Laboratory and field measurements of HONO emissions from agricultural soils in Guangdong of China 

<p>Laboratory studies indicated that soil could produce considerable nitrous acid (HONO) emissions, which is the main primary source of hydroxyl radical (OH) in the troposphere. However, very few field observations of HONO emission from soil were reported. In order to relate laboratory results and field measurements, we measured HONO emissions from 7 representative agricultural soils (rice, vegetables, orchards, peanuts, potatoes, sugarcane and maize) in Guangdong under controlled laboratory conditions, and took flux measurements on 2 of them (rice and vegetables) by dynamic chambers in the field. Generally, release rates of HONO from the seven soils increased with temperature and varied with soil moisture, and the optimum release rates can be reached under specific values of water-filled pore space (WFPS), which is considered to be beneficial to nitrification. The seven soils' optimum release rates ranged from 1.24 to 43.19 ng kg<sup>-1</sup> s<sup>-1</sup>, and the Q<sub>10</sub> (It is defined as the multiple of the increase of soil gas emission rate when the temperature increases by 10℃) ranged from 1.03 to 2.25. Formulas were deduced from the lab results to express HONO emissions for every soil. Flux measurements on two soils varied around -1 to 4 ng N m<sup>-2</sup> s<sup>-1</sup>, and both showed similar diurnal variations with peaks around noontime and very low even negative values during nighttime. There were good correlations between HONO fluxes and soil temperature (R<sup>2</sup>=0.5). Furthermore, irrigation enhanced the HONO emission substantially. However, a large discrepancy existed between soil HONO emissions measured in lab and low HONO fluxes in field. More investigations are needed to explain the paradox.</p>