The adverse effects of acidic ultrafine particles (AUFPs) have been widely recognized in scientific communities. Two methods have previously been developed to measure AUFPs, but there are certain drawbacks. Thus, the aim of the study was to develop an easier, more rapid and more accurate measurement system for semi-automatic measurement of AUFPs in the atmosphere. The new measurement system was developed by integrating a diffusion sampler (DS) with three quartz crystal microbalances (QCM), namely QCM + DS system. The QCM detectors were coated with a nano-film of metal (metal-QCM detectors) and then placed inside the DS at three sampling spots for collection and detection of ultrafine particles (UFPs). The frequency changes obtained from the metal-QCM detectors were converted into the weights of deposited particles and used to determine the proportions of AUFPs in UFPs through the removal process of non-AUFP particles. Prior to sampling, the sensitive response of the QCM system and collection efficiencies of the QCM + DS system were calibrated using standard acidic and non-acidic particles. Reactions between the AUFPs and nano-film of metal were guaranteed by confirming much lower than one-layer deposition of particles on the detectors based on theoretical calculation and experimental results. Finally, the QCM + DS system was validated in a field measurement by comparing the results with those obtained from the previously developed method and a commercial measurement system (i.e. SMPS). All the three methods showed good agreements in measuring AUFPs and UFPs concentrations, indicating the reliability of the QCM + DS system for the quantification of ambient UFPs and AUFPs.