Volcanic ash ice nucleation activity is variably reduced by aging in water and sulfuric acid: the effects of leaching, dissolution, and precipitation.

William D Fahy,Leif G Jahn,Ryan C Sullivan,Benjamin J Murray,Elena C Maters,Rona Giese Miranda,Michael P Adams

doi:10.1039/d1ea00071c

Abstract

Volcanic ash nucleates ice when immersed in supercooled water droplets, giving it the potential to influence weather and climate from local to global scales. This ice nucleation activity (INA) is likely derived from a subset of the crystalline mineral phases in the ash. The INA of other mineral-based dusts can change when exposed to various gaseous and aqueous chemical species, many of which also interact with volcanic ash in the eruption plume and atmosphere. However, the effects of aqueous chemical aging on the INA of volcanic ash have not been explored. We show that the INA of two mineralogically distinct ash samples from Fuego and Astroni volcanoes is variably reduced following immersion in water or aqueous sulfuric acid for minutes to days. Aging in water decreases the INA of both ash samples by up to two orders of magnitude, possibly due to a reduction in surface crystallinity and cation availability accompanying leaching. Aging in sulfuric acid leads to minimal loss of INA for Fuego ash, which is proposed to reflect a quasi-equilibrium between leaching that removes ice-active sites and dissolution that reveals or creates new sites on the pyroxene phases present. Conversely, exposure to sulfuric acid reduces the INA of Astroni ash by one to two orders of magnitude, potentially through selective dissolution of ice-active sites associated with surface microtextures on some K-feldspar phases. Analysis of dissolved element concentrations in the aged ash leachates shows supersaturation of certain mineral species which could have precipitated and altered the INA of the ash. These results highlight the key role that leaching, dissolution, and precipitation likely play in the aqueous aging of volcanic ash with respect to its INA. Finally, we discuss the implications for understanding the nature and reactivity of ice-active sites on volcanic ash and its role in influencing cloud properties in the atmosphere.

Highlights

Volcanic ash from explosive eruptions can serve as ice-nucleating particles (INPs) in the atmosphere and may be important far from mineral dust sources of INPs such as arid-40 and semi-arid regions.[1,2,3,4] While global annual mean emissions of volcanic ash (176–256 Tg a-1 5) are an order of magnitude lower than those of mineral dust (1000-2000 Tg a-1),[6] ash loadings in 42 the atmosphere regularly meet or exceed mineral dust loadings on regional scales, and can even surpass the global annual mean dust emissions during a single large event (e.g., >5000 Tg44 released during the 1991 Pinatubo eruption, Philippines).[7]
We propose that formation of silica-like surface layers during aqueous leaching[62,65,67] of 320 volcanic ash is one of the main drivers of chemical aging deactivating ice nucleation activity (INA) in these ash samples
We have shown that the effects of aqueous chemical aging in H2O or H2SO4 on the INA

Summary

Introduction

Volcanic ash from explosive eruptions can serve as ice-nucleating particles (INPs) in the atmosphere and may be important far from mineral dust sources of INPs such as arid-40 and semi-arid regions.[1,2,3,4] While global annual mean emissions of volcanic ash (176–256 Tg a-1 5) are an order of magnitude lower than those of mineral dust (1000-2000 Tg a-1),[6] ash loadings in 42 the atmosphere regularly meet or exceed mineral dust loadings on regional scales, and can even surpass the global annual mean dust emissions during a single large event (e.g., >5000 Tg44 released during the 1991 Pinatubo eruption, Philippines).[7]. Volcanic ash from explosive eruptions can serve as ice-nucleating particles (INPs) in the atmosphere and may be important far from mineral dust sources of INPs such as arid-. 46 (>63 μm) rapidly sediment to the ground, smaller super- and submicron particles have been reported to remain airborne for up to 6 months following an eruption and spread 1000s of 48 kilometers from the source volcano.[8,9]. Several primary minerals have 56 been implicated as the ice-active phases in volcanic ash in previous studies, including alkali (Na/K-rich) feldspar, plagioclase (Na/Ca-rich) feldspar, and pyroxene.[14,15,16,17] K-feldspar 58 (hereafter used to refer to K-rich alkali feldspar), Na/Ca-feldspar, and quartz are recognized as ice-active phases in mineral dust,[19,20,21,22,23,24,25,26,27] while secondary minerals in dust such as kaolinite, illite and 60 montmorillonite may nucleate ice.[20,24,28,29,30,31,32] Ice nucleation on mineral surfaces is thought to occur at specific nanoscale sites[33,34,35,36] due to some combination of chemical and physical features, 62 for instance relating to crystal structure, microscale texture and resulting geometry, and chemical functionalities such as hydroxyl groups.[37,38,39,40,41,42]

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