Long-term exposure of outdoor power transmission and distribution equipment to varying levels of water vapor and solid particles in the atmospheric environment often leads to diminished insulation strength, consequently posing safety hazards. This paper delves into a systematic exploration of the impact of water vapor and solid particles in impure air on the DC breakdown characteristics of air gaps. Through the design and implementation of a tailored discharge chamber and air filtration system, the study scrutinizes the influence of these factors on the DC breakdown characteristics of a needle-plate gap. The findings reveal that, under conditions of humidity saturation, the frequency breakdown voltage of the needle-plate gap escalates with rising water vapor concentration. With the passage of solid particles, the breakdown voltage of the needle-plate gap exhibits an upward trend correlated with increasing solid particle concentration. Conversely, larger particle sizes tend to induce a reduction in breakdown voltage. Additionally, while the relative dielectric constant of solid particulate matter generally maintains the breakdown voltage at a stable level, there is a slight inclination towards decrease.