Observational Evidence of Unknown NOx Source and Its Perturbation of Oxidative Capacity in Bermuda's Marine Boundary Layer.

Nitrogen oxides (NOx) are key intermediates in the atmospheric cycling of reactive nitrogen, the spatiotemporal distribution of which modulates ozone (O3) production. Field campaigns were conducted at the Tudor Hill Marine Atmospheric Observatory, Bermuda, in the spring and summer of 2019 to explore atmospheric cycling of NOx and its modulation of photochemical O3 production in the marine boundary layer. In aged, clean marine air, an atypical NO2 diel profile with a solar noon peak of 69 ± 5 pptv was recorded, challenging the classic U‐shaped diel profile with a solar noon valley characterized by fast photolysis and oxidation consumption in the daytime. This result indicated an unknown daytime NOx source excluded from the current near‐explicit chemical model, which underestimated the solar noon NOx level by 20–56 pptv and source rate by 9.7–33.5 pptv hr−1, considering the upper and lower limits of total OH reactivity and halogen photochemistry in the marine boundary layer. The observed HONO level accounted for ∼56% of the unknown NOx source, implying an unknown NOx regeneration pathway with HONO as an intermediate. The photochemical nature of the unknown NOx source maximized perturbation of photochemical OH and O3 production. The O3 abundance and production rate were underestimated by 2–4 ppbv and 28%–80%, respectively, and the OH abundance and source rate were 7%–55% and 21%–57% lower than the estimated levels with the constraint of NOx, respectively. The unknown NOx source requires urgent revision of the current understanding of reactive nitrogen cycling and the oxidative capacity of the clean marine atmosphere. Plain Language Summary: This study presents measurements and a chemical budget analysis of NO2, O3, and OH in the clean marine boundary layer at the Tudor Hill Marine Atmospheric Observatory, Bermuda. The high solar noon abundance and atypical diel profile of NO2 challenged the classic U‐shape, indicating an unknown daytime NOx source. Assessments using a near‐explicit MCM chemical model (v3.3.1) yielded an unknown NOx source by 9.7–33.5 pptv hr−1 at solar noon and demonstrated that the photochemical nature of the unknown NOx source maximized perturbation of the O3 and OH budgets. In addition to halogen photochemistry, the unknown NOx source necessitates revision of the photochemistry in the clean marine boundary layer. Key Points: The atypical NO2 diel profile against its classic U‐shape indicates an unknown daytime source of NOx in the marine boundary layerThe unknown daytime NOx source, in addition to halogen photochemistry, greatly perturbs NOx, O3, and OH budgetsThe unknown NOx source appears to be due in part to a missing HONO source, that is, NOx is regenerated with HONO as an intermediate [ABSTRACT FROM AUTHOR]