Measuring tropospheric HNO3: Problems and prospects for nylon filter and mist chamber techniques

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 atmospheric HNO₃ concentrations determined with the two techniques. We hypothesize that unidentified (organic?) nitrogen species in the atmosphere react for form NO₃^- 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-NO₃^- levels are low. In light of 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.