A new quantification procedure was developed for the evaluation of micro X-ray fluorescence (XRF) data-sets obtained from individual particles, based on iterative Monte Carlo (MC) simulation. Combined with the high sensitivity of synchrotron radiation induced XRF spectroscopy, the method was used to obtain quantitative information down to trace-level concentrations from micron-sized particulate matter. The detailed XRF simulation model was validated by comparison of calculated and experimental XRF spectra obtained for glass microsphere standards, resulting in uncertainties in the range of 3-10 % for the calculated elemental sensitivities. The simulation model was applied for the quantitative analysis of X-ray tube and synchrotron radiation induced scanning µ-XRF spectra of individual coal and wood fly ash particles originating from different Hungarian power plants. By measuring the same particles by both methods the major, minor and trace-element composition of the particles were determined. The uncertainty of the MC based quantitative analysis scheme is estimated to be in the range of 5-30 %.