Formation and Impacts of Atmospheric Aerosols

Abstract

In this study, research has been conducted to investigate the formation and impacts of aerosols in the atmosphere. Experimental studies were performed to elucidate the role of organic functionality in the nucleation and growth of nanoparticles. The experiments were performed by utilizing a flow reactor and different organic acids to represent the distinct functionality of organic molecules, i.e., from monocarboxylic, dicarboxylic, to tricarboxylic acids and with/without the presence of hydroxyl functional groups and methyl functional groups. Gaseous species in the flow reactor were monitored by ion-drift chemical ionization mass spectrometry, aerosol size and number concentration after the flow reactor were measured by differential mobility analyzer, condensation particle counter, and particle size magnifier, and the chemical composition of nanoparticles was analyzed using thermal desorption ion-drift chemical ionization mass spectrometry. To corroborate with the laboratory measurements, theoretical calculations were performed to determine the bonding energies of the organic acid–H₂SO₄ dimers and to estimate the structures of the critical nuclei of the water-H₂SO₄-organic acid nucleation systems. Both experimental and theoretical results demonstrate that the impacts of the organic species on new particle formation are strongly dependent on the functionality of the organic molecules. Larger carboxylic functional acids (i.e., dicarboxylic and tricarboxylic acids) exhibit enhanced aerosol nucleation, because of the formation of strong hydrogen bonding with H₂SO₄. However, organic acids without the presence of hydroxyl functional groups do not facilitate the growth of newly formed nanoparticles. Organic acids with the hydroxyl functional groups enhance nucleation and growth of nanoparticles, because of increased intermolecular hydrogen bonding interactions. Finally, an animal model was used to investigate the effects of prenatal exposure to ultrafine particulate matter (smaller than 0.1 μm or PM₂.₅) on two common strains of mice. The time-mated mice were housed in a particle filtered air chamber (clean) and an exposure chamber (polluted) with added aerosols for 6 hours a day from gestation day 0 to 18. The aerosol mass concentration of 100 μg m⁻³ were used for daily exposure of 6 hours, corresponding to a 24-hour average PM₂.₅ concentration of 25 μg m⁻³ (the recommended limit for PM₂.₅ by the World Health Organization). Following birth, the offspring of both clean and polluted chambers were challenged with either phosphate buffered saline (PBS) or house dust mite (HDM) in PBS, from 0 to 4 weeks of age, and 72 hours after the final dose they were euthanized on postnatal day 31. Mice exposed in utero to PM₂.₅ and challenged with HDM did not respond with as robust airway inflammation as clean air exposed mice challenged by HDM, indicating an immunosuppressive effect in the lungs. The results reveal that in utero ultrafine PM exposure increases infant susceptibility to respiratory infection and impacts overall long-term pulmonary health.