Concentrations and size-distributions of water-soluble inorganic and organic species on aerosols over the Arctic Ocean observed during the US GEOTRACES Western Arctic Cruise GN01

Highlights

• Sea-salt aerosol species dominated but less so in the high Arctic.

• Non-sea-salt sulfate, MSA and ammonium predominately in fine-mode particles.

• Aerosol nitrate dominated by coarse-mode particles over Bering Strait.

• Organic species contributed up to 15% (by mass) of anions in high Arctic samples.

Abstract

Marine aerosols affect the climate directly and indirectly as well as serving as tracers of biogenic processes. Here we present the results from size-segregated and bulk aerosol samples collected from 2nd August to 10^th October 2015 during the US GEOTRACES Arctic Ocean (GN01) expedition. Samples were analyzed for concentrations of major water-soluble organic and inorganic species, including acetate, formate, methanesulfonate (MSA), oxalate, propionate, chloride, nitrate, sulfate and major cations (Na⁺, K⁺, Mg²⁺, Ca²⁺, NH₄⁺). Back-trajectory analysis was performed to categorize the wind patterns into three types; type 1, originating from the North Pacific and the Bering Sea, type–2, consisting entirely of marine Arctic air, and type 3, consisting of marine Arctic air mixed with air masses from the surrounding continents. Sea-salt was the major aerosol component, dominating in the coarse mode, 1.8–5.6 μm. Non-sea-salt sulfate and MSA were predominantly present in the fine mode, 0.18–0.32 μm, and MSA was associated with type 1 and type 2 air masses. The results from this study provide useful information on the origins and chemical processes involving these aerosol species over the Arctic in summer and help elucidate the significance of natural sources as contributors for the formation of cloud condensation nuclei.

Introduction

The Arctic region has witnessed intense warming, at a rate almost twice the global average rate over the last 20–30 years; an effect known as Arctic amplification (Serreze and Barry, 2011; Flato et al., 2013; van Wijngaarden, 2015). Presumably related to this warming, the average annual sea-ice extent decreased by 3.6% per decade between 1976 and 2007 (Meier et al., 2007). Results from satellite observations reveal a ~77% decrease in the Arctic planetary albedo over the past 40 years (Pistone et al., 2014). Although model studies claim that cloudiness has a small effect on Arctic amplification (Screen and Simmonds, 2010; Ghatak and Miller, 2013), it is a major source of uncertainty in understanding recent trends and predicting the future pattern of climate change in the Arctic (Vavrus, 2004; Künzli et al., 2005; Inoue et al., 2006; Walsh et al., 2009; Kay et al., 2016).

Certain aerosol particles can serve as cloud condensation nuclei (CCN), influencing the radiative properties of clouds. Aerosol particle size and the solubility of aerosol species control the activation of CCN at different levels of supersaturation, but the effects of aerosols on cloud dynamics and climate change in the Arctic have not yet been thoroughly examined in climate models due to limited aerosol data from observations over the Arctic (Walsh et al., 2005; Chang et al., 2011; Carslaw et al., 2013).

Over the open ocean, sulfate and ammonium aerosols in particular play important roles as CCN (Galloway et al., 1982; Quinn et al., 1987; Vong et al., 1988) and are crucial for altering the absorption properties of aerosols (Lim et al., 2018). Non-sea-salt-sulfate (NSS-sulfate) particles from natural and anthropogenic sources primarily exist in the fine mode and are generally produced by gas-to-particle conversion (Murphy et al., 1998). Dimethyl sulfide (DMS), produced by phytoplankton through the enzymatic cleavage of dimethyl sulfonio-propionate (DMSP), is a major oceanic source of sulfur to the atmosphere (Prospero and Savoie, 1991; Bates et al., 1992). Alongside this biogenic production of NSS-sulfate, methanesulfonate (MSA) is also produced by the oxidation of DMS derived from phytoplankton, and the ratio of MSA/NSS-sulfate is used as a metric for determining the biogenic contribution to NSS-sulfate (Savoie and Prospero, 1989; Li et al., 1993b; Gondwe et al., 2003; Chen et al., 2012). Several marine sources, such as zooplankton metabolism and the decay of organic material, contribute to gas-phase ammonia (Quinn et al., 1988; Galloway et al., 1995; Bouwman et al., 1997; Adams et al., 1999; Wentworth et al., 2016), which in turn leads to the formation of fine-mode aerosol ammonium (Du et al., 2010).

Water-soluble organic species (WSOS) on aerosols are also potential CCN (Novakov and Penner, 1993; Yu, 2000; Sun and Ariya, 2006). Previous work shows that WSOS can account for a significant fraction (~20–50%) of the total organic mass in aerosols (Cadle and Groblicki, 1982; Yu and Schauer, 2002; Liya et al., 2005), mostly on fine-mode particles (Zappoli et al., 1999). Previous results from both polar regions show that water-soluble low molecular weight carboxylic acids (such as acetic, formic, and oxalic acids) constitute a significant fraction (~20–70%) of the total organic carbon in aerosol (Kawamura et al., 1995; Saxena and Hildemann, 1996; Decesari et al., 2000; Xu et al., 2013). Several primary sources, including vegetation and biomass burning, directly emit organics into the air (Kawamura et al., 1985), whereas oxidation of biogenic olefins, hydrocarbons, and isoprene can act as potential secondary sources (Chebbi and Carlier, 1996).

Although many previous studies on Arctic aerosols focused on inorganic species (Barrie and Hoff, 1985; Li et al., 1993b; Suzuki et al., 1995; Norman et al., 1999; Quinn et al., 2009; Frossard et al., 2011; Zhan et al., 2014, 2017), there are comparatively few reports on WSOS (Kawamura et al., 1995, 2010; Narukawa et al., 2003; Fu et al., 2009). Moreover, unlike during winter and early spring seasons, the summertime Arctic air is relatively isolated from pollutants from the surrounding continental landmasses and hence summer provides a favorable period for studying the chemistry of aerosols originating from marine sources (Leck and Persson, 1996; Quinn et al., 2007). Therefore, to document their distribution it is important to characterize the water-soluble inorganic and organic components simultaneously in summertime Arctic aerosols.

As part of the US GEOTRACES Arctic Ocean (GN01) expedition, shipboard aerosol sampling was carried out over the Arctic Ocean during August–October 2015. We highlight the spatial distribution of the water-soluble inorganic and organic aerosol species, which include sea-salt, NSS-sulfate, ammonium, nitrate, MSA, and low molecular weight carboxylic acids. For mass-size distributions, only inorganic components and MSA are presented here. This information adds new insight on the chemical and physical properties of aerosols in the marine atmospheric boundary layer over the Arctic Ocean.