Abstract. The real-time measurements of NH3 and trace gases were conducted, in conjunction with semi-continuous measurements of water-soluble ions in PM2.5 at a rural site in the North China Plain (NCP) from May to September 2013 in order to better understand of chemical characteristics for ammonia, and of the impact on formation of secondary ammonium aerosols in the NCP. Extremely high NH3 and NH4+ concentrations were observed after a precipitation event within 7–10 days following urea application. Elevated NH3 levels coincided with elevated NH4+, suggesting that NH3 plays a vital role in enhancing particulate ammonium. For the sampling period, the average oxidation/conversion ratios for SO42− (SOR), NO3− (NOR) and NH4+ (NHR) were estimated to be 64 %, 24 % and 30 %, respectively. The increased NH3 concentrations mainly from agricultural activities, coincided with the prevailing meteorological conditions could promote the secondary transformation, resulting in higher hourly SOR, NOR and NHR. The concentrations of NH3, NH4+, and NHR had clear diurnal variations, which could be attributed to their sources, meteorological conditions, and formation mechanisms. The back trajectory analysis indicates that the transport of air masses from the North China Plain region contributed to the atmospheric NH3 variations, and both regional sources and long-distance transport from southeast played important roles in the observed ammonium aerosol at rural site in the NCP. The findings of this study are expected to facilitate developing future NH3 emission control policies for the North China Plain.