Atmospheric reactive nitrogen in typical croplands and intensive pig and poultry farms in the North China Plain

Reactive nitrogen (N_r) is the main precursor of atmospheric haze pollution, which leads to excessive nitrogen deposition. With prevention and control measured for air pollution in recent years, reducing N_r from agricultural sources has recently been suggested. Currently, field measurements focusing on atmospheric N_r in farmland and animal farms are still limited and are mainly focused on ammonia (NH₃), with little attention being paid to other N_r components. In order to characterize the potential impact of agricultural activities on atmospheric N_r concentrations, this study carried out simultaneous field measurements of major N_r species, including gaseous NH₃, nitric acid (HNO₃), particle ammonium (p-NH₄⁺) and nitrate (p-NO₃^-), in two farmlands (Xianghe farmland and Luancheng farmland in Hebei Province), a pig farm and a poultry farm in the North China Plain using the active denuder sampling system. The results showed that the average concentrations of NH₃ and p-NH₄⁺ inside the pig house (1 250.9 and 76.6 μg·m^-3, respectively) were significantly higher (P < 0.05) than those outside the pig house (378.5 and 4.2 μg·m^-3, respectively). However, the average concentrations of HNO₃ and p-NO₃^- inside the pig house (10.3 and 20.8 μg·m^-3, respectively) were comparable to those outside the pig house (9.8 and 22.1 μg·m^-3, respectively). In contrast, the measured N_r concentrations were similar both inside and outside the poultry house for all N_r species (overall ranged from 6.2 to 10.7 μg·m^-3) except for NH₃, which was significantly higher (P < 0.05) inside (197.7 μg·m^-3) compared to outside (77.3 μg·m^-3). In general, atmospheric N_r concentrations in the pig farm were significantly higher than those in the poultry farm (P < 0.05). The average air concentrations of NH₃, p-NH₄⁺, HNO₃ and p-NO₃^- in the farmland were 21.4, 1.9, 4.4 and 5.5 μg·m^-3, respectively, which were significantly lower than those outside the pig and poultry farms (P < 0.05). From the perspective of chemical composition, N_r mainly existed in the form of NH₃-N (accounting for more than 80%) in pig, poultry and farmland environments, which indicated that the N_r produced by agricultural activities in the North China Plain mainly diffused outward in the form of NH₃ instead of transforming into particles locally. Detailed investigations on the transmission mechanisms and transport pathways of N_r in the atmosphere are urgently needed to provide scientific support for the implementation of effective emission mitigation measures.