Atmospheric Implications Research Articles

Hydration of acetic acid-dimethylamine complex and its atmospheric implications

Atmospheric aerosols are closely related to weather, climate and human health, and New particle formation (NPF) is a major source of atmospheric aerosols. Currently, some field observations and experiments indicate that acetic acid (HOAc) could be involved in NPF events. However, mechanism of acetic acid nucleation is still unclear. In this study, the low-lying structures and thermodynamics of acetic acid (HOAc)-dimethylamine (DMA)-water (W) system were studied at PW91PW91/6–311++G (3df, 3pd) level. We found that acetic acid forms relatively stable clusters with dimethylamine, and that proton transfer enhances the strength of the hydrogen bond in (HOAc) (DMA) (H2O)n (n = 2–4) clusters. Temperature has an important effect on the distribution of isomers, especially for (HOAc) (DMA) (H2O)2 clusters. Besides, all the isomers contribute to the nucleation of clusters. The various RH has a negligible effect on the hydrate distribution. However, the non-hydrated clusters are always dominant and they are easy to form stable cluster, as seen from the comparison of hydrate distributions and cluster formation rates. The above analyses indicate that (HOAc) (DMA) is relatively stable and some larger clusters based on (HOAc) (DMA) may participate in new particle formation.

Theoretical Investigation of the Photoexcited NO2 +H2 O reaction at the Air-Water Interface and Its Atmospheric Implications.

The atmospheric role of photochemical processes involving NO2 beyond its dissociation limit (398 nm) is controversial. Recent experiments have confirmed that excited NO2 * beyond 420 nm reacts with water according to NO2 * +H2 O→HONO+OH. However, the estimated kinetic constant for this process in the gas phase is quite small (k≈10-15 -3.4×10-14 cm3 molecule-1 s-1 ) suggesting minor atmospheric implications of the formed radicals. In this work, ab initio molecular dynamics simulations of NO2 adsorbed at the air-water interface reveal that the OH production rate increases by about 2 orders of magnitude with respect to gas phase, attaining ozone reference values for NO2 concentrations corresponding to slightly polluted rural areas. This finding substantiates the argument that chemistry on clouds can be an additional source of OH radicals in the troposphere and suggests directions for future laboratory experimental studies.

Cover Feature: Theoretical Investigation of the Photoexcited NO2+H2O reaction at the Air–Water Interface and Its Atmospheric Implications (Chem. Eur. J. 61/2019)

Cover Feature: Theoretical Investigation of the Photoexcited NO₂+H₂O reaction at the Air–Water Interface and Its Atmospheric Implications (Chem. Eur. J. 61/2019)

Halogen activation and radical cycling initiated by imidazole-2-carboxaldehyde photochemistry

Abstract. Atmospheric aerosol particles can contain light-absorbing organic compounds, also referred to as brown carbon (BrC). The ocean surface and sea spray aerosol particles can also contain light-absorbing organic species referred to as chromophoric dissolved organic matter (CDOM). Many BrC and CDOM species can contain carbonyls, dicarbonyls or aromatic carbonyls such as imidazole-2-carboxaldehyde (IC), which may act as photosensitizers because they form triplet excited states upon UV–VIS light absorption. These triplet excited states are strong oxidants and may initiate catalytic radical reaction cycles within and at the surface of atmospheric aerosol particles, thereby increasing the production of condensed-phase reactive oxygen species (ROS). Triplet states or ROS can also react with halides, generating halogen radicals and molecular halogen compounds. In particular, molecular halogens can be released into the gas phase, which is one halogen activation pathway. In this work, we studied the influence of bromide and iodide on the photosensitized production and release of hydroperoxy radicals (HO2) upon UV irradiation of films in a coated wall flow tube (CWFT) containing IC in a matrix of citric acid (CA) irradiated with UV light. In addition, we measured the iodine release upon irradiation of IC ∕ CA films in the CWFT. We developed a kinetic model coupling photosensitized CA oxidation with condensed-phase halogen chemistry to support data analysis and assessment of atmospheric implications in terms of HO2 production and halogen release in sea spray particles. As indicated by the experimental results and confirmed by the model, significant recycling of halogen species occurred via scavenging reactions with HO2. These prevented the full and immediate release of the molecular halogen (bromine and iodine) produced. Recycling was stronger at low relative humidity, attributed to diffusion limitations. Our findings also show that the HO2 production from BrC or CDOM photosensitized reactions can increase due to the presence of halides, leading to high HO2 turnover, in spite of low release due to the scavenging reactions. We estimated the iodine production within sea salt aerosol particles due to iodide oxidation by ozone (O3) at 5.0×10-6 M s−1 assuming O3 was in Henry's law equilibrium with the particle. However, using an O3 diffusion coefficient of 1×10-12 cm2 s−1, iodine activation in an aged, organic-rich sea spray is estimated to be 5.5×10-8 M s−1. The estimated iodine production from BrC photochemistry based on the results reported here amounts to 4.1×10-7 M s−1 and indicates that BrC photochemistry can exceed O3 reactive uptake in controlling the rates of iodine activation from sea spray particles under dry or cold conditions where diffusion is slow within particles.

