Fast and Efficient Atmospheric NO₂ Collection for Isotopic Analysis by a 3D-Printed Denuder System

Being major species of atmospheric reactive nitrogen, nitrogen oxides (NO = NO + NO₂) have important implications for ozone and OH radical formation in addition to nitrogen cycles. Stable nitrogen isotopes (δ¹⁵N) of NO have been sought to track NO emissions and NO chemical reactivities in the atmosphere. The current atmospheric NO collection methods for isotopic analysis, however, largely suffer from unverified collection efficiency and/or low collection speed (<10 L/min). The latter makes it difficult to study δ¹⁵N(NO) in pristine regions with low NO concentrations. Here, we present a three-dimensional (3D)-printed honeycomb denuder (3DP-HCD) system, which can effectively collect atmospheric NO₂ (a major part of NO) under a variety of laboratory and field conditions. With a coating solution consisting of 10% potassium hydroxide (KOH) and 25% guaiacol in methanol, the denuder system can collect NO₂ with nearly 100% efficiency at flow rates of up to 70 L/min, which is 7 times higher than that of the existing method and allows high-resolution (e.g., diurnal or finer resolution) NO₂ collection even in pristine sites. Besides, the δ¹⁵N of NO₂ collected by the 3DP-HCD system shows good reproducibility and consistency with the previously tested method. Preliminary results of online NO oxidation by a chrome trioxide (CrO₃) oxidizer for simultaneous NO and NO₂ collection are also presented and discussed.