Strong acidic gases such as CO2, SO2, and NO2 are harsh air pollutants with major human health threatening factors, and as such, developing new tools to monitor and to quickly sense these gases is critically required. However, it is difficult to selectively detect the acidic air pollutants with single channel material due to the similar chemistry shared by acidic molecules. In this work, three acidic gases (i.e., CO2, SO2, and NO2) are selectively discriminated using single channel material with precise moiety design. By changing the composition ratio of primary (1), secondary (2), and tertiary (3) amines of polyethylenimine (PEI) on CNT channels, unprecedented high selectivity between CO2 and SO2 is achieved. Using in situ FT-IR characterizations, the distinct adsorption phenomenon of acidic gases on each amine moiety is precisely demonstrated. Our approach is the first attempt at controlling gas adsorption selectivity of solid-state sensor via modulating chemical moiety level within the single channel material. In addition, discrimination of CO2, SO2, and NO2 with the single channel material solid-state sensor is first reported. We believe that this approach can greatly enhance air pollution tracking systems for strong acidic pollutants and thus aid future studies on selective solid-state gas sensors.