Insights into the complete and partial photooxidation of black carbon in surface waters

Abstract

Increasing wildfire activity in the Alaskan Arctic may result in new sources of black carbon (BC) to arctic watersheds. Black carbon, primarily comprised of condensed aromatics, is one of the most chemically recalcitrant fractions of organic carbon. However, lateral transfer of particulate and dissolved BC from soils to sunlit surface waters is increasingly suggested to result in the photochemical mineralization of BC to CO₂. While sunlight can also partially photooxidize aromatic compounds in surface waters, producing compounds with a higher O/C than the parent compound, this degradation pathway has not yet been identified for either particulate or dissolved BC. To address knowledge gaps on the photochemical degradation of particulate and dissolved BC, we quantified the complete and partial photooxidation of particulate and dissolved BC derived from arctic biomass as photochemical CO₂ production and O₂ consumption relative to dark controls. Concurrently, we investigated shifts in the chemical composition of dissolved BC following exposure to sunlight using UV-visible absorbance, fluorescence spectroscopy, and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). The chemical and physical properties of BC produced from charring arctic biomass were similar to BC produced by wildfires in terrestrial ecosystems based on elemental analysis and FT-ICR MS. Based on the concentration of light-absorbing carbon in each fraction, dissolved BC was disproportionately more susceptible to complete and partial photooxidation compared to particulate BC. Upon exposure to sunlight, the predominant fate of dissolved BC was partial photooxidation, while a smaller fraction of dissolved BC was photomineralized to CO₂. Shifts in both the optical and mass spectrometry spectra suggested that condensed aromatics likely comprised the fraction of dissolved BC that was completely and partially photooxidized. To further refine the meaning of sunlight as a sink for aquatic BC, the reactivity of partially oxidized photoproducts of BC in the aquatic organic carbon pool must be determined.

Wildfire Altering Terrestrial Precursors of Disinfection Byproducts in Forest Detritus

“…47 These condensed aromatics, which can account for more than half of the DOM structure in the extract, impart the DOM with lower H/C and O/C ratios and a much higher double bond equivalent. 47,48 The ash extracts possessed lower reactivity in forming THM and HAA suggesting that the condensed aromatics were likely less reactive THM and HAA precursors. Although high specific THM and HAA FP often positively link to high DOM aromaticity (or high SUVA 254 ), 49 SUVA 254 could not explain the DOM reactivity in THM or HAA formation across the three DOM sources in this study (r 2 = 0.06, P = 0.12−0.19; Figure S3, Supporting Information).…”

Production and Composition of Pyrogenic Dissolved Organic Matter From a Logical Series of Laboratory-Generated Chars

“…This method does not target aliphatic or other non-condensed compounds in pyDOM, as they likely do not form BPCA compounds upon nitric acid oxidation. Aliphatic and O-containing components are likely to be more soluble than the condensed aromatic fraction, thus may represent a large portion of pyDOM, as directly identified using FTICR-MS (Podgorski et al, 2012;Ward et al, 2014). It is also unclear if the BPCA method quantifies pyC, exclusively.…”

Black Carbon: Sources, Mobility and Fate in Freshwater Systems

“…However, since an increase in both CDOM concentration and DBC concentration was observed for LMW fractions after photodegradation (Table 5.1), it is suggested that smaller aromatic photo-products were indeed generated from the decomposition of CDOM and DBC in HMW fractions for this particular DOM sample. Although the present study is corroborated by previous observations where DBC is preferentially removed through photo-reactive processes vs. bulk DOC Ward et al, 2014), it was also discovered that DBC is associated with HMW fractions is more photo-labile than DBC in LMW fractions ( Fig. 5.2).…”