Absorption of Near UV Light by HNO3/NO3– on Sapphire Surfaces

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• 作者：Manuvesh Sangwan ; William R. Stockwell ; Devoun Stewart ; Lei Zhu
• 刊名：Journal of Physical Chemistry A
• 出版年：2016
• 出版时间：May 12, 2016
• 年：2016
• 卷：120
• 期：18
• 页码：2877-2884
• 全文大小：375K
• 年卷期：0
• ISSN：1520-5215

文摘

We have determined absorption of the near UV light (290–345 nm) by nitric acid (HNO₃) deposition on sapphire window surfaces as a function of the HNO₃ pressure, by using Brewster angle cavity ring-down spectroscopy. Apparent monolayer HNO₃ surface absorption cross sections have been obtained; they range between (1.7 ± 1.1) × 10^–19 and (0.29 ± 0.03) × 10^–19 cm²/molecule. When nitric acid cross section values on sapphire surfaces were divided by those on fused silica surfaces for which only molecular HNO₃ adsorption was reported, a new absorption band appeared in the 320–345 nm region. The shape of this absorption band is similar to that reported for surface nitrate (NO₃^–) at quartz/water interfaces, but is red-shifted by about 10 nm. Our study suggests that a small percentage (<7%) of adsorbed HNO₃ formed by HNO₃ deposition on sapphire surfaces is dissociated into surface nitrate on the time scale of about 5–7 min. Background transmission changes in the 320–350 nm region after exposing clean sapphire surfaces with many repeated HNO₃ deposition/evacuation cycles are consistent with surface nitrate formation. We obtained nitrate surface absorption cross section data over 320–350 nm range. We also modeled photolysis rates of HNO₃/NO₃^– on urban grimes. Atmospheric implications of the results are discussed.