Aerosol Acidity Measurement Using Colorimetry Coupled With a Reflectance UV-Visible Spectrometer

摘要

In this study, aerosol acidity was measured using colorimetry integrated with a Reflectance UV-Visible spectrometer (C-RUV). An inorganic aerosol comprising NH₄ +−H+−SO₄ 2−−H₂O was generated using an atomizer and introduced into a 2-m3 indoor Teflon film chamber. The produced aerosol was collected on the Teflon-coated glass fiber filter dyed with metanil yellow (MY) as an indicator for measuring proton concentrations in aerosol. A calibration curve for measuring aerosol acidity using the C-RUV technique was obtained through the relationship between the absorbance of the UV-Visible spectrum of the filter sample vs. theoretically calculated proton concentrations using the E-AIM Model II. To develop the C-RUV method under various humidities, the aerosol filter sample was mounted inside a small optical flow chamber controlled for humidity in situ. The humidity effect on the equilibrium thermodynamics of the indicator was theoretically described by inclusion of excess acidity (X) into the calibration curve. The calibration curve obtained from relatively highly acidic aerosols (e.g., H₂SO₄, NH₄H₃(SO₄)₂, and (NH₄)₇H₁₃(SO₄)₁₀) was extrapolated to estimate proton concentrations for weakly acidic aerosols (e.g., NH₄HSO₄ and (NH₄)₃H(SO₄)₂), which is more relevant to the ambient aerosol acidity but has been poorly predicted with typical inorganic thermodynamic models due to the limited experimental data. The C-RUV technique of this study permits one to estimate aerosol acidity for a variety of compositions of the NH₄ +−H+−SO₄ 2−−H₂O system including both ammonia-poor and ammonia-rich sulfate aerosols.