm in diameter). The results also showed that a sharp increase in the atmospheric DMS mixing ratio during Arctic phytoplankton bloom events was directly associated with the formation of sub-micrometer SO42– aerosols, and their subsequent growth to climate-relevant particles. Most importantly, two independent estimates of the formation of DMS-derived SO42– aerosols, calculated using the stable S isotope ratio and non-sea-salt SO42–/methanesulfonic acid ratio, respectively, were in close agreement, providing compelling evidence that the contribution of biogenic DMS to the formation of aerosol particles was substantial during the Arctic phytoplankton bloom period