以紫外光吸收光譜法測量克里奇中間體與硝酸的反應動力學

Abstract

克里奇中間體為大氣中重要的物種，其反應性高，可與水蒸氣、二氧化硫、有機酸或無機酸等大氣分子發生反應。硝酸 (HNO3) 為常見的無機酸，且為空氣中的汙染物之一，可經由大氣中之氮氧化物 (NOx) 循環反應生成，且與人類的活動息息相關，在大氣中的含量可達到ppb的等級。本研究的目的為探討克里奇中間體與硝酸的反應。先前，Foreman等人研究了最簡單的克里奇中間體 (CH2OO) 與HNO3的反應，其結果顯示此反應非常快速，反應速率常數之數量級為10-10 cm3 s-1，但其定量HNO3的方法可能有很大的誤差，造成其速率常數可能不準確。而在本文中，我們利用相對可靠的紫外光吸收光譜法定量HNO3以及進一步研究與比較四種具有不同取代基的克里奇中間體與硝酸之反應動力學，此四種克里奇中間體分別為CH2OO、(CH3)2COO、methyl vinyl ketone oxide (MVKO) 與methacrolein oxide (MACRO)，其中後兩者具有乙烯基取代，可藉由共振而穩定其結構。我們的結果顯示，CH2OO、(CH3)2COO、MVKO及MACRO與硝酸的反應速率常數分別為 (1.51 ± 0.45) × 10−10、(3.54 ± 1.06) × 10−10、(3.93 ± 1.18) × 10−10 及 (3.0 ± 1.3) × 10−10 cm3 s−1，這些反應如同克里奇中間體與其他酸類反應一樣都相當快速，與先前的研究結果一致，然而對比文獻中CH2OO + HNO3的反應速率常數，我們認為其高估了約3.6倍。此外，不同於克里奇中間體與水蒸氣的反應，克里奇中間體與HNO3的反應並未觀察到顯著的取代基效應。對於CH2OO與(CH3)2COO的反應而言，在100至400 Torr之壓力範圍內亦未觀察到顯著的壓力效應。本文亦探討了克里奇中間體與HNO3的反應在大氣中的重要性，我們的實驗結果顯示，在氮氧化物汙染嚴重及乾燥的地區，克里奇中間體與硝酸的反應以及其產物可能是值得關注的。但此反應對於除去在大氣中之HNO3的貢獻可能並不顯著。

Criegee intermediates (CIs) are very reactive species and can react with atmospheric molecules such as water, SO2, and organic/inorganic acids. Nitric acid (HNO3) is a common inorganic acid and also one of the air pollutants. HNO3 is formed from the NOx cycle in the atmosphere. In this work, we focus on the reactions between CIs and HNO3. Previously, Forman et al. have studied the kinetics of CH2OO reaction with HNO3. Their results show that the reaction is very fast with a rate coefficient on the order of 10-10 cm3 s-1. But their experimental method to determine the concentration of HNO3 might cause a large uncertainty in the reported rate coefficient. Thus, here we utilize a relatively reliable UV absorption spectroscopy to measure the concentration of HNO3 and to further investigate the kinetics of HNO3 reactions with four CIs, which have different substitute groups. These four CIs are CH2OO, (CH3)2COO, methyl vinyl ketone oxide (MVKO) and methacrolein oxide (MACRO); the last two CIs are vinyl substituted and resonance stabilized. Our results show that the rate coefficients are (1.51 ± 0.45) × 10−10, (3.54 ± 1.06) × 10−10, (3.93 ± 1.18) × 10−10, and (3.0 ± 1.3) × 10−10 cm3 s−1 for HNO3 reactions with CH2OO, (CH3)2COO, MVKO, and MACRO, respectively. All of these reactions are extremely fast, consistent with previous observations of CI reactions with acids. The rate coefficient of CH2OO + HNO3 in the literature is, however, overestimated by a factor of 3.6. We conclude that the reactivity of CIs toward HNO3 has no significant substituent effect, which is different from that of CIs + water. In addition, we observed no pressure dependence in reactions of CH2OO and (CH3)2COO with HNO3 under a pressure range of 100−400 Torr. Here, we also discuss the atmospheric impact of CI reactions with HNO3. Our results indicate that, in dry area with severe NOx pollution, the reactions of CI + HNO3 and their products may be worthy of attention. Even though these reactions may be insignificant in the removal processes of HNO3 in the atmosphere.