| Summary: | 博士 === 國立臺灣大學 === 環境工程學研究所 === 89 === The goal of this dissertation is to understand the budget of the atmospheric chlorine after releasing from the sea salt aerosol, and its impacts on the atmospheric chemistry. The first approach of this research was to estimate the magnitude and the source of chlorine, and then a chemical transport model was applied to quantify the magnitude and the distribution of the atmospheric chlorine, and it impacts on the atmospheric chemistry.
The chlorine related observations indicated that HCl concentrations range from less than 100 to 2800 ppt, and Cl2 ranges from slightly less than 50 to 580 ppt in Taiwan. According to the concentrations of these chlorine related species, the calculated concentration of chlorine atom peaks up at noon, with the concentrations vary from 104 to 106 molecules/cm3. The existence of chlorine atoms in the atmosphere can affect the concentration of hydroxyl radical, with the maximum enhancement of about 120% at noon.
Simulation results indicated that most of the atmospheric chlorine was in the form of chloride in aerosol and HCl was the most abundant gas-phase chlorine species. The magnitude of gas phase chlorine follows the order of: HCl > Cl2 >HOCl >ClNO2> ClO> ClNO3>ClNO, except that Cl2 has a very low concentration during the daytime, and can be lower than ClNO2. Among these gas phase chlorine species, the concentrations of HCl、HOCl and ClO rise in the daytime and drop at night; while the concentrations of Cl2 and ClNO2 are reversed.
NOx concentration was enhanced if chlorine chemistry was considered in the model. Also, the time when peak NOx occurred shifted when chlorine mechanisms were included. The magnitude of O3 was not influenced, but there was a delay of the time variation. This is because the lifetime of NOx was slightly extended after chlorine mechanisms were considered, resulting in a delay of the time variation of NOx and thus O3.
Some sensitivity studies were performed to further understand chlorine mechanisms in this research. First the sensitivity of aerosol emission magnitude and the PH value of sea salt were tested. The results indicated that these two factors have minor impacts on the previous modeling calculations. Sensitivity studies of both initial and boundary conditions of the aerosol models were also considered by using theoretical calculations. The results of varying initial condition showed that it takes about a time period of 2.3 and 4.6 times the lifetime of a chemical species in order to reduces the influences of initial conditions to less than 10% and 1% respectively. For most species with lifetime less than one day, 2 days of initiation can reduce the influences of initial conditions to less than 10%. For the boundary conditions, this study found that for short-life species, for example, Isoprene, about 70 km from the boundary can reduces the influences of boundary conditions to less than 10%. For longer lifetime species, for example, O3, about 1000 km from the boundary is required to reduce the boundary influences to less than 10%.
In summary, distance to the coast, pollution level, and atmospheric conditions all will affect the chlorine concentration in the atmosphere. Chlorine atoms have larger impacts on the coastal areas. The influences of chlorine atoms on other chemical species were larger in highly polluted areas. This may due to the completing of chlorine atoms with hydroxyl radicals for hydrocarbon oxidation, or for oxidation with nitrogen oxides, although these impacts on O3 concentration were minor. For most of air quality modeling focusing on O3 problems, in considering the representative of the results, the complexity of the mechanisms, the accuracy of the calculation and the burden of the computer facilities, neglecting the chlorine chemistry is recommended in this study.
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