Spatial–temporal patterns of inorganic nitrogen air concentrations and deposition in eastern China

Abstract. Five-year (2011–2015) measurements of gaseous NH₃, NO₂, and HNO₃ and particulate NH₄⁺ and NO₃⁻ in air and/or precipitation were conducted at 27 sites in the Nationwide Nitrogen Deposition Monitoring Network (NNDMN) to better understand spatial and temporal (seasonal and annual) characteristics of reactive nitrogen (N_r) concentrations and deposition in eastern China. Our observations reveal annual average concentrations (16.4–32.6 µg N m⁻³), dry deposition fluxes (15.8–31.7 kg N ha⁻¹ yr⁻¹), and wet/bulk deposition fluxes (18.4–28.0 kg N ha⁻¹ yr⁻¹) based on land use, ranked as urban > rural > background sites. Annual concentrations and dry deposition fluxes of each N_r species in air were comparable at urban and background sites in northern and southern regions, but were significantly higher at northern rural sites. These results, together with good agreement between spatial distributions of NH₃ and NO₂ concentrations determined from ground measurements and satellite observations, demonstrate that atmospheric N_r pollution is heavier in the northern region than in the southern region. No significant inter-annual trends were found in the annual N_r dry and wet/bulk N deposition at almost all of the selected sites. A lack of significant changes in annual averages between the 2013–2015 and 2011–2012 periods for all land use types suggests that any effects of current emission controls are not yet apparent in N_r pollution and deposition in the region. Ambient concentrations of total N_r exhibited non-significant seasonal variation at all land use types, although significant seasonal variations were found for individual N_r species (e.g. NH₃, NO₂, and pNO₃⁻) in most cases. In contrast, dry deposition of total N_r exhibited a consistent and significant seasonal variation at all land use types, with the highest fluxes in summer and the lowest in winter. Based on sensitivity tests by the GEOS-Chem model, we found that NH₃ emissions from fertilizer use (including chemical and organic fertilizers) were the largest contributor (36 %) to total inorganic N_r deposition over eastern China. Our results not only improve the understanding of spatial–temporal variations of N_r concentrations and deposition in this pollution hotspot, but also provide useful information for policy-makers that mitigation of NH₃ emissions should be a priority to tackle serious N deposition in eastern China.