Reactive Uptake of ClNO2 on Aqueous Bromide Solutions

Springtime Nitrogen Oxide-Influenced Chlorine Chemistry in the Coastal Arctic

Atomic chlorine (Cl) is a strong atmospheric oxidant that shortens the lifetimes of pollutants and methane in the springtime Arctic, where the molecular halogens Cl₂ and BrCl are known Cl precursors. Here, we quantify the contributions of reactive chlorine trace gases and present the first observations, to our knowledge, of ClNO₂ (another Cl precursor), N₂O₅, and HO₂NO₂ in the Arctic. During March - May 2016 near Utqiaġvik, Alaska, up to 21 ppt of ClNO₂, 154 ppt of Cl₂, 27 ppt of ClO, 71 ppt of N₂O₅, 21 ppt of BrCl, and 153 ppt of HO₂NO₂ were measured using chemical ionization mass spectrometry. The main Cl precursor was calculated to be Cl₂ (up to 73%) in March, while BrCl was a greater contributor (63%) in May, when total Cl production was lower. Elevated levels of ClNO₂, N₂O₅, Cl₂, and HO₂NO₂ coincided with pollution influence from the nearby town of Utqiaġvik and the North Slope of Alaska (Prudhoe Bay) Oilfields. We propose a coupled mechanism linking NO_x with Arctic chlorine chemistry. Enhanced Cl₂ was likely the result of the multiphase reaction of Cl^-_(aq) with ClONO₂, formed from the reaction of ClO and NO₂. In addition to this NO_x-enhanced chlorine chemistry, Cl₂ and BrCl were observed under clean Arctic conditions from snowpack photochemical production. These connections between NO_x and chlorine chemistry, and the role of snowpack recycling, are important given increasing shipping and fossil fuel extraction predicted to accompany Arctic sea ice loss.

ClNO2 Production from N2O5 Uptake on Saline Playa Dusts: New Insights into Potential Inland Sources of ClNO2

Nitryl chloride (ClNO₂), formed when dinitrogen pentoxide (N₂O₅) reacts with chloride-containing aerosol, photolyzes to produce chlorine radicals that facilitate the formation of tropospheric ozone. ClNO₂ has been measured in continental areas; however, the sources of particulate chloride required to form ClNO₂ in inland regions remain unclear. Dust emitted from saline playas (e.g., dried lakebeds) contains salts that can potentially form ClNO₂ in inland regions. Here, we present the first laboratory measurements demonstrating the production of ClNO₂ from playa dusts. N₂O₅ reactive uptake coefficients (γ_N2O5) ranged from ∼10^-3 to 10^-1 and ClNO₂ yields (φ_ClNO2) were >50% for all playas tested except one. In general, as the soluble ion fraction of playa dusts increases, γ_N2O5 decreases and φ_ClNO2 increases. We attribute this finding to a transition from aerosol surfaces dominated by silicates that react efficiently with N₂O₅ and produce little ClNO₂ to aerosols that behave like deliquesced chloride-containing salts that generate high yields of ClNO₂. Molecular bromine (Br₂) and nitryl bromide (BrNO₂) were also detected, highlighting that playas facilitate the heterogeneous production of brominated compounds. Our results suggest that parameterizations and models should be updated to include playas as an inland source of aerosol chloride capable of efficiently generating ClNO₂.

Compilation and evaluation of gas phase diffusion coefficients of halogenated organic compounds

Organic halogens are of great environmental and climatic concern. In this work, we have compiled their gas phase diffusivities (pressure-normalized diffusion coefficients) in a variety of bath gases experimentally measured by previous studies. It is found that diffusivities estimated using Fuller's semi-empirical method agree very well with measured values for organic halogens. In addition, we find that at a given temperature and pressure, different molecules exhibit very similar mean free paths in the same bath gas, and then propose a method to estimate mean free paths in different bath gases. For example, the pressure-normalized mean free paths are estimated to be 90, 350, 90, 80, 120 nm atm in air (and N₂/O₂), He, argon, CO₂ and CH₄, respectively, with estimated errors of around ±25%. A generic method, which requires less input parameter than Fuller's method, is proposed to calculate gas phase diffusivities. We find that gas phase diffusivities in He (and air as well) calculated using our method show fairly good agreement with those measured experimentally and estimated using Fuller's method. Our method is particularly useful for the estimation of gas phase diffusivities when the trace gas contains atoms whose diffusion volumes are not known.

