Heterogeneous reactions of NO₂ and HNO₃ on oxides and mineral dust: A combined laboratory and modeling study

Abstract

Abstract. This study combines laboratory measurements and modeling analysis to quantify the role of heterogeneous reactions of gaseous nitrogen dioxide and nitric acid on mineral oxide and mineral dust particles in tropospheric ozone formation. At least two types of heterogeneous reactions occur on the surface of these particles. Upon initial exposure of the oxide to NO2 there is a loss of NO2 from the gas phase by adsorption on the particle surface, i.e., NO2(g) --> NO2(a). As the reaction proceeds, a reduction of gaseous NO2 to NO, NO2 (g) --> NO (g) is found to occur. Initial uptake coefficients ¾0 for NO2 on the surface of these particles have been measured at 298 K using a Klmdsen cell reactor coupled to a mass spectrometer. For the oxides studied, ct,¾-A1203, ct,¾-Fe203, TiO2, SiO2, CaO, and MgO, ¾0 ranges from < 4 x 10 © for SiO2 to 2 x 10 -5 for CaO with most values in the 10 -6 range. For authentic samples of China 10ess and Saharan sand, similar reactivity to the oxides is observed with ¾0 values of 2 x 10 -6 and 1 x 10 -6, respectively. For HNO3 the reactivity is 1-2 orders of magnitude higher. Using these laboratory measurements, the impact of heterogeneous reactions of NO2 and HNO3 on mineral dust in tropospheric ozone formation and on O3-precursor relationships is assessed using a time-dependent, multiphase chemistry box model. Simulations with and without heterogeneous reactions were conducted to evaluate the possible influence of these heterogeneous reactions on ambient levels. Results show that values of the initial uptake for NO2 and HNO3, adjusted for roughness effects, must be greater than 10 -4 to have an appreciable impact on NOx, HNO3, and 03 concentrations for the conditions modeled here.Thus the measured uptake coefficients for NO2 on dry surfaces are just below the lower limit to have an impact on the photochemical oxidant cycle, while the heterogeneous reactivity of HNO3 is sufficiently large to have an effect. Under conditions of high mineral dust mass loadings and/or smaller size distributions the importance of these reactions (both NO2 and HNO3) is expected to increase.

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

“…These values are slightly larger than the values recommended by Crowley et al 21 According to the model results, the g t,ini should be greater than 10 À4 to observe an appreciable impact on NO x and O 3 concentrations for the interactions of NO 2 with mineral dust. 5 Thus, the small uptake coefficients measured at ambient RH in this work imply that heterogeneous reactions of NO 2 on mineral dust might be unimportant for NO x and O 3 concentrations. At the present date, secondary inorganic aerosol (SIA) has attracted much attention for China's air pollution because SIA often abruptly increases during severe haze days.…”

“…S3 (ESI †), a Langmuir adsorption isotherm for water vapor and the Langmuir-Hinshelwood reaction mechanism for NO 2 on these particles can adequately describe the influence of RH on the uptake coefficient. 6 and from 7 Â 10 À7 to 7.7 Â 10 À6 for NO 2 on hematite under dry conditions, 5,19 respectively. At high RH (32-74%), the average g t (10 s) of NO 2 on kaolin was (4.42 AE 1.17) Â 10 À8 , and it was (2.83 AE 0.84) Â 10 À7 on hematite.…”

“…The concentration of NO 2 in this work (1.6 Â 10 À4 mbar) was slightly lower than that used by Angelini et al, 6 while it was higher than Underwood's. 5 Additionally, in DRIFTS experiments, the data collected within the first several minutes are usually used to calculate the uptake coefficients, whereas the time resolution of a Knudsen cell-mass spectrometer is often higher than 1 s. Thus, a lower time resolution should be achieved in Angelini's work, 6 while the time resolution was higher in Underwood's work. 5,19 Therefore, the g t (10 s) of NO 2 at 0% RH estimated in this work is larger than Angelini's results (8.1 AE 0.2 Â 10 À8 at 2.3 Â 10 À4 mbar) on kaolin, 6 while it is smaller than that of Underwood's results (7.7 Â 10 À6 at 5.3 Â 10 À6 mbar) on 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%).…”

Interaction of NO&lt;sub&gt;2&lt;/sub&gt; with TiO&lt;sub&gt;2&lt;/sub&gt; surface under UV irradiation: measurements of the uptake coefficient

“…The dark reaction of NO 2 with TiO 2 surface was investigated in three studies using a Knudsen cell reactor coupled to a quadrupole mass spectrometer (Underwood et al, 1999(Underwood et al, , 2001Setyan et al, 2009). …”