Multi-Hazard Mitigation Plan: 08. Lava And Vog

Home

Chapter 8:Hazard Analysis—Lava and VOG the recently formed pit) there can also be contributions (of water vapor, hydrogen sulfide, and sulfur trioxide) from lower temperature hydrothermal systems that surround the magma conduit below and around Halema'uma'u. Soon after the gases discharge from the magma or the conduit surfaces, they begin to cool and undergo a variety of internal reactions and reactions with the atmosphere. While in the gas phase all the above compounds are transparent but, with cooling, sulfur trioxide will rapidly combine with water vapor(from the plume or the atmosphere) to form a sulfuric acid (H2SO4) aerosol. Under relatively dry conditions, the sulfuric acid aerosol can be distinguished by its bluish cast; when abundant water is present, a dense white plume is formed. HCl and HF will also condense onto solid particulates, if the latter are present, or form aerosol droplets by combining with condensing water vapor. Under dry atmospheric conditions, the plume will drift downwind and mix with the air column which will decrease the dew point in the discharge plume and the aerosol droplets will dehydrate and disperse. Under moist conditions, the plume will remain hydrated and the aerosols will remain much more visible. Mixing with air will also lead to reactions between atmospheric oxygen and the sulfur compounds sulfur dioxide and hydrogen sulfide. Sulfur dioxide is the more reactive of the two, with reported half-lives as short as a few tens of minutes to as long as several days depending on the chemical constituents in the air mass (e.g. urban air pollution, marine air), as well as the intensity of ultraviolet radiation. Given the absence of significant sulfur dioxide in the vog plume by the time it reaches Kona, it's likely that the half-life in the Hawaii air mass is no more than a few hours. The oxidation of any hydrogen sulfide present in the plume occurs at a much slower rate where the half-life of HZS in the atmosphere may be a day or more. The oxidation of sulfur dioxide will result in the formation of additional sulfur trioxide/sulfuric acid aerosol (because sulfur trioxide is extremely hygrosocopic, it will extract nearly any and all available water vapor from the atmosphere to form the acid aerosol) to contribute to the dry haze in the downwind plume. The oxidation process will continue until all of the sulfur dioxide is converted to sulfuric acid. Mixing of the plume with the atmosphere also brings the plume constituents into contact with ammonia that is derived from biogenic decay processes occurring in tropical soils. In the presence of water, ammonia (NH3) forms a weak base (NH40H) that will react very rapidly with the sulfuric acid aerosols to form ammonium sulfate [(NH4) 2SO41, and with the hydrochloric and hydrofluoric acids to form ammonium chloride (NH4C1) and ammonium fluoride (NH4F) respectively. Although these ammonia salts are not as hygroscopic as sulfur trioxide, they are sensitive to atmospheric moisture levels and the optical density of the plume will vary depending on the relative humidity in the ambient air. Whereas the non-reactive gas phase components of the plume (e.g. carbon dioxide) will gradually disperse, the aerosols are subject to both gravitational settling, through a process called dry deposition, as well as scrubbing from the atmosphere by rainfall. Although the aerosols can serve as a source of condensation nuclei for raindrop formation, some studies 8-5 Hawaii County Multi-Hazard Mitigation Plan