Abstract
Abstract. Free tropospheric aerosol was sampled at the Pico Mountain Observatory located at 2225 m above mean sea level on Pico Island of the Azores archipelago in the North Atlantic. The observatory is located ~ 3900 km east and downwind of North America, which enables studies of free tropospheric air transported over long distances. Aerosol samples collected on filters from June to October 2012 were analyzed to characterize organic carbon, elemental carbon, and inorganic ions. The average ambient concentration of aerosol was 0.9 ± 0.7 μg m−3. On average, organic aerosol components represent the largest mass fraction of the total measured aerosol (60 ± 51%), followed by sulfate (23 ± 28%), nitrate (13 ± 10%), chloride (2 ± 3%), and elemental carbon (2 ± 2%). Water-soluble organic matter (WSOM) extracted from two aerosol samples (9/24 and 9/25) collected consecutively during a pollution event were analyzed using ultrahigh-resolution electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. Approximately 4000 molecular formulas were assigned to each of the mass spectra in the range of m/z 100–1000. The majority of the assigned molecular formulas had unsaturated structures with CHO and CHNO elemental compositions. FLEXPART retroplume analyses showed the sampled air masses were very aged (average plume age > 12 days). These aged aerosol WSOM compounds had an average O/C ratio of ~ 0.45, which is relatively low compared to O/C ratios of other aged aerosol. The increase in aerosol loading during the measurement period of 9/24 was linked to biomass burning emissions from North America by FLEXPART retroplume analysis and Moderate Resolution Imaging Spectroradiometer (MODIS) fire counts. This was confirmed with biomass burning markers detected in the WSOM and with the morphology and mixing state of particles as determined by scanning electron microscopy. The presence of markers characteristic of aqueous-phase reactions of phenolic species suggests that the aerosol collected at the Pico Mountain Observatory had undergone cloud processing before reaching the site. Finally, the air masses of 9/25 were more aged and influenced by marine emissions, as indicated by the presence of organosulfates and other species characteristic of marine aerosol. The change in the air masses for the two samples was corroborated by the changes in ethane, propane, and ozone, morphology of particles, as well as by the FLEXPART retroplume simulations. This paper presents the first detailed molecular characterization of free tropospheric aged aerosol intercepted at a lower free troposphere remote location and provides evidence of low oxygenation after long-range transport. We hypothesize this is a result of the selective removal of highly aged and polar species during long-range transport, because the aerosol underwent a combination of atmospheric processes during transport facilitating aqueous-phase removal (e.g., clouds processing) and fragmentation (e.g., photolysis) of components.
Highlights
The low scientific understanding of the properties and transformations of atmospheric aerosol is a key source of uncertainty in determining the anthropogenic climate forcing through the aerosol direct, semi-direct, and indirect effects (IPCC, 2013)
Our findings confirm the dominance of Organic aerosol (OA) in the total aerosol mass fraction of remote atmospheres, followed by sulfate and nitrate aerosol
The subtle differences in the composition of the two samples were attributed to the sampling of somewhat different air masses
Summary
The low scientific understanding of the properties and transformations of atmospheric aerosol is a key source of uncertainty in determining the anthropogenic climate forcing through the aerosol direct, semi-direct, and indirect effects (IPCC, 2013). Aged and processed longrange transported ambient aerosol are of particular interest and have chemical composition characteristic of global aerosol (Ramanathan et al, 2001). Ambient watersoluble organic matter (WSOM) is estimated to account for up to 80 % of the OA mass (Saxena and Hildemann, 1996; Sun et al, 2011), but these species are still not well characterized on a molecular level (Reemtsma, 2009). Molecular characterization of WSOM is important for understanding its role in fundamental processes such as aerosol light absorption and scattering (Dinar et al, 2006; Shapiro et al, 2009; Nguyen et al, 2013), chemical reduction / oxidation mediation (Kundu et al, 2012), and water uptake and reactivity (Ervens and Volkamer, 2010; Ervens et al, 2011). Analytical methods capable of characterizing OA at the molecular level are necessary to tackle these problems
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