Measurement of ambient aerosol and nitrogen dioxide (NO2) is important as they are major pollutants from the burning of fossil fuel and biomass. In the present work, a differential photoacoustic spectrometer (D-PAS) was developed for simultaneous, online measurements of aerosol optical absorption and NO2 concentration. A novel photoacoustic resonator was designed and employed in the D-PAS for controlling a large flow rate, improving response time, and keeping the flow noise at a low level. The detection limits of 1.0 Mm−1 and 0.87 ppb for aerosol absorption and NO2 concentration measurements were achieved with a lock-in amplifier time constant of 1 s. The D-PAS accuracy was demonstrated by performing a long-time, continuous measurement of aerosol, and NO2 in ambient air. The measured results of NO2 are consistent with the NOx analyzer and environmental monitoring station results.