Comment on “Investigations on HONO formation from photolysis of adsorbed HNO3 on quartz glass surfaces” by S. Laufs and J. Kleffmann, Phys. Chem. Chem. Phys., 2016, 18, 9616

Abstract

Laufs and Kleffmann observed that HNO₃ surface photolysis rates resemble that of HNO₃ in the gas phase after depositing HNO₃ in air at ∼50% relative humidity onto quartz glass surfaces. They questioned the dry HNO₃ coverage (1.1 × 10¹⁴ molecules per cm² after depositing ∼15 mTorr HNO₃ on silica at 0% humidity) used to derive our previously published HNO₃ near-UV surface absorption cross sections. We directly determined the HNO₃ coverage on a quartz surface using a quartz crystal microbalance (QCM). A similar HNO₃ monolayer coverage obtained by QCM confirms that our estimated HNO₃ coverage is reasonable. We also obtained an NO₂ quantum yield from the 308 nm HNO₃ photolysis on fused silica. In this Comment, we provide an explanation of the variance in HNO₃ surface photolysis rates by clarifying the effects arising from important differences in the HNO₃ coverage on quartz/silica in the presence of humidity versus those in the absence of humidity.