An automated dynamic chamber system for exchange flux measurement of reactive nitrogen oxides (HONO and NO) in farmland ecosystems of the Huaihe River Basin, China.

An automated dynamic chamber system was first developed to simultaneously measure the HONO flux and NOx flux. The new dynamic chamber system was applied to field observation, and the HONO and NO exchange flux of farmland in the Huaihe River Basin was obtained for the first time. The performance of the dynamic chamber system was verified in the field. In the field observation, the diurnal variations of the HONO fluxes and NO fluxes before and after a rainfall event exhibited two different trends. Before the rainfall and in the latter stage after the rainfall, the maxima of the HONO fluxes and NO fluxes occurred in the morning, then decreased gradually. However, during the early stage after the rainfall, the HONO fluxes and NO fluxes gradually increased in the morning and reached their maximum values in the afternoon. During the measurement period, the maximum HONO flux was 7.69 ng N m s and the maximum NO flux was 34.52 ng N m s. There was no significant correlation between HONO flux and temperature before the rainfall and in the latter stage after the rainfall period, although the correlation coefficient (R) between HONO flux and temperature reached 0.78 in the early stage after the rainfall period, and the R between NO flux and HONO flux reached more than 0.6 before and after rainfall periods. The HONO flux of fresh soil samples were the same order of magnitude as that of field observations. The field results indicate that soil emissions are an important source of atmospheric HONO during the crop growth stage. Negative NO fluxes were found in most observation periods, and there were significant negative linear correlations between NO fluxes and atmospheric NO concentrations. The R between ambient NO concentration and NO flux was 0.79, and the compensation point of NO was 5 ppbv.