PP-29-038 Background/Aims: Hexavalent chromium (Cr-VI) has been listed as one of the core hazardous air pollutants by the US Environmental Protection Agency, whereas trivalent chromium (Cr-III) is an essential nutritional trace element for humans. Thus, it is crucial to investigate the environmental factors affecting the transformation between the 2 oxidation states. Methods: A 24-hour exposure study was undertaken in a controlled environmental facility in the absence of sunlight by pulling clean air or clean air mixed with O3, SO2, and NO2 through filters, which were pre-collected with diesel particles (DPM) or secondary organic aerosol (SOA). Isotope spiking (53Cr-VI and 50Cr-III) was used to monitor the redox change of chromium species within the exposure process. The exposure factors included O3, SO2, NO2, airborne particle type, temperature, and humidity. ANOVA test (P = 0.05) was used. Results: After exposure to clean air, neither significant reduction nor oxidation was found. Compared to control, exposure to 160 ppb SO2 promoted significant reduction of Cr-VI in DPM (40.8 ± 3.5%, n = 3, P = 0.0022) and SOA (41.9 ± 6.8%, n = 3, P < 0.0001). Conclusion: The results suggested SO2 as a potential reductant for ambient Cr-VI. Our study also suggested that O3 may promote reduction of Cr-VI while NO2 may induce oxidation of Cr-III in the presence of ambient organics. Another finding is that Cr-III is easily oxidized in SOA than DPM, probably because of the organic composition and liquid microenvironment of SOA. Further study will be undertaken to understand the mechanisms behind the transformation we found in the controlled environmental facility tests.