Photoenhanced uptake of NO₂ on mineral dust: Laboratory experiments and model simulations

Abstract

Mineral dust contains material such as TiO2 that is well known to have photocatalytic activity. In this laboratory study, mixed TiO2‐SiO2, Saharan dust and Arizona Test Dust were exposed to NO2 in a coated wall flow tube reactor. While uptake in the dark was negligible, photoenhanced uptake of NO2 was observed on all samples. For the mixed TiO2‐SiO2, the uptake coefficients increased with increasing TiO2 mass fraction, with BET uptake coefficients ranging from 0.12 to 1.9 × 10−6. HONO was observed from all samples, with varying yields, e.g., 80% for Saharan dust. Three‐dimensional modeling indicates that photochemistry of dust may reduce the NO2 level up to 37% and ozone up to 5% during a dust event in the free troposphere.

Measurements of nitrous acid (HONO) in urban area of Shanghai, China

“…This includes different surfaces (ground and aerosols) such as the photocatalytic conversion of NO 2 on mineral dust (Ndour et al 2008), the dark heterogeneous conversion of NO 2 on suspended soot particles (Ammann et al 1998;Arens et al 2001), the heterogeneous hydrolysis of NO 2 (Finlayson-Pitts et al 2003), the photosensitized reduction of NO 2 on organic surfaces (George et al 2005;Stemmler et al 2006), the photolysis of adsorbed nitric acid (HNO 3 ) (Zhou et al 2003) and nitrate (NO 3 − ) (Zhou et al 2001), the HNO 3 conversion on primary organic aerosols (Ziemba et al 2010) and from soilemitted nitrite (Su et al 2011). Direct emissions from combustion processes (i.e.…”

Influence of relative humidity on heterogeneous kinetics of NO2 on kaolin and hematite

“…[15][16][17] The uptake coefficient (g) is one of the key parameters in modelling studies. Several studies have measured the g of NO 2 on dust from Saharan or Asian source regions 5,18 and surrogate materials including CaCO 3 , 7 Al 2 O 3 , Fe 2 O 3 , CaO, MgO, TiO 2 5,19 and clay minerals. 6 The g of NO 2 on these samples varies from 10 À4 to 10 À9 depending on the substrates and experimental conditions, 20 and was recommended to be 1.2 Â 10 À8 by Crowley et al 21 Except for a few reaction systems, such as reactions of NO 2 on TiO 2 , 18,22,23 Saharan sand, 23 and CaCO 3 , 7 however, most of these g values were measured under dry conditions due to the limitations of the low pressure reactors used, 5,19 or at a single point of relative humidity (g was measured for NO 2 on TiO 2 and Saharan sand at RH = 25%).…”

“…6 The g of NO 2 on these samples varies from 10 À4 to 10 À9 depending on the substrates and experimental conditions, 20 and was recommended to be 1.2 Â 10 À8 by Crowley et al 21 Except for a few reaction systems, such as reactions of NO 2 on TiO 2 , 18,22,23 Saharan sand, 23 and CaCO 3 , 7 however, most of these g values were measured under dry conditions due to the limitations of the low pressure reactors used, 5,19 or at a single point of relative humidity (g was measured for NO 2 on TiO 2 and Saharan sand at RH = 25%). 18,23 However, g has been found to show a strong dependence on humidity for some reactions. Both the increase and the decrease of g have been observed as a function of RH.…”

“…20 Besides surface nitrates and nitrites, 19,[29][30][31] HONO is also detected as a gaseous product. 6,11,18 Under ultra-violet irradiation, the HONO yield (y HONO ) is 75%, 33% and 80% for reactions of NO 2 on pure TiO 2 , 22 1% TiO 2 /SiO 2 and Saharan sand, 18 respectively. It has been postulated that the surface acidity, microstructure and other unknown factors control the production of HONO.…”

“…It has been postulated that the surface acidity, microstructure and other unknown factors control the production of HONO. 18 However, the HONO yields for heterogeneous reactions of NO 2 on kaolin and hematite are as yet unclear, and how the RH affects the HONO yield for the reactions of NO 2 on these materials is also unknown.…”