Aerosol measurements were carried out to determine particle formation and characteristics produced in a 4-stroke, turbo-charged 1 MW diesel engine operating with high ash-content heavy fuel oil with and without a Mg-based additive. The mass size distributions are bimodal (modes at 0.1 and 10 mum, aerodynamic size) without additive and have three modes (additional mode at 2 mum) with the additive. It was found that the 2 mum mode was generated by magnesium together with some vanadium, nickel and sulphur. The primary particles are formed by nucleation of the volatilised fuel oil ash species that further grow by condensation and agglomeration. The 10 mum mode particles are mainly re-entrained from deposits and fuel residue particles of different sizes. Primary particle size is about 40-100 nm as observed in the SEM and TEM micrographs. It appears that the fine particles (0.1 mum mode) are more spheroidal and catenulate with the additive than without. (C) 2002 Elsevier Science Ltd. All rights reserved.