A particle concentration measurement system includes a first window and a second window in a housing. The first window and the second window respectively define a first end and a second end of an interaction region between the first window and the second window. The system also includes a particle-laden gas inlet through a wall of the housing between the first window and the second window. The particle-laden gas inlet introduces particle-laden gas from an environment that includes particles mixed with other materials into the interaction region. A first set of clean gas inlets through the wall of the housing are at different radial positions of the housing and at a first axial location of the housing, and a second set of clean gas inlets through the wall of the housing are at different radial positions of the housing and at a second axial location of the housing.