Skip to content

Size and Salt Matter: Key Ionic Controls on S-Scavenging by Water Droplets

Luke Brown , Mark Jellinek

Abstract

Hunga Tonga-Hunga Ha'apai 2022 (HTHH) is arguably the largest explosive eruption in the satellite era and it is marked by a near absence of climate effects from sulfate aerosols. Space-borne and in-situ cloud measurements and chemical analyses of fresh proximal tephra deposits support sulfur scavenging by volcanic particles as a major process influencing the longevity of HTHH sulfate aerosols. A key underlying control may be the extent of seawater entrainment during magma-water interactions to deposit sea salts on ash surfaces and change the ash hygroscopicity. We hypothesize that the growth of many heavy cloud droplets on sea-salt-coated ash particles led to a rapid removal of volcanic SO2 during HTHH by droplet scavenging and aqueous reaction with dissolved seawater Ca2+.     From preliminary calculations we identify a positive feedback between aqueous sulfate production and cloud droplet growth. CaSO4 production in droplets depletes dissolved SO2, encouraging further uptake of SO2(g).~ ~Sulfur uptake into seawater droplets can be diffusion- or kinetic-limited depending on the ratio between the particle Reynolds number and the Damköhler number. In contrast, the absorption of sulfur into entrained droplets produced during eruptions through freshwater is limited by the solubility.      Initial results suggest that these differences can explain the wide variability of stratospheric sulfur injection efficiency observed for eruptions through freshwater and seawater. We provide a lookup table of scavenging efficiency for atmospheric climate models, and suggest considering source water salinity when predicting volcanic effects on climate.