Disclosed is a method and system for reducing the nitrogen oxide off-gas concentration in a nitric acid plant operated under pressure and equipped with a residual gas purification first reactor configured to remove nitrogen oxides from the off-gas during steady-state operation of the plant, and a second reactor configured to remove nitrogen oxides from the off-gas during a start-up and/or shut down of the plant. The method includes, during start-up and/or shut-down of the nitric acid plant, passing pressurized nitrogen-oxide-containing off-gas from the nitric acid plant and a gaseous reducing agent for the nitrogen oxides into the second reactor charged with a catalyst, to reduce the NOx content in the off-gas by at least catalytic reduction. Using the process and system, a colorless start-up and shut-down of nitric acid plants is possible and the nitrogen oxide content in the off-gas during start-up and/or shutdown can be substantially lowered.