Particle number concentrations and size distributions were measured near a heated wafer in a low pressure, SiH4:O-2:He chemical vapor deposition reactor. The wafer was heated to 450 degrees C and the pressure was varied from 650-3250 Pa. The measurements were compared with 2D numerical simulations of particle formation, growth, and transport. A key feature of the model is that pressure-dependent cluster sticking coefficients are needed to explain the pressure-dependence of the aerosol size distribution. The pressure dependence of the calculated and measured aerosol size distributions are in good agreement. At a height of 2 cm above the wafer and at a reactor pressure, P, of 650 Pa, the simulated particle concentration is 10(4) [No./cm(3)], increasing to about 10(8) [No./cm(3)] at 1950 Pa, indicating that particle nucleation rates depend on approximately P-9. Particle formation rates were shown to become negligible below about P = 650 Pa. At P = 2600 Pa the size distribution was unimodal at a height of 2 cm above the wafer and became trimodal at a height of 7 cm above the wafer. This trimodal size distribution occurs because particle nucleation occurs both near the center of the wafer as well as near the edge of the wafer, where the temperature gradients are steep.