Abstract: In-situ measurement of ambient ammonia (NH₃) and water-soluble ions in PM_2.5 is conducted at Gucheng, a rural site, from June to August in 2013. Gucheng is an integrated experiment site on ecological and meteorological observation belonging to Chinese Academy of Meteorological Sciences. This station is influenced by high NH₃ emissions from fertilizer use and animal production in surrounding areas. Ammonia and other trace gases are observed by DLT-100 Ammonia Analyzer and a set of commercial instruments during summer of 2013. Hourly concentrations of the water-soluble inorganic ions in PM_2.5 are also measured with the Ambient Ion Monitor (URG 9000 Series, USA). Concentrations of NH₃ at Gucheng range from 0.9×10^-9 to 862.9×10^-9, with the average of 43.9×10^-9±65.9×10^-9. In summer, high temperatures favor ammonia volatilization from fertilizer applied to the cropland. NH₃ concentrations increase sharply after fertilizer application in July for summer maize. Mean concentrations of SO₂, NO_X and O₃ are 4.3×10^-9±5.5×10^-9, 13.2×10^-9±6.8×10^-9 and 42.4×10^-9±31.5×10^-9 during the sampling period. The ammonia shows a significant diurnal variation during the sampling period. NH₃ concentration maximum occurs at 0900 BT and the minimum at 1900 BT. The sulfate, ammonium and nitrate are dominant ions in PM_2.5 with their average concentration being (20.46±13.62), (19.77±33.24) μg·m^-3 and (11.34±9.14) μg·m^-3, respectively. Ammonium shows significant positive correlations with NH₃ concentration. To understand the relationship between particulate ions and their respective precursors, sulfur oxidation ratio (SOR), nitrogen oxidation ratio (NOR) and ammonia conversion ratio (NHR) are investigated. SOR and NOR represent the oxidation ratio of sulfate and nitrate, and NHR represents the conversion ratio of ammonium. Higher SOR and NHR have important effects on the conversion of SO₂ to SO₄^2- and NH₃ to NH₄⁺. The dependence of inorganic PM_2.5 on NH₃ levels suggest that controlling NH₃ emission from agricultural sources could be an efficient way to reduce secondary inorganic particle pollution on North China Plain.