A measure of the specific rate of disappearance of small ions due to either 1) union with neutral Aitken nuclei to form new large ions; or 2) union with large ions of opposite sign to form neutral Aitken nuclei. Dimensionally, a combination coefficient is identical with the physically similar recombination coefficient. Both types of combination coefficients exhibit mean values of the order of 10−5cm3s−1 at sea level, while the recombination coefficients are typically about one order of magnitude smaller. See recombination, small-ion combination.