Photochemistry of Atmospheric Dust: Ozone Decomposition on Illuminated Titanium Dioxide

Abstract

The ozone decomposition onto mineral surfaces prepared with traces of solid TiO2 in a matrix of SiO2 in order to mimic mineral dust particles has been investigated using a coated-wall flow-tube system at room temperature and atmospheric pressure. The ozone uptake coefficients were measured both under dark conditions and irradiation using near UV-light. While uptake in the dark was negligible, a large photoenhanced ozone uptake was observed. For TiO2/SiO2 mixtures under irradiation, the uptake coefficients increased with increasing TiO2 mass fraction (from 1 to 3 wt %), and the corresponding uptake coefficient based on the geometric surfaces ranged from 3 x 10(-6) to 3 x 10(-5). The uptake kinetics was also observed to increase with decreasing ozone concentration between 290 and 50 ppbv. Relative humidity influenced the ozone uptake on the film, and a reduced ozone loss was observed for relative humidity above 30%. The experimental results suggest that under atmospherically relevant conditions the photochemistry of dust can represent an important sink of ozone inside the dust plume.

Sulfur isotope fractionation during heterogeneous oxidation of SO<sub>2</sub> on mineral dust

“…Similarly, McCabe et al (2006) measured 17 O of sulfate aerosol in Alert, Canada, and proposed that the importance of transition metal-catalysed oxidation was underestimated in winter, when concentrations of Fe and Mn are approximately doubled due to transport of polluted air masses. Norman et al (1999) found that δ 34 S of non-sea salt sulfate aerosol at Alert was lower in winter than in summer, which is consistent with the results of McCabe et al (2006) when the fractionation factor for transition metal catalysis measured in this study is considered. Although the suite of transition metals from a polluted source will be different to those from a dust source, this study has shown that the identity of the transition metals involved in catalysis does not affect the isotopic fractionation.…”

“…Humidity regenerates the reactive capacity of dust for SO 2 uptake (Judeikis et al, 1978;Ullerstam et al, 2002), possibly due to the increased mobility of surface ions which leads to the re-exposure of active sites (Al-Hosney and Grassian, 2005). Uptake and decomposition of ozone, which increases the basicity and oxidising capacity of the surface, is highest when irradiated and at around ∼35 % RH (Hanisch and Crowley, 2003;Nicolas et al, 2009). Many components of dust, particularly iron and titanium oxides, are photosensitive and show increased uptake and oxidation due to the formation of election-hole pairs Rubasinghege et al, 2010).…”

Heterogeneous reaction of N<sub>2</sub>O<sub>5</sub> with illite and Arizona test dust particles

“…The loss of surface reactivity of mineral dust at increasing RH has been previously observed for other trace gases (e.g. H 2 O 2 ) (Pradhan et al, 2010) and O 3 (Mogili et al, 2006b;Nicolas et al, 2009).…”

Heterogeneous reaction of N<sub>2</sub>O<sub>5</sub> with airborne TiO<sub>2</sub> particles and its implication for stratospheric particle injection

“…Tropospheric mineral dust aerosols have a large impact on direct and indirect radiative forcing (Balkanski et al, 2007;Cziczo et al, 2013), and their heterogeneous reactions with several trace gases can significantly influence tropospheric photochemistry (Dentener et al, 1996) and modify the composition of dust particles (Sullivan et al, 2007). TiO 2 , an important component in natural mineral dust particles (Hanisch and Crowley, 2003;Usher et al, 2003), is of particular interest because heterogeneous reactivity towards some trace gases (e.g., NO 2 , O 3 ) is significantly enhanced under illuminated conditions (Ndour et al, 2008;Nicolas et al, 2009). TiO 2 is also a well-established photocatalyst for a wide range of reactions (Linsebigler et al, 1995;Nakata and Fujishima, 2012), including the conversion of NO x species Ndour et al, 2008).…”