The mechanisms of HONO emissions from soil: A review

Nitrous acid (HONO) significantly contributes to atmospheric hydroxyl radical (OH·) and also influences atmospheric oxidation capacity and air quality. Comparison of HONO concentrations measured in a field campaign and by modeling showed a large unknown HONO source during daytime. Studies have shown that the unknown HONO source can be attributed to soil emissions, a major source of atmospheric HONO. The mechanisms may be taking the form of chemical equilibrium between soil nitrite and H⁺, reactive uptake and displacement by soil, emissions by ammonia-oxidizing bacteria (AOB) and other micro-organisms, or surface reaction between hydroxylamine and H₂O. Therefore, HONO flux from soils is controlled by soil nitrite concentration, pH, AOB abundance, soil minerals, soil moisture and C/N ratio. The mechanism of HONO emissions from soil has remained a point of hot discussion and few results have been reported from China. Here, we introduced the background of HONO emissions from soil, reviewed studies on the mechanisms of HONO emissions from soil and the related driving factors. This review was a relevant support for research on reducing nitrogen loss, enhancing nitrogen use efficiency, and evaluating the effects of nitrogen fertilization on environmental and urban air quality.