α-蒎烯與臭氧反應產生粒子現象之探討

Abstract

本研究利用實驗與模式模擬，探討室溫下α-蒎烯與臭氧反應之新生粒子現象在不同臭氧濃度及相對濕度下的表現及其原因，並且推論反應中產生的自由基與另外加入的人為排放(SO2)在此反應中扮演之角色。本實驗使用掃描式電動度粒徑分析儀(SMPS)測量反應所產生之粒徑譜，初始α-蒎烯濃度(19.3 ppm與15.4 ppm)遠高於初始臭氧濃度(0.04 - 0.12 ppm)，相對溼度為< 1 %、36 %及54 %。為了探討在不同環境下的化學反應及物理過程如何影響氣膠量及粒徑分布，本研究使用氣相化學盒子模式(Box model)與粒徑譜模式(Particle spectral model)進行產物量與粒徑譜之模擬，並估算產物的飽和蒸氣壓與其核化、凝結速率等參數。 實驗結果顯示初始臭氧濃度於0.05至0.12 ppm之範圍內，氣膠數量與質量濃度皆隨著初始臭氧濃度而提升，推測是因氣態的低揮發性產物在初始臭氧濃度為約0.05 ppm之條件下可達到飽和並形成氣膠，由模式估計本研究之低揮發性產物的飽和蒸氣壓約在3.7 × 10^-10至1.6 × 10^-8 bar之間。當提高相對濕度至36 %與54 %時，氣膠的質量濃度隨著水氣量有下降之趨勢，此現象可能是由於HO2•H2O複合物之形成，消耗了系統中的HO2自由基，使產物氧化程度減少並導致形成之氣膠量下降。當氣流中添加了3900 ppm的OH自由基移除劑，甲醇，新生粒子現象即明顯被抑制，推測OH自由基對於氣膠之生成有重大的影響；模式結果顯示，本系統中最多有27 %之HO2自由基與79 %之OH自由基分別被水氣與甲醇消耗。以6.3 ppm之二氧化硫代表人為排放加入系統後，可測量到很多小粒子生成，藉由模式得知二氧化硫與OH自由基反應產生了0.35 ppb之硫酸，推測硫酸加強了系統的核化現象，導致小粒子大量生成；以模式估算核化速率後發現，硫酸在本系統中的核化速率遠超過同樣濃度的硫酸與水之雙組份系統的核化速率，因此推測實驗觀測到之粒子生成現象可能是硫酸-水-有機物之多組份核化所造成的結果。 本研究對於α-蒎烯與臭氧在不同環境下進行之化學反應提出了較明確的看法，尤其是在自由基的部分，此研究結論有可能適用於其他有機物質之反應；另外，本研究對於核化與凝結之物理過程進行了模擬與測試，在估算大氣中的氣膠數量與質量濃度之應用上提供了可能的參考方向。

In this study, the new particle formation from the ozonolysis of α-pinene as a function of initial ozone concentration ([O3]i) and relative humidity (RH) was studied using a scanning mobility particle sizer spectrometer (SMPS) to monitor the size distribution of submicrometer particles at room temperature. How the radicals produced from the chemical reactions and the impact of the addition of anthropogenic emissions (SO2) on the particle formation were investigated. The applied initial concentration of α-pinene (19.3 ppm and 15.4 ppm) was much higher than [O3]i (0.04 - 0.12 ppm), and RH was controlled at < 1 %, 36 % and 54 %. A box model was constructed to simulate the concentration of products with possible chemical reactions while a particle spectral model was applied to simulate the particle size distribution, with the adjustment of physical processes and chemical kinetic parameters of the products such as the saturation vapor pressure, nucleation and condensation rate of products. The results showed a positive correlation of the produced SOA to [O3]i in both number and mass concentration for [O3]i in the range of 0.05 - 0.12 ppm. It is likely due to the produced low-volatility products reaching the saturation point at [O3]i = 0.05 ppm. The saturation vapor pressure was estimated to be 3.7 × 10^-10 - 1.6 × 10^-8 bar by model simulation. For a given [O3]i, SOA mass concentration showed a decreasing trend with RH. It is surmised that water vapor may react with HO2 to form HO2•H2O, which decreases the overall oxidation of α-pinene-O3 system. With the addition of 3900 ppm of methanol vapor, a scavenger of OH, the new particle formation was then almost inhibited. By simulations, it was estimated that at most 27 % of HO2 radical and 79 % of OH radical were consumed by water and methanol vapor, respectively. With the addition of 6.3 ppm of SO2, one of the major anthropogenic emissions, a significant enhancement of smaller particles in number and mass concentration was observed likely due to the formation of H2SO4 from the reaction of SO2¬ with OH radical. By model simulation, it was estimated to have 0.35 ppb of H2SO4, which might lead to significant nucleation rate. A significant faster nucleation rate from our experimental system than that of H2SO4-H2O binary system with the same concentration of H2SO4 and H2O might suggest the importance of the produced organic species for the multi-component nucleation of H2SO4-H2O-organic system. This study illustrated the new particle formation from the ozonolysis of α-pinene at different environments and suggested the importance of radicals, which might be extended to other organic compound systems. The nucleation and condensation processes from the model simulations might provide other regional models the possible physical and chemical parameters required to estimate the number and mass concentration of aerosols formed in such processes in real atmosphere.