Correspondence: Min Qin (mqin@aiofm.ac.cn)

Nitrous acid (HONO) and nitrogen oxides (NO_x) play a crucial role in tropospheric photochemistry by contributing to the hydroxyl radical (OH) and influencing atmospheric oxidization capacity. Recent research has found that soil HONO emissions are considered to be the main source of atmospheric HONO. Here, an aerodynamic gradient (AG) method combined with a BroadBand Cavity Enhanced Absorption Spectrometer (BBCEAS) system was developed to measure HONO and NOx emission flux from agricultural fields in the Huaihe River Basin. Measurements were taken at two different heights and included various agricultural management activities such as rotary tillage, flood irrigation, fertilization, transplanting rice seedlings, and top-dressing. For HONO and NO, upward fluxes were observed (0.07 ± 0.22 and 0.19 ± 0.53 nmol /(m^2·s)), while NO₂ was deposited to the ground (-0.37 ± 0.47 nmol /(m^2·s)). The maximum emission fluxes of HONO and NO occurred at around 24°C, which is close to the optimal temperature (25°C) for soil microbial nitrification processes. This indicates that surface microbial processes may contribute to gas emissions. Specifically, during rotary tillage, continuous peaks in HONO flux and NO flux were observed. P_OH(HONO)_net and P_OH(O₃)_net were 1.42 ppb/h and 1.35 ppb/h, respectively, with HONO and O₃ photolysis accounted for 51% and 49% of the total yield. The peak of P_OH(HONO)_net coincides with the peak of HONO emissions from agricultural fields, revealing the significant contribution of agricultural HONO emissions to atmospheric oxidizing capacity. After irrigation in agricultural fields, the increase in soil moisture content (~80% water filled pore space) restricts oxygen availability, thereby suppressing the HONO emission. Overall, this study provides valuable insights into the dynamics of soil HONO and NO_x emissions in agricultural fields, shedding light on their environmental implications and the role of agricultural activities in atmospheric chemistry.

Acknowledgments: This work was supported by the National Natural Science Foundation of China (U21A2028) and the National key Reearch and Development Program of China (2022YFC3701100).