A re-assessment of aerosol size distributions from Masaya volcano (Nicaragua)

Cascade impactors were used to sample volcanic aerosol from Masaya (Nicaragua) in 2007, 2009 and 2010. Differences were found in the size distributions of volcanic aerosol between these recent campaigns and with a campaign in 2001: (1) SO42− showed modes in both the fine (<1 μm; with low Na+/K+) and coarse (>1 μm; with high Na+/K+) fractions in all of the recent campaigns despite being unimodal in 2001 (<1 μm); (2) The modal diameters for SO42− roughly doubled in 2009, compared to 2007 or 2010; (3) total Cl− was depleted in volcanic aerosol compared to background aerosol in all the more recent campaigns but was enriched in 2001. Other aspects of the volcanic aerosol appear to be persistent, such as a fine SO42−–H+–Na+–K+ mode, which was the most abundant mode in all campaigns, and a coarse Cl−–F−–Mg2+–Ca2+ mode of lower abundance. Water uptake and speciation in the aerosol were investigated using the equilibrium model, ISORROPIA II. Results show that the coarse SO42−-rich mode deliquesces at lower relative humidity (40% RH) than the fine SO42−-rich mode (50% RH) due to increased Na+/K+ in the former. The aerosol was predicted to be dry at ambient relative humidity in 2009 and dominated by NaHSO4, KHSO4, CaSO4 and MgSO4. In contrast, model results predict a liquid aerosol at ambient relative humidity in 2010. These results indicate that aerosol emissions from a volcano can vary in ionic composition and even more so in physical speciation (i.e., salts or solutions). These observations are set against a near-constant magmatic gas composition at Masaya, which highlights the significance of atmospheric and dynamic factors in the formation of volcanic aerosols.