Studies conducted at the EPA facility in Research Triangle Park, NC and at a field study in Southern California have demonstrated the capability for the semi-continuous determination of particulate nitrate (and other water soluble ionic species). Two instruments, a R and P 8400N particulate nitrate monitor and an ion chromatography (IC)-based prototype monitor developed at Texas Tech University (TTU), were evaluated both in the laboratory using a simulated ambient aerosol and in the field (Rubidoux, CA) during a three week joint ambient comparison study with Brigham Young University (BYU) and the South Coast Air Quality Monitoring District (SCAQMD). During the initial 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 and these problems are currently being addressed. Both instruments were then transported to the SCAQMD Rubidoux field site and operated for a period of three weeks (July 1-21, 2003).