Unveiling the underestimated direct emissions of nitrous acid (HONO)

Abstract

Gaseous nitrous acid (HONO) is a critical source of hydroxyl radicals (OH) in the troposphere. While both direct and secondary sources contribute to atmospheric HONO, direct emissions have traditionally been considered minor contributors. In this study, we developed δ 15 N and δ 18 O isotopic fingerprints to identify six direct HONO emission sources and conducted a 1-y case study on the isotopic composition of atmospheric HONO at rural and urban sites. Interestingly, we identified that livestock farming is a previously overlooked direct source of HONO and determined its HONO to ammonia (NH 3 ) emission ratio. Additionally, our results revealed that spatial and temporal variations in atmospheric HONO isotopic composition can be partially attributed to direct emissions. Through a detailed HONO budget analysis incorporating agricultural sources, we found that direct HONO emissions accounted for 39~45% of HONO production in rural areas across different seasons. The findings were further confirmed by chemistry transport model simulations, highlighting the significance of direct HONO emissions and their impact on air quality in the North China Plain. These findings provide compelling evidence that direct HONO emissions play a more substantial role in contributing to atmospheric HONO than previously believed. Moreover, the δ 15 N and δ 18 O isotopic fingerprints developed in this study may serve as a valuable tool for further research on the atmospheric chemistry of reactive nitrogen gases.

Observation-Based Diagnostics of Reactive Nitrogen Recycling through HONO Heterogenous Production: Divergent Implications for Ozone Production and Emission Control

“…Ground sources of HONO have been recognized as potentially important contributors. , The HONO isotope observations in China show significant influences of fertilizer applications and livestock farms . Ground conversion from NO 2 is found to be a minor HONO source in that study.…”

“…HONO is produced through the gas-phase reaction of nitric oxide (NO) and OH (R1) . Direct emissions from combustion activities, such as vehicle exhaust and biomass burning (BB) can also contribute to HONO levels. − In recent years, observations in a variety of locations have found other daytime HONO sources to be important. , Soil emissions of HONO have been found to be potentially important for regions with livestock farming or after fertilization, highlighting the need of detailed quantification in future studies . Moreover, various HONO production pathways have been proposed based on field measurements and laboratory experiments, with heterogeneous reactions highlighted as crucial contributors. − For example, studies have demonstrated the importance of heterogeneous nitrogen dioxide (NO 2 ) conversion on ground surfaces (eq S3) as an important nocturnal HONO source .…”

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.…”

Characterization of nitrous acid and its potential effects on secondary pollution in warm-season of Beijing urban areas

“…The release from soil nitrite is one important primary source of HONO (Su et al, 2011;Wu et al, 2019), the biological soil crusts can accelerate the HONO emission in drylands (Weber et al, 2015), fertilization behavior of agricultural fields remarkably enhance the emission of HONO (Xue et al, 2021), and denitrification is one major HONO production pathway in boreal agricultural fields (Bhattarai et al, 2021). Meanwhile, livestock farming is also identified as one important primary emission source of HONO (Zhang et al, 2023a). (b) Homogeneous reaction of OH +NO, which usually occurs in polluted areas, the contribution of this recombination pathway to HONO may be significant during daytime with high NO and OH concentration (Gu et al, 2022).…”