A comprehensive kinetic study of a potential daytime nitrous acid (HONO) source reaction, the photoenhanced reduction reaction of the nitrogen dioxide (NO2) on acidic humic acid (HA), was completed using a wetted-wall flow tube (WWFT) (Fickert et al.: J. Phys. Chem. A. 102, 10689, 1998) photoreactor integrated with a high sensitivity HONO analyser (Wall et al.: J. Atmos. Chem. 55, 31–54, 2006; Huang et al.: Atmos. Environ. 36, 2225–2235, 2002). The nature of this reaction, is of great interest since recently observed, unpredictably high HONO daytime concentrations demand its ordinarily proposed heterogeneous source to proceed 60 times more rapidly at noon than during the night (Kleffmann et al.: ChemPhysChem 8, 1137–1144, 2007). This study investigated the nature of the reduction reaction with simulated colloidal HA aqueous solutions characteristic of anaerobic environmental conditions, varying in acidity, concentration and composition. Typical urban NO2 levels were investigated. Increasing photoenhanced HONO production with weakening solution acidity was detected due to increased deprotonation of the carboxyl groups within the humic acid. It was deduced that the acidic HA substrate contains numerous feasible chromophoric sensitizer units capable of photochemically reducing NO2 to HONO, owing to its ‘biofilm’ (Donlan, 2002) function under UV exposure. The mechanism was found to be more effective for HA standards with higher levels of ‘bioactivity’ (refractivity). Using a complex mathematical model developed, incorporating both chemistry and diffusion, reaction probability datasets were produced from the experimental data, providing evidence that this is, indeed, an environmentally important daytime HONO surface source reaction. The parameters required to scale up the data of the photoreactor to that of a regional rural/urban scale were assessed.
Journal of Atmospheric Chemistry – Springer Journals
Published: Aug 12, 2016
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