During July 1977 and again in February 1978 in St. Louis, Missouri, episodes of about 3-day duration occurred in which sulfuric acid persisted as part of the regional haze aerosol. These sulfuric acid episodes were monitored with 15-min time resolution by a thermal analysis--flame photometric detection particulate sulfur monitor. The measurement technique detects the concentration of total particulate sulfur as well as the fraction as H2SO4. However, it cannot distinguish between (NH4)2SO4 and NH4HSO4; thus it is not specific for the detection of total hydrogen ion concentration associated with particulate sulfur. In the summer, July--August 1977, total particulate sulfur average was 3.8 μgS/m3, and the H2SO4 (measured as sulfur) average was below 0.5 μgS/m3, while in the winter, February--March 1978, it was 3.2 and 0.8 μgS/m3, respectively. During the summer episode the H2SO4 (as sulfur) peaked at about 10 μgS/m3, compared to a total particulate sulfur peak concentration of 20 μgS/m3. During the winter episode the H2SO4 concentration peaked at about 4 μgS/m3, compared to a particulate sulfur peak concentration of 6 μgS/m3. Daily average particulate sulfur and light scattering from July--August 1977 and February--March 1978 show a strong correlation (r>0.8). There were several general differences between the summer and winter data which suggest chemical and physical changes in the atmospheric aerosol. Analysis of the summer data revealed overall diurnal patterns in particulate sulfur and acidity, but these patterns did not show up with the winter data. The strong correlation between light scattering and particulate sulfur indicates that sulfur compounds play a prominent role in visibility degradation by fine particles.