Estimation Of Vapor Pressure Research Articles

A novel model for the estimation of water vapor pressure and temperature considering diurnal variations in China

Water vapor pressure (e) and temperature (T) are essential tropospheric parameters in the Global Navigation Satellite System (GNSS) high-precision positioning navigation and GNSS meteorology. Current models for water vapor pressure and temperature exhibit insufficiently detailed vertical profiling and fail to effectively capture diurnal variations. For two tropospheric parameters, e and T, the empirical models of meteorological parameters in China, CET-H models, were established based on the fifth-generation European Center for Medium-Range Weather Forecasts Reanalysis (ERA5) reanalysis data and the elaborate diurnal variation from 2017 to 2019. The 2020 ERA5 atmospheric reanalysis data and radiosonde data are used as reference values and compared with the current GPT3 model with higher accuracy. The results show that: (1) The BIAS and Root Mean Square Error (RMSE) of e obtained by the CET-H model at the ERA5 surface are 0.18 hPa and 2.65 hPa, respectively, higher by 42.4 % compared to the GPT3 model. The BIAS and RMSE of T obtained by the CET-H model at the ERA5 surface are −0.90 K and 3.63 K, respectively, higher by 21 % compared to the GPT3 model. The atmospheric layer mean values of both models are superior to the GPT3 model in all seasons across different latitude bands. (2) The BIAS and RMSE of e obtained by the CET-H model at the radiosonde station are −0.05 hPa and 3.17 hPa, higher by 43 % compared to the GPT3 model. The BIAS and RMSE of T obtained by the CET-H model at the radiosonde station are 4.49 hPa and 6.80 hPa, higher by 15 % compared to the GPT3 model. The atmospheric layer mean values of both models are superior to the GPT3 model in all seasons across different latitude bands. Therefore, the new model will provide a new reference for GNSS meteorology.

The influence of functional groups on the pyrolysis of per- and polyfluoroalkyl substances

Thermal treatment is a widely used remediation strategy for PFAS-contaminated materials such as soil, spent sorbents, and domestic waste. To better understand the effectiveness and environmental impact of thermal treatments for PFAS-contaminated materials, a fundamental understanding of PFAS thermal degradation mechanisms is required. This work aims to study the pyrolysis of six representative PFAS compounds, all of which have an eight-carbon length but with different functional groups. To assess the thermal stability and pyrolysis products of these six PFAS compounds, evolved gas analysis (EGA) was performed using thermogravimetric analysis/differential scanning calorimetry (TGA/DSC) coupled with an infrared spectrometer (IR) and a mass spectrometer (MS), as well as pyrolysis-GCMS (Pyr-GCMS). The EGA data demonstrates that compounds with lower estimated vapor pressures were generally found to be more thermally labile, and the presence of an ionic bond necessitates higher temperatures for pyrolysis. Pyrolysis at 900 °C yielded a variety of fluorinated organic compounds at significant signals. Tetrafluoroethene constituted the majority of the Pyr-GCMS signal for all the compounds. Moreover, a significant fraction of detected products was unable to be identified, underscoring a need for better tools to help with the identification of unknowns. Pyrolysis can occur through random chain scission, scission of the functional group, scission of the terminal CF3 group, and HF elimination. Some compounds may undergo more complete beta-scission to produce smaller pyrolysis products compared to others. The presence of acidic protons within the functional group can help facilitate HF elimination, whereas the salt form of a PFAS compound is less likely to undergo HF elimination. Termination of radical intermediates can either be recombination with a CF3 radical or hydrogen abstraction (H-abstraction). Observed hydrogen-substituted products indicate that functional groups with higher hydrocarbon character may lead to more H-abstraction terminated products. Overall findings show that the functional group of a PFAS may decrease or increase its thermal stability and lead to different profiles of pyrolysis products.

