In Situ Aerosol Acidity Measurements Using Uv-Visible Spectrometerand And Its Application To The Ambient Air

INTRODUCTION

The atmosphere's condensed phase is made up of suspended liquid or solid particles called atmospheric aerosols, which are distinguished from cloud droplets by their significantly smaller sizes (usually less than 10 μm). The atmospheric aerosol's pH can change a particle's optical and water-uptake characteristics, promote specific chemical reactions that result in the formation of secondary aerosol (additional condensed phase mass from lower volatility species), and solubilize metals that can act as important nutrients in ecosystems with limited nutrient availability or induce oxidative stress upon inhalation.

Though aerosol acidity is important for climate and health, the size and complexity of aerosols (>99% of which are less than 1 μm) have hindered our basic understanding of pH. Within a single atmospheric particle, there can be hundreds to thousands of distinct chemical species, varying water content, high ionic strengths, and different phases (liquid, semisolid, and solid). Making aerosol analysis even more challenging, atmospheric particles are constantly evolving through heterogeneous reactions with gases and multiphase chemistry within the condensed phase. Based on these challenges, traditional pH measurements are not feasible, and, for years, indirect and proxy methods were the most common way to estimate aerosol pH, with mixed results. However, aerosol pH needs to be incorporated into climate models to accurately determine which chemical reactions are dominant in the atmosphere. Consequently, in situ measurements that probe pH in atmospherically relevant particles are sorely needed.

Recent advances in measuring aerosol acidity directly have been studied: acid-conjugate base method, polymer degradation method, ... An even simpler and low-cost approach to average (or bulk) pH aggregation for particles with a different size range down to a diameter of a few hundred nanometers is the colorimetry method integrated with a reflectance UV-Visible spectrometer (C-RUV).

METHODS

Using colorimetry coupled with a reflectance method through UV-Visible spectrometer to measure aerosol acidity (H⁺ molarity, mol/L) using the aerosol filter sample. In this method, The inorganic aerosol comprising NH4⁺-H⁺-SO₄²-H₂O was sampled on the Teflon-coated glass fiber filter dyed by metanil yellow (MY) as an indicator. The proton concentration within the aerosol may thus be measured in terms of the color change of the indicator on the filter. Because of its high sensitivity, the C-RUV method can significantly reduce sampling time. The C-RUV also requires a short analysis time because it does not require solvent extraction and uses an optical method that needs no column separation. After, the absorption spectrum of the filter sample was measured using the C-RUV technique across a VIS wavelength range from 400 to 800 nm.

The maximum of the absorption spectrum for non-protonated MY appears at a wavelength of 420 nm and for protonated MY (MYH⁺) the maximum is seen at 545 nm (Figure 1). Based on the Beer-Lambert Law, the concentration of MY on the filter is linearly related to its UV absorbance. Thus, the UV absorbance of MYH⁺ (A ∗ 545 nm) and that of the MY interacting with acids in aerosol (A ∗ 420 nm) can be used to estimate the ionization ratio of MY in the aerosol system.

Figure 1: Absorbance spectra of the acidic aerosol collected on the MY dyed filter.

Typically, to consider the non-ideality of aerosols, aerosol ion concentrations obtained by IC have been integrated with inorganic thermodynamic models to predict proton concentrations in aerosols. There are several inorganic thermodynamic models used to predict aerosol water content and proton concentration, e.g., SCAPE2 (Meng et al. 1995), ISORROPIA (Nenes et al. 1998), and E-AIM (Clegg et al. 1998). To provide reference values of proton concentrations for the C-RUV method, the E-AIM Model II was used for the aerosol system because it has been shown to give the most accurate predictions of proton concentrations. The proton concentration predicted by the E-AIM Model II method is compared with the proton concentration calculated by the C-RUV method shown in Figure 2 with a concentration ratio of [H₂SO₄]/([NH₃]+[H₂SO₄]) (FSA) are different. A good linear fit (R² = 0.95) was observed between the E-AIM Model II prediction and the C-RUV method.

Figure 2: The correlation of H+ concentrations predicted by E-AIM Model II vs. H+ concentrations measured using the C-RUV technique.

APPLICATIONS

Aerosol in situ acidity measurement is a critical technique in various fields, providing insights and data that drive research and practical applications. Here are some notable applications:

• Environmental Science: In air quality monitoring, in situ acidity measurements help monitor the presence of acidic aerosols, which can contribute to air pollution and acid rain. At the same time, understanding the acidity of aerosols aids in modeling their impact on atmospheric chemistry and climate, influencing policies and measures to address climate change in research studies. climate change research.
• Health and Medicine: Helps in assessing the health impacts of air pollution by providing data on the acidity of particulate matter (PM), which can be harmful to respiratory and cardiovascular health.
• Agriculture: Acidic aerosols can affect soil pH and plant health. In situ acidity measurements enable farmers to take corrective actions to protect crops from acid deposition, ensuring better yield and soil health.
• Atmospheric chemistry: Accurate in situ acidity data enhance the precision of atmospheric chemical transport models, which are used to predict the distribution and transformation of pollutants. Furthermore, studying the acidity of aerosols helps in understanding the reaction kinetics of atmospheric particles, providing insights into processes like secondary organic aerosol formation.
• Industrial Applications: Industries can use in situ acidity measurements to monitor and control the emissions of acidic aerosols, ensuring compliance with environmental regulations and reducing environmental impact.
• Materials Science: Measuring the acidity of aerosols can help in studying the corrosion rates of materials, leading to the development of more durable and corrosion-resistant materials. Understanding how acidic aerosols interact with different surfaces can inform the design of better coatings and materials for various applications.

Figure 3: The effects that aerosols have in the atmosphere, and on terrestrial and marine ecosystems and human health.

BUILD SYSTEM

INTINS can provide a complete system for this application. The Ocean SR4 UV-VIS spectrometer is a high-performance spectrometer with high-speed spectral acquisition and excellent signal-to-noise ratio performance for diverse applications. This small-footprint instrument unlocks UV-VIS signature data from 190-1100 nm and entrance slit options in widths of 5 µm to 200 µm. The SR4 spectrometer is compact, versatile, and compatible with Ocean Insight light sources and accessories.

The most important thing about choosing a light source for reflection is to find one with strong output over the wavelength range of interest. In this application, the light source needs to be observed in the Visible spectrum from 400 nm - 800 nm. Our product - Tungsten halogen light source - is suitable to meet these standards. Ocean Insight's HL-2000 series offers models ranging from 380nm – 2400 nm and varies from high power models (HL-2000-HP Light Source) to long life models (HL-2000-FHSA-LL Light Source and HL -2000-LL Light Source), both meet your application requirements.

Bifurcated optical fiber assemblies have two fibers at the common end and break out into two legs at the other end. The fiber type (i.e., its most efficient transmission range) used on each leg can be the same or different, depending on your application needs. UV-Visible bifurcated fibers are high OH fibers that transmit efficiently from 300-1100 nm. Bifurcated fibers are good for routing equal amounts of light from a single source to two different locations, or from a single sample to two spectrometers configured for different wavelength ranges.