Ambient aerosol pH during the CleanCloud PIANO campaign inferred from thermodynamic analysis and spectroscopic pH sensors

The pH of atmospheric aerosols plays a central role in multiphase chemistry, secondary aerosol formation, gas–particle partitioning, and aerosol toxicity among other processes. Despite its importance, ambient aerosol pH remains poorly constrained, as most estimates rely on indirect thermodynamic inferences from bulk aerosol composition, which induces uncertainties associated with equilibrium assumptions and measurement limitations. On the other hand, pH-responsive substrates can provide direct aerosol pH measurements, fast and reliably. Most of these materials are sensitive to protonation that can be quantified by spectroscopic techniques. In this work, ambient aerosol acidity during the CleanCloud PIANO field campaign in Summer 2025 to Spring 2026 was investigated by combining thermodynamic pH inference with direct spectroscopic measurements using pH-responsive sensors.

The field campaign commenced in summer 2025 and is ongoing, with completion expected in spring 2026, in Patras, Greece. Daily aerosol samples (PM_2.5) were collected on quartz filters using a high-volume sampler. Two different pH-responsive substrates were mounted on top of each filter. The first, consisted of polymer-based sensors made from phase-inverted polybenzimidazole (PBI) membranes, whose protonation response was analyzed by Raman spectroscopy. The second substrate employed the low-molecular-weight imidazole probe 2-mercaptobenzimidazole (2-MBI), applied on the filters and analyzed using surface-enhanced Raman spectroscopy (SERS). In both cases, aerosol pH was quantified using laboratory-derived calibration curves, providing a daily pH over the sampling period. Additionally, the other half of the filter was used for offline chemical analysis.

Real time measurements were conducted using an aerosol mass spectrometer (AMS), an ammonia (NH₃) monitor, and a VOCUS chemical ionization time-of-flight mass spectrometer (VOCUS-CI-TOF) to characterize the gas and particle phases. Thermodynamic aerosol pH was inferred with ISORROPIA-lite using 30-min averaged AMS inorganic composition (SO₄²⁻, NO₃⁻, NH₄⁺, Cl⁻), relative humidity, and temperature. Calculations were performed in forward mode under the metastable aerosol assumption, and the resulting pH was aggregated to 24-h averages for comparison with the substrate-based spectroscopic measurements. The spectroscopic and thermodynamic pH estimates show consistent temporal behavior and comparable acidity levels.

This combined observational framework provides complementary and independent constraints on ambient aerosol acidity with diverse techniques. It also demonstrates the potential of Raman-based pH sensors deployed on common aerosol samplers to augment thermodynamic pH estimates in field studies.

This work was supported by the CleanCloud project funded by the EC Horizon Europe Call “Improved knowledge in cloud-aerosol interaction” (HORIZON-CL5-2023-D1-01-04).