Bulk transport and Brownian diffusion alone are not sufficient to explain the fact that tidal fresh air reaches the alveolar membrane. Convective mixing is also involved. However, up to now it is unknown which convective mixing mechanism(s) may contribute to gas transport in the lungs. To study gas dispersion, the bolus technique is a usual means. Gas boli however undergo also Brownian diffusion thus masking the mechanical dispersive motion to be studied. Therefore aerosol boli have proved as a useful tracer for such studies. However, it is uncertain up to which extent the intrinsic motion of the aerosol particles contributes to the dispersion of the bolus. In this paper a method is introduced which aims at estimating this contribution.