Weak interactions within nitryl halide heterodimers

Nitryl halides (XNO₂, X = F, Cl, Br and I) are versatile molecules that exhibit several types of interactions within XNO₂:YNO₂ heterodimers mainly governed by dispersion.

Halogen, chalcogen and pnictogen interactions in (XNO2)2homodimers (X = F, Cl, Br, I)

XNO₂(X = F, Cl, Br and I) homodimers present a large variety of interactions. A combination of pnictogen and chalcogen is stronger than single halogen bonds.

Competition and interplay between σ-hole and π-hole interactions: a computational study of 1:1 and 1:2 complexes of nitryl halides (O2NX) with ammonia

Quantum calculations at the MP2/cc-pVTZ, MP2/aug-cc-pVTZ, and CCSD(T)/cc-pVTZ levels have been used to examine 1:1 and 1:2 complexes between O(2)NX (X = Cl, Br, and I) with NH(3). The interaction of the lone pair of the ammonia with the σ-hole and π-hole of O(2)NX molecules have been considered. The 1:1 complexes can easily be differentiated using the stretching frequency of the N-X bond. Thus, those complexes with σ-hole interaction show a blue shift of the N-X bond stretching whereas a red shift is observed in the complexes along the π-hole. The SAPT-DFT methodology has been used to gain insight on the source of the interaction energy. In the 1:2 complexes, the cooperative and diminutive energetic effects have been analyzed using the many-body interaction energies. The nature of the interactions has been characterized with the atoms in molecules (AIM) and natural bond orbital (NBO) methodologies. Stabilization energies of 1:1 and 1:2 complexes including the variation of the zero point vibrational energy (ΔZPVE) are in the ranges 7-26 and 14-46 kJ mol(-1), respectively.

Uptake of gas-phase nitrous acid by pH-controlled aqueous solution studied by a wetted wall flow tube

Uptake kinetics of gas phase nitrous acid (HONO) by a pH-controlled aqueous solution was investigated by using a wetted wall flow tube. The gas phase concentration of HONO after exposure to the aqueous solution was measured selectively by the chemical ionization mass spectrometer in a high sensitive manner. The uptake rate of the gaseous HONO was found to depend on the pH of the solution. For the uptake by neutral and alkaline solutions, the gas phase concentration was observed to decay exponentially, suggesting that the uptake was fully limited by the gas phase diffusion. On the other hand, the uptake by the acidic solution was found to be determined by both the gas phase diffusion and the liquid phase processes such as physical absorption and reversible acid dissociation reaction. The decay was analyzed by the rate equations using the time dependent uptake coefficient involving the saturation of the liquid surface. While the uptake processes by the solution at pH = 2-3 were well described by those calculated using the physical and chemical parameters reported for the bulk, the uptake rates by the solution at 4 < pH < 7 deviate from the calculated ones. The present result can suggest that the pH at the liquid surface is lower than that in the bulk liquid, which is responsible for the additional resistance of mass transfer from the gas to the liquid phase.

Heterogeneous reactions of HOI, ICl and IBr on sea salt and sea salt proxies

The heterogeneous chemistry of HOI, ICl and IBr on sea salt and sea salt proxies has been studied at 274 K using two experimental approaches: a wetted wall flow tube coupled to an electron impact mass spectrometer (WWFT-MS) and an aerosol flow tube (AFT) coupled to a differential mobility analyser (DMA) and a chemical ionisation mass spectrometer (CIMS). Uptake of all three title molecules into bulk aqueous halide salt films was rapid and controlled by gas phase diffusion. Uptake of HOI gave rise to gas-phase ICl and IBr, with the latter being the predominant product whenever Br(-) was present. Only partial release of IBr was observed due to high solubility of dihalogens in the film. ICl uptake gave the same yield of IBr as HOI uptake. Uptake of ICl on NaBr aerosol was accommodation limited with alpha = 0.018 +/- 0.004 and gas phase IBr product has a yield of 0.6 +/- 0.3. The results show that HOI can act as a catalyst for activation of bromine from sea-salt aerosols in the marine boundary layer, via the reactions: HOI(aq) + Cl + H--> ICl(aq) + H(2)O(l) and ICl(aq) + Br--> IBr(aq) + Cl.