Simulation of Gas‐Particle Partitioning of Semi‐Volatile n‐Alkanes and PAHs in Nanjing, China, and Denver, United States: Effects of Vapor Pressure and Surface Adsorption Estimation

AbstractThe concentration data of semi‐volatile n‐alkanes and PAHs in the gas phase and in PM2.5 were obtained from Nanjing and Denver. First, the gas‐particle partitioning coefficients of the target compounds were calculated and compared with the predictions based on the equilibrium absorptive partitioning theory. Although the vapor pressure (poL) estimation method was selected to improve the agreement between measured and predicted partitioning coefficients, n‐alkanes in Denver and PAHs in both cities exhibited stronger sorption to PM2.5 than predicted. By including the adsorption mechanism, the average logarithms and temporal variations of the partitioning coefficients were well simulated in both Nanjing and Denver. The partitioning of n‐alkanes in Nanjing can be primarily explained by the absorption mechanism, while the adsorption mechanism dominates that for n‐alkanes in Denver and PAHs in both cities. To obtain an optimal simulation, the poL of n‐alkanes and PAHs was estimated using the SPARC and SIMPOL—two group contribution methods, respectively, and the difference between desorption and vaporization enthalpies was set between 2 and 3 kcal mol−1 for n‐alkanes in Denver and PAHs in both cities. Therefore, the estimation of poL and surface adsorption should be carefully considered when parameterizing the gas‐particle partitioning of non‐polar organic compounds in future field and modeling studies.

Ab Initio Prediction of Vapor Pressure for Diverse Atomic Layer Deposition Precursors.

Understanding the saturated vapor pressure (Pvap) is vital for evaluating atomic layer deposition (ALD) precursors, as it directly influences the ALD temperature window and, by extension, the processability of compounds. The early estimation of vapor pressure ranges is crucial during the initial stages of novel precursor design, reducing the reliance on empirical synthesis or experimentation. However, predicting vapor pressure through computer simulations is often impeded by the scarcity of suitable empirical force fields for molecular dynamics simulations. This challenge is further compounded by the diverse chemical substances and the introduction of new elements into modern ALD processes, necessitating robust force fields that can accommodate metals, organics, and halides. In response, this study introduces a novel approach utilizing a quantum mechanically derived force field for the prediction of vapor pressure across a wide spectrum of potential ALD precursors. This approach enables the creation of system-specific force fields through parametrization based on ab initio calculations for a single molecule. We develop a comprehensive workflow to simulate both liquid and gaseous equilibrium phases, allowing the calculation of vapor pressure across a wide temperature range. Our methodology has been validated with a diverse set of ALD precursors, demonstrating its robustness in predicting Pvap at specified temperatures. The approach yields a Pearson's correlation coefficient (R2) greater than 0.9 on a logarithmic scale and a root-mean-squared deviation in self-solvation-free energies as low as 1.3 kcal mol-1. This innovative workflow, which does not require any prior experimental data, marks a significant advancement in the computer-aided design of novel ALD precursors, paving the way for accelerating developments in technology.

Automating methods for estimating metabolite volatility.

The volatility of metabolites can influence their biological roles and inform optimal methods for their detection. Yet, volatility information is not readily available for the large number of described metabolites, limiting the exploration of volatility as a fundamental trait of metabolites. Here, we adapted methods to estimate vapor pressure from the functional group composition of individual molecules (SIMPOL.1) to predict the gas-phase partitioning of compounds in different environments. We implemented these methods in a new open pipeline called volcalc that uses chemoinformatic tools to automate these volatility estimates for all metabolites in an extensive and continuously updated pathway database: the Kyoto Encyclopedia of Genes and Genomes (KEGG) that connects metabolites, organisms, and reactions. We first benchmark the automated pipeline against a manually curated data set and show that the same category of volatility (e.g., nonvolatile, low, moderate, high) is predicted for 93% of compounds. We then demonstrate how volcalc might be used to generate and test hypotheses about the role of volatility in biological systems and organisms. Specifically, we estimate that 3.4 and 26.6% of compounds in KEGG have high volatility depending on the environment (soil vs. clean atmosphere, respectively) and that a core set of volatiles is shared among all domains of life (30%) with the largest proportion of kingdom-specific volatiles identified in bacteria. With volcalc, we lay a foundation for uncovering the role of the volatilome using an approach that is easily integrated with other bioinformatic pipelines and can be continually refined to consider additional dimensions to volatility. The volcalc package is an accessible tool to help design and test hypotheses on volatile metabolites and their unique roles in biological systems.

