Measurement report: Exchange fluxes of HONO over agricultural fields in the North China Plain

Abstract

Nitrous acid (HONO) is a crucial precursor of tropospheric hydroxyl radicals, but its sources are not fully understood. Soil is recognized as an important HONO source, but the lack of measurements of soil–atmosphere HONO exchange flux (FHONO) has led to uncertainties in modeling its atmospheric impacts and understanding the reactive nitrogen budget. Herein, we conduct FHONO measurements over agricultural fields under fertilized (FHONO-NP, normal fertilization and irrigation) and non-fertilized (FHONO-CK, normal irrigation but no fertilization) treatments. Our results show that nitrogen fertilizer use causes a remarkable increase in FHONO-NP. FHONO-NP exhibits distinct diurnal variations, with an average noontime peak of 152 ng N m−2 s−1. The average FHONO-NP within 3 weeks after fertilization is 97.7±8.6 ng N m−2 s−1, around 2 orders of magnitude higher than before fertilization, revealing the remarkable promotion effect of nitrogen fertilizer on HONO emissions. We also discuss other factors influencing soil HONO emissions, such as meteorological parameters and soil properties/nutrients. Additionally, we estimate the HONO emission factor of 0.68±0.07 % relative to the applied nitrogen during the whole growing season of summer maize. Accordingly, the fertilizer-induced soil HONO emission is estimated to be 22.3 and 60.8 Gg N yr−1 in the North China Plain (NCP) and mainland China, respectively, representing a significant reactive nitrogen source. Furthermore, our observations reveal that soil emissions sustain a high level of daytime HONO, enhancing the atmospheric oxidizing capacity and aggravating O3 pollution in the NCP. Our results indicate that to mitigate regional air pollution effectively, future policies should consider reactive nitrogen emissions from agricultural soils.