and evolution of organic aerosol involves simultaneous changes to both carbon oxidation state 39 and carbon number (n C ).

Abstract. Despite the acknowledged relevance of aerosolderived water-soluble organic carbon (WSOC) to climate and biogeochemical cycling, characterization of aerosol WSOC has been limited. Electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI FT-ICR MS) was utilized in this study to provide detailed molecular level characterization of the high molecular weight (HMW; m/z>223) component of aerosol-derived WSOC collected from rural sites in Virginia and New York, USA. More than 3000 peaks were detected by ESI FT-ICR MS within a m/z range of 223-600 for each sample. Approximately 86% (Virginia) and 78% (New York) of these peaks were assigned molecular formulas using only carbon (C), hydrogen (H), oxygen (O), nitrogen (N), and sulfur (S) as elemental constituents. H/C and O/C molar ratios were plotted on van Krevelen diagrams and indicated a strong contribution of lignin-like and lipid-like compounds to the aerosol-derived WSOC samples. Approximately 1-4% of the peaks in the aerosol-derived WSOC mass spectra were classified as black carbon (BC) on the basis of double bond equivalents calculated from the assigned molecular formulas. In addition, several high-magnitude peaks in the mass spectra of samples from both sites corresponded to molecular formulas proposed in previous secondary organic aerosol (SOA) laboratory investigations indicating that SOAs are important constituents of the WSOC. Overall, ESI FT-ICR MS provides a level of resolution adequate for detailed compositional and source information of the HMW constituents of aerosolderived WSOC.

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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.

3Evidence suggests that reactive oxygen species (ROS) are important in 4 transforming the chemical composition of the large pool of terrestrially-derived dissolved 5 organic matter (DOM) exported from land to water annually. However, due to the 6 challenges inherent in isolating the effects of individual ROS on DOM composition, the 7 role of ROS in the photochemical alteration of DOM remains poorly characterized. In 8 this work, terrestrial DOM was independently exposed to singlet oxygen ( 1 O 2 ), and 9 superoxide (O 2 − under controlled laboratory conditions. Using ultra-high resolution mass 10 spectrometry to track molecular level alterations of DOM by ROS, these findings suggest 11 exposure to 1 O 2 (generated using Rose Bengal and visible light) removed formulas with 12an O/C > 0.3, and primarily resulted in DOM comprised of formulas with higher oxygen 13 content, while O 2 − exposure (from KO 2 in DMSO) removed formulas with O/C < 0.3 14 and produced aliphatic formulas (H/C > 1.5). Comparison of DOM altered by ROS in 15 this study to riverine and coastal DOM showed that (20-80%) overlap in formulas, 16providing evidence for the role of ROS in shaping the composition of DOM exported 17 from rivers to oceans. 18