A modelling study of multiphase chemistry in cloud droplets and deliquescent particles in China

PolyU Library Call No.: [THS] LG51 .H577P CEE 2015 Guo153 pages :color illustrations ;30 cmHigh concentrations of particulate matters and associated heavy haze have become a serious environmental issue in many urban areas in China. Interactions of gases, particles and cloud/fog play important roles in the fate of trace gases and the physical-chemical character of aerosols, and consequently contribute to haze formation and climate change. Yet, compared with gas-phase reactions, the heterogeneous or multiphase chemistry has been investigated to a much smaller extent because of the huge complexity.In the present work, a multiphase model with detailed microphysical and aqueous chemistry mechanisms was utilized to study the multiphase chemistry in cloud droplets and deliquescent particles. Based on the intensive field observations of gas, aerosol, cloud waters at a mountain site (Mt. Tai, China) in 2007-2008 and a suburban site (Tung Chung, Hong Kong) in 2011-2012, model simulations were performed to investigate several poorly understood heterogeneous processes, including the aqueous phase HxOy-TMI reaction and nitrogen chemistry as well as the sulfate production in aqueous phase under some unique conditions of China. The comprehensive cloud composition data at Mount Tai were analyzed in terms of seasonal variation, impact of different sources regions and sulfate formation using a tracer method. This is the most recent characterization study of cloud composition in heavily polluted North China.Both gas and aqueous phase budgets of HxOy species (OH, HO₂ and H₂O₂) have been quantitatively evaluated. The simulations suggest differences in aqueous HxOy budget between cloud droplet and deliquescent aerosol scenarios. The cloud droplets are found to efficiently uptake nearly all HxOy species from gas to aqueous phase after cloud formation. In comparison, while the deliquescent aerosols (at Tung Chung) are important sinks of HO₂ radical and H₂O₂ but act as a net source of OH radical. These results suggest significant contribution from heterogeneous processes to the loss of H₂O₂ in gas phase and that models without considering the chemical processes in aqueous particles or misrepresenting the metal concentrations may result in large uncertainty in determining the chemical budget of H₂O₂. For atmospheric nitrogen chemistry, the heterogeneous processes of HONO, N₂O₅ and NO3 radical are of particular importance. Motivated by the challenge that existing mechanisms tend to underestimate the ambient HONO concentrations, HONO formation is investigated using a multiphase model. Two aqueous nitrogen reactions, which are rarely mentioned but potentially play important roles in HONO formation, are emphatically studied. Model simulations show significant N₂O₅ uptake and ClNO₂ production rate on deliquescent aerosols at night time. Br-ion in aerosols is found to strongly suppress the heterogeneous production of ClNO₂. This implies that current models that only consider Cl in studying N₂O₅ hydrolysis may be inadequate.To summarize, this study adds to the increasing body of literature demonstrating the importance of multiphase processes in atmospheric chemistry, and provide a quantitative description of aqueous-phase reactions affecting gas-phase species concentrations and aerosol compositions. The approach introduced herein can be applicable to other atmospheric systems and facilitate further understanding of physical-chemical interactions among gas-phase species, aerosols, fog and clouds.Department of Civil and Environmental EngineeringPh.D., Dept. of Civil and Environmental Engineering, The Hong Kong Polytechnic University, 2015Doctorat