Influence of Biomass Burning on Dicarboxylic Acid Composition and Secondary Formation in PM _2.5 over Manaus, Brazil

Urban PM2.5 samples were collected in Manaus, a major city in the Amazon rainforest, to analyze dicarboxylic acids (DCA) (C2–C9), black carbon (BC), levoglucosan (LEV), and sulfate (SO42–) during daytime and nighttime, in periods with and without smoke over the city from biomass burning (BB). Oxalic acid (C2) was the dominant DCA, followed by malonic acid (C3) and succinic acid (C4). The 12.5% nighttime increase in oxalic acid (C2) and higher C2/TDCA ratio at night indicate ongoing secondary formation, not just accumulation from a lower boundary layer. BC showed a moderate correlation with C4, and, since no correlation was observed between BC and LEV, this suggests that fossil fuel combustion–a recognized source of BC–is one of the possible primary sources of C4 in the studied atmosphere. The phthalic acid (Ph) to azelaic acid (C9) ratio (Ph/C9), an indicator of the dominant oxidative pathway for DCA formation, was lowest at night, and C9 exhibited a strong correlation with levoglucosan (LEV) (r = 0.89), suggesting that BB serves as a major source of primary C9 and may also provide precursors that contribute to the secondary formation of PM2.5. During smoke periods, LEV increased by 479%, while C2 and C3 increased by 247% and 259%, and the ratios of C2/C4 and C2/SO42– increased by 40% and 259%, respectively. Biomass burning has modified the composition of DCAs, possibly influencing aerosol hygroscopicity and their potential to act as cloud condensation nuclei, with possible implications for regional climate. This study highlights the critical role of biomass burning in the formation of organic aerosols and their influence on oxidative pathways, as well as their broader environmental impacts.