Direct measurement of the acidity (pH) of ambient aerosol particles/droplets has long been a challenge for atmospheric scientists. A novel and facile method was introduced recently by Craig et al. (2018), where the pH of size-resolved aerosol droplets was directly measured by two types of pH-indicator papers (pH ranges: 0–2.5 and 2.5–4.5) combined with RGB-based colorimetric analyses using a model of G−B (G minus B) vs. pH². Given the wide pH range of ambient aerosols, we optimize the RGB-based colorimetric analysis on pH papers with a wider detection range (pH ∼ 0 to 6). Here, we propose a new model to establish the linear relationship between RGB values and pH: pH_predict = $a\cdot R_{\text{normal}}+b\cdot G_{\text{normal}}+c\cdot B_{\text{normal}}$. This model shows a wider applicability and higher accuracy than those in previous studies and is thus recommended in future RGB-based colorimetric analyses on pH papers. Moreover, we identify one type of pH paper (Hydrion^® Brilliant pH dip stiks, lot no. 3110, Sigma-Aldrich) that is more applicable for ambient aerosols in terms of its wide pH detection range (0 to 6) and strong anti-interference capacity. Custom-made impactors are used to collect lab-generated aerosols on this type of pH paper. Preliminary tests show that, with a collected particle size range of ∼ 0.4–2.2 µm, the pH paper method can be used to predict aerosol pH with an overall uncertainty ≤ 0.5 units. Based on laboratory tests, a relatively short sampling time (∼ 1 to 4 h) is speculated for pH prediction of ambient aerosols. More importantly, our design of the impactors minimizes potential influences of changed environmental conditions during pH paper photographing processes on the predicted aerosol pH. We further show that the routinely adopted way of using pH color charts to predict aerosol pH may be biased by the mismatch between the standard colors on the color charts and the real colors of investigated samples. Thus, instead of using the producer-provided color charts, we suggest an in situ calibration of pH papers with standard pH buffers.

中文翻译：

---

pH指示纸上的多因素比色分析：直接测定环境气溶胶pH值的优化方法

长期以来，直接测量环境气溶胶颗粒/液滴的酸度（pH）一直是大气科学家的一项挑战。Craig等人最近提出了一种新颖而简便的方法。（2018），通过两种类型的pH指示纸（pH范围：0–2.5和2.5–4.5）直接结合使用基于RGB的比色分析（使用G - B模型）来测量尺寸分辨气溶胶液滴的pH （G减去B）与pH ^2的关系。鉴于环境气溶胶的pH值范围很宽，我们在检测范围更广（pH约为 0至6）的pH试纸上优化了基于RGB的比色分析 。在这里，我们提出了一个新的模型来建立RGB值和pH之间的线性关系：pH值_预测 = $\mathrm{一种}\cdot {\mathrm{[R}}_{\text{正常}}+b\cdot G_{\text{正常}}+C\cdot {乙}_{\text{正常}}$。与以前的研究相比，该模型具有更广泛的适用性和更高的准确性，因此建议在以后的pH纸上基于RGB的比色分析中使用。此外，我们确定一种类型的pH试纸的（氢离子^®马斯亮pH值浸stiks，批号。3110，Sigma-Aldrich公司），其更适用于环境的气溶胶在其宽的pH检测范围的术语（0〜6）和强反干扰能力。定制的撞击器用于在这种类型的pH纸上收集实验室产生的气溶胶。初步测试表明，与一个收集粒径范围〜  0.4-2.2 微米时，pH纸方法可用于预测气雾剂pH值与总不确定度≤  0.5单位。根据实验室测试，采样时间相对较短（ 推测约1-4小时）可预测环境气溶胶的pH值。更重要的是，我们设计的撞击器可最大程度减少pH纸照相过程中环境条件变化对预计气溶胶pH的潜在影响。我们进一步表明，使用pH比色表来预测气溶胶pH值的常规方法可能会因比色表上标准颜色与所研究样品的真实颜色之间不匹配而产生偏差。因此，我们建议使用标准pH缓冲液对pH试纸进行原位校准，而不是使用生产商提供的颜色图表。