Depth-Dependent Dissociation of Nitric Acid at an Aqueous Surface: Car−Parrinello Molecular Dynamics

) is small, potentially allowing reconstruction of past shifts in tropospheric oxidation pathways from ice cores. Assuming a Rayleigh-type process we find fractionation constants ε of −60±15‰, 8±2‰ and 1±1‰, for δ 15 N, δ 18 O and 17 O, respectively. A photolysis model yields an upper limit for the photolytic fractionation constant 15 ε of δ 15 N, consistent with lab and field measurements, and demonstrates a high sensitivity of 15 ε to the incident actinic flux spectrum. The photolytic 15 ε is process-specific and therefore applies to any snow covered location. Previously published 15 ε values are not representative for conditions at the Earth surface, but apply only to the UV lamp used in the reported experiment (Blunier et al., 2005;Jacobi et al., 2006). Depletion of oxygen stable isotopes is attributed to photolysis followed by isotopic exchange with water and hydroxyl radicals. Conversely, 15 N enrichment of the NO

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“…Thomas et al, 2007;Wang et al, 2009). The pH and ionic strength control acid dissociation but are not well characterized in the QLL.…”

Section: Mechanisms Of Isotopic Fractionation In Snowmentioning

confidence: 99%

Photolysis imprint in the nitrate stable isotope signal in snow and atmosphere of East Antarctica and implications for reactive nitrogen cycling

et al. 2009

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“…25 Both theory and experiment suggest that nitric acid is not fully dissociated at the interface of aqueous solutions. [26][27][28][29][30][31][32][33][34] Indeed, recent results suggest that undissociated HNO 3 interacts very weakly with water. 33,34 Because of the presence of significant concentrations of water vapor in the lower atmosphere, there is an abundance of water available on surfaces such as roads, buildings, etc., as well as in airborne particles.…”

Section: Introductionmentioning

confidence: 99%

Experimental and theoretical studies of the interaction of gas phase nitric acid and water with a self-assembled monolayer

Moussa

Stern

Raff

et al. 2013

Phys. Chem. Chem. Phys.

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Experimental and theoretical studies of the interaction of gas phase nitric acid and water with a self-assembled monolayer surface of a germanium infrared-transmitting attenuated total reflectance (ATR) crystal that was coated with a thin layer of silicon oxide (SiO x ). The SAM was exposed at 298 AE 2 K to dry HNO 3 in a flow of N 2 , followed by HNO 3 in humid N 2 at a controlled relative humidity (RH) between 20-90%. For comparison, similar studies were carried out using a similar crystal without the SAM coating. Changes in the surface were followed using Fourier transform infared spectroscopy (FTIR). In the case of the SAMcoated crystal, molecular HNO 3 and smaller amounts of NO 3 À ions were observed on the surface upon exposure to dry HNO 3 . Addition of water vapor led to less molecular HNO 3 and more H 3 O + and NO 3 À complexed to water, but surprisingly, molecular HNO 3 was still evident in the spectra up to 70% RH.This suggests that part of the HNO 3 observed was initially trapped in pockets within the SAM and shielded from water vapor. After increasing the RH to 90% and then exposing the film to a flow of dry N 2 , molecular nitric acid was regenerated, as expected from recombination of protons and nitrate ions as water evaporated. The nitric acid ultimately evaporated from the film. On the other hand, exposure of the SAM to HNO 3 and H 2 O simultaneously gave only hydronium and nitrate ions. Molecular dynamics simulations of defective SAMs in the presence of HNO 3 and water predict that nitric acid intercalates in defects as a complex with a single water molecule that is protected by alkyl chains from interacting with additional water molecules. These studies are consistent with the recently proposed hydrophobic nature of HNO 3 . Under atmospheric conditions, if HNO 3 is formed in organic layers on surfaces in the boundary layer, e.g. through NO 3 or N 2 O 5 reactions, it may exist to a significant extent in its molecular form rather than fully dissociated to nitrate ions. IntroductionNitric acid is considered to be the end-product of oxidation of oxides of nitrogen in the atmosphere. 1 This acid reacts with ammonia and amines to form solid or aqueous phase nitrate particles, and can also be taken up into other types of atmospheric particles. Removal of gaseous HNO 3 and particulate nitrates takes place primarily through wet and dry deposition, in what were once considered to be terminal removal processes. However, there are now a number of laboratory and field studies that suggest that nitric acid can be converted back into gaseous oxides of nitrogen such as NO, NO 2 and HONO. 2-8Nitric acid is a strong acid, but it requires a cluster of at least four water molecules to dissociate in the gas phase. [9][10][11] In bulk aqueous solutions and on ice, nitric acid can form hydrates with 1, 2 or 3 water molecules, depending on the concentration. 12-24 Such hydrates, as well as the HNO 3 dimer, have also been seen on solid surfaces during the hydrolysis of NO 2 at

