H2SO4/(NH4)2SO4 background aerosol: optical detection in St. Louis region

Atmospheric H₂SO₄ present as droplets of the free acid and the half neutralized acid, NH₄HSO₄ mixed with other substances have proved difficult to identify by traditional air sampling methods and ionic analyses. Knowledge of their existence as a major or minor fraction of atmospheric aerosol is important to studies of health effects of air pollution, atmospheric optics, acid rain and to understanding the global sulfur cycle. The in situ method reported here utilizes a light scattering/humidity relation to detect reaction of the aerosol with NH₃ to form the deliquescent salt (NH₄)₂SO₄. Field measurements in and near Saint Louis, Missouri, confirm that H₂SO₄ and/or NH₄HSO₄ are frequently present as a major fraction (ca. 1/2 or more) of the submicrometer particles, and that (NH₄)₂SO₄ also is frequently found. At one site 35 km W.S.W. of the Arch, over 98 per cent of the measurements were dominated by H₂SO₄ or its neutralization products and showed no dependence on wind direction or synoptic condition, thus indicating a regional as opposed to urban behavior. While the results to date are qualitative and semi-quantitative in nature, it appears possible to utilize gas phase titration with NH₃ to quantify the method.