A temperature-controlled ash deposition probe was applied to collect temporally resolved deposit samples in a 100 kW rated down-fired pilot-scale combustor under two atmospheres: (1) air condition and (2) 70% O-2 as inlet gas (named as OXY70). The mineral compositions, particle size distributions, and partitioning of arsenic in the inside and outside deposits were measured. The properties of the ash aerosols were also given to analyze the effect of the atmosphere on arsenic partitioning. Results showed that Ca, Fe, S, and Al were enriched in the inside deposits during coal combustion, while the major contributor to arsenic enrichment in the inside deposits was the interactions between Ca and arsenic. Meanwhile, the smaller particles in the inside deposits also played a positive role in arsenic enrichment as a result of their larger specific surface area and more active cation sites for arsenic adsorption. As the atmosphere changed from air to OXY70 conditions, a remarkable increase of the Ca content in the sub-micrometer particles was observed, which enhanced the enrichment of arsenic in the inside deposits. Meanwhile, larger proportions of aerosol nuclei and arsenic vapor were generated under a higher flame temperature; thus, the arsenic concentrations in the inside and outside deposits were both higher for the OXY70 case. Furthermore, the "nucleation" mode of ash aerosols disappeared as a result of the enhanced association behavior of Fe and aluminosilicate to form super-micrometer particles as well as more Na2SO4 nuclei being scavenged by silicate or composite silicate with larger sizes.