Strong HONO Emissions from Fertilized Soil in the North China Plain Driven by Nitrification and Surface Water Exchange

Abstract Soil was proposed as an important nitrous acid (HONO) source but has not so far been demonstrated by field flux measurements and its atmospheric impacts are highly uncertain. Through field flux measurements we found strong soil HONO emission flux (F HONO ) enhanced by nitrogen fertilizer use for agricultural fields. F HONO showed distinct diurnal variations with noontime peaks up to hundreds of ng-N m -2 s -1 . With soil samples where field measurements took place, our laboratory experiments subsequently found that the release of soil nitrification-originated nitrite as HONO was largely accelerated by surface water exchange. This process was parameterized by a modified Arrhenius equation that reproduced field flux observations well. With the consideration of F HONO , the box model could well reproduce the observed ambient HONO and found that F HONO could significantly accelerate ozone production, highlighting the amplified role of nitrogen fertilizer in exacerbating regional ozone pollution. Moreover, we found that nitrification inhibitors could reduce HONO emissions by ~90%, indicating their great potential to mitigate air pollution. Furthermore, we estimated yearly HONO emission of 0.25 Tg-N from global fertilizer use for agriculture, representing a considerable reactive nitrogen source and significantly affecting atmospheric oxidizing capacity and air quality on a global scale.