Particulate matter with aerodynamic diameter less than 2.5 micrometer (PM_2.5) has aroused a heated debate in China recently. But do you know how particulate matter comes from? In fact, one of the main contributors of particulate matter is secondary inorganic particles (e.g. (NH₄)₂SO₄, NH₄NO₃, and NH₄Cl), which is the reaction of ammonia (NH₃) and acidic species (e.g. H₂SO₄, HNO₃, HCl).

As the most abundant basic gas in the atmosphere, NH₃ mainly comes from agricultural emission sources such as domestic animals, nitrogen (N) fertilizer application and biomass burning. NH₃ is dangerous for two reasons: first, once emitted to the atmosphere, it will deposit onto earth surface in a large quantity as dry deposition; second, once caused particulate matter, it will not only be harmful for human health and air visibility but also affect climate change. All in all, the deposition of NH₃ and particulate ammonium (NH₄⁺) in natural or semi-natural ecosystems may cause eutrophication, soil acidification and loss of biodiversity.

Using passive samplers and particulate samplers, a team of researchers from China Agricultural University, the Institute of Subtropical Agriculture, Chinese Academy of Sciences (ISA),and University of Hohenheim spent 3 years to estimate NH₃ and NH4+ at four rural and two suburban sites in the North China Plain (NCP).

The researchers found that atmospheric NH₃ and particulate NH₄⁺ concentrations were very high at six agricultural sites in NCP, with annual mean concentrations of NH3 and NH₄⁺ of 15.6 and 12.4 μg m^-3, respectively. “This result reflects high NH₃ emission intensities from agriculture.” said lead researcher Dr SHEN Jianlin from ISA. “Both NH₃ and particulate NH₄⁺ concentrations showed distinct spatio-temperal variations. Compared with suburban sites, higher NH₃ and NH₄⁺ concentrations were found at rural sites due to fertilizer N induced higher NH₃ emission intensities. Moreover, for the seasonal variations, NH₃ concentrations were highest in summer and lowest in winter at both rural and suburban sites, while highest NH4+ concentrations occurred in winter and summer at all six sites.”

The researchers also found that Particulate NH₄⁺ showed significant positive correlations with particulate SO₄^2- and NO_3-at the six sites. The annual mean concentrations of secondary inorganic particles (sum of NH₄⁺, SO₄^2-and NO_3-) 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 NH_3. 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.

The research was supported by the “One Hundred Talents Program” from Chinese Academy of Sciences, the National Natural Science Foundation of China (NSFC) (41071151 and 40771188), the Innovative Group Grants from NSFC (30821003), and the Sino-German project (DFG Research Training Group, GK1070).

The study entitled “Atmospheric ammonia and particulate ammonium from agricultural sources in the North China Plain” has been published in Volume 45, Issue 28, September 2011 of Atmospheric Environment, details could be found at http://www.sciencedirect.com/science/article/pii/S1352231011001658