This paper describes atmospheric pressure-ion drift chemical ionization mass spectrometry (AP-ID-CIMS) for monitoring of ambient trace species. Operation of the drift tube at atmospheric pressure allows a significantly longer ion-molecule reaction time and eliminates dilution of the ambient samples, while a well-defined electric field inside the drift tube provides the benefits to confine the flight path and velocity of reagent/product ions, to break down ion clusters, and to control the ion-molecule reaction time. The AP-ID-CIMS exhibits advantages over the conventional low pressure ID-CIMS and flow tube AP-CIMS, improving the detection sensitivity by 3 orders of magnitude and a factor of 3, respectively. We demonstrate that the AP-ID-CIMS allows quantification of sulfuric acid concentrations and is capable of detecting gaseous sulfuric acid with a detection limit of less than 10(5) molecules cm(-3), on the basis of 3sigma of the baseline noise and an integration time of 12 s. A field evaluation of the AP-ID-CIMS is presented for ambient H(2)SO(4) measurements.