The present work investigates the effects of thermophoresis and transverse magnetic field on aerosol particle transport and deposition onto a horizontal plate in the presence of a natural convective flow. Micro to nano sized particles (particle diameter in the range 1 nm(-1) mu m) are considered. A similarity solution for the fluid flow field is formulated for natural convection with and without magnetohydrodynamic (MHD) effects above a heated horizontal plate as well as beneath a cold horizontal plate. The mechanisms of particle deposition include the effects of free convection, Brownian diffusion and thermophoresis. Numerical results for the velocity and temperature fields of the fluid, and, the concentration profile and the deposition velocity of the particles are obtained and presented graphically as a function of pertinent parameters. The importance of Cunningham correction on the concentration profile and deposition velocity of ultrafine (sub-micron to nanometer) particles is demonstrated. It is found that the thicknesses of both the hydrodynamic boundary layer and thermal boundary layer increase with an increase in magnetic field parameter. The deposition velocity decreases with an increase in particle diameter d(p) (i.e an increase in particle Schmidt number Sc), usually decreases with an increase in magnetic field parameter zeta, and increases with an increase in the value of the coefficient of diffusion due to temperature gradient D-T. It is shown that for the cold plate facing downward, the thermal drift of particles assists Brownian diffusion. For the heated plate facing upward, the thermal drift away from the surface decreases the overall deposition velocity which decreases drastically above a certain particle size. (C) 2013 Elsevier Ltd. All rights reserved.