Acidity is a critical characteristic of aerosol particles that impacts many atmospheric chemical reactions. However, the measurement of aerosol acidity remains challenging because of the high ionic strength of atmospheric aerosol particles. In this study, we developed and evaluated a direct method for measuring the pH of atmospheric aerosol samples using pH testing paper. We first examined the color responses of six pH testing papers under various ionic strengths and selected the best performing paper for further testing at higher ionic strength. This paper was then tested under realistic high-ionic-strength conditions created using hygroscopic equilibrium under high and constant relative humidity (RH). A KNO₃ saturated solution was used to maintain an RH of 92% in an airtight box. Under these conditions, the hygroscopic constituents in aerosol particles form droplets of aqueous solutions that possess high ionic strength. On the basis of these results, a method for the direct pH measurement of aerosol particle samples was developed. The pH of the atmospheric aerosol samples was measured using this method under high RH to demonstrate its versatility. Our results indicate that this method requires more than approximately 12 μg per spot of fine (PM_0.2–2.0 in this study) particles to form a detectable droplet. Compared with previous methods, our direct pH measurement method is simpler, more convenient, and reliable for wide application without excess dilution in practice, and it may be used to obtain important basic information required to study atmospheric aerosols.

酸度是影响许多大气化学反应的气溶胶颗粒的关键特征。然而，由于大气气溶胶颗粒的高离子强度，气溶胶酸度的测量仍然具有挑战性。在本研究中，我们开发并评估了一种使用 pH 试纸测量大气气溶胶样品 pH 值的直接方法。我们首先检查了六种 pH 试纸在不同离子强度下的颜色响应，并选择了性能最佳的试纸在更高的离子强度下进行进一步测试。然后在使用高和恒定相对湿度 (RH) 下的吸湿平衡创建的实际高离子强度条件下对本文进行了测试。一个 KNO ₃饱和溶液用于在密闭箱中保持 92% 的 RH。在这些条件下，气溶胶颗粒中的吸湿成分形成具有高离子强度的水溶液液滴。在这些结果的基础上，开发了一种直接测量气溶胶颗粒样品 pH 值的方法。使用该方法在高 RH 下测量大气气溶胶样品的 pH 值以证明其多功能性。我们的结果表明，该方法每个细粒（PM _0.2–2.0在本研究中）粒子形成可检测的液滴。与以前的方法相比，我们的直接 pH 测量方法更简单、更方便、更可靠，在实践中无需过度稀释即可广泛应用，可用于获取研究大气气溶胶所需的重要基础信息。