Computer-controled. scanning electron microscopy has been used to characterize airborne particles (> 0.6 mum) chemically and morphologically. Two major types of particles were identified, based on the chemistry. 'Sulfur bearing' particles were assigned to shotcrete used in the tunnel, while 'silicate' particles represent a geological source. In addition, 'iron-rich' and 'calcium-rich' particles were found in minor amounts. Individual particles that had chemically been attributed to the two main sources were further characterized morphologically. A shape factor of the particles was used to distinguish the two sources. A linear model using the shape factor-size distributions of the sources was developed to determine the contribution of 'sulfur bearing' and 'silicate' particles to the aerosol. Results from the model are in excellent agreement with results based on chemical classification of single particles. We suggest that this concept of determining morphological profiles and subsequent modeling can be applied to more complex systems. (C) 2003 Elsevier Ltd. All rights reserved.