The measurement of atmospheric NH3 was conducted by mean of passive samplers from September 2009 to December 2010 at Lin'an regional background station located in the economically blooming Yangtze River Delta (YRD) region in eastern China. NH4+ in fine particles was also measured in 2010 at this site. The NH3 concentration ranged from 0.1 to 41.8 ppb, with the annual average of 16.5 +/- 11.2 ppb in 2010. The daily NH4+ concentrations ranged from 0.02 to 19.2 mu g/m(3), with an annual average of 4.3 +/- 3.5 mu g/m(3). NH3 concentrations were highest in summer and lowest in winter, showing positive correlations with agricultural activities and temperature. The highest concentrations of NH4+ were in autumn coinciding with the period of active open burning of agricultural residues. The mean mass ratio of NH3/NHx is estimated to be 0.8 +/- 0.1 during 2010, indicating that NHx was mainly influenced by local sources around Lin'an. The air mass back trajectory analysis suggests that both local sources and long-distance transport played important roles in the observed ammonium aerosol at Lin'an. High NHx deposition in this regional background station suggests the urgency of reducing NH3 emission in the YRD region.