Aerosol optical depths and lidar measurements were obtained under the plume of Hawaii Kilauea Volcano on August 17, 2001, ~9 km downwind from the erupting Pu'u O'o vent. Measured aerosol optical depths (at 500 nm) were between 0.2--0.4. Aerosol size distributions inverted from the spectral sun photometer measurements suggest the volcanic aerosol is present in the accumulation mode (0.1--0.5 micron diameter), which is consistent with past in situ optical counter measurements. The aerosol dry mass flux rate was calculated to be 53 Mg d-1. The estimated SO2 emission rate during the aerosol measurements was ~1450 Mg d-1. Assuming the sulfur emissions at Pu'u O'o vent are mainly SO2 (not aerosol), this corresponds to a SO2 half-life of 6.0 hours in the atmosphere.