Quantifying the Nitrogen Sources and Secondary Formation of Ambient HONO with a Stable Isotopic Method

Abstract

Nitrous acid (HONO) is a reactive gas that plays an important role in atmospheric chemistry. However, accurately quantifying its direct emissions and secondary formation in the atmosphere as well as attributing it to specific nitrogen sources remains a significant challenge. In this study, we developed a novel method using stable nitrogen and oxygen isotopes (δ15N; δ18O) for apportioning ambient HONO in an urban area in North China. The results show that secondary formation was the dominant HONO formation processes during both day and night, with the NO2 heterogeneous reaction contributing 59.0 ± 14.6% in daytime and 64.4 ± 10.8% at nighttime. A Bayesian simulation demonstrated that the average contributions of coal combustion, biomass burning, vehicle exhaust, and soil emissions to HONO were 22.2 ± 13.1, 26.0 ± 5.7, 28.6 ± 6.7, and 23.2 ± 8.1%, respectively. We propose that the isotopic method presents a promising approach for identifying nitrogen sources and the secondary formation of HONO, which could contribute to mitigating HONO and its adverse effects on air quality.

Additional HONO and OH Generation from Photoexcited Phenyl Organic Nitrates in the Photoreaction of Aromatics and NOx

“…So far, HONO sources include direct emissions (e.g., emission from soil), − gas-phase homogeneous reactions, heterogeneous NO 2 conversion, , and photolysis of nitric acid, nitrate, and nitroaromatic. , Recent studies have underscored the existence of strong unknown HONO sources in forests and agricultural regions, ,, as well as in urban areas, particularly its daytime sources. − Of particular interest is the potential role of motor vehicle exhaust in ambient HONO levels, considering that motor vehicles have been reported to emit HONO directly − as well as a cocktail of volatile organic compounds (VOCs), NO x , and other reactive substances. The interaction between these components under various atmospheric conditions could give rise to novel pathways for the HONO formation.…”