Isomerization and Degradation of Levoglucosan via the Photo-Fenton Process: Insights from Aqueous-Phase Experiments and Atmospheric Particulate Matter

Abstract

So far, studies on the conversion of stereochemistry under photo-Fenton conditions and their atmospheric implication are still rare. Here, we found that the biomass burning marker, the chiral compound levoglucosan (L), undergoes oxidative degradation under photo-Fenton conditions and can be isomerized into mannosan (M) and galactosan (G) simultaneously. Among the formic acid, acetic acid, and oxalic acid in the degradation products of levoglucosan, it was found that the yield of formation of formic acid in the photo-Fenton pathway can be as high as 86%. It is worth noting that both levoglucosan and its isomers are present in the atmosphere and their concentrations are strongly correlated. At the same time, the range of their concentration ratios, L/(G + M), measured in the photo-Fenton experiments in the laboratory was found to agree well with that measured in atmospheric PM 2.5 samples. However, the sources of L, G, and M in the atmosphere are complex, and the photo-Fenton reaction may be an essential pathway for the distribution of L, G, and M in the atmosphere.

Impacts of Chemical Degradation on the Global Budget of Atmospheric Levoglucosan and Its Use As a Biomass Burning Tracer

“…Particulate levoglucosan can be scavenged by cloud and rain, or it can dissolve in aqueous aerosols, which then activate into cloud droplets. Once in the aqueous phase, levoglucosan can be oxidized by OH and NO 3 radicals. , To the best of our knowledge, the aqueous removal of levoglucosan has thus far only been measured in bulk, dilute solutions. , A previous box model calculation using these measured aqueous reaction rates showed the lifetime of levoglucosan against aqueous oxidation in wet aerosols to be significantly longer than that in cloudwater, even at 90% relative humidity . As such, in our model we only account for the aqueous oxidations of levoglucosan in clouds.…”

“…11,17 To the best of our knowledge, the aqueous removal of levoglucosan has thus far only been measured in bulk, dilute solutions. 11,17 A previous box model calculation using these measured aqueous reaction rates showed the lifetime of levoglucosan against aqueous oxidation in wet aerosols to be significantly longer than that in cloudwater, even at 90% relative humidity. 11 As such, in our model we only account for the aqueous oxidations of levoglucosan in clouds.…”

“…However, there are few reported values of ER levoglucosan,OC , and they vary greatly by fuel types and combustion conditions. − Also, gaseous precursors emitted from the biomass burning source may produce secondary OC during transport . Most importantly, laboratory experiments and field measurements have shown that atmospheric levoglucosan is chemically reactive, ,− resulting in its faster removal from the atmosphere relative to OC. , Previous studies have not considered the chemical degradation of levoglucosan when using eq for aerosol source apportionment and likely underestimated the contributions of biomass burning at receptor sites.…”

“…Laboratory experiments showed that levoglucosan can be oxidized in the gas and aqueous phases ,, or be heterogeneously oxidized on aerosol surfaces. − ,, Significant degradation of atmospheric levoglucosan has also been observed in the field. ,, Levoglucosan is semivolatile; after its pyrogenic production, it mostly condenses onto fine particles with only 6–20% in the gas phase. , The atmospheric lifetime of levoglucosan against gas-phase oxidation (defined as the burden of levoglucosan divided by the loss rate due to gas-phase oxidation) is estimated to be 26 days under typical atmospheric OH levels ([OH] g = 2.0 × 10 6 molecules cm –3 ) . In comparison, the aqueous-phase and heterogeneous removal of levoglucosan from the atmosphere are likely much faster.…”

“…Laboratory experiments showed that levoglucosan can be oxidized in the gas 16 and aqueous phases 1,11,17 or be heterogeneously oxidized on aerosol surfaces. [12][13][14]19,20 Significant degradation of atmospheric levoglucosan has also been observed in the field.…”