What is it about?

In this study, we delved into the origins of a particular air pollutant called nitrous acid (HONO). To trace these origins, we employed a distinctive tool known as stable isotopic fingerprints (δ15N and δ18O). We investigated six potential sources where HONO emanates from and conducted a year-long analysis of its composition in both rural and urban environments. Interestingly, our investigations unveiled that livestock rearing, involving animals like pigs and chickens, contributes to the presence of HONO in the air. We also quantified the proportion of HONO released compared to another gas called ammonia (NH3). Furthermore, our discoveries indicated that the isotopic structure of HONO in the air fluctuates depending on the source and time of emission. By meticulously examining all potential sources of this pollution, we concluded that direct emissions from activities like farming are responsible for around 39~45% of the pollution in rural regions throughout the year. To substantiate our findings, we employed computer models, which corroborated the pivotal role of these direct sources in influencing the air quality in the North China Plain.

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Why is it important?

Hydroxyl radicals (OH) are crucial oxidizers in the atmosphere, playing a significant role in various chemical processes. During the daytime, OH forms mainly from the breakdown of HONO caused by sunlight. Historically, we believed that most HONO originated from secondary sources, like chemical reactions in the air, and paid less attention to direct emissions. This study employs distinctive indicators to trace the origins of HONO, revealing the impact of sources we previously overlooked, such as direct emissions, on the air we inhale. This new insight transforms our understanding of HONO's sources and equips us with valuable tools for future investigations into air quality.


This extensive collaborative work spanned six years, from the conception of the idea to its publication. The research sheds light on previously underestimated sources of direct HONO emissions, which could potentially worsen both outdoor and indoor air quality. The distinctive isotopic fingerprints of these direct HONO sources offer a new and valuable tool for future studies in this field.

Shandong University

Read the Original

This page is a summary of: Unveiling the underestimated direct emissions of nitrous acid (HONO), Proceedings of the National Academy of Sciences, August 2023, Proceedings of the National Academy of Sciences,
DOI: 10.1073/pnas.2302048120.
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