Measurements of SO2 emission rates and concentrations and of particle distribution, size, shape, and composition were made in quiescent volcanic plumes emitted into the troposhere from Po¿s and Arenal volcanos, Cost Rica, and Colima volcano, Mexico, SO2 emission rates were 700¿180 metric tons per day (t/d) for Po¿s, 210¿30 t/d for Arenal, and 320¿50 t/d for Colima. The concentrations of SO2 calculated from the COSPEC/lidar data were 5-380 ppb. Concentrations of SO2 measured directly by flame photomery were 10-250 ppb. Particles collected in the plumes with a quartz crystal microbalance impactor were mostly less than 3μm in diameter and consisted of droplets of dilute sulfur-bearing solutions and minor amounts of larger silicate particles coated with a sulfur-bearing film or crust. Total particle concentrations were 4.7 μg/m3 for Po¿s and 18.8 μg/m3 for Colima. Comparison of concentrations of SO2 in the plumes with gas samples collected at fumaroles on the ground suggests that the plumes are diluted by the atmosphere by factors of up to 105.