PARTICLES IN TROPOSPHERIC VOLCANIC PLUMES: SIZE, COMPOSITION AND POTENTIAL ENVIRONMENTAL IMPACT

Persistently degassing volcanoes contribute substantially to the aerosol burden of the troposphere. Volatiles known to be released from degassing volcanoes include acidic gases (such as sulfur dioxide, hydrogen fluoride and hydrogen chloride), and toxic trace metals (including mercury, cadmium and lead). The continuous release of such species from active volcanoes may lead to significant short- and medium-range environmental impacts, but these remain relatively poorly studied.

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 cations and anions were determined in 12 size bins. The near-source size distributions showed major modes at 0.5 micron (sulfate, hydrogen and ammonium ions); 0.2 and 5.0 micron (chloride) and 2.0-5.0 micron (fluoride). Potassium and sodium ion distribution mirrored the sulfate size-resolved concentrations closely, suggesting that these were transported primarily as potassium sulfate and sodium sulfate in acidic solution, while magnesium and calcium ions peaked in size populations both less than, and greater than 1 micron. The estimated flux of volcanic sulfate leaving the crater was 0.07 kg per second. Effects of relative humidity were studied by comparing daytime (transparent plume) and nighttime (condensed plume) results. Enhanced particle growth rates were observed in the nocturnal plume as well as preferential scavenging of soluble gases, such as hydrogen chloride, by condensed water. Neutralization of the acidic aerosol by background ammonia was observed at the crater rim and to a greater extent 15 km downwind.

We discuss the implications of these results for the understanding of the chemistry of tropospheric volcanic plumes, and their potential environmental impact.