A simple model for nithttime D region under disturbed conditions that treats negative ions as a group has been developed. Good quantitative agreement is obtained between the major positive ions of the model and nighttime data for a PCA event by Narcisi et al. (1972a). Atomic oxygen, water vapor, and nitric oxide concentrations necessary to fit the data deduced. The derived [O> and [H2O> profiles are shown to be in harmony with theoretical predictions by others. The deduced [H2O> mixing ratio is 1 part per million by volume within a factor of 3 accuracy, there being some indication that the ratio is declining with altitude above about 80 km. An upper limit for [NO> of about 100 parts per billion by volume is determined, and a rough estimate for [NO2>/[NO> is given. The major three-body NO+ loss process appears to be NO++CO2+N2→NO+⋅CO2+N2. It is pointed out that the reaction O4++M→O2++O2+M may be important, especially if mesospheric temperatures are higher than normal as a result of a man-made or natural disturbance.