In high-NOx environments, nitrogen oxides (NOx) and nitrous acid (HONO) are coupled in their internal cycling, i.e., HONO formation by heterogeneous reactions of NO2 on ambient surfaces and gas-phase reactions between NO and OH, as well as HONO recycling back to NO via photolysis. As air masses are transported away from source regions, internal cycling is suppressed since NOx ages to form more oxidized reservoirs, referred to as NOz. In low-NOx environments, external cycling of NOx, the processes regenerating NOx from NOz, dominate the production of NOx. Given the catalytic role of NOx in radical chain propagation, external cycling further promotes the formation of hydroxyl radical, the most important oxidant in the atmosphere. However, the external cycling of NOx has not been fully validated due to its large variability and potentially large uncertainties in its kinetics and mechanisms across the high- to low-NOx atmospheres.

To solve this dilemma, we suggest synthesizing observational and model evidence, summarizing the fundamental characteristics of external cycling, and quantifying its impact on the oxidative capacity of the atmosphere. Based on a valuable dataset concerning NOx photochemical cycling collected onboard the aircraft platform, this study reveals two characteristics of the external cycling, i.e. unexpectedly high ratios of HONO/NO2 and NO2/NOz, and atypical diurnal concentration profiles of HONO and NO2 lacking of noontime valleys, neither of which can be predicted by the chemical transport model, GEOS-Chem. HONO as an intermediate of the external cycling perfectly rationalizes the observed high HONO/NO2 ratio and the atypical diurnal profiles of HONO and NO2. Moreover, the external cycling of NOx is accelerated by the accumulation of NOz as NOx ages in the atmosphere, which also rationalizes the two observational characteristics. Perturbation on the budget of HONO and NOx by external cycling is found to increase as NOx concentration decreases. As such, external cycling of NOx compensates for NOx aging and therefore sustains the distribution of NOx in the low-NOx atmospheres. Consequently, hydroxyl radicals in such environments increased by up to 41%, compared with the models omitting the external cycling. The results indicate the dominant role of external cycling in the chemical budget of reactive nitrogen in low-NOx atmospheres and its significant impact on oxidant photochemistry.

This study provides new insights into reactive nitrogen chemistry by synthesizing observational evidence across high- to low-NOx atmospheres rather than attempting to establish the kinetics or the dominant mechanism. We suggest that field measurements, especially for low-NOx atmospheres, are in urgent need to establish spatial and temporal distributions of reactive nitrogen species and facilitate our understanding of reactive nitrogen chemistry and its impact on atmospheric oxidative capacity.

This work has been recently published in Nature Communications under the title “Synthesizing evidence for the external cycling of NOx in high- to low-NOx atmosphere” (https://www.nature.com/articles/s41467-023-43866-z). Professor Chunxiang Ye is the first and the corresponding author of this paper. This research is supported by the National Natural Science Foundation of China (Grants No. 41875151).