The paper presents an assessment of changes in air emissions and impacts on air quality resulting from the modernization of flue gas cleaning and evacuation in two Polish coal-fired power plants. These modernizations were associated with the construction of flue gas desulphurisation systems and use for discharging desulphurized gases separate stacks with a height of 120 m. On the basis of analyzes and atmospheric dispersion modeling, it was found that the changes in the effective height of emitters (the apparent point of emission), due to additional reducing of flue gas volume in existing chimneys, caused that important ecological effects of the reduction of air pollutant emissions did not translate into a similar reduction in their concentrations in the air.

English title: Assessment of the effects of modernization of exhaust gas treatment and evacuation systems on selected examples.