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

Being major species of atmospheric reactive nitrogen, nitrogen oxides (NOx = NO + NO2) have important implications for ozone and OH radical formation in addition to nitrogen cycles. Stable nitrogen isotopes (δ15N) of NOx have been sought to track NOx emissions and NOx chemical reactivities in the atmosphere. The current atmospheric NOx 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(NOx) in pristine regions with low NOx concentrations. Here, we present a three-dimensional (3D)-printed honeycomb denuder (3DP-HCD) system, which can effectively collect atmospheric NO2 (a major part of NOx) 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.