During full-scale sorbent injection tests for mercury removal at coal-fired power plants (CFPPs), a common trend has been observed in which the overall mercury removal efficiency reaches a performance plateau as the sorbent injection rate increases. Two common explanations for this trend are (1) the increased sorbent injection rate induces increased rates of particle agglomeration within sorbent feeding lines, which eventually shifts the particle size distribution to larger sizes and reduces the available particle surface area for heterogeneous oxidation and adsorption, and (2) the increased sorbent injection rate limits the available acid gas concentration for heterogeneous oxidation of Hg0 to Hg2+. In this experimental study, an in situ measurement technique based on the principles of light extinction is used to assess the degree of particle agglomeration. The present in situ measurement technique applies a simple inline HeNe laser, and a sorbent feeder fluidizes powdered activated carbon (PAC) and drives the suspension through a length of rubber tubing to simulate the sorbent feeding process. Each experimental trial yields a time-dependent function of the light extinction ratio, I(t)/I0, for the time-dependent evolution of the PAC suspensions. Observed differences in I(t)/I0 for suspensions ejected directly from the feeder as compared to those directed through the tubing indicate changes in the overall particle size distribution. A numerical model of the experiment provides excellent agreement and facilitates interpretation of the experimental results.