Nontrivial Impact of Relative Humidity on Organic New Particle Formation from Ozonolysis of cis-3-Hexenyl Acetate

The impact of relative humidity (RH) on organic new particle formation (NPF) from the ozonolysis of biogenic volatile organic compounds (BVOCs) remains an area of active debate. Previous reports provide contradictory results, indicating both the depression and enhancement of NPF under conditions of high RH. Herein, we report on the impact of RH on NPF from the dark ozonolysis of cis-3-hexenyl acetate (CHA), a green-leaf volatile (GLV) emitted by vegetation. We show that RH inhibits NPF by this BVOC, essentially shutting it down at RH levels > 1%. While the mechanism for the inhibition of NPF remains unclear, we demonstrate that it is likely not due to increased losses of CHA to the humid chamber walls. New oxidation products dominant under humid conditions are proposed that, based on estimated vapor pressures (VPs), should enhance NPF; however, it is possible that the vapor phase concentration of these low-volatility products is not sufficient to initiate NPF. Furthermore, the reaction of C3-excited state Criegee intermediates (CIs) with water may lead to the formation of small carboxylic acids that do not contribute to NPF. This hypothesis is supported by experiments with quaternary O3 + CHA + α-pinene + RH systems, which showed decreases in total α-pinene-derived NPF at ~0% RH and subsequent recovery at elevated RH.

Phase Transition Thermodynamic Properties Of 2-Methylquinoline, 2-Chloroquinoline And 2-Phenylquinoline

Derivatives of quinoline are widely utilized in both industries and in healthcare. To understand the quinolines' quality and stability in usage, it is crucial to study their phase transition chemical thermodynamic characteristics. In this work, the phase transition thermodynamic characters of 2-methylquinoline (quinaldine), 2-chloroquinoline, and 2-phenylquinoline were investigated. Moreover, the sublimation/vaporization enthalpy of the compounds were determined the solution calorimetry-additivity scheme approach at 298.15 K. The solution calorimetry was applied to measure solution enthalpies of the compounds in benzene solvent at 298.15 K. While, the solvation enthalpy of the compounds were calculated additivity scheme approach. In addition, the transpiration method applied to estimate vapor pressure to temperature dependency to 2-Chloroquinoline. In consequence, the vapor pressure values with respect to temperature variation was determined to 2-Chloroquinoline compound for the first time. As a result, the phase transition chemical thermodynamic properties; enthalpy, entropy, and Gibbs energy for 2-methylquinoline, 2-chloroquinoline and 2-phenylquinoline were determined from crystalline/liquid to gas phase. Furthermore, in this work the thermochemical characteristics values of the studied compounds exhibited higher accuracy to those in literature data. Finally, the phase transition thermodynamically studied on 2-position of the quinoline compound, where it substituted to methyl, chloro and phenyl groups.

