Model Evaluation of New Techniques for Maintaining High-NO Conditions in Oxidation Flow Reactors for the Study of OH-Initiated Atmospheric Chemistry

Oxidation flow reactors (OFRs) efficiently produce OH radicals using low-pressure Hg-lamp emissions at lambda = 254 nm (OFR254) or both lambda = 185 and 254 nm (OFR185). OFRs under most conditions are limited to studying low-NO chemistry (where RO2 + HO2 dominates RO2 fate), even though substantial amounts of initial NO may be injected. This is due to very fast NO oxidation by high concentrations of OH, HO2, and O-3. In this study, we model new techniques for maintaining high-NO conditions in OFRs, that is, continuous NO addition along the length of the reactor in OFR185 (OFRI8S-cNO), recently proposed injection of N2O at the entrance of the reactor in OFR254 (OFR254-iN(2)O), and an extension of that idea to OFR185 (OFR185-iN(2)O). For these techniques, we evaluate (1) fraction of conditions dominated by RO2 + NO while avoiding significant nontropospheric photolysis and (2) fraction of conditions where reactions of precursors with OH dominate over unwanted reactions with NO3. OFR18S-iN(2)O is the most practical for general high-NO experiments because it represents the best compromise between experimental complexity and performance upon proper usage. Short lamp distances are recommended for OFR185-iN(2)O to ensure a relatively uniform radiation field. OFR185-iN(2)O with low O-2 or using Hg lamps with higher 185 nm-to-254 nm ratio can improve performance. OFR18S-iN(2)O experiments should generally be conducted at higher relative humidity, higher UV, lower concentration of non-NOy external OH reactants, and percent-level N2O. OFR18S-cNO and OFR185-iN(2)O at optimal NO precursor injection rate (similar to 2 ppb/s) or concentration (similar to 3%) would have satisfactory performance in typical field studies where ambient air is oxidized. Exposure estimation equations are provided to aid experimental planning. This work enables improved high-NO OFR experimental design and interpretation.