Hydration of glycolic acid sulfate and lactic acid sulfate: Atmospheric implications

Organosulfates constitute an important component of secondary organic aerosols. Due to their polar and hydrophilic nature, organosulfates have the ability to bind with water and their water content may substantially affect the hygroscopicity of aerosols. However, there is poor information on how they interact with water and how their hydrates are distributed in the gas-phase. This study uses electronic structure calculations to investigate the structures and thermodynamics of water uptake by glycolic acid sulfate and lactic acid sulfate in the gas-phase, and the equilibrium distributions of their hydrates at varying degrees of humidity and different temperatures. Given the demonstrated diverse role of neutral and charged species in aerosol formation, the hydration of these organosulfates, both electrically neutral and negatively charged (deprotonated) were investigated. We find that electrically neutral glycolic acid sulfate is more extensively hydrated than its deprotonated counterpart, forming relatively high concentrations of mono-, di-, and tri-hydrates at most tropospheric temperatures and relative humidity, while the latter mostly forms the monohydrate at similar conditions. The opposite situation is observed for lactic acid sulfate where the deprotonated form is the most extensively hydrated, despite their hydrates are formed at low concentrations. However, due to high dissociation constants of organosulfates coupled to their increasing partition to the particle phase with increasing relative humidity, the hydrates of electrically neutral organosulfates studied here would mostly partition into the particle phase and reinforce the effects of their corresponding deprotonated counterparts in altering the hygroscopic properties of aerosol particles. This study can serve as a basis to evaluate the hydration pattern of other organosulfates and assess their implications in aerosol formation.

Tropospheric degradation of 2-fluoropropene (CH3CF[dbnd]CH2) initiated by hydroxyl radical: Reaction mechanisms, kinetics and atmospheric implications from DFT study

Degradation of hydrofluoro-olefins (HFOs) with oxidants plays a significant role in the troposphere. Thus, we have investigated detail theoretical calculations of hydroxyl radical (•OH) initiated oxidation of 2-fluoropropene (CH3CFCH2) using M06-2X/6-311++G(d,p) level of theory. Here, we have considered different possible H-abstraction and OH addition for the degradation of CH3CFCH2 molecule. The potential energy analysis shows that OH-addition channels are more dominant than H-abstraction channels. The calculated reaction enthalpies (ΔrH°) and Gibbs free energies (ΔrG°) also suggest that OH-addition reaction channels are more favourable than H-abstraction channels. The overall rate coefficients for CH3CFCH2 + •OH reaction is calculated within the temperature range of 250–450 K. The observed overall rate coefficient (2.01 × 10−11 cm3 molecule−1 s−1) at 298 K for the titled reaction is found to be in good agreement with the earlier reported experimental rate coefficient. The calculated percentage branching ratio shows that the contribution of OH-addition to α-carbon and β-carbon of CH3CFCH2 molecule are 85.10% and 14.20% to the overall rate coefficient while H-abstractions have a negligible contribution. Based on the kinetics calculations, the atmospheric lifetime of the titled molecule is found to be 0.6 days. Further, we have also explored the degradation pathways of OH-addition product radicals and found acetyl fluoride (CH3CFO) and formaldehyde (HCHO) are the end degradation products.

