The heterogeneous reactivity of gaseous nitrogen dioxide on mineral oxide particles was investigated. In particular, spectroscopic and kinetic measurements have been made to investigate surface reactions of NO2 on Al2O3, Fe2O3, and TiO2 at 298 K. Both gas-phase and surface-bound products are formed from the reaction of NO2 with these mineral oxide particles. At low coverages, FT-IR spectra of the mineral oxide surface exposed to gaseous NO2 show absorptions due to surface nitrite, specifically a chelating nitrito species. As the coverage increases, the surface becomes populated with surface nitrate bonded in several different bonding coordinations (monodentate, bidentate, and bridging). The predominant gas-phase product is NO, although there is a small amount (<1%) of detectable N2O. A Knudsen cell reactor coupled to a quadrupole mass spectrometer was used to measure the uptake coefficient, γ, for NO2 on these oxide particles and to characterize gas-phase product formation. The Knudsen cell data showed NO to be the major gas-phase product with a delay in the onset of NO production. There was little production of N2O and no gas-phase HONO or HNO3 was detected. By monitoring the reaction until completion, the ratio of NO2 reacted to NO produced was determined to be ∼2:1. These results complement the FT-IR data and suggest a two-step mechanism in which NO2(g) is initially adsorbed as a nitrite species which subsequently reacts with additional NO2 to form surface nitrate and gas-phase NO. Finally, the initial uptake coefficient was determined from the Knudsen cell data for NO2 on Al2O3, Fe2O3, and TiO2. Because NO2 can diffuse into the underlying layers of these oxide particles, the use of a geometric area does not give accurate values of the uptake coefficient. Gas diffusion must be taken into account to more accurately determine the uptake coefficient.

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.

Abstract. It has been postulated that the reaction of nitric acid with calcium carbonate, namely, CaCO3(s) + 2HNO3(g) -0 Ca(NO3)2(s) + CO2(g) + H20(g), plays an important role in the atmosphere. In this study, transmission FTIR spectroscopy, diffuse reflectance UV-visible spectroscopy, transmission electron microscopy and a Knudsen cell reactor coupled to a quadrupole mass spectrometer have been used to investigate the heterogeneous reactivity of HNO3 on CaCO3 at 295 K as a function of relative humidity. Transmission FTIR spectroscopy was used to probe both gas-phase and adsorbed products and showed that the reaction of HNO3 and CaCO3 is limited to the surface of the CaCO3 particle in the absence of adsorbed water. However, in the presence of water vapor, the reaction is greatly enhanced and is not limited to the surface of the particle producing both solid calcium nitrate and gaseous carbon dioxide. The enhanced reactivity of the particles is attributed to the presence of a layer of adsorbed water on the particle surface. The amount of adsorbed water on the particle surface is strongly dependent on the extent of the reaction. This can be understood in terms of the increased hydrophilicity of calcium nitrate as compared to calcium carbonate. Data from experiments using a mass-calibrated Knudsen cell reactor showed the stoichiometry for the reaction determined from gas-phase species deviated from that expected from the balanced equation. Water adsorption on the particle surface and gases dissolved into the water layer appear to be the cause of this discrepancy. The measured uptake coefficient accounting for the BET area of the sample is determined to be 2.5 + 0.1 x 10 -4 for HNO3 on CaCO3 under dry conditions and is found to increase in the presence of water vapor. Atmospheric implications of the results presented here are discussed. This current laboratory study is motivated by the above modeling studies. Here we attempt to discern whether mineral aerosols containing CaCO3 would be a sufficient sink for HNO3 that could reduce the HNO3 to NO• ratio in the atmosphere. The following issues concerning the heterogeneous reactivity of HNO3 on CaCO3 particles are addressed in this study:(1) does nitric acid react with CaCO3 according to reaction (1)?; (2) is the reaction of nitric acid limited to the surface of the CaCO3 particles?; (3) does water adsorbed on the particle surface play a key role in the reaction?; (4) do the physicochemical properties of the particle change after reaction with HNO37; (5) is the reaction 29,053

This study provides the first in-situ characterization of the products in the heterogeneous reaction 2NO2(g) + H2O(a) → HONO(g) + HNO3(a). Transmission FT-IR spectroscopy and UV−vis spectroscopy were used to probe the heterogeneous chemistry of gas-phase nitrogen dioxide on hydrated SiO2 particles. At 298 K, gaseous nitrogen dioxide reacts with adsorbed water on the surface of SiO2 particles to form adsorbed nitric acid and gas-phase nitrous acid. FT-IR spectroscopy of the hydrated SiO2 surface following reaction with NO2 provides definitive identification of the adsorbed nitric acid product. The reaction does not occur when gas-phase nitrogen dioxide is reacted on dehydrated silica particles. The gas-phase product nitrous acid was detected with both FT-IR and UV−vis spectroscopy. Although homogeneous and heterogeneous reactions can contribute to the observed HONO signal, flow cell measurements can be used to distinguish between heterogeneous and homogeneous reaction pathways leading to HONO production.

One of the most important applications of the Knudsen cell reactor is in determining heterogeneous reaction kinetics of potentially important atmospheric reactions. Knudsen cell measurements involving gas reactions on atmospherically relevant particle surfaces, including salt, carbon black, soot, and mineral dust, are often obtained using powdered samples. In this study, we have investigated the importance of gas diffusion into the underlying layers of powdered samples when determining kinetic parameters from Knudsen cell experiments. In particular, we show that the use of the geometric surface area of the sample holder is, in general, not justified in determining initial uptake coefficients or reaction probabilities because the interrogation or probe depth of gas-phase molecules into the bulk powder can be anywhere from tens to thousands of layers deep. One problem encountered by current models used to account for gas diffusion into the underlying layers is that the diffusion constant of the gas through the powdered sample must be known. Typically, diffusion constants for gases into powdered samples are unknown and are difficult to measure or accurately calculate. One way to circumvent this problem is to use thin samples such that the thickness of the sample is less than the interrogation depth of the gas-phase molecules. Under these conditions, the observed initial uptake coefficient is directly proportional to the surface area of the entire sample. This region is termed the linear mass-dependent regime and can be experimentally accessed for many, but not all, heterogeneous reactions. Several examples discussed here include heterogeneous reaction of NO 2 on γ-and R-Al 2 O 3 , R-Fe 2 O 3 , carbon black; HNO 3 on CaCO 3 ; and acetone on TiO 2 .