Theoretical study of the mechanism of NO2 production from NO + ClO

The reaction of NO with ClO has been studied theoretically using density-functional and wave function methods (B3LYP and CCSD(T)). Although a barrier for cis and trans additions could be located at the RCCSD(T) and UCCSD(T) levels, no barrier exists at the B3LYP/6-311+G(d) level. Variational transition state theory on a CASPT2(12,12)/ANO-L//B3LYP/6-311+G(d) surface was used to calculate the rate constants for addition. The rate constant for cis addition was faster than that for trans addition (cis:trans 1:0.76 at 298 K). The rate constant data summed for cis and trans addition in the range 200-1000 K were fit to a temperature-dependent rate in the form kdi) = 3.30 x 10(-13)T(0.558) exp(305/T) cm3.molecule(-1).s(-1), which is in good agreement with experiment. When the data are fit to an Arrhenius plot in the range 200-400 K, an activation barrier of -0.35 kcal/mol is obtained. The formation of ClNO2 from ONOCl has a much higher activation enthalpy from the trans isomer compared to the cis isomer. In fact, the preferred decomposition pathway from trans-ONOCl to NO2 + Cl is predicted to go through the cis-ONOCl intermediate. The trans --> cis isomerization rate constant is kiso = 1.92 x 10(13) exp(-4730/T) s(-1) using transition state theory.

Nitrogen dioxide multiphase chemistry: uptake kinetics on aqueous solutions containing phenolic compounds

The uptake coefficients of NO2 on aqueous solutions containing guaiacol, syringol and catechol were determined over the pH range from 1 to 13 using the wetted wall flowtube technique. The measured uptake coefficients were used to determine the rate coefficients for the reaction of the physically dissolved NO2 with the neutral and deprotonated forms of phenolic compounds listed above. These organic compounds are ubiquitous not only in biomass burning plumes but also in soils, where they form part of the building blocks of humic acids. The NO2 uptake kinetics on solutions containing guaiacol, syringol or catechol were observed to be strongly pH dependent with uptake coefficients increasing from below 10(-7), under acidic conditions, to more than 10(-5) at pH values above 10. This behaviour illustrates the difference of reactivity between the neutral phenolic species and the phenoxide ions. The corresponding second order rate coefficients were typically observed to increase from 10(5) M(-1) s(-1) for the neutral compounds to a minimum of 10(8) M(-1) s(-1) for the phenoxide ions.

Temperature Dependence of the Mass Accommodation Coefficients of 2-Nitrophenol, 2-Methylphenol, 3-Methylphenol, and 4-Methylphenol on Aqueous Surfaces

The uptake of 2-nitrophenol, 2-methylphenol, 3-methylphenol, and 4-methylphenol on aqueous surfaces was investigated between 278 and 303 K, using the wetted-wall flow tube technique coupled with UV absorption spectroscopic detection. The uptake coefficients gamma were found to be independent of the aqueous phase composition and of the gas-liquid contact times. In addition, the uptake coefficients and the derived mass accommodation coefficients alpha show a negative temperature dependence in the temperature range studied. The mass accommodation coefficients decrease from 5.2 x 10(-3) to 8.3 x 10(-4), from 5.0 x 10(-3) to 3.1 x 10(-4), from 6.7 x 10(-3) to 7.3 x 10(-4), and from 1.2 x 10(-2) to 5.9 x 10(-4) for 2-nitrophenol, 2-methylphenol, 3-methylphenol, and 4-methylphenol, respectively. These results are used to discuss the incorporation of these species into the liquid using the nucleation theory. These data combined with the Henry's law constants were used to estimate the partitioning of the phenolic compounds between gaseous and aqueous phases and the corresponding atmospheric lifetimes under clear sky (tau(gas)) and cloudy conditions (tau(multiphase)) have then been derived.