Effect of photodegradation on molecular size distribution and quality of dissolved black carbon

The effects of photodegradation on the molecular size distribution and composition of dissolved black carbon (DBC) were explored using a surface water dissolved organic matter (DOM) sample from a terrigenous-influenced, fire-impacted Everglades area canal. The original and photodegraded DOM samples were fractionated using size exclusion chromatography and DBC was quantified via benzene polycarboxylic acid (BPCA) analysis. Size fractionation revealed that DBC was unequally distributed along the DOM molecular weight (MW) continuum and was preferentially associated with high MW (HMW) fractions. The photo-decomposition of HMW DBC generated less condensed DBC photo-products that preferentially re-associated with, and became enriched in, low MW (LMW) DOM size fractions. DBC composition in whole and size-fractionated DOM, as determined from relative BPCA distributions, was not considerably altered with short term photodegradation. This indicated that the size of the conjugated aromatic ring structure may drive the association of DBC compounds with different DOM MW fractions. HMW DBC was also more photo-labile than LMW DBC, which suggests that DBC associated with DOM over a range of size fractions may not exhibit the same degree of photo-reactivity, thereby resulting in different environmental fates for pyrogenic OM.