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 λ
= 254 nm (OFR254) or both λ = 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 O3. 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 (OFR185-cNO), recently
proposed injection of N2O at the entrance of the reactor
in OFR254 (OFR254-iN2O), and an extension of that idea to OFR185 (OFR185-iN2O). 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. OFR185-iN2O
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-iN2O
to ensure a relatively uniform radiation field. OFR185-iN2O with low O2 or using Hg lamps with higher 185 nm-to-254
nm ratio can improve performance. OFR185-iN2O experiments should generally
be conducted at higher relative humidity, higher UV, lower concentration
of non-NOy external OH reactants, and
percent-level N2O. OFR185-cNO and OFR185-iN2O at optimal
NO precursor injection rate (∼2 ppb/s) or concentration (∼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.