Formation of ultrafine particles during coal combustion is believed to be governed by the solid-vapor-particle process of evaporative inorganic species. In this work, a down-fired flat-flame burner with a two-stage dilution sampling system were used to characterize the particulate matter formed in the early stage of high-sodium coal combustion. It was found that 33.9% of Na and 33.1% of Mg in the coal were enriched in the ultrafine particles through the solid-vapor-particle process at 1500 K, 20% O-2. The control mechanisms of homogeneous condensation and surface reaction were identified with elemental particle size distributions. Below 1500 K, either increasing the oxygen concentration or the ambience temperature lead to a higher ultrafine particle yield, which was caused by the intensification of gas-phase release. At 1700 K, however, the gas-to-particle conversion process became the control step, resulting in reduced particle yield with lower molar fraction of Na, Mg and S. This change in particle elemental composition is correlated to the thermodynamic behavior of the Na- Mg-Ca-Si-S-Cl system, which is of great importance during the gas-to-particle conversion process at high temperature. (C) 2016 Elsevier Ltd. All rights reserved.