Nucleation and Growth of Atmospheric Nanoparticles at Molecular Scale
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Atmospheric aerosols are fine liquid droplets or solid particles of various chemical compositions suspended in the air. They influence the Earth radiation budget, impact cloud formation, cause or enhance diseases on humans, and change photochemical chemistry and partitioning of trace gas species. Atmospheric aerosols are classified into two categories, primary and secondary, on the basis of their formation mechanisms. Although a large portion of atmospheric aerosols is secondary, the mechanisms for secondary aerosol formation remain highly uncertain, preventing the development of physically based representations of their formation in atmospheric models. So far it is known that secondary aerosol formation consists two consecutive steps, nucleation to form critical nucleus and subsequent growth of freshly nucleated nanoparticles. Unfortunately, our current knowledge of these two steps is very limited. In the current study, the dicarboxylic acids (organic acid) assisted nucleation is investigated both experimentally and theoretically. First, nucleation and partitioning theories are presented as the theoretical framework for data analysis and explanation. Subsequently, quantum chemistry calculations are performed to evaluate the hydrogen bonding strength of dicarboxylic acids with common atmospheric nucleation precursors, including sulfuric acid, water, ammonia, and amines. Then, succinic acid (dicarboxylic acid) assisted nucleation experiment is carried out to assess the nucleation enhancement ability of dicarboxylic acids. Next, the growth contributions from epoxides vapors are determined using a combination of Nano-tandem differential mobility analyzer (n-TDMA) and thermo desorption ion drift chemical ionization mass spectrometer (TD-ID- CIMS). Finally, the hygroscopicity and CCN properties of atmospheric polymers are characterized. Our results show that dicarboxylic acids bind strongly with sulfuric acid and enhance nucleation rate by 5-13 times with a concentration of 1 ppb. Dicarboxylic acids also react with amines under hydration to form non-volatile aminium carboxylate ion pairs, which contribute to nanoparticles growth. The n-TDMA and TD-ID-CIMS results show that epoxides contribute to freshly nucleated nanoparticle (sulfuric acid nanoparticles) growth through forming non-volatile organosulfates and oligomers, which subsequently changes the cloud-forming properties of aerosols.