3-formylchromones: Vapor pressures and standard molar enthalpies, entropies and Gibbs energy of sublimation

In present work, the estimation of vapor pressures of 3-formylchromone and 3-formyl-6-methylchromone by the isothermal thermogravimetry method is presented. Briefly, this method consists of measuring the rate of mass loss as a function of temperature in a cylindrical cell and with a flow of nitrogen. To derive the vapor pressure, the Langmuir equation was used, from which the vaporization constant was calculated using anthracene as a calibrating compound. Subsequently, the vapor pressures 3-fromylchromone and 3-formyl-6-methylchromone were estimated. From the vapor pressure, the enthalpy of sublimation was calculated using the Clausius-Clapeyron equation for each compound at its mean sublimation temperature. Finally, the enthalpy of sublimation was calculated at T = 298.15 K. The entropy and Gibbs energy of sublimation at the same temperature were determined. In order to test the validity of the method, the vapor pressures of phenanthrene and pyrene were calculated. The values obtained by isothermal thermogravimetry agree with those reported by other techniques.

Volatility of Secondary Organic Aerosol from β-Caryophyllene Ozonolysis over a Wide Tropospheric Temperature Range.

We investigated secondary organic aerosol (SOA) from β-caryophyllene oxidation generated over a wide tropospheric temperature range (213-313 K) from ozonolysis. Positive matrix factorization (PMF) was used to deconvolute the desorption data (thermograms) of SOA products detected by a chemical ionization mass spectrometer (FIGAERO-CIMS). A nonmonotonic dependence of particle volatility (saturation concentration at 298 K, C298K*) on formation temperature (213-313 K) was observed, primarily due to temperature-dependent formation pathways of β-caryophyllene oxidation products. The PMF analysis grouped detected ions into 11 compound groups (factors) with characteristic volatility. These compound groups act as indicators for the underlying SOA formation mechanisms. Their different temperature responses revealed that the relevant chemical pathways (e.g., autoxidation, oligomer formation, and isomer formation) had distinct optimal temperatures between 213 and 313 K, significantly beyond the effect of temperature-dependent partitioning. Furthermore, PMF-resolved volatility groups were compared with volatility basis set (VBS) distributions based on different vapor pressure estimation methods. The variation of the volatilities predicted by different methods is affected by highly oxygenated molecules, isomers, and thermal decomposition of oligomers with long carbon chains. This work distinguishes multiple isomers and identifies compound groups of varying volatilities, providing new insights into the temperature-dependent formation mechanisms of β-caryophyllene-derived SOA particles.

A generalized alpha function of Peng-Robison equation of state for non-polar, weakly polar and polar compounds

A novel alpha function for Peng-Robinson equation of state was proposed and generalized with acentric factor and dipole moment for predicting thermodynamic properties of non-polar, weakly polar, and polar compounds. The parameters of new alpha function were fitted with vapor pressures of 70 compounds. Six different methods were investigated for the correlation of parameters of new alpha function and Heyen alpha function. The generalized new alpha function passed the consistency test. The results indicated that the predictive accuracy of generalized new alpha function and generalized Heyen alpha function was improved for the estimation of vapor pressure of 11 kinds of compounds, with the average relative deviations (ARDs) being 2.60% and 2.76%. The ARDs of the two generalized alpha functions were 2.04% and 2.09% for the enthalpy of vaporization. However, the generalized new alpha function and the other alpha functions had great deviations for the prediction of liquid volumes and isobaric heat capacities. The alpha function that was generalized with acentric factor and reduced dipole moment was more accurate than that was generalized with acentric factor, especially for the prediction of vapor pressure and enthalpy of vaporization of polar compounds.

Validation and Bias Correction of Forecast Reference Evapotranspiration for Agricultural Applications in Nevada

Accurate estimates of reference evapotranspiration (ET0) are critical for estimating actual crop evapotranspiration and agricultural water use. This study uses observations from the Nevada Integrated Climate and Evapotranspiration Network (NICE Net) to validate forecasts of ET0 and its driving variables from the National Weather Service’s National Digital Forecast Database (NDFD). Daily NDFD ET0 at lead times of 1 to 6 days were compared against 18 NICE Net stations. Correlations between NDFD and observations generally ranged between 0.4 and 0.9, with lower correlations at longer leads and a notable drop in skill during July and August. Systematic arid biases (high bias for temperatures and low bias for humidity) were found in NDