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

Abstract

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 (δ¹⁵N) and oxygen isotope anomaly (Δ¹⁷O) 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), δ¹⁵N-HONO, and Δ¹⁷O-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 δ¹⁵N-HONO, and 0.6 ± 0.3 ‰ and 6.2 ± 2.0 ‰ for Δ¹⁷O-HONO, respectively. Constrained by Δ¹⁷O and δ¹⁵N 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.