On the latitudinal variation of the semiannual oscillation in received solar radiation and temperature

This paper characterizes the semiannual oscillation (SAO) in the received solar radiation (RSR) as a function of latitude for Earth, as a paradigm of a planet with an inclined rotational axis, and studies atmospheric implications. The latitude variations of the RSR SAO amplitude and phase are complex yet systematic. To investigate atmospheric response to the SAO in received solar radiation, we focus on Earth, examining monthly average surface temperature data over a wide latitude range. The latitude variation of surface temperature SAO amplitude and phase show marked similarity to those of the RSR - possible evidence of a residual RSR footprint in the atmospheric temperature and conflicting with current empirical representations of zonal temperature SAO phase and amplitude. For RSR and surface temperature as functions of latitude: (1) properties of SAO are independent of orbital eccentricity, which contributes only a constant global component to local annual oscillation (AO); (2) SAO peaks near equinox at lower latitudes and near solstices at higher latitudes, transitioning in phase over 40° − 60° latitude; and (3) SAO strengths (amplitude relative to local AO amplitude) of RSR and surface temperature are largest near the poles and equator. Also discussed is the presence of SAO in the global average surface temperature while SAO is negligible in the global average RSR. The approximate altitude independence of latitude variations in RSR SAO and the similarity of local surface temperature SAO suggest investigation of a simplified working hypothesis of similar behavior of observed local temperature SAO at higher altitude.

Hygroscopic properties of saline mineral dust from different regions in China: geographical variations, compositional dependence and atmospheric implications

Hygroscopic properties of saline mineral dust from different regions in China: geographical variations, compositional dependence and atmospheric implications

Cl-Atom-Initiated Atmospheric Degradation of Saturated Cyclic Hydrocarbons: Kinetic and Mechanistic Investigation.

The temperature-dependent reaction kinetics of methyl cyclohexane (MCH) and methyl cyclopentane (MCP) with Cl atoms were experimentally explored via the relative rate technique. Gas chromatography coupled with a flame ionization detector was employed to follow the reactant as well as the reference compound concentrations during the course of reaction. Gas chromatography coupled with mass spectrometry and gas chromatography coupled with infrared spectroscopy were used as the diagnostic tools for the detection and identification of the products in the title reaction. The rate coefficients for the reaction of Cl atoms with methyl cyclohexane and methyl cyclopentane were measured in the temperature range of 283-363 K at 760 Torr using isoprene and propylene as reference compounds. To support the experimental results, computational calculations were performed over the temperature range of 200-400 K using canonical variational transition state theory coupled with small curvature tunneling (CVT/SCT), conventional transition state theory (CTST) coupled with Wigner tunneling corrections, and CVT coupled with Wigner tunneling methods using the MP2 level of theory with 6-31G(d,p) as the basis set. The temperature-dependent rate coefficients were measured for the reaction of methyl cyclohexane and methyl cyclopentane with Cl atoms to be k(MCH) = [(4.48 ± 0.75) × 10-11]exp[(604 ± 25)/T] cm3 molecule-1 s-1 and k(MCP) = [(5.71 ± 0.66) × 10-12]exp[(1819 ± 669)/T] cm3 molecule-1 s-1, respectively. The measured rate coefficients at 298 K for the reactions of Cl atoms with methyl cyclohexane and methyl cyclopentane are k298KMCH = (3.36 ± 0.34) × 10-10 cm3 molecule-1 s-1 and k298KMCP = (2.25 ± 0.24) × 10-10 cm3 molecule-1 s-1, respectively. The conceivable atmospheric degradation mechanism for the reaction of methyl cyclohexane as well as methyl cyclopentane with Cl atoms was projected based on the products obtained during the reaction. Atmospheric implications, cumulative atmospheric lifetimes, thermochemistry, ozone formation potentials, and branching ratios of these molecules were also calculated and have been reported in this article.

