Role of ambient ammonia in particulate ammonium formation at a rural site in the North China Plain

The real-time measurements of NH 3 and trace gases were conducted, in conjunction with semi-continuous measurements of water-soluble ions in PM 2.5 at a rural site in the North China Plain (NCP) from May to September 2013 in order to better understand chemical characteristics of ammonia and the impact of secondary ammonium aerosols on formation in the NCP. Extremely high NH 3 and NH 4 + concentrations were observed after a precipitation event within 7–10 days following urea application. Elevated NH 3 levels coincided with elevated NH 4 + , indicating that NH 3 likely influenced particulate ammonium mass. For the sampling period, the average conversion ∕ oxidation ratios for NH 4 + (NHR), SO 4 2− (SOR), and NO 3 − (NOR) were estimated to be 0.30, 0.64, and 0.24, respectively. The increased NH 3 concentrations, mainly from agricultural activities and regional transport, coincided with the prevailing meteorological conditions. The high NH 3 level with NHR of about 0.30 indicates that the emission of NH 3 in the NCP is much higher than needed for aerosol acid neutralisation, and NH 3 plays an important role in the formation of secondary aerosols as a key neutraliser. The hourly data obtained were used to investigate gas–aerosol partitioning characteristics using the thermodynamic equilibrium model ISORROPIA-II. Modelled SO 4 2− , NO 3 − , and NH 3 values agree well with the measurements, while the modelled NH 4 + values largely underestimate the measurements. Our observation and modelling results indicate that strong acids in aerosol are completely neutralised. Additional NH 4 + exists in aerosol, probably a result of the presence of a substantial amount of oxalic and other diacids.