HCI in Rocket Exhaust Clouds: Atmospheric Dispersion, Acid Aerosol Characteristics, and Acid Rain Deposition

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Abstract
NASA is examining Space Shuttle launch impacts. Solid rocket exhaust includes ˜60 tons HCL and ˜87 tons alumina particles emitted below 2.5 km, of which 50-80% forms an altitude stabilized exhaust cloud (EC). Several 60% smaller Titan-Ill EC were sampled by aircraft for this study. Three distinct features are presented: (a) An analysis of HCL (gaseous plus aqueous) data traces. Total range of peak HCL was 25-0.5 ppm (3-300 min) for 8 EC. Power-law decays of peak HCL applied. Calculated HCL dispersions for 7 standard meteorologies are also shown, (b) An analysis of simultaneous HCL (g), HCL (g + aq) data for 2 EC. Vapor-liquid HCL/H2O equilibria were calculated for a flat surface aqueous aerosol. HCL partitioning varied with EC dilution and H2O content. HCL (aq) and aqueous mass fraction maximized early at >3 molal and >0.1 mg/g air. Calculated H2O (g + aq) compared favorably with independent EC measurements, (c) An analysis of wet deposition after EC interception at ˜30 min by a convective storm. A 28 km2 acid chloride (1 < pH < 3) footprint was defined. In conclusion, (a) HCL dispersion in large EC tends to follow power-law decay, but HCL concentration may vary widely (100 times after 1 h) with meteorology, (b) HCL (g/aq) and H2O (g/aq) partitioning is consistent with equilibrated acid aerosol compositions, and (c) localized deposition of highly acidic rain may occur sometimes.

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