Pollution profiles of volatile organic compounds from different urban functional areas in Guangzhou China based on GC/MS and PTR-TOF-MS: Atmospheric environmental implications

Ozone is currently the main atmospheric pollutant in the Pearl River Delta region, China. The atmospheric volatile organic compounds (VOCs) are key participators in the formation of ozone. Thus, the pollution profiles of ambient VOCs in Guangzhou city were measured using both offline (gas chromatography-mass spectrometry (GC/MS)) and online (proton transfer reaction time-of-flight mass spectrometer (PTR-TOF-MS)) methods. Three different functional areas were selected as vehicle detection station (VSD), roadside of city expressway (RCE), and Higher Education Mega Center (HEMC). GC/MS measurement results demonstrated that the total concentration of VOCs (TVOCs) with mean values of 55.0, 44.9 and 24.5 ppb at VSD, RCE and HEMC sites, respectively. At VSD site, the dominant group is aromatic hydrocarbons (accounting for 41.4% of TVOCs), in which toluene accounts for the largest proportion (17.6%). In contrast, aliphatic hydrocarbons were the dominant group at RCE (50.2% of TVOCs) and HEMC (47.4% of TVOCs), in which n-hexane was the major compound (18.4% and 21.6% at RCE and HEMC sites, respectively). Comparatively, the mean TVOC levels analyzed with PTR-TOF-MS were much higher at all studied sites, because large quantity of oxygenated VOCs (OVOCs accounting for 72.0%, 76.3% and 73.5% of TVOCs at VSD, RCE and HEMC sites, respectively) could be detected, but not by GC/MS. Among OVOCs, methanol was found as the most abundant component at all three studied sites. The assessment of atmospheric photochemical activity showed that the average contributions to ozone formation potential (OFP) from VOCs were 8.5 × 102, 7.3 × 102 and 8.2 × 102 μg m−3 at VSD, RCE and HEMC sites, respectively, and OVOCs were the dominant contributors for OFP. The result can provide the references for local government to eliminate VOCs and attenuate ozone pollution.

Cl atom initiated tropospheric chemistry of ethyl butyrate

Temperature dependent rate coefficients for the reaction of ethyl butyrate with Cl atom were measured using relative rate (RR) technique. Gas chromatography with Flame Ionization Detector (GC-FID) was used to follow the concentrations of the reactants during experiments. The rate coefficients were computed using CVT/SCT/ISPE method. The product analysis for the title reaction was carried out using GC-MS and GC-IR as analytical tools. Based on the obtained products, the probable degradation reaction mechanism for the reaction of ethyl butyrate with Cl atom in the troposphere was proposed. Thermochemistry, branching ratios, atmospheric implications and cumulative lifetimes were also calculated.

The effects of surfactants on the heterogeneous uptake of sulfur dioxide on hematite

