Detection and quantification of water-based aerosols using active open-path FTIR

Abstract

Aerosols have a leading role in many eco-systems and knowledge of their properties is critical for many applications. This study suggests using active Open-Path Fourier Transform Infra-Red (OP-FTIR) spectroscopy for quantifying water droplets and solutes load in the atmosphere. The OP-FTIR was used to measure water droplets, with and without solutes, in a 20 m spray tunnel. Three sets of spraying experiments generated different hydrosols clouds: (1) tap water only, (2) aqueous ammonium sulfate (0.25–3.6%wt) and (3) aqueous ethylene glycol (0.47–2.38%wt). Experiment (1) yielded a linear relationship between the shift of the extinction spectrum baseline and the water load in the line-of-sight (LOS) (R2 = 0.984). Experiment (2) also yielded a linear relationship between the integrated extinction in the range of 880–1150 cm−1 and the ammonium sulfate load in the LOS (R2 = 0.972). For the semi-volatile ethylene glycol (experiment 3), present in the gas and condense phases, quantification was much more complex and two spectral approaches were developed: (1) according to the linear relationship from the first experiment (determination error of 8%), and (2) inverse modeling (determination error of 57%). This work demonstrates the potential of the OP-FTIR for detecting clouds of water-based aerosols and for quantifying water droplets and solutes at relatively low concentrations.

Electric-Field-Induced Effects on the Dipole Moment and Vibrational Modes of the Centrosymmetric Indigo Molecule

“…Another possible application of the concepts discussed in the present work may be the characterization of the surface field on, e.g., liquid water droplets, which has been reported in the literature. , Current technology allows one to generate in a controlled way sprays of a few micron-sized electrically charged droplets, which could be investigated by Raman spectroscopy . Also IR spectroscopy was successfully used to characterize water aerosols, with dissolved molecules . Therefore, the observation of the activation in the IR of Raman-active modes of a dissolved probe molecule would be a proxy of the local field at the droplet surface, where the central symmetry of the probe molecule is broken by the surface dipoles.…”

Chemical Characterization of Aerosol Particles Using On-Chip Photonic Cavity Enhanced Spectroscopy

“…The present method shows a proof-of-concept of cavity enhanced on-chip aerosol spectroscopy. Figure 7: Absorption spectrum of the N-methyl aniline between 1460 nm to 1600 nm [10]. (b) The absorption spectrum of the N-methyl aniline aerosol obtained from the relative change of quality factor of the resonance peaks in the transmission spectrum of the racetrack resonant cavity.…”

“…Physical refers to estimating the particle size and count in the test medium, and the latter relates to the chemistry of the particles. For decades, the physical characterization of aerosol particles has been performed using free space optical and electrical methods that involve evaluation of light scattering and electrical mobility properties of the medium. , For chemical characterization, commonly used methods are based on Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, and fluorescent imaging . These techniques for chemical characterization are either are too bulky for field testing or have poor sensitivity when miniaturized to a hand-held form factor. , …”

Testing a Drop of Liquid Using Smartphone LiDAR

“…These techniques have numerous applications including measurement of cerebral blood flow and volume [41], identification and quality analysis of pharmaceutical substances [32], and polymer identification for microplastics analysis [25]. FTIR techniques have been used to measure and quantify the amount of water droplets and other solutes sprayed into the air [29]. NIRS can be used to distinguish between liquids like water, alcohol, benzene and other chemicals at volumes as low as 2.5µl in a capillary tube [4,37].…”