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

Abstract Soil was proposed as an important nitrous acid (HONO) source but has not so far been demonstrated by field flux measurements. Here we conducted field flux measurements and found large HONO emission fluxes (FHONO) after the fertilization of agricultural fields in the North China Plain. FHONO 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 enhanced by soil surface water evaporation. This process was parameterized by a modified Arrhenius equation that reproduced field flux observations well. We estimated yearly 0.25 Tg-N/year HONO for global agricultural fertilizer-derived emissions, representing a considerable reactive nitrogen source and significantly affecting atmospheric oxidizing capacity and air quality. Furthermore, nitrification inhibitors could reduce HONO emissions by 90%, indicating their great potential to mitigate regional air pollution.