Inorganic chemical composition of PM2.5 emissions from the combustion of six main tree species in subtropical China

Branches and leaves of six prevalent tree species in sub-tropical China (Masson pine, Oleander, Chinese fir, Camphor, Eucalyptus and Phoebe bournei) were burned in a self-designed biomass combustion system to determine the emission characteristics and chemical composition of the particulate matter emitted. The carbonaceous content and water-soluble ions of PM2.5 were analyzed by an Elementar elemental analyzer and ion chromatography in liquid. The results demonstrate that different combustion conditions have significant effects on emission factors, carbonaceous components and water-soluble ion emissions of PM2.5 released from the combustion of branches and leaves. The average emission factor of PM2.5 (EFPM2.5) of leaves in smoldering and flaming stages were 28.61 ± 4.90 g/kg and 10.91 ± 3.32 g/kg, respectively, while the average EFPM2.5 of branches in these stages were 21.09 ± 2.86 g/kg and 9.74 ± 2.76 g/kg. Compared to flaming, more PM2.5 was emitted during the smoldering stage from both branches and leaves for six different tree species, while leaves released greater PM2.5 than branches, and conifer trees contributed more to PM2.5 emissions than broad-leaved trees. In addition, findings demonstrate that the smoldering stage accelerates the release of OC (organic carbon), while the release of EC (elemental carbon) is promoted by the flaming stage. Findings also demonstrate that the ratio between OC, EC and PM2.5 varies significantly under different combustion conditions. Both tree species and combustion conditions were identified as important influencing factors for the content and emission factors of water-soluble ions in PM2.5. Ten kinds of water-soluble ions were detected in PM2.5, with K+ occupying the highest percentage of emissions content and highest emission factor, followed by Cl−, F− and SO42−. With these findings, this study contributes to further understanding of the impact of forest fires on the atmospheric environment and ecosystem in subtropical China, as well as provides reference for other regions of the world.