Characterization and tracer-based source apportionment of PM₂.₅ and organic carbon with hourly time-scale air quality considerations

Abstract

Atmospheric fine particulate matter (PM_2.5) plays an important role in the global climate and adversely affects human health. Identifying and quantifying the contributions of various sources are crucial to develop more targeted control strategies of PM pollution. Compared with the traditional offline source apportionment (e.g., positive matrix factorization, PMF), the combination of online instruments with the organic tracer-based source apportionment achieves the identification of individual organic dominant sources down to hourly time scale, providing a more precise source profile of PM_2.5. This thesis work presents comprehensive field measurements of ambient PM_2.5, its major chemical components, and molecular organic tracers by deploying an array of online instruments. Work in this thesis aims to investigate the characteristics of aerosol PM_2.5 and their potential sources based on elemental and organic molecular markers on hourly basis. This thesis consists of four major parts: (1) A case study was conducted to comparatively evaluate PM_2.5 components and sources in suburban Shanghai before and during the outbreak of COVID-19 by using an array of online instruments. Fourteen PMF factors were resolved, including nine primary sources (i.e., firework burning, industrial emission, coal combustions, residual oil combustions, cooking emission, PAHs-rich, biomass burning, dust, vehicle emission), and five secondary sources (i.e., sulfate-rich, nitrate-rich, SOA_I, SOA_II and SOA_III). We observed the decreased secondary inorganic and organic formation despite the overall increasing oxidation capacity in suburban sites, which was contrary to those observed in megacities. Cross-comparison in organic carbon (OC) source apportionment in this study with that in urban Shanghai revealed the complex interaction of SOA formation with precursor reduction, oxidation capacity and meteorological parameters. (2) Wintertime PM_2.5 and OC sources in suburban Hong Kong during the winter of 2020 were examined based on hourly chemical measurements covering major chemical components, organic molecular and elemental source tracers. Seven primary sources (i.e., firework burning, industrial & coal combustion, sea salt, residual oil combustion, cooking emission, biomass burning, dust & vehicle emission) and six secondary sources (i.e., sulfate-rich, nitrate-rich, β-CA & Naphthalene derived SOA, toluene derived SOA, biogenic SOA and secondary biomass burning) were resolved by PMF model. Variation in source contributions under different air masses revealed the Pearl River Delta region and coastal areas to the north are main origins. We found that secondary sources (e.g., sulfate-rich, nitrate-rich, etc) were the dominant contributor to PM_2.5 whereas OC was more associated with primary sources. The hour-by-hour examination of most polluted and firework event hours enhanced understanding on dynamic variations of haze and firework burning evolution. (3) Springtime PM_2.5 and OC in suburban Hong Kong were characterized, including individual precursor-specific SOA molecular tracers derived from various sources including biomass burning, anthropogenic (i.e., monoaromatics, naphthalene/methylnaphthalenes) and biogenic VOCs (i.e., isoprene, α-pinene and β-caryophyllene). An in-depth analysis was conducted on four city-scale pollution episodes characterized by high levels of air pollutants of PM_2.5, PM₁₀, and O₃. We have clearly identified distinct characteristics among the four episodes in chemical evolution of individual precursor-specific SOA molecular tracers and in major sources contributing to rising in PM_2.5 levels. (4) A recently developed novel Bayesian Inference approach was applied to estimate primary organic carbon (POC) and secondary organic carbon (SOC) based on measurement data of PM_2.5 major components (i.e., OC, elemental carbon, and major secondary inorganic ions) in suburban Hong Kong from 16 July 2020 to 31 December 2021. The simulation results showed good agreement with elemental and organic tracer based-PMF results. The characteristics of carbonaceous aerosols including multiple temporal variations and major influencing factors under varied PM pollution conditions were discussed in detail. A total of 65 city-wide PM_2.5 episodes were identified, with SOC contributions varying from 10-66%. The case studies of summer typhoon episodes and winter haze episodes revealed different formation processes of SOC.

Date of Award	2024
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	Jianzhen YU (Supervisor)