Surfactants are often found to be associated with atmospheric particles. They can change the physicochemical properties of these particles, and thereby impact their chemical reactivity. However, their influences on heterogeneous reactions of atmospheric trace gases have not been paid much attention. In this study, the impacts of different surfactants on the heterogeneous conversion of SO2 on hematite were investigated by using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), including cationic cetyltrimethylammonium bromide (CTAB), anionic sodium dodecyl sulfate (SDS) and nonionic polyethylene glycol tert-octylphenyl ether (Triton X-100). Results indicated that heterogeneous reactivities of hematite-surfactant mixtures were significantly different due to the presence of different surfactants. Under dry conditions, heterogeneous reactivities decreased with the increase of their concentrations, but the higher reactivities were observed at an appropriate concentration range of CTAB and SDS when compared to pure hematite, showing their promoting effects at this concentration range. Under the condition of 58% relative humidity, the heterogeneous reactivities increased first and then decreased with the increasing mass fractions of CTAB and SDS, but pure hematite showed the highest reactivity. While the mixtures containing Triton X-100 had stronger ability to suppress sulfate formation under 58% RH than under dry condition, though these mixtures under 58% RH presented same trend as under dry condition. Water molecules can renew surface reactive sites and promote sulfate formation at an appropriate mass fraction of CTAB or SDS, water molecules can also promote the adsorption mode transition of Triton X-100 from adsorption through terminal OH groups to adsorption through ethoxyl groups, resulting in more surface active sites being covered and a drastically decrease in the reactivity. A possible mechanism for the impact of surfactants on the heterogeneous conversion of SO2 on hematite is proposed, and atmospheric implications based on these results are discussed.

Theoretical Studies of the Gas-Phase Reactions of S-Methyl Methanesulfinothioate (Dimethyl Thiosulfinate) with OH and Cl Radicals: Reaction Mechanisms, Energetics, and Kinetics.

The mechanisms, energetics, and kinetics of the gas-phase reactions of terrestrial plant-derived dimethyl thiosulfinate (DMTS) with the atmospheric oxidants OH and Cl radicals were investigated using high-level ab initio calculations. The results show that the addition of OH and Cl radicals to the sulfinyl [-S(═O)] of DMTS, followed by S(═O)-S single bond cleavage to form methanesulfinic acid + CH3S• and methanesulfinyl chloride + CH3S•, respectively, are the more dominant reactions. The barrier heights for the reactions with OH and Cl radicals were found to be -5.6 and -12.7 kcal/mol relative to the energies of the starting reactants, respectively, when computed at the CCSD(T)/aug-cc-pVTZ//M06-2X/6-311++G(3df,3pd) level of theory. The rate constants for all possible pathways of DMTS + •OH/•Cl reactions were investigated using the MESMER kinetics code over the temperature range between 200 and 300 K. The calculated global rate constants for the DMTS + •OH and DMTS + •Cl reactions at 300 K were found to be 1.42 × 10-11 and 3.72 × 10-11 cm3 molecule-1 s-1, respectively. In addition, the thermochemistry of all possible paths and branching ratios was determined. The atmospheric chemistry implications of the DMTS + •OH/•Cl reactions are discussed.

Mechanistic Insight into the Reaction of Organic Acids with SO3 at the Air–Water Interface

AbstractThe gas‐phase reaction of organic acids with SO3 has been recognized as essential in promoting aerosol‐particle formation. However, at the air–water interface, this reaction is much less understood. We performed systematic Born–Oppenheimer molecular dynamics (BOMD) simulations to study the reaction of various organic acids with SO3 on a water droplet. The results show that with the involvement of interfacial water molecules, organic acids can react with SO3 and form the ion pair of sulfuric‐carboxylic anhydride and hydronium. This mechanism is in contrast to the gas‐phase reaction mechanisms in which the organic acid either serves as a catalyst for the reaction between SO3 and H2O or reacts with SO3 directly. The distinct reaction at the water surface has important atmospheric implications, for example, promoting water condensation, uptaking atmospheric condesation species, and incorporating “SO42−” into organic species in aerosol particles. Therefore, this reaction, typically occurring within a few picoseconds, provides another pathway towards aerosol formation.

Mechanistic Insight into the Reaction of Organic Acids with SO3 at the Air-Water Interface.

