Uptake of nitrogen dioxide (NO
) on acidic aqueous humic
acid (HA) solutions as a missing daytime nitrous acid
(HONO) surface source
K. J. Wall
G. W. Harris
Received: 28 January 2016 /Accepted: 24 June 2016 /
Published online: 12 August 2016
Springer Science+Business Media Dordrecht 2016, corrected publication July/2017
Abstract A comprehensive kinetic study of a potential daytime nitrous acid (HONO) source
reaction, the photoenhanced reduction reaction of the nitrogen dioxide (NO
) 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 NO
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 NO
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.
J Atmos Chem (2017) 74:283–321
The original version of this article was revised: Mistakes were introduced during the production process. Please
refer to the Erratum article for the complete list of changes.
Electronic supplementary material The online version of this article (doi:10.1007/s10874-016-9342-8)
contains supplementary material, which is available to authorized users.
* G. W. Harris
Department of Chemistry and the Centre for Atmospheric Chemistry, York University, 4700 Keele
St., Toronto M3J 1P3 ON, Canada