Work

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

Year: 2023

Type: article

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. To address this, we conduct long-period FHONO measurements over agricultural fields under fertilized (FHONO-NP) and non-fertilized (FHONO-CK) treatments. Our results show that nitrogen fertilizer use causes a remarkable increase in FHONO-NP and it exhibits distinct diurnal variations, with an average noontime peak of 152 ng N m-2 s-1. The average FHONO-NP within three weeks after fertilization is 97.7 ± 8.6 ng N m-2 s-1, around two orders of magnitude higher than before fertilization, revealing the remarkable promotion effect of nitrogen fertilizer on HONO emissions. We also discuss other factors that influence 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 0.06 and 0.16 Tg N yr-1 in the North China Plain (NCP) and 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 in order to effectively mitigate regional air pollution, future policies should consider reactive nitrogen emissions from agricultural soils.

Source: Atmospheric chemistry and physics

DOI: 10.5194/acp-23-15733-2023