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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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Discussion papers
https://doi.org/10.5194/amt-2019-394
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/amt-2019-394
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 16 Oct 2019

Submitted as: research article | 16 Oct 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Measurement Techniques (AMT).

Multifactor colorimetric analysis on pH-indicator papers: an optimized approach for direct determination of ambient aerosol pH

Guo Li1, Hang Su1, Nan Ma2, Guangjie Zheng1,a, Uwe Kuhn1, Meng Li1, Thomas Klimach1, Ulrich Pöschl1, and Yafang Cheng1 Guo Li et al.
  • 1Multiphase Chemistry Department, Max Planck Institute for Chemistry, Mainz, Germany
  • 2Institute for Environmental and Climate Research, Jinan University, Guangzhou, China
  • anow at: Department of Energy, Environmental & Chemical Engineering, Center for Aerosol Science & Engineering, Washington University, St. Louis, Missouri 63130, USA

Abstract. 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) versus 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: pHpredict = a × Rnormal + b × Gnormal + c × Bnormal. 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 Nr. 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. The determined minimum sample mass (~ 180 μg) highlights its potential to predict aerosol pH with a high time resolution (e.g., ≤ 1 hour). 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 chart, we suggest an in-situ calibration of pH papers with standard pH buffers.

Guo Li et al.
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Short summary
Aerosol acidity plays an important role in regulating the chemistry, health and ecological effect of aerosol particles. However, a direct measurement of aerosol pH is very challenging because of its fast transition and equilibrium with adjacent environments. Therefore, most early studies have to use modeled pH, resulting in intensive debates about model uncertainties. Here we developed an optimized approach to measure aerosol pH by using pH indicator papers combined with RGB-based colorimetry.
Aerosol acidity plays an important role in regulating the chemistry, health and ecological...
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