Heterogeneous Reactions of Gaseous HNO₃ and NO₂ on the Clay Minerals Kaolinite and Pyrophyllite

Abstract

Airborne clay mineral particles have long atmospheric lifetimes due to their relatively small size. To assess their impact on trace atmospheric gases, we investigated heterogeneous reactions on prototype clay minerals. Diffuse reflectance infrared spectroscopy identified surface-adsorbed products formed from the uptake of gaseous nitric acid and nitrogen dioxide on kaolinite and pyrophyllite. For kaolinite, a 1:1 phyllosilicate, HNO3 molecularly adsorbed onto the octahedral aluminum hydroxide and tetrahedral silicon oxide surfaces. Also detected on the aluminum hydroxide surface were irreversibly adsorbed monodentate, bidentate, bridged, and water-coordinated nitrate species as well as surface-adsorbed water. Similar adsorbed products formed during the uptake of NO2 on kaolinite at relative humidity (RH) of 0%, and the reaction was second order with respect to reactive surface sites and 1.5 +/- 0.1 for NO2. Reactive uptake coefficients, calculated using Brunauer, Emmett, and Teller surface areas, increased from (8.0 +/- 0.2) x 10(-8) to (2.3 +/- 0.4) x 10(-7) for NO2 concentrations ranging from 0.56 x 10(13) to 8.8 x 10(13) molecules cm(-3). UV-visible spectroscopy detected gaseous HONO as a product for the reaction of NO2 on wet kaolinite. The uptake of HNO3 on pyrophyllite, a 2:1 phyllosilicate, resulted in stronger signal for nitric acid molecularly adsorbed on the silicon oxide surface compared to kaolinite. Monodentate, bridged, and water-coordinated nitrate species bound to aluminum sites also formed during this reaction indicating that reactive sites on edge facets are important for this system. The uptake of NO2 on pyrophyllite, gammaBET = (7 +/- 1) x 10(-9), was significantly lower than kaolinite because NO2 did not react with the dominant tetrahedral silicon oxide surface. These results highlight general trends regarding the reactivity of tetrahedral silicon oxide and octahedral aluminum hydroxide clay surfaces and indicate that the heterogeneous chemistry of clay aerosols varies with mineralogy and cannot be predicted by elemental analysis.

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

“…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 decline of the initial g with RH can be ascribed to competitive adsorption between H 2 O and NO 2 , similar to the competitive adsorption of O 3 , NO 2 or COS with H 2 O on mineral oxides. 22,24,25 It has been found that a monolayer of H 2 O on kaolin is achieved at 13% RH, 6 and at 10% RH on hematite. 24 The initial g of H 2 O on mineral dust (4.2 AE 0.7 Â 10 À2 ) 42 is much larger than that of NO 2 , as discussed above.…”

“…It should be noted that kaolin consists of neutral layers containing one octahedral aluminium hydroxide sheet bonded to one tetrahedral silicon oxide sheet through a plane of shared oxygen atoms. 6 Although kaolin has a low shrink-swell capacity compared to montmorillonite, it swells slightly with increased water adsorption, 43 which should make more reactive sites (M-O or M-OH) available. Thus, an increase of g t (SS) and the integrated uptake capacity was observed when the RH was higher than 32%.…”

“…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%). 18,23 However, g has been found to show a strong dependence on humidity for some reactions.…”

“…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.…”