Measurements of Nitrous Acid (HONO) Direct Emission from Rice Paddy Soil and Its Contribution to Atmospheric HONO Concentration

Abstract

It has been recently reported that the direct emission of nitrous acid (HONO) from soils can be a significant source of HONO especially during the day when a sufficient HONO source has not been found. In this study, the direct HONO emission from rice paddy soil was measured in the field during the mid-summer drainage period. To investigate the impact of fertilization, the system with a traditional amount of fertilization (the chemical nitrogen fertilizer system) and a non-fertilized system (the control system) were prepared. The maximum flux was 40.4 ng/m²/s which was observed after the soil oxidation reduction potential (Eh) had increased from negative values to positive values. The flux was higher during the daytime (3.5±0.9 ng/m²/s) than the nighttime (1.0±0.3 ng/m²/s). This cannot be explained only by the shift in the chemical equilibrium. The chemical nitrogen fertilizer system had a greater HONO flux than the control system on average. The reason could be that the chemical nitrogen fertilizer system has a lower soil moisture content. It was found that the direct emission amount from rice paddy soils may have a non-negligible impact on the atmospheric HONO concentrations.