Formation of secondary inorganic aerosols by power plant emissions exhausted through cooling towers in Saxony |
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Authors: | Detlef Hinneburg Eberhard Renner Ralf Wolke |
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Institution: | (1) Leibniz Institute for Tropospheric Research, Permoserstr. 15, 04318 Leipzig, Germany |
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Abstract: | Background, aim, and scope The fraction of ambient PM10 that is due to the formation of secondary inorganic particulate sulfate and nitrate from the emissions of two large, brown-coal-fired
power stations in Saxony (East Germany) is examined. The power stations are equipped with natural-draft cooling towers. The
flue gases are directly piped into the cooling towers, thereby receiving an additionally intensified uplift. The exhausted
gas-steam mixture contains the gases CO, CO2, NO, NO2, and SO2, the directly emitted primary particles, and additionally, an excess of ‘free’ sulfate ions in water solution, which, after
the desulfurization steps, remain non-neutralized by cations. The precursor gases NO2 and SO2 are capable of forming nitric and sulfuric acid by several pathways. The acids can be neutralized by ammonia and generate
secondary particulate matter by heterogeneous condensation on preexisting particles.
Materials and methods The simulations are performed by a nested and multi-scale application of the online-coupled model system LM-MUSCAT. The Local
Model (LM; recently renamed as COSMO) of the German Weather Service performs the meteorological processes, while the Multi-scale
Atmospheric Transport Model (MUSCAT) includes the transport, the gas phase chemistry, as well as the aerosol chemistry (thermodynamic
ammonium–sulfate–nitrate–water system). The highest horizontal resolution in the inner region of Saxony is 0.7 km. One summer
and one winter episode, each realizing 5 weeks of the year 2002, are simulated twice, with the cooling tower emissions switched
on and off, respectively. This procedure serves to identify the direct and indirect influences of the single plumes on the
formation and distribution of the secondary inorganic aerosols.
Results and conclusions Surface traces of the individual tower plumes can be located and distinguished, especially in the well-mixed boundary layer
in daytime. At night, the plumes are decoupled from the surface. In no case does the resulting contribution of the cooling
tower emissions to PM10 significantly exceed 15 μgm−3 at the surface. These extreme values are obtained in narrow plumes on intensive summer conditions, whereas different situations
with lower turbulence (night, winter) remain below this value. About 90% of the PM10 concentrations in the plumes are secondarily formed sulfate, mainly ammonium sulfate, and about 10% originate from the primarily
emitted particles. Under the assumptions made, ammonium nitrate plays a rather marginal role.
Recommendations and perspectives The analyzed results depend on the specific emission data of power plants with flue gas emissions piped through the cooling
towers. The emitted fraction of ‘free’ sulfate ions remaining in excess after the desulfurization steps plays an important
role at the formation of secondary aerosols and therefore has to be measured carefully. |
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Keywords: | Air pollution Cooling towers Plume rise PM10 Power plant emissions Secondary aerosols Sulfate aerosol |
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