Source investigation and mass spectrometric characterization of humic-like substances in atmospheric aerosols

Abstract

Humic-like substances (HULIS) is a class of unresolved organic compounds in atmospheric aerosols. HULIS is operationally defined by its isolation procedure in this work to be the hydrophobic part of the water-soluble organic carbon (WSOC) fraction. HULIS account for a significant fraction of PM_2.5. This thesis work aims to improve the scientific understanding of HULIS in PM_2.5, focusing on source apportionment and molecular characterization of HULIS in the atmosphere of the Pearl River Delta (PRD), China. The main findings are summarized below: Positive matrix factorization (PMF) analyses were used to investigate sources of HULIS carbon (HULIS-C) at three sites in the Pearl River Delta (PRD): an urban site Guangzhou (GZ) and a suburban site Nansha (NS) in the PRD, and an urban site Tsuen Wan (TW) in Hong Kong, utilizing PM_2.5 chemical composition and source markers data. HULIS-C sources were found to include secondary formation processes, biomass burning (BB), coal combustion and ship emission which was identified for the first time as a significant source of HULIS-C. Secondary formation processes contributed 42-79% (seasonal average) of ambient HULIS-C in different seasons. BB emissions contributed 6-27% (seasonal average), with the highest percentages in winter. Coal combustion could be apportioned at GZ and NS, contributing 6-20% (seasonal average). Ship emission was revealed to be an important source in summer (38% of HULIS-C at NS and 26% at TW). Vehicle emissions were found to contribute little to HULIS-C. Sources of HULIS were also investigated using radiocarbon (¹⁴C) analysis for unambiguous differentiation of fossil and contemporary carbon origins. A set of 8 samples in winter and 9 samples in summer was selected from the 184 PM_2.5 samples based on PMF results. The contemporary and fossil fractions of primary and secondary HULIS-C were calculated by coupling ¹⁴C analysis and PMF results. On average, fossil carbon sources contributed 60±15% to HULIS-C, and contemporary carbon sources contributed 40±15%. In the winter samples, 81% of fossil source HULIS-C was borne from secondary processing, which is hypothesized as the precursors from fossil fuel burning transported in by the northerly wind in winter. In the summer samples, 87% of contemporary HULIS-C was borne from secondary processing mostly as biogenic secondary organic aerosol. This work highlights the importance of both anthropogenic and biogenic precursor emissions for HULIS in PM_2.5 in PRD. The molecular composition of HULIS was characterized using Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS), an ultrahigh resolution mass spectrometry (UHRMS). Based on the accurate mass measurements obtained with UHRMS, molecular formulas were assigned and potential compound groups are identified. HULIS from two BB source samples (rice straw and sugarcane leaves) and ten GZ and NS ambient samples were analyzed. CHOS- and CHONS- formulas (possibly organosulfates (OS) and nitrooxy-OS) dominated the ambient samples, while CHO- formulas dominated BB source samples. CHN+ formulas (possibly alkaloids) were abundant in rice straw burning. Ambient samples had significantly fewer CHN+ formulas. The differences between the ambient and BB source samples highlight the importance of secondary formation of HULIS. OS homologous series of different DBE values were discovered, indicating OS precursors from saturated alkanes, biogenic VOCs and anthropogenic aromatic VOCs. Half of CHN+ formulas in ambient samples were also found in rice straw burning, indicating biomass burning could be a source contributing to this group of compounds in ambient samples. Size distribution of CHOS- and CHONS- (0.056-18 μm) in HULIS was investigated to provide hints of formation pathways of OS. Size-segregated samples were analyzed by the Orbitrap MS (also an UHRMS) coupled with negative ESI. OS derived from biogenic volatile organic compounds (e.g. isoprene, α-/β-pinene, limonene) share a common characteristic with sulfate in that the droplet mode dominated, peaking in either the 0.56-1.0 or 1.0-1.8 μm size bin, reflecting sulfate as their common precursor. Most of these OSs have a minor coarse mode, accounting for 0-45%. The presence of OSs on coarse particles is hypothesized to be a result of OSs on small particle (<0.32 μm) coagulating with coarse particles. Comparison of formulas identified by FT-ICR and Orbitrap MS was conducted by analyzing BB HULIS samples and a GZ ambient HULIS sample on both instruments. Fewer formulas were detected by Orbitrap than FT-ICR for the GZ ambient sample. It was revealed that un-optimized mass detection sensitivity and inability to differentiate multiple formulas as the causes for the smaller number of formulas obtained with Orbitrap.

Date of Award	2017
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology