Size-resolved characterisation of soluble ions in the particles in the tropospheric plume of Masaya volcano, Nicaragua: Origins and plume processing

We present the first application of a multi-stage impactor to study volcanic particle emissions to the troposphere from Masaya volcano, Nicaragua. Concentrations of soluble SO42-, Cl-, F-, NO3-, K+, Na+, NH4+, Ca2+ and Mg2+ were determined in 11 size bins from similar to0.07 mum to > 25.5 mum. The near-source size distributions showed major modes at 0.5 mum (SO42-, H+, NH4+); 0.2 mum and 5.0 mum (Cl-) and 2.0-5.0 mum (F-). K+ and Na+ mirrored the SO42- size-resolved concentrations closely, suggesting that these were transported primarily as K2SO4 and Na2SO4 in acidic solution, while Mg2+ and Ca2+ presented modes in both <1 μm and > 1 mum particles. Changes in relative humidity were studied by comparing daytime (transparent plume) and night-time (condensed plume) results. Enhanced particle growth rates were observed in the night-time plume as well as preferential scavenging of soluble gases, such as HCl, by condensed water. Neutralisation of the acidic aerosol by background ammonia was observed at the crater rim and to a greater extent approximately 15 km downwind of the active crater. We report measurements of re-suspended near-source volcanic dust, which may form a component of the plume downwind. Elevated levels of SO42-, Cl-, F-, H+, Na+, K+ and Mg2+ were observed around the 10 mum particle diameter in this dust. The volcanic SO42- flux leaving the crater was similar to0.07 kg s(-1).