The IMPROVE (Interagency Monitoring of Protected Visual Environments) network collects fine particles on nylon filters for analysis of ions, including NO 3 -. An annular anodized aluminum denuder coated with Na 2 CO 3 and glycerin (to prevent drying and improve efficiency) precedes the filters to remove nitric acid. The denuders are changed once each year during annual maintenance, but otherwise are not serviced. If the efficiency of the denuder for removing nitric acid decreases during the year, it could allow nitric acid to be collected on the nylon filter. This would result in a positive artifact, as the nitric acid would be interpreted as particulate nitrate. Several changes have been made to the IMPROVE ion module since the first samples were collected. It's possible that these changes may have affected collection of nitrate, also. To examine the efficiency of the denuder in several configurations for removing nitric acid, we collected daily samples for four consecutive weeks at Brigantine National Wildlife Refuge, Grand Canyon National Park, and San Gorgonio Wilderness. At San Gorgonio, we collected samples during two separate four-week periods. The results of this study show that the denuder in the IMPROVE ion module effectively and efficiently removes nitric acid.