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“…The many studies indicating a preference for the molecular form of nitric acid at a water surface rather than the deprotonated NO 3 À anion offer an additional driving force for reaction immediately following autoionization. 6,[43][44][45][46][47] Based on gas phase calculations, Liu and Goddard suggest that NO 2 hydrolysis likely occurs through direct formation of the cis asymmetric dimer followed by isomerization to the trans isomer circumventing the high symmetric to asymmetric barrier. 18 They found no barrier for formation of the cis isomer and the barrier for the cis-trans isomerization was predicted to be 3 kcal mol À1 above the cis minimum.…”

mentioning

confidence: 99%

Reaction of a charge-separated ONONO2 species with water in the formation of HONO: an MP2 Molecular Dynamics study

Varner

Finlayson-Pitts

Gerber

2014

Phys. Chem. Chem. Phys.

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À) with water is modelled in ONONO 2 Á (H 2 O) 4 clusters. Molecular Dynamics simulations using second-order Møller-Plesset perturbation (MP2) theory support the feasibility of the reaction of a charge-separated species to produce HONO and nitric acid.It is well known that nitrous acid (HONO) plays a major role in the chemistry of the atmosphere, e.g. as a source of hydroxyl (OH) radicals. 1,2 While heterogeneous reactions of NO 2 with water on tropospheric surfaces are believed to be important in generating HONO, the actual mechanisms involved are not known with certainty. On a chemical basis, the involvement of the NO 2 dimer seems reasonable, 3 although other mechanisms have been proposed for production of HONO both in the dark and under irradiation. [4][5][6][7][8][9][10][11] Initial steps of one proposed mechanism 3 include NO 2 dimerization to form N 2 O 4 and autoionization of asymmetric N 2 O 4 , i.e. ONONO 2 , to form (NO + )(NO 3 À ).ONONO 2 -(NO + )(NO 3 À )Reaction of the (NO + )(NO 3 À ) ion pair with water to form HONO and nitric acid (HONO 2 ) is the final step. A number of studies have addressed the formation of NO 2 dimers in both the symmetric (with a central NN bond) and asymmetric (ONONO 2 ) forms, and the interconversion between the two. [12][13][14][15][16][17][18] While the symmetric form of the dimer is favoured, there are pathways that can generate the asymmetric trans-ONONO 2 form that is the most likely precursor to the ion pair and subsequently HONO. 12,13,15,16,18 de Jesus Medeiros and Pimentel, in a study of the dimerization of NO 2 and the symmetric to asymmetric isomerization of the dimer in water clusters, predicted direct HONO formation from the NO 2 Á(H 2 O) n + NO 2 reaction and from the transition state for symmetric to asymmetric isomerization. 15 These modifications of the proposed mechanism described above bypass formation of (NO + )(NO 3 À ) through autoionization or, in the case of the NO 2 Á(H 2 O) n + NO 2 reaction, formation of the NO 2 dimer. Further exploration of the (NO + )(NO 3 À ) path will allow for comparison between potential routes and provide additional insight into the detailed mechanism that converts NO 2 in the presence of water to HONO. This is particularly useful for understanding other important reactions of NO 2 on surfaces in the presence of water, such as the reaction with HCl to form ClNO, since such chemistry likely shares common intermediates with NO 2 hydrolysis. [19][20][21] In ab initio Molecular Dynamics [AIMD], the reaction trajectory is propagated classically on the ab initio potential surface providing a means for studying reactive processes and the influence of the immediate environment. 22 The system wave function or electron density is calculated at each time step, i.e. on-the-fly, for determination of the energy, the interatomic forces and other properties. Partial charges are an obvious metric to investigate charge separation in the reaction of an ion pair as charge characteristics can indicate a shift in the nature of the interactio...

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Depth-Dependent Dissociation of Nitric Acid at an Aqueous Surface: Car−Parrinello Molecular Dynamics

Cited by 54 publications

References 102 publications

Photolysis imprint in the nitrate stable isotope signal in snow and atmosphere of East Antarctica and implications for reactive nitrogen cycling

Photolysis imprint in the nitrate stable isotope signal in snow and atmosphere of East Antarctica and implications for reactive nitrogen cycling

Experimental and theoretical studies of the interaction of gas phase nitric acid and water with a self-assembled monolayer

Reaction of a charge-separated ONONO2 species with water in the formation of HONO: an MP2 Molecular Dynamics study

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