Characterization of Water Soluble Organic Aerosols: Looking for the molecular identity of HULIS

Mukherjee, Arya (2019) Characterization of Water Soluble Organic Aerosols: Looking for the molecular identity of HULIS. Masters thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

A class of organic molecules extracted from atmospheric aerosol particles and isolated from fog and cloud water has been termed HUmic-LIke Substances (HULIS) due to a certain resemblance to terrestrial and aquatic humic and fulvic acids. They play a major role in light absorption, cloud formation and restricting the photochemistry in the troposphere. While sharing some important features such as polyacidic nature, accumulating evidence suggests that atmospheric HULIS differ substantially from terrestrial and aquatic humic substances. Major differences between HULIS and humic substances, including smaller average molecular weight, lower aromatic moiety content, greater surface activity, better droplet activation ability, as well as others, have been highlighted in existing literature. Water-soluble organic matters (WSOM) were extracted from the total suspended particulate matters collected at Mohanpur, India during summer, post-monsoon and winter. Two fractions of humic-like substances, showing neutral and acidic behaviour (HULIS-n and HULIS-a, respectively), and the remaining highly polar part (HPWSOM) were fractionated from WSOM using solid phase extraction. The optical properties and chemical structural characteristics of the organic matters were investigated using UV-vis and Fluorescence spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, Nuclear Magnetic Resonance (NMR) spectroscopy and liquid chromatography-mass spectrometry. The optical studies show that (1) WSOM absorbs light at UV and near UV visible wavelengths (250 -400 nm) and HULIS-n is the fraction containing most of the light absorbing compounds. The HP-WSOM fractions absorb light below 250 nm. (2) Winter samples show higher light absorbance compared to the other seasons, suggesting presence of strong fluorophores in particulate matter probably originating from local biomass burning emissions. The FT-IR, NMR and LC-MS studies indicate that (1) HULIS-n was abundant in aliphatic and small aromatic structures and hydroxyl groups (primarily C10−C12) and by branched structures; (2) HULIS-a and HP-WSOM contained relatively large amounts of low-molecular-weight carboxylic acids and alcohols (primarily C4−C10); and (3) WSOM and subsequent HULIS fractions were relatively abundant in amines and amides. (4) Winter samples contain more aromatic species than the rest of the seasons, which supports the data obtained from optical studies and points toward the contribution of local biomass burning. This comprehensive study on atmospheric HULIS is the first of its kind in India.

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