Comparative contributions of primary emission and secondary production of HONO from unfertilized soil in Eastern China

• A dual-isotope constraint approach utilizing Δ 17 O-HONO and δ 15 N-HONO is developed to quantify the emission pathways of HONO from soil. • In fertilized soil, bacterial processes contributed 47 ± 10 % of the total soil HONO emissions through nitrification and 48 ± 10 % through denitrification. • Heterogeneous hydrolysis of NO 2 on the soil surface contributed nearly half of the total soil HONO emissions in unfertilized soil. Nitrous acid (HONO), as a crucial precursor of hydroxyl radical (OH) in the atmosphere, dominates the atmospheric oxidizing capacity. Soil microbial activity is a considerable HONO emission source. However, soil can also provide a reaction interface for the secondary production of HONO, but its importance has not been quantitatively demonstrated in the field environment. In this study, observations of stable nitrogen isotope (δ 15 N) and oxygen isotope anomaly (Δ 17 O) of HONO released from farmland soil were conducted to identify the relative importance of primary and secondary HONO sources. The results showed that the HONO emission flux ( F HONO ), δ 15 N-HONO, and Δ 17 O-HONO exhibited differences between intensive fertilization and idle farmland soils, which were 43.9 ± 11.8 ng m −2 s −1 and 4.8 ± 2.7 ng m −2 s −1 for F HONO , −24.3 ± 4.2 ‰ and −19.1 ± 6.3 ‰ for δ 15 N-HONO, and 0.6 ± 0.3 ‰ and 6.2 ± 2.0 ‰ for Δ 17 O-HONO, respectively. Constrained by Δ 17 O and δ 15 N observations of HONO from farmland soil using a Bayesian isotope mixing model, we quantified that 95 % of the F HONO during the intensive fertilization period was contributed by soil bacterial activities, including 47 ± 10 % by nitrification and 48 ± 10 % by denitrification. In idle farmland, where the effects of fertilization are not evident, primary emissions originating from bacterial activities accounted for 48 % to 62 % of the HONO emission flux. The remaining 38 % to 52 % of F HONO was attributed to the heterogeneous hydrolysis of NO₂ occurring on the soil surface. The results highlighted the important contribution of secondary HONO production on the soil surface, a factor that should be considered in model framework for the simulation of soil HONO emissions.