# Possible Missing Sources of Atmospheric Glyoxal Part I: Phospholipid Oxidation from Marine Algae
## 3 Results and Discussion
### SSML Analysis with Marine Aerosol Reference Tank (MART)  
During a smaller mesocom experiment conducted in a marine aerosol reference tank (MART) [52] prior to the start of IMPACTS, foam was collected from atop the SSML at the end of a phytoplankton bloom and analyzed by LC-HR-MS/MS. Pero oxidized phospholipids and their corresponding short-chain oxidation products were identified. Specifically, a series of diacyl phosphatidylcholine were characterized, which are uniquely abundant in eukaryotes, thus allowing for distinction between algal- and bacterial-origins. The presence of these phospholipids suggests a potential mechanism by which marine glyoxal is formed from the oxidation and cleavage of the PUFA. For example, Figure 3a outlines a potential mechanism by which glyoxal can be produced from the oxidation and cleavage of the PUFA chain of 1-palmitoyl-2-archidonoyl-3-glycerophosphocholine (PAPC). In this mechanism, a radical initiator (**R\(\bullet\)**) is used to represent the activity of a non-ozone oxidizing agent. Upon hydrogen abstraction, triplet oxygen (\({}^{3}\)O\({}_{2}\)) can add to the chain along with a hydrogen atom to form a peroxyl adduct. Heterolytic (Hock) cleavage of the O-O bond occurs resulting in the formation of hexanal. Glyoxal can then be produced as a secondary oxidation product of malondialdehydedehyde upon repeat of the same chemistry. The reaction terminates when allylic hydrogens are no longer available for abstraction. Figure 2 contains the assigned spectra for PAPC (\(m/z\) 782) (b), its hydroxylated form 1-palmitoyl-2-(\(\omega\)6-hydroxy-archidonoyl)-3-glycerophosphocholine (HO-PAPC, \(m/z\) 798) (c), and the short-chain oxidation product 1-palmitoyl-2-(5-hydroxy-8-oxo-6-octenoic acid)-3-glycerophosphocholine (HOOA-PC, \(m/z\) 650) (d), which correspond to structures (1), (2b), and (6), respectively, of the proposed mechanism. Although structure (2a) was not found, the presence of (2b), a 1-palmitoyl-2-(\(\omega\)7- hydroperoxy-linoleoyl)-3-glycerophosphocholine (HP-PLPC; C16:0)/C18:2 \(\Delta\) 9,12; \(m/z\) 790, Figure 4a) analogue of PAPC, and a 5-HP-PAPC isomer (Figure 4b) confirms that radical initiated oxidation of phospholipids in the SSML can and does occur. Unlike the wave channel, the air that passed over the water surface in the MART was not scrubbed; thus, ozone (and other oxidants) was also present and reacted as evidenced by the C16:0/C18:2 \(\Delta\) 9,12 ozonide-containing phosphotidylcholines (Oz-PLPC, \(m/z\) 806) that were characterized (Figure 4c). In this case, the ozonide products likely contributed to the radical initiated mechanism, and this was in keeping with the finding that showed 85-90% of the ozone adduct decomposes into H\({}_{2}\)O\({}_{2}\)[59]. The absence of glyoxal during the decline of the chlorophyll \(a\) peaks during IMPACTS suggests that the oxidants needed to promote this chemistry come primarily in the atmosphere not the SSML. These data demonstrate for the first time that more than one oxidation pathway to the formation of marine glyoxal is possible, and that they can occur not only simultaneously, but synergistically within the same system. It should be noted that the presence of 5-HP-PAPC suggests that some oxidation events may occur endogenously. When intracellular peroxidation of PAPC occurs, it does so almost exclusively on C5 for unclear reasons [60]. Its degradation would yield a long-chain PUF aldehyde that would be highly susceptible to further oxidation and decomposition either in the SSML or within the SSA, given its liberation from the charged phosphocholine headgroup [61]. It should also be noted there is some precedence in mammalian cells for hydratase-facilitated oxidation in vivo of 9Z- and 12Z-unsaturated C16 and C18 fatty acids to yield 10-hydroxy and 10,13-dihydroxy derivatives, respectively. However, the enzyme active site is highly selective for non-ester-bound substrates [62].