Particle charge evolution equation is considered in the presence of diffusion and drifts combined with bipolar and radioactive charging in the vicinity of absorbing surfaces (walls). Boundary conditions to be prescribed for solving the mean charge evolution equation have been derived. It is shown that the mean charge J(x) at a distance x from the surface (x = 0) satisfies the boundary condition partial derivativeJ/partial derivativex \(x=0) = U(0)/2D(0) [alphaJ(0) + sigma(0)(2)], where U(x) and D(x) are the drift velocity and diffusion coefficients respectively, 1 (0) for radioactive (non-radioactive) aerosols, and sigma(0)(2) is the variance of the Boltzmann charge distribution. Using this. specific cases of charge build-up of radioactive particles undergoing turbulent diffusion near walls is examined. Radioactive particle charges are found to increase both for zero and negative electric fields, although the increases are far smaller than those expected from stationary-state formulae. The results are further discussed. (C) 2002 Elsevier Science Ltd. All rights reserved.