Measurement of HONO Flux Using the Aerodynamic Gradient Method over an Agricultural Field in the Huaihe River Basin, China.

To investigate nitrous acid (HONO) levels and potential HONO sources above crop rotation fields. The HONO fluxes were measured by the aerodynamic gradient (AG) method from 14 December 2019 to 2 January 2020 over an agricultural field in the Huaihe River Basin. The ambient HONO levels were measured at two different heights (0.15 and 1.5 m), showing a typical diurnal cycle with low daytime levels and high nighttime levels. The upward HONO fluxes were mostly observed during the day, whereas deposition dominated at night. The diurnal variation of HONO flux followed solar radiation, with a noontime maximum of 0.2 nmol/(m 2 .sec). The average upward HONO flux of 0.06 ?? 0.17 nmol/(m 2 .sec) indicated that the agricultural field was a net source for atmospheric HONO. The higher HONO/NO 2 ratio and NO 2 -to-HONO conversion rate close to the surface suggested that nocturnal HONO was formed and released near the ground. The unknown HONO source was derived from the daytime HONO budget analysis, with an average strength of 0.31 ppbV/hr at noontime. The surface HONO flux, which was highly correlated with the photolysis frequency J (NO 2 ) ( R 2 = 0.925) and the product of J (NO 2 ) x NO 2 ( R 2 = 0.840), accounted for -23% of unknown daytime HONO source. The significant correlation between HONO fluxes and J (NO 2 ) suggests a light-driven HONO formation mechanism responsible for the surface HONO flux during daytime.