Seasonal variations and interactions of N-containing gases and particles over a coniferous forest, Saxony, Germany

Ambient concentrations of NH3, HNO3, HNO2, HCl, SO2, particulate NH4+, NO3-, and SO42- were simultaneously measured from October 2001 to April 2003 at the Oberbärenburg research site (Erzgebirge, Germany). Seasonal variations of gas phase concentrations were often related to meteorological parameters and specific source strength. The particulate species (PM2.5) displayed weak seasonal trends, but periods with low mixing heights resulted in significantly elevated concentrations.Mean NH3, HNO2, HNO3, and SO2 concentrations in air (μg m−3) were about 0.48, 0.30, 0.80, and 5.9, respectively. With a PM2.5-mass fraction of 11.3 μg m−3, the Erzgebirge was moderately polluted. Average particle bound NH4+- (1.5 μg m−3) and NO3-- (2.2 μg m−3) concentrations were 2–3-times higher than the corresponding gas phase concentrations. In summer, preferentially (NH4)2SO4 existed, while the dominance of NH4NO3-aerosol increased in spring. The thermodynamic equilibrium constant for the system NH3/HNO3/NH4NO3 agrees well with the measured concentration product considering the co-existence of SO42-. The particulate phase is often favoured under ambient temperature and humidity.Nearly all species exhibited maximum concentrations from February to April 2003, during a stable high-pressure system. In March and April 2003, the secondary ions contributed more than 90% to the PM2.5 fraction. This was particularly due to increased NO3-- concentrations. The formation of NO3- was supported by a large supply of free NH3.