The Taiwan air quality model incorporating a dust module was applied to calculate masses of supermicron (diameter greater than 1 ¿m) and submicron particles (diameter less than 1 ¿m) and their dust fractions during the ACE-Asia airborne experiments over the northwestern Pacific. The results showed that the calculated vertical profiles of supermicron particle concentrations matched reasonably well with the observations obtained from 19 research aircraft missions. During dust storm events and at dust concentrated altitudes, the calculated dust fractions in the supermicron particles were usually greater than 90%, and the dust was concentrated in the lower troposphere mainly below 6 km. Without dust storm, dust was still the major component of the supermicron particles above boundary layer. In contrast to supermicron particles, the model results showed that the major component of the submicron particles observed during aircraft experiments was mostly from pollution. The calculated vertical profiles of submicron particle concentrations were sensitive to the emission inventory of air pollutants over east Asia. The correlation between observed anthropogenic volatile organic compound and submicron particles were used to identify the pollution fraction in the submicron particles, and the results were consistent with the model calculations of dust fraction. The model results showed that the dust fractions in the submicron particles were usually less than 28% in the boundary layer. During dust storm events the dust fractions were usually greater than 40% but can be as low as 24% when significant amount of pollutants were present.