Measurement report: Surface exchange fluxes of HONO during the growth process of paddy fields in the Huaihe River Basin, China

Abstract. Significant amounts of nitrous acid (HONO) released from soil affect the tropospheric atmosphere, as a major precursor of hydroxyl radical. However, the scarcity of soil–atmosphere HONO exchange flux has constrained the comprehension of emission processes and reactive nitrogen budget. Herein, we performed measurements of HONO and NO_x fluxes over paddy fields in the Huaihe River Basin for the first time. The entire experiment experienced various agricultural management activities, including rotary tillage, flood irrigation, fertilization, paddy cultivation and growth, and top-dressing. HONO and NO exhibited upward fluxes, while NO₂ deposited to the ground, with average hourly fluxes of 0.07 ± 0.22, 0.19 ± 0.53 and -0.37 ± 0.47 nmol m^-2 s^-1, respectively. During paddy cultivation, the flooded environment with a higher water-filled pore space (~80 %) significantly suppressed the HONO emission, and the fertilization did not have a significant promoting impact on HONO fluxes.

During the rotary tillage, continuous peaks were observed in HONO and NO flux, which exhibited a significant correlation (R = 0.77). Moreover, a significant correlation (R = 0.60) between HONO flux and the product of J(NO₂) × NO₂ was also observed during the daytime. The results suggest that both soil release mechanisms from biological processes and light-driven NO₂ conversion are likely active, and together influence the diurnal pattern of HONO flux. Source analysis revealed that the unknown HONO source (P_unknown) exhibited a diurnal pattern with higher daytime and lower nighttime values. Sensitivity tests demonstrated that photo-enhanced NO₂ conversion on the ground could effectively explain P_unknown, and the nighttime HONO flux rates ranging from 0.32 ppbv h^-1 to 0.79 ppbv h^-1 were fully capable of explaining the nighttime P_unknown. Our study emphasized the variability of HONO fluxes across various agricultural management activities, as well as the importance of heterogeneous NO₂ conversion on the ground surface and soil emissions in HONO production.