Aggregates (composed of large numbers of primary particles) are produced in many engineering environments. One convenient characterization is the fractal dimension, the exponent describing how the number of primary particles in each aggregate scales with radial distance from its center of mass. We describe a finite-analytic pseudo-continuum prediction of the normalized accessible surface area of an isothermal quasi-spherical fractal aggregate containing N (>> 1) primary particles, on the surfaces of which a first-order chemical process occurs. Results are displayed for specific fractal dimensions (2.5, 2.18, and 1 8) frequently observed in aggregating systems. An effective Thiele modulus is used to develop an efficient and accurate scheme for predicting/correlating the effectiveness factor for an aggregate containing N primary particles in terms of aggregate fractal dimension, reaction probability, and Knudsen number. Our methods now allow calculations of the accessible surface area of populations of aggregates, provided pdf-(N, D(f), ...)- is known for the populations of interest.