Evolution of effective density of submicron aerosol particles and effects of firework displays and weather conditions on particle effective density in a typical valley city in northwestern China were evaluated by fitting the APS data to the SMPS data in the overlapping size region of the two instruments during September 2012 to August 2013. The monthly mean effective densities for submicron aerosol particles were found to vary from 1.3 g cm–3 to 1.6 g cm–3 depending on the month of year. In addition, the ranges of average submicron particle effective density were different on workdays, Saturdays, Sundays and holidays in different seasons with minimums in the morning and afternoon, especially observed in morning rush hours of weekdays, and a maximum during midday. The precipitation and strong wind resulted in particle effective densities increased by 0.1 g cm–3 and 0.08 g cm–3, respectively, while effective densities of submicron aerosol particles decreased by 0.21 g cm–3 due to the effect of dust. During the periods mainly affected by firework displays, the effective densities of submicron aerosol particles were 1.19 g cm–3, which was much lower than those for firework-unaffected days, and the submicron particle effective densities decreased by 0.29 g cm–3 due to firework displays.