Project No: 21201214

Title: Black carbon in urban atmosphere: size and coating from near-source to ambient environment

PI: Prof. Ning, Zhi


Black carbon (BC) particles are major components of emissions from fossil fuel combustion and biomass burning. It is a strong light absorbing material in the atmosphere, changing earth’s radiative balance and affecting the climate and visibility. Locally in Hong Kong and Southern China, black carbon has been linked with summer monsoon patterns impacting the regional climate. BC is also a component of diesel exhaust emissions, recently classified by World Health Organization as a Category I Carcinogen, and has been often mixed with other toxic constituents, such as polycyclic aromatic hydrocarbons (PAHs), and associated with various adverse health effects. Large fleets of diesel vehicles locally in Hong Kong contribute to 50% of total mileage and produce black carbon in highly packed urban environment with large population exposed to the roadside pollutants. Atmospheric BC can be readily modified in the atmosphere causing changes in their physical size and chemical nature of mixing materials, which affects their lung deposition regions and their ability to absorb solar radiation. Although there have been laboratory and modelling investigations in the past, there exist very few real-world investigations on the topics especially in urban areas, and the lack of such knowledge also considerably hinders efforts to assess their impact on local air quality, regional and global climate forcing and on human health.

The objectives of the proposed work are to: (1). Investigate the evolution and dynamics of BC size distribution in urban environments and the factors that influence them in near-source and well mixed air over diurnal and seasonal time scales; (2). Determine the chemical characteristics of BC mixing and coating as functions of the particle size in fresh, mixed and aged aerosols in urban environment; (3). Explore the diurnal and seasonal pattern of particle bound surface carcinogenic PAHs as coating on particles and its impact on black carbon mixing state; and (4). Investigate the impact of chemical properties of soot particle coating on the light absorption properties of fresh, mixed and aged aerosols.

The proposed work will integrate traditional filter based approaches for chemical characterization of black carbon mixture, and new approaches of continuous BC size measurement developed by our group to determine the dynamic evolution of BC in the atmosphere. The complementary approaches will be implemented in typical urban atmosphere from fresh to mixed and aged aerosols with diurnal and seasonal variations. Controlled parametric investigation by thermally removing semi-volatile particle coating will also be applied for direct comparative study on the physicochemical nature of aerosol coating. The proposed study builds upon the application of existing knowledge and technologies, and also represents a major effort on the advancement of new scientific approach pioneered by our group, which allows the determination of black carbon size and aerosol coating in temporal and spatial scales in greater detail than have been possible in prior investigations.

With the successful implementation of the proposed work, we will depict a clearer picture of the evolution of black carbon size and the physicochemical nature of the particle coatings during their transformation processes in urban atmosphere. The results from the work will provide more refined inputs of black carbon in global and regional climate models to shorten the gap between observed and simulated output for better climate prediction. A better understanding and estimation of exposure to black carbon and PAHs by the population in roadside and urban environment in Hong Kong and other cities is also expected. What we achieve and learn both from observations and methodical improvements in the proposed work in Hong Kong can be applied to address these issues regionally and on a much larger scale.