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Measurements of SO2 emission rates and concentrations and of particle distribution, size, shape, and composition were made in quiescent volcanic plumes emitted into the troposphere from Poás and Arenal volcanos, Costa Rica, and Colima volcano, Mexico. SO2 emission rates were 700±180 metric tons per day (t/d) for Poás, 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 photometry were 10–250 ppb. Particles collected in the plumes with a quartz crystal microbalance impactor were mostly less than 3 μm 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 Poás 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.

Studies of the chemical composition, size distributions and concentrations of stratospheric aerosol particles have shown that the particlea conskt of ammonium sulfate and ammonium persulfate, and may have liquid aeaociated with them in the stratosphere. The average particle size distribution function waa found to approximate a log-normal distribution over the range of radius 0.1 to 1.0 microns. The number concentrations found were variable but generally of the order of 0.05 cm-a at 18 km altitude. The corresponding masa concentrations are of the order of lo-* pg cm-a. Combination of the presently determined size distribution with estimates of the size distribution of Aitken nuclei in the stratosphere indicates that the overall distribution of partculate matter in the lower stratosphere may be bimodal. This bimodality indicates that other processes in addition to coagulation must occur if the particles with radius < 0.1 micron give rise to particles with radius > 0.1 micron.

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