The killing of microorganisms by atmospheric pressure low-temperature plasma is mainly achieved by the effects of ultraviolet light（UV）

high-energy particle etching

and strong reactive active substances. When using plasma to kill microorganisms in aqueous solutions

the effects of UV and ion etching are feeble due to the liquid barrier

and the killing effect is mainly dependent on the reactive substances in the aqueous solution. In our study

medium-blocking discharge plasma was used to treat E. coli in aqueous solution

and the killing effect of plasma on bacteria at different depths were compared by colony dilution counting. To investigate the different effects of long and short-lived active particles on the killing effect

pH indicator was used to visualize the diffusion process of long-lived acidic substances. The experimental results show that even if the plasma action was extended from 1 min to 10 min

the plasma could only kill E. coli in the surface layer of the aqueous solution instantly

and the killing effect is independent of the diffusion depth of the long-lived active material. Furthermore

the killing of E. coli in the deep layer of the solution requires a period of standing solution after plasma treatment

which can also inhibit or even kill E. coli in solution. Short-lived active substances have a rapid and robust killing effect on bacteria

but their lifetimes and diffusion distances in solution are short

and the effective distance of the killing effect is limited. Long-lived active particles can act on larger water bodies

but they require a longer action time and cannot achieve immediate and rapid killing. This study provides an experimental basis and design reference for the use of low-temperature plasma for disinfection in different settings.