Nitrogen dioxide multiphase chemistry: Uptake kinetics on aqueous solutions containing phenolic compounds

Abstract

The uptake coefficients of NO₂ on aqueous solutions containing guaiacol, syringol and catechol were determined over the pH range from 1 to 13 using the wetted wall flowtube technique. The measured uptake coefficients were used to determine the rate coefficients for the reaction of the physically dissolved NO₂ with the neutral and deprotonated forms of phenolic compounds listed above. These organic compounds are ubiquitous not only in biomass burning plumes but also in soils, where they form part of the building blocks of humic acids. The NO₂ uptake kinetics on solutions containing guaiacol, syringol or catechol were observed to be strongly pH dependent with uptake coefficients increasing from below 10⁻⁷, under acidic conditions, to more than 10⁻⁵ at pH values above 10. This behaviour illustrates the difference of reactivity between the neutral phenolic species and the phenoxide ions. The corresponding second order rate coefficients were typically observed to increase from 10⁵ M⁻¹ s⁻¹ for the neutral compounds to a minimum of 10⁸ M⁻¹ s⁻¹ for the phenoxide ions.