No abstract

Polar ice sheets hold a significant pool of the world's carbon reserve and are an integral component of the global carbon cycle. Yet, organic carbon composition and cycling in these systems is least understood. Here, we use ultrahigh resolution mass spectrometry to elucidate, at an unprecedented level, molecular details of dissolved organic matter (DOM) in Antarctic snow. Tens of thousands of distinct molecular species are identified, providing clues to the nature and sources of organic carbon in Antarctica. We show that many of the identified supraglacial organic matter formulas are consistent with material from microbial sources, and terrestrial inputs of vascular plant-derived materials are likely more important sources of organic carbon to Antarctica than previously thought. Black carbon-like material apparently originating from biomass burning in South America is also present, while a smaller fraction originated from soil humics and appears to be photochemically or microbially modified. In addition to remote continental sources, we document signals of oceanic emissions of primary aerosols and secondary organic aerosol precursors. The new insights on the diversity of organic species in Antarctic snowpack reinforce the importance of studying organic carbon associated with the Earth's polar regions in the face of changing climate.

This study describes a method for evaluating the reproducibility of replicate mass spectra acquired for complex natural organic matter (NOM) samples analyzed by electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry, with regard to both peak detection and peak magnitude. Because studies of NOM characterization utilize not only the emergence and disappearance of peaks but also changes in relative peak magnitude, it is important to establish that the differences between samples are significantly larger than those between sample replicates. Here, a method is developed for correcting strict signal-to-noise thresholds, along with a new scheme for assessing the reproducibility of peak magnitudes. Furthermore, a systematic approach for discerning when the comparison of samples by the presence or absence of peaks is appropriate and when it is necessary to compare based on the relative magnitude of the peaks is presented. A variety of 10 different types of NOM samples are analyzed in duplicate or triplicate instrumental injections or experimental extractions. A framework for these procedures is provided, and acceptable reproducibility levels are recommended.

Snow overlays the majority of Antarctica and is an important repository of dissolved organic matter (DOM). DOM transformations by supraglacial microbes are not well understood. We use ultrahigh resolution mass spectrometry to elucidate molecular changes in snowpack DOM by in situ microbial processes (up to 55 days) in a coastal Antarctic site. Both autochthonous and allochthonous DOM is highly bioavailable and is transformed by resident microbial communities through parallel processes of degradation and synthesis. DOM thought to be of a more refractory nature, such as dissolved black carbon and carboxylic-rich alicyclic molecules, was also rapidly and extensively reworked. Microbially reworked DOM exhibits an increase in the number and magnitude of N-, S-, and P-containing formulas, is less oxygenated, and more aromatic when compared to the initial DOM. Shifts in the heteroatom composition suggest that microbial processes may be important in the cycling of not only C, but other elements such as N, S, and P. Microbial reworking also produces photoreactive compounds, with potential implications for DOM photochemistry. Refined measurements of supraglacial DOM and their cycling by microbes is critical for improving our understanding of supraglacial DOM cycling and the biogeochemical and ecological impacts of DOM export to downstream environments.

Organic aerosols (OA) are universally regarded as an important component of the atmosphere that have far-ranging impacts on climate forcing and human health. Many of these impacts are related to OA molecular characteristics. Despite the acknowledged importance, current uncertainties related to the source apportionment of molecular properties and environmental impacts make it difficult to confidently predict the net impacts of OA. Here we evaluate the specific molecular compounds as well as bulk structural properties of total suspended particulates in ambient OA collected from key emission sources (marine, biomass burning, and urban) using ultrahigh resolution mass spectrometry (UHR-MS) and proton nuclear magnetic resonance spectroscopy ( 1 H NMR). UHR-MS and 1 H NMR show that OA within each source is structurally diverse, and the molecular characteristics are described in detail. Principal component analysis (PCA) revealed that (1) aromatic nitrogen species are distinguishing components for these biomass burning aerosols; (2) these urban aerosols are distinguished by having formulas with high O/C ratios and lesser aromatic and condensed aromatic formulas; and (3) these marine aerosols are distinguished by lipid-like compounds of likely marine biological origin. This study provides a unique qualitative approach for enhancing the chemical characterization of OA necessary for molecular source apportionment.