The Contribution of Fine Particle Sulfates to Light Scattering in ST. Louis

In this study the extent to which sulfates produce visibility reduction in the St. Louis area was examined. The contribution of fine aerosol sulfates to total light scattering was calculated from measured sulfur size distributions and Mie theory of light scattering. The aerosol was collected during both a summer and a winter season. Short sampling intervals (10 minutes to 3 hours) permitted observation of the fine structure of the sulfur size distribution and concentration, and involved only small changes in meteorology. Size separation was accomplished with an 8 stage low pressure impactor which yielded size fractions in the range of 4 (mu)m (GREATERTHEQ) d (GREATERTHEQ) 0.05 (mu)m. Elemental sulfur was measured by flash volatilization and flame photometric detection. All fine sulfur was assumed to be in the form of (NH(,4))(,2)SO(,4). Water which might be associated with this deliquescent aerosol was estimated. The measured light scattering coefficient, b(,scat), and (NH(,4))(,2)SO(,4) were extremely well correlated (r = 0.97) in the summer, but not as well correlated (r = 0.63) in the winter. Conversely, b(,scat) and non-sulfate fine mass were well correlated (r = 0.85) in the winter, but poorly correlated (r = 0.24) in the summer. The light scattering efficiency of (NH(,4))(,2)SO(,4) plus associated water calculated from Mie theory and size distributions was 4.2 m('2)/g for the summer aerosol, but only 2.0 m('2)/g for the dry winter (NH(,4))(,2)SO(,4). Two basic types of sulfur size distributions were observed. The most common type observed during the summer had a major sulfur mass peak between 0.5 (mu)m and 1.0 (mu)m in diameter. This size particle scatters light more efficiently than other particles. Its prevalence in the summer aerosol accounts for the high scattering efficiency of sulfates in the summer aerosol when compared to winter sulfate aerosol, which did not have such prominent mass peaks between 0.5 (mu)m and 1.0 (mu)m in diameter. Calculation of individual sample scattering efficiencies indicated that during the summer, the (NH(,4))(,2)SO(,4) fraction of the total particle light scattering increased with decreasing visibility. No such pattern was observed for the winter samples. The combination of large sulfate particles and high sulfate loadings led to the dominance of sulfates in visibility reduction during the summer.