Study of Inlet Materials for Sampling Atmospheric Nitric Acid

The adsorption of nitric acid (HNO 3 ) from a flowing gas stream is studied for a variety of wall materials to determine their suitability for use in atmospheric sampling instruments. Parts per billion level mixtures of HNO 3 in synthetic air flow through tubes of different materials such that >80% of the molecules interact with the walls. A chemical ionization mass spectrometer with a fast time response and high sensitivity detects HNO 3 that is not adsorbed on the tube walls. Less than 5% of available HNO 3 is adsorbed on Teflon fluoropolymer tubing after 1 min of HNO 3 exposure, whereas >70% is lost on walls made of stainless steel, glass, fused silica, aluminum, nylon, silica-steel, and silane-coated glass. Glass tubes exposed to HNO 3 on the order of hours passivate with HNO 3 adsorption dropping to zero. The adsorption of HNO 3 on PFA Teflon tubing (PFA) is nearly temperature-independent from 10 to 80 °C, but below −10 °C nearly all HNO 3 that interacts with PFA is reversibly adsorbed. In ambient and synthetic air, humidity increases HNO 3 adsorption. The results suggest that Teflon at temperatures above 10 °C is an optimal choice for inlet surfaces used for in situ measurements of HNO 3 in the ambient atmosphere.