Secondary sulfate is internally mixed with sea spray aerosol and organic aerosol in the winter Arctic

Abstract

Abstract. Few measurements of aerosol chemical composition have been made during the winter–spring transition (following polar sunrise) to constrain Arctic aerosol–cloud–climate feedbacks. Herein, we report the first measurements of individual particle chemical composition near Utqiaġvik (Barrow), Alaska, in winter (seven sample days in January and February 2014). Individual particles were analyzed by computer-controlled scanning electron microscopy with energy dispersive X-ray spectroscopy (CCSEM-EDX, 24 847 particles), Raman microspectroscopy (300 particles), and scanning transmission X-ray microscopy with near-edge X-ray absorption fine structure spectroscopy (STXM-NEXAFS, 290 particles). Sea spray aerosol (SSA) was observed in all samples, with fresh and aged SSA comprising 99 %, by number, of 2.5–7.5 µm diameter particles, 65–95 % from 0.5–2.5 µm, and 50–60 % from 0.1–0.5 µm, indicating SSA is the dominant contributor to accumulation and coarse-mode aerosol during the winter. The aged SSA particles were characterized by reduced chlorine content with 94 %, by number, internally mixed with secondary sulfate (39 %, by number, internally mixed with both nitrate and sulfate), indicative of multiphase aging reactions during transport. There was a large number fraction (40 % of 1.0–4.0 µm diameter particles) of aged SSA during periods when particles were transported from near Prudhoe Bay, consistent with pollutant emissions from the oil fields participating in atmospheric processing of aerosol particles. Organic carbon and sulfate particles were observed in all samples and comprised 40–50 %, by number, of 0.1–0.4 µm diameter particles, indicative of Arctic haze influence. Soot was internally mixed with organic and sulfate components. All sulfate was mixed with organic carbon or SSA particles. Therefore, aerosol sources in the Alaskan Arctic and resulting aerosol chemical mixing states need to be considered when predicting aerosol climate effects, particularly cloud formation, in the winter Arctic.

Aerosol Mixing State: Measurements, Modeling, and Impacts

“…can be probed, STXM-NEXAFS has been used to analyze a range of sources, including sea spray aerosol (Ault et al, 2013b;Kirpes et al, 2018), biomass burning (Fraund et al, 2017) SOA (Takahama et al, 2007), and soot . For higher-Z elements (e.g., metals) X-ray absorption near edge spectroscopy (XANES) is also available at some synchrotrons (De Santiago et al, 2014;Oakes et al, 2012).…”

Processes Controlling the Composition and Abundance of Arctic Aerosol

“…As discussed in section , sea salt concentrations peak in the winter–spring at ground stations, and areas of open water (i.e., leads, polynas) in ice‐covered regions are likely contributors to PMA during this time of year (e.g., Deshpande & Kamra, ; Kirpes et al, ; Leck et al, ; May et al, ; Nilsson et al, ; Scott & Levin, ; Weinbruch et al, ). Kirpes et al () show that freshly emitted PMA is a significant contributor to wintertime aerosol at Utqiaġvik (Figure ). Multi‐year observations at Utqiaġvik indicate that sea salt is produced when open leads are present and wind speeds are greater than 4 m/s (May et al, ).…”

“…Size‐resolved number fractions of particle types observed with CCSEM‐EDX, for sample periods influenced by (a) the Arctic Ocean (26 February 2014, 4,490 particles) and (b) Prudhoe Bay (27 January 2014, 1,475 particles), from Figure 5 in Kirpes et al (). 48‐hr backward trajectories (NOAA HYSPLIT) are shown for (c) 26 February and (d) 27 January.…”

“…In addition, the first wintertime observations of aerosol mixing state, at Utqiaġvik, illustrated that PMA was a large contributor to submicron and supermicron particle numbers (Kirpes et al, ). Aged PMA contributed significantly and was both internally mixed with secondary sulfate and depleted in chloride (Kirpes et al, ). These observations demonstrate the importance of heterogeneous and multiphase chemistry on Arctic PMA in both dark and light seasons.…”

“…Smaller cloud droplets were observed in clouds affected by local pollution sources compared to clouds present in cleaner conditions (Maahn et al, ). Significant impact of Prudhoe Bay Oil Field emissions has been observed on aerosol composition at Utqiaġvik (Gunsch et al, ; Kirpes et al, ). Submicron combustion‐derived particles and aged PMA were more prevalent in air masses from Prudhoe Bay compared to clean marine air masses (Figure ).…”