Sulfur aerosols in the Arctic, Antarctic, and Tibetan Plateau: Current knowledge and future perspectives

Abstract

Sulfur aerosols, mainly composed of sulfate and methanesulfonic acid (MSA), significantly affect the Earth’s radiation balance, biogeochemical cycles and ecosystems, especially in the polar regions with vulnerable environments. To better understand the relationship between anthropogenic activities and climate change, a comprehensive review is presented, covering sulfate and MSA concentrations and isotope composition from 18 sites in the Arctic, 22 sites in the Antarctic and 25 sites in the Tibetan Plateau. The spatio-temporal variability of sulfur aerosols and the potential factors controlling their concentrations are summarized, sulfur isotopes are used to identify the importance of anthropogenic vs. natural inputs, and ice cores are employed to reconstruct the paleo-evolution of atmospheric sulfates. Finally, this review discusses the need for future research on organosulfur aerosols, the mixing state of sulfur aerosols, their deposition fluxes and velocities, potential emissions by biomass burning, and the anticipated trends in sulfur aerosol concentrations in the Arctic, Antarctic, and Tibetan Plateau.

Introduction

Climate change is a critical issue globally, as stated by the key findings of the IPCC Special Report that highlights its impacts on natural and human systems (IPCC, 2018). Aerosols represent one of the greatest sources of uncertainty in climate modelling (Tomasi et al., 2015). Aerosols are solid or liquid particles suspended in the atmosphere. Their composition is complex, with key aerosol groups including sulfates, organic carbon, black carbon (i.e. elemental carbon), nitrates, mineral dust and sea salt. While most aerosols contribute to warming, sulfate aerosols have a net cooling effect (Charlson et al., 1992) and strongly change radiative forcing (−0.81 Wm⁻² to −0.55 Wm⁻²) (Penner et al., 1998). Sulfates account for 11 to 59% of sub-micrometer aerosols at multiple surface locations in the Northern Hemisphere (Jimenez et al., 2009).

Sulfur-containing aerosols can be divided into inorganic sulfate (sea-salt sulfate (ss-sulfate) and non-sea-salt sulfate (nss-sulfate)) and organosulfur compounds (MSA, organosulfates and others). Sulfur-containing aerosols, whether natural or anthropogenic, originate from primary or secondary emissions of gaseous precursors and particulate matter (Fig. 1). Ss-sulfate is introduced into the atmosphere as a primary particle, but nss-sulfate and most of the organosulfur compounds are predominantly formed by secondary aerosol processes (Seinfeld and Pandis, 2016). A majority of organosulfur compounds are attributed to biogenic origins (Stone et al., 2012) and MSA can serve as a tracer of atmospheric biogenic sulfur (Yli-Tuomi et al., 2003). Sulfate and MSA, the main components of sulfur-oxidized compounds, have been widely studied recently (Table 1).

Numerous gaseous sulfur compounds can be oxidized into sulfate in the atmosphere (Fig. 1), including hydrogen sulfide (H₂S), carbon disulfide (CS₂), carbonyl sulfide (COS), methyl mercaptan (CH₃SH), dimethyl sulfide (CH₃SCH₃), dimethyl disulfide (CH₃SSCH₃) (Likens et al., 2002) and sulfur dioxide (SO₂). For example, dimethyl sulfide (DMS), emitted by phytoplankton activity from the global ocean (Mulvaney et al., 1992), can be oxidized into SO₄^2- and MSA aerosols (Li and Barrie, 1993; Mahmood et al., 2019; Mungall et al., 2017) and consequently plays an important role in the global biogeochemistry of sulfur (Simó, 2001). Globally, the largest sources of sulfate are estimated to come from anthropogenic emissions (122Tg/a), but the sulfate budget is also influenced by biogenic (57 Tg/a) and volcanic (21 Tg/a) inputs (Andreae and Rosenfeld, 2008).

Sulfur aerosols impact global and regional climates, ecosystems, and human health (Smith et al., 2011; Wang et al., 2016). Sulfur-containing aerosols significantly affect the Earth’s radiation balance, both directly through solar radiation scattering and indirectly by modifying cloud properties (Charlson et al., 1987; Zhao and Garrett, 2015) (Fig. 1). Modeling studies estimated that the fraction of in-cloud sulfate production increased by 2% since 1985 due to anthropogenic emissions, leading to a mean global cooling of about 0.1 K (Tsai et al., 2010). Recent reductions in anthropogenic sulfate loading in the Arctic have contributed to a net warming of the Arctic surface of +0.27 ± 0.04 K during 1980–2010 (Breider et al., 2017). Organosulfur components also play a potentially important role in altering aerosol properties by nucleophilic substitution reactions (Darer et al., 2011) and radical-radical mechanisms (Perri et al., 2010). Deposition of sulfate, the main acid component of aerosols (Weber et al., 2016), could also amplify soil and aquatic acidification (McDonnell et al., 2014), causing harmful effects on ecosystems. Atmospheric inputs of acidic byproducts from the dissociation of sulfuric acid (H₂SO₄) take part in biogeochemical reactions (Doney et al., 2007). Moreover, when inhaled, these particles can be taken-up by cells in the lungs, the circulatory system and organs (Kennedy, 2007) and cause adverse human health effects (McDonald et al., 2010).

Anthropogenic emission sources are extremely low in Antarctica and such environment can be considered areas where we can still find preindustrial-like conditions (Schmale, 2017), conversely mankind is having a profound impact on the fragile Arctic environment (Barbante et al., 2017). The atmosphere in the Arctic and Antarctic regions has generally been considered the cleanest on the planet (Legrand and Mayewski, 1997). The Tibetan Plateau, also referred to as the third pole (Qiu, 2008) with a mean elevation of 4.5 km a.s.l., is another important cryosphere region in the low-to-middle latitude region. These three areas are located far away from populated/industrial areas and represent particularly vulnerable environments (Kang et al., 2007; Lange et al., 2018; McClintock et al., 2008). These regions are most sensitive to climate change, are warming faster than other regions in the world (Immerzeel et al., 2010; Levasseur, 2013; McClintock et al., 2008), and are crucial areas that are undergoing unprecedented changes in their climate and environment (Li et al., 2020). The recent IPCC Special Report reaffirms that Arctic warming is two to three times higher than the global average (IPCC, 2018). Global warming has the potential to rapidly affect terrestrial, freshwater, and marine systems with widespread impacts in the Arctic (Post et al., 2009), and leads to unprecedented ecological shifts in the Antarctic Peninsula (McClintock et al., 2008). The role of polar regions has a strong representation on effects of regional and transported emissions abundance (Petäjä et al., 2020).

This review discusses the achieved progress on sulfur aerosols research in recent years, using observational data from various geographical sites and ambient conditions in the Arctic, Antarctic, and the Tibetan Plateau. A systematic evaluation of sulfur aerosols in such unique areas improves our knowledge about their sources and transport mechanisms. By synthesizing the spatio-temporal variability of their abundances and chemical and isotopic compositions, this work also provides a comprehensive dataset for evaluating the role of sulfur aerosols on climate in the future. Furthermore, this study will help policy-makers to better design and implement effective regulatory and remediation strategies in the near future.