Abstract: Understanding the emission factors of fine particulate matter (PM_2.5) released by forest fuel combustion is important for revealing the impacts of forest fire on atmosphere and ecosystem. Water-soluble ions are important components of fine particulate matter, with great significance to the formation of particulate matter. A self-designed biomass combustion system was used to simulate the combustion of three components (trunks, branches, barks) and their surface dead fuel (litter, semi-humus, humus) of five tree species (Quercus mongolica, Betula platyphylla, Larix gmelinii, Betula dahurica, Populus davidiana) and branches of three shrub species (Corylus heterophylla, Lespedeza bicolor, Rhododendron dauricum) in Great Xing'an Mountains in Inner Mongolia. The water-soluble ion emission factors (Na⁺, NH₄⁺, K⁺, Mg²⁺, Ca²⁺, F^-, Cl^-, NO₃^-, NO₂^-, SO₄^2-) in PM_2.5 under two combustion conditions (smoldering and flaming) were measured by ISC1100 ion chromatograph. The results showed that for the water-soluble ion detected in PM_2.5 from combustion of all types of materials, K⁺, Cl^- and Na⁺ were the main components in smoldering, while K⁺, Cl^- and SO₄^2- were the main components in flaming. There was significant difference in the total amount of water-soluble ions in PM_2.5 from the same type of material under different combustion conditions. During the smoldering period, the emission factor of water-soluble inorganic ions in PM_2.5 of shrub branches was higher than that of flaming. The cation to anion ratio in PM_2.5 was 1.26 for all trees, 1.12 for surface dead fuel of trees, and 2.0 for branch of shrub, indicating that the particulate matter was alkaline. Forest fires in Great Xing'an Mountains could not result in ecosystem acidification by releasing water-soluble ions.

Key words: water-soluble ion, forest, combustion state, PM_2.5, emission characteristics