Translucent gas conditions in all models (see Table 3). Upper left: pure gas phase chemistry. Upper right: gas and surface chemistry and inactive non-thermal desorption efficiency a_RRK = 0. Lower left: gas and surface chemistry and active non-thermal desorption with the typical efficiency a_RRK = 0.01. Lower right: gas and surface chemistry and high active non-thermal desorption (a_RRK = 0.1). The observed abundances and upper limits are indicated with solid and dashed horizontal lines, respectively, following the respective species colour code. The dot-dashed and dotted lines for NH represent the estimated X(NH for the T_K = 30 K and 50 K models. If the reactive desorption mechanism is active with the typical a_RRK = 0.01, each NH, NH₂ and NH₃ species that is formed on the grain through a hydrogenation reaction has a probability of 9.3 × 10^-3, 7.6 × 10^-3, and 5.2 × 10^-3, respectively, to desorb into the gas phase. There it will become available for detection and for follow-up reactions. Experiments by Dulieu et al. (2013) indicate that this type of non-thermal desorption could be much more efficient on bare grains than a_RRK = 0.01. As shown, our models are not very sensitive to the exact value of the desorption probability, since the model with a_RRK = 0.1 gives very similar results to the model using a_RRK = 0.01.