Seasonal variation of ammonia and ammonium aerosol at a background station in the yangtze river Delta Region, China

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 μg/m3, with an annual average of 4.3 ± 3.5 μg/m³. 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/NH_x is estimated to be 0.8 ± 0.1 during 2010, indicating that NH_x 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 NH_x deposition in this regional background station suggests the urgency of reducing NH₃ emission in the YRD region.