Aerosol acidity, defined as pH, is a critical property that influences the formation, evolution, and health and climate effects of atmospheric aerosol particles. Direct measurement of ambient aerosol pH, however, remains challenging for atmospheric scientists. Here, based on the method of colorimetric analysis on aerosol-loaded pH-indicator papers, we develop a new device that can achieve in situ and real-time measurement of the pH of ambient aerosol droplets at a fixed relative humidity of 90%. The design of this device provides a more convenient and efficient method than the pH paper methods used in previous studies by allowing automated sample switching and data recording. The size range of ambient aerosols that are collected on pH papers is determined to be ∼0.4-2.5 μm. In addition, a standardized calibration procedure is established using pH buffer standards. An intensive ambient measurement was performed with the device to examine its operational stability as well as the reliability of the measured results. The measured aerosol pH reveals good agreement with those calculated by two commonly used thermodynamic models, i.e., ISORROPIA and E-AIM. These comparisons further demonstrate that the thermodynamic models are capable of predicting aerosol pH with a reasonable range of uncertainty.