HONO Emission From Rice Paddies and its Impact on Atmospheric HONO Budget

Soil emissions play a key role in regulating the atmospheric nitrous acid (HONO) budget and thus atmospheric chemistry. However, there are large uncertainties related to the HONO budget due to a lack of field measurements to support mechanistic studies. Here, we performed flux measurements of HONO emissions from rice paddies in Shanghai and evaluated their potential impact on the atmospheric HONO budget. Our results show distinct roles of rice paddies between daytime and nighttime. Unfertilized rice paddies emit HONO during the daytime with an average flux of 0.68 ± 1.37 ng N m⁻² s⁻¹, which corresponds to ∼10% of the total atmospheric HONO production for a daytime mixing layer height of 100 m. However, rice paddies act as a sink for HONO at night due to deposition and the high solubility of HONO in paddy water with an average flux of −0.30 ± 0.73 ng N m⁻² s⁻¹. Our correlation analyses reveal that water evaporation is a crucial factor influencing HONO emissions from rice paddies. Furthermore, the average HONO flux in rice paddies after nitrogen fertilization increased up to 2.24 ng N m⁻² s⁻¹ and contributed ∼40% to atmospheric HONO production. HONO emissions from rice paddies were estimated to be 0.10 ± 0.25 kg N ha⁻¹ over the entire cultivation cycle (∼150 days) with an emission factor of 1.3 × 10⁻³ relative to the applied nitrogen. Our results highlight the crucial impact of rice paddies on the atmospheric HONO budget especially in regions and/or during periods when combustion sources have limited influence.