The object under study is a long plasma jet (up to 15 cm) generated by a steady-state dc discharge in N2 at atmospheric pressure. Successful usage of long afterglow plasma jet for distant treatment of 3-D object of a complicated shape with deep slots was demonstrated. This paper focuses on a role of trace impurities in plasma-forming gas on chemical composition of reactive species in far afterglow plasma. This role was studied numerically by example of the additives in high-purity nitrogen contained in high-pressure N2 tank. It is shown that small oxygen admixtures (at 10 ppm level) to N2 can be more effective in production of atomic oxygen than ambient air. Besides, trace admixtures of water vapor and hydrocarbons (at parts per million level) lead to full change in a composition of reactive species in far afterglow in comparison with the set of primary species generated by plasma source.