Molecular distributions of dicarboxylic acids, ketocarboxylic acids and α-dicarbonyls in biomass burning aerosols: implications for photochemical production and degradation in smoke layers

摘要

Aerosols in the size class <2.5 μm (6 daytime and9 nighttime samples) were collected at a pasture site in Rond?nia,Brazil, during the intensive biomass burning period of 16–26 September 2002as part of the Large-Scale Biosphere-Atmosphere Experiment in Amazonia –Smoke, Aerosols, Clouds, Rainfall and Climate (LBA-SMOCC). Homologousseries of dicarboxylic acids (C₂–C₁₁) and related compounds(ketocarboxylic acids and α-dicarbonyls) were identified using gaschromatography (GC) and GC/mass spectrometry (GC/MS). Among the speciesdetected, oxalic acid was found to be the most abundant, followed bysuccinic, malonic and glyoxylic acids. Average concentrations of totaldicarboxylic acids, ketocarboxylic acids and α-dicarbonyls in theaerosol samples were 2180, 167 and 56 ng m−3, respectively. These are2–8, 3–11 and 2–16 times higher, respectively, than those reported in urbanaerosols, such as in 14 Chinese megacities. Higher ratios of dicarboxylicacids and related compounds to biomass burning tracers (levoglucosan andK+) were found in the daytime than in the nighttime, suggesting theimportance of photochemical production. On the other hand, higher ratios ofoxalic acid to other dicarboxylic acids and related compounds normalized tobiomass burning tracers (levoglucosan and K+) in the daytime provideevidence for the possible degradation of dicarboxylic acids (≥C₃)in this smoke-polluted environment. Assuming that these and relatedcompounds are photo-chemically oxidized to oxalic acid in the daytime, andgiven their linear relationship, they could account for, on average, 77%of the formation of oxalic acid. The remaining portion of oxalic acid mayhave been directly emitted from biomass burning as suggested by a goodcorrelation with the biomass burning tracers (K+, CO and EC_a) andorganic carbon (OC). However, photochemical production from other precursorscould not be excluded.