Production of HONO from Heterogeneous Uptake of NO2 on Illuminated TiO2 Aerosols Measured by Photo-Fragmentation Laser Induced Fluorescence

Abstract:

In polluted environments an important source of OH is the photolysis of nitrous acid (HONO). In a recent study in central Beijing, HONO photolysis accounted for 83 % of the production of new primary radicals (Slater et al., 2020). Current tropospheric chemistry models underestimate the concentration of HONO suggesting a missing source. Specifically, there is a large uncertainty in the production of HONO from heterogeneous sources and therefore the role of aerosols under both dark and illuminated conditions. In order to investigate the potential missing source of HONO from illuminated aerosols and determine its atmospheric relevance, a highly sensitive photo-fragmentation laser induced fluorescence (PF-LIF) instrument has been constructed and coupled to an aerosol flow tube system.

We will present a short overview of the PF-LIF instrument and results from experiments investigating the reactive uptake coefficient of NO₂ to form HONO onto TiO₂ aerosols. Dust aerosols, containing 0.1-10 % TiO₂, are transported from the Gobi desert to urban areas of China where high NO_x levels have been observed (Saliba et al., 2014;Hanisch and Crowley, 2003). In these areas, photo-catalysed HONO production on aerosol surfaces may be important. The change in reactive uptake coefficient as a function of NO₂ concentration suggested a mechanism dependent on 2 NO₂ molecules adsorbing to the surface to form one HONO molecule, likely via the dimerization of NO₂ and subsequent isomerisation to form trans-ONO-NO₂, from which HONO is produced (Finlayson-Pitts et al., 2003;de Jesus Madeiros and Pimentel, 2011;Liu and Goddard, 2012;Varner et al., 2014). We will present the observed trends in HONO concentration and uptake coefficient as a function of NO₂ concentration, and investigate possible mechanisms for HONO formation using a zero-dimensional box model. In addition, we will present experiments showing production of HONO from illumination of mixed nitrate/TiO₂ aerosols in the absence of NO₂, the results of which could be significant in low NO_x environments in the presence of mixed dust/nitrate aerosols, e.g. in oceanic regions off the coast of West Africa, or in continental regions effected by the outflow from the Gobi desert.