Matthew Ninneman, Sarah Lu, Xianliang Zhou, James Schwab.Atmospheric implications of the results are discussed. We also modeled photolysis rates of HNO 3/NO 3 – on urban grimes. We obtained nitrate surface absorption cross section data over 320–350 nm range. Background transmission changes in the 320–350 nm region after exposing clean sapphire surfaces with many repeated HNO 3 deposition/evacuation cycles are consistent with surface nitrate formation. Our study suggests that a small percentage (<7%) of adsorbed HNO 3 formed by HNO 3 deposition on sapphire surfaces is dissociated into surface nitrate on the time scale of about 5–7 min. The shape of this absorption band is similar to that reported for surface nitrate (NO 3 –) at quartz/water interfaces, but is red-shifted by about 10 nm. When nitric acid cross section values on sapphire surfaces were divided by those on fused silica surfaces for which only molecular HNO 3 adsorption was reported, a new absorption band appeared in the 320–345 nm region. Apparent monolayer HNO 3 surface absorption cross sections have been obtained they range between (1.7 ± 1.1) × 10 –19 and (0.29 ± 0.03) × 10 –19 cm 2/molecule. We have determined absorption of the near UV light (290–345 nm) by nitric acid (HNO 3) deposition on sapphire window surfaces as a function of the HNO 3 pressure, by using Brewster angle cavity ring-down spectroscopy.
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