Lidar observations of Kasatochi volcano aerosols in the troposphere and stratosphere

The eruption of Kasatochi volcano on 7-8 August 2008 injected material into the troposphere and lower stratosphere of the northern midlatitudes during a period of low stratospheric aerosol background concentrations. Aerosols from the volcanic plume were detected with a lidar in Halifax, Nova Scotia (44.64 degrees N, 63.59 degrees W) 1 week after the eruption and for the next 4 months thereafter. The volcanic origin of the plume is established using the FLEXPART Lagrangian particle transport model for both the stratosphere and troposphere. The stratospheric plume descended 47.1 +/- 2.8 m/d on average as it dispersed, corresponding to a cooling rate of 0.60 +/- 0.07 K/d. The descent rate was the same for the tropopause (within statistical uncertainties). The top of the plume remained steady at about 18 km altitude and was likely sustained by vertical eddy diffusion from large-scale horizontal mixing. The lower boundary of the plume descended with the tropopause. The integrated aerosol backscatter between 15 and 19 km altitude was relatively constant at about 8 x 10(-5) sr(-1) for 532 nm wavelength. Observations and modeling of Kasatochi aerosols in the middle and lower troposphere indicate a possible ground impact. The volcanic contribution to surface PM2.5 did not exceed 5 mu g/m(3) at the measurement site.