Studies conducted at the U.S. Environmental Protection Agency facility in Research Triangle Park, NC, and at a field study in Southern California have demonstrated the capability for the semicontinuous determination of particulate nitrate (and other water-soluble ionic species). Two instruments, an R&P Series 8400N Ambient Particulate Nitrate Monitor (8400N) and an ion chromatography (IC)-based prototype monitor developed at Texas Tech University, were evaluated both in the laboratory using aqueous standards and simulated ambient aerosols and in the field during a 3-week July 1-21, 2003) joint ambient comparison study with Brigham Young University and the South Coast Air Quality Management District (SCAQMD). The field study was conducted at the SCAQMD Rubidoux field site near Riverside, CA. During initial (before the field study) laboratory studies, both instruments were responsive to changes in the simulated aerosol concentration. However, potential problems were discovered involving both instruments during the laboratory-based studies both before and after the Rubidoux study, and these problems are currently being addressed. During the 3-week field study period, 15-minute average particulate nitrate concentrations approaching 30 microg/m3 were observed. Because of malfunctioning IC components (concentrator columns) of the Texas Tech University prototype monitor, limited data were obtained from this instrument during the 3-week sampling period. Initial ambient comparisons show general agreement between the 8400N and IC-based prototype instrument for the semicontinuous determination of ambient particulate nitrate at lower nitrate concentrations (<15 microg/m3) and an underdetermination by the 8400N at higher concentrations (>15 microg/m3). A decreased molybdenum converter efficiency in the pulse analyzer of the 8400N discovered during postfield study laboratory evaluation may explain the negative bias observed at higher nitrate concentrations. Semicontinuous results obtained from U.S. Environmental Protection Agency-operated instruments were averaged and compared with integrated sampler results obtained by Brigham Young University and SCAQMD.