The gas-phase reaction of organic acids with SO3 has been recognized as essential in promoting aerosol-particle formation. However, at the air-water interface, this reaction is much less understood. We performed systematic Born-Oppenheimer molecular dynamics (BOMD) simulations to study the reaction of various organic acids with SO3 on a water droplet. The results show that with the involvement of interfacial water molecules, organic acids can react with SO3 and form the ion pair of sulfuric-carboxylic anhydride and hydronium. This mechanism is in contrast to the gas-phase reaction mechanisms in which the organic acid either serves as a catalyst for the reaction between SO3 and H2 O or reacts with SO3 directly. The distinct reaction at the water surface has important atmospheric implications, for example, promoting water condensation, uptaking atmospheric condesation species, and incorporating "SO4 2- " into organic species in aerosol particles. Therefore, this reaction, typically occurring within a few picoseconds, provides another pathway towards aerosol formation.

Photo-Oxidation Reaction Kinetics and Mechanistics of 4-Hydroxy-2-butanone with Cl Atoms and OH Radicals in the Gas Phase.

The temperature-dependent rate coefficients for the gas-phase reaction of 4-hydroxy-2-butanone (4H2BN) with Cl atoms and OH radicals were explored experimentally using relative rate technique and computational methods. The concentrations of the reactants as well as products were followed using gas chromatography (GC) with the flame ionization detector, GC/mass spectrometry, and GC/infrared spectroscopy as analytical techniques. Formaldehyde was obtained as the major product during the title reaction. The kinetics of 4H2BN with Cl atoms and OH radicals were measured over the temperature range of 298-363 K at 760 Torr in the N2 atmosphere using C3H8, C2H4, isopropanol, and n-propanol as reference compounds. The temperature-dependent rate coefficients for the reaction of 4H2BN with Cl atoms and OH radicals were obtained as kExpt( T) = [(1.52 ± 0.86) × 10-26] T5 exp[(2474 ± 450)/ T] cm3 molecule-1 s-1 and kexpt( T) = [(2.09 ± 0.24) × 10-12] exp[-(409 ± 15)/ T] cm3 molecule-1 s-1, respectively. Theoretical calculations were carried out at the M062X/6-31G(d,p) and M06-2X/6-31+G(d,p) level of theories, and the rate coefficients for H abstraction reactions were evaluated using the canonical variational transition state theory with the inclusion of small-curvature tunneling correction over the temperature range of 200-400 K. The rate coefficients obtained over the studied temperature range were used to fit the data, and the Arrhenius expression was obtained to be kCl(Theory) (200-400 K) = (6.10 × 10-25) T4.42 exp(2397/ T) cm3 molecule-1 s-1, kOH(Theory) (200-400 K) = (1.13 × 10-19) T2.27 exp(1505/ T) cm3 molecule-1 s-1, respectively, for the reactions of Cl atoms and OH radicals with 4H2BN. The possible reaction mechanism proposed based on the obtained products for the title reaction, thermochemistry, branching ratios, and atmospheric implications and cumulative lifetime of 4H2BN were also explored in this study.

Kinetics and mechanisms of the gas-phase reactions of OH radicals with three C15 alkanes

The reaction constants and products for the reactions of OH radicals with different C15 alkanes, were determined by using Proton-Transfer-Reaction Mass Spectrum (PTR-MS) in the smog chamber. Rate coefficients (in units of 10−11 cm3 molecule−1 s−1) k1(pentadecane) = 1.72 ± 0.18,k2(2,6,10-trimethyldodecane) = 1.90 ± 0.20, k3(nonylcyclohexane) = 2.09 ± 0.22 were first obtained by the relative rate method using toluene and m-xylene as reference compounds at (298 ± 1)K and atmospheric pressure. In addition, the products of these reactions were measured by PTR-MS simultaneously. The results show that the major gas phase products observed in the OH radicals reactions were aldehydes and ketones for C15 alkanes. The possible reaction mechanisms were speculated and the atmospheric implications were also discussed.

Influence of Ammonia and Water on the Fate of Sulfur Trioxide in the Troposphere: Theoretical Investigation of Sulfamic Acid and Sulfuric Acid Formation Pathways.

