Typical time evolution of the average populations of certain species from MC (solid lines), HME to the 2nd order (dotted lines), and RE (dashed lines). Note that the Monte Carlo has been repeated twice. The y-axis is the number of each species in a volume containing exactly one grain. To translate it into abundance relative to H nuclei, it should be multiplied by 2.8 × 10^-12. Physical parameters used: T = 20 K, n = 2 × 10⁵ cm^-3, grain radius = 0.1 μm.