Using a kinetic model, we estimate the time-needed (t0) to establish a pseudo-steady state nucleation when sulfuric acid concentration (2SO4>) suddenly increases to a certain fixed level. We find that t0 is roughly inversely proportional to 2SO4>. The assumption of instantaneously achieved pseudo-steady state concentration of critical clusters, on which the classical nucleation theories are based, is no longer valid when t0 is longer than the time period that 2SO4> can be treated as constant. Under the lower marine atmospheric conditions considered in this study, the nucleation rate predicted by ion-mediated nucleation theory is very sensitive to 2SO4> when 2SO4> is low (<~7 ¿ 106/cm3) but such a sensitivity reduces significantly as 2SO4> increases, which is quite different from that predicted with classical binary and ternary homogeneous nucleation theories.