We performed a series of laboratory and field measurements to evaluate the mist chamber technique for determining tropospheric HNO3 concentrations. Both the mist chamber and standard Nylon filter techniques exhibit high collection efficiency and excellent agreement measuring HNO3 vapors from a permeation source. When simultaneously sampling ambient air in eastern Virginia the Nylon filter measured an average of 70% higher HNO3 concentration than the mist chamber technique. Our results indicate that O3 causes a low-level positive artifact interference in HNO3 measurements performed with the filter technique. This O3-induced error is small, however, compared to the large difference between atmospoheric HNO3 concentrations determined with the two techniques. We hypothesize that unidentified (organic?) nitrogen species in the atmosphere react for form NO-3 on the filter and this phenomenon may interfere with Nylon filter measurements of HNO3 vapor. These potential interferences did not appear to affect measurements of HNO3 with the mist chamber method, but we are continuing studies to evaluate potential interference from various atmospheric species. Because the mist chamber technique employs an aerosol prefilter, it is probably best suited for measuring HNO3 concentrations when particle-NO-3 levels are low. In light or our preliminary results, we caution users of the Nylon filter method to continue to critically evaluate this technique for specificity in determining HNO3 vapor phase concentrations in the atmosphere, particularly at low concentrations typical of the remote troposphere.