Atmospheric ammonia and particulate ammonium from agricultural sources in the North China Plain

Atmospheric ammonia (NH3) and particulate ammonium (NH4+) were measured using passive samplers and particulate samplers at four rural and two suburban sites in the North China Plain (NCP) from August 2006 to September 2009. The annual mean concentrations of NH3 and NH4+ in PM10 across the sites were 15.6 and 12.4 μg m−3, respectively. Higher concentrations of NH3 and NH4+ were found at the rural sites compared with the suburban sites. NH3 concentrations were highest in summer and lowest in winter, showing positive correlations with N fertilizer application rates and temperature in each season. Particulate NH4+ showed significant positive correlations with particulate SO42− and NO3− at the six sites. The annual mean concentrations of secondary inorganic particles (sum of NH4+, SO42− and NO3−) accounted for 26.1–41.3% of the PM10 concentration across all sites. The formation of inorganic NH4+ particles was limited mainly by the availability of H2SO4 and HNO3 and partly by NH3. The total NHx deposition in the NCP averaged 43.5 kg N ha−1 yr−1, accounting for 84.1% of the mean NH3 emission intensity. The serious particulate NH4+ pollution and high NHx deposition suggest the urgency of reducing NH3 emission from agricultural sources in the NCP.