Reaction of ammonia with SO3 as a potential source of sulfamic acid in the troposphere has been investigated by means of electronic structure and chemical kinetic calculations. Besides, the hydrolysis reaction, which is known to be a major atmospheric decay channel of SO3, has also been investigated. The catalytic effects of ammonia and water on both the reactions have been studied. Rate coefficients for all the studied reaction channels were calculated using the transition state theory employing pre-equilibrium approximation. Calculated rate coefficients for a number of catalyzed hydrolysis and ammonolysis processes were found to be much higher (by ∼105 to ∼109 times) than the gas kinetic limit at ambient temperature. With decrease in temperature because of negative temperature dependence of rate coefficients, that difference became even larger (up to ∼1016 times). Therefore, in order to remove the discrepancies, rate coefficients for all the studied reaction channels were calculated by means of the master equation. The results showed marked improvements, with only one channel showing a slightly higher rate coefficient above the gas kinetic limit. The rate coefficients for catalyzed channels obtained from the master equation also showed negative temperature dependence, albeit to a much smaller extent. The uncatalyzed ammonolysis reaction, similar to the corresponding hydrolysis, was found to be too slow to have any practical atmospheric implication. For both reactions, ammonia-catalyzed pathways have higher rate coefficients than water-catalyzed ones. Between hydrolysis and ammonolysis, the latter showed a higher rate coefficient. In spite of that, ammonolysis is expected to have negligible contribution in the tropospheric loss process of SO3 because of large difference in concentration values between water and ammonia in the troposphere.

Impact of neutral and acidic species on cycloalkenes nucleation

Non-covalent hydrogen bond interactions between the π cloud of cycloalkenes and three atmospheric common nucleation precursors (H2S, H2O, and MeOH) have been investigated using DFT and CCSD(T). The structures and the energies of the 1:1 and 1:2 adducts were computed with the B3LYP-D3 method. The analysis of the investigated electronic properties and geometric parameters shows that cyclohexene is a stronger hydrogen bond acceptor than cyclopentene, then followed by 1,4-cyclohexadiene and 1,3-cyclohexadiene. Comparable red shifts of the OH-/SH-stretching vibrational frequencies were noticed for the studied clusters. Increasing the ring size enhances the hydrogen bond interaction, and increasing the π delocalization decreases the hydrogen bond interactions. This is further confirmed by Bader’s quantum theory of atoms in molecules. The nonadditivity effects were observed in the trimolecular complexes. All the complexes were analyzed by energy decomposition analysis to divide the interaction energy into individual components. Furthermore, the dipole moments and atmospheric implications were also investigated.

Atmospheric implications of large C2-C5 alkane emissions from the U.S. oil and gas industry.

Emissions of C2-C5 alkanes from the U.S. oil and gas sector have changed rapidly over the last decade. We use a nested GEOS-Chem simulation driven by updated 2011NEI emissions with aircraft, surface and column observations to 1) examine spatial patterns in the emissions and observed atmospheric abundances of C2-C5 alkanes over the U.S., and 2) estimate the contribution of emissions from the U.S. oil and gas industry to these patterns. The oil and gas sector in the updated 2011NEI contributes over 80% of the total U.S. emissions of ethane (C2H6) and propane (C3H8), and emissions of these species are largest in the central U.S. Observed mixing ratios of C2-C5 alkanes show enhancements over the central U.S. below 2 km. A nested GEOS-Chem simulation underpredicts observed C3H8 mixing ratios in the boundary layer over several U.S. regions and the relative underprediction is not consistent, suggesting C3H8 emissions should receive more attention moving forward. Our decision to consider only C4-C5 alkane emissions as a single lumped species produces a geographic distribution similar to observations. Due to the increasing importance of oil and gas emissions in the U.S., we recommend continued support of existing long-term measurements of C2-C5 alkanes. We suggest additional monitoring of C2-C5 alkanes downwind of northeastern Colorado, Wyoming and western North Dakota to capture changes in these regions. The atmospheric chemistry modeling community should also evaluate whether chemical mechanisms that lump larger alkanes are sufficient to understand air quality issues in regions with large emissions of these species.