To provide additional field data for assessing urban flow and dispersion models, the Mock Urban Setting Test (MUST) field experimental program was conducted at U.S. Army Dugway Proving Ground in September 2001 to give simultaneous measurements of the instantaneous wind velocity fluctuations using sonic anemometers and of the instantaneous plume concentration fluctuations using fast-response photoionization detectors within a regular array of building-like obstacles. In this program, a set of field experiments was conducted to investigate the dispersion of a neutrally-buoyant plume of propylene tracer released within an extensive array of prismatic obstacles, each of which measured 12.2 m by 2.6 m by 2.6 m. The focus of the present study is to determine the shapes of the crosswind and vertical profiles of plume concentration fluctuation statistics required to define the extent of the hazard zone using four lateral and one vertical sampling arrays of fast-response photoionization detectors. Horizontal and vertical profiles of a number of concentration statistics were extracted; namely, mean concentration, concentration variance, fluctuation intensity, peak-to-mean concentration ratio, and concentration time and length scales of dominant motions in the plume. The downwind evolution of the concentration probability density function for a plume dispersing within the obstacle array is examined. The behaviour of the concentration power spectra obtained at different heights on the vertical sampling tower both below and above the canopy top is investigated in relationship to the turbulence structures within and above the canopy flow that influence the molecular mixing of the plume material.