H2SO4 VAPOR-PRESSURE OF SULFURIC-ACID AND AMMONIUM-SULFATE SOLUTIONS

Few measurements of H2SO4 vapor pressure have been made for sulfuric a cid in the temperature and concentration ranges of atmospheric interes t because of the very low pressures involved (below 10(-4) Pa, or 10(- 6) torr); no such measurements appear to have been made for sulfuric a cid solutions neutralized with ammonia, This work presents measurement s of H2SO4 vapor pressure for aqueous sulfuric acid solutions between 55 and 77 wt % H2SO4 (corresponding to about 5-25% relative humidity), ammonium sulfate solids at low humidities, and partially neutralized sulfate solutions with [NH4+]:[SO4=] ratios between 0.13 and 1.0. The vapor pressure data collected over sulfuric acid solutions generally a gree with the predictions of Ayers, et al. [1980], although positive d eviation was observed for the more dilute solutions. The good agreemen t between this measurement and previous efforts by absolute techniques suggests that the evaporative coefficient for the H2SO4-H2O system is near unity. H2SO4 vapor pressures over solid ammonium sulfate were me asured between 27 degrees C and 60 degrees C; the data were fitted to ln p = A/T + B, with A = -5928 and B = -3.77. The H2SO4 vapor pressure s of mixed H2SO4-H2O-(NH4)(2)SO4 solutions dropped significantly as th e [NH4+]:[SO4=] ratio exceeded 0.5. The results suggest that ammonia c ould very effectively stabilize molecular clusters of sulfuric acid an d water in the atmosphere against evaporation, leading to rates of new particle formation higher than those predicted by binary H2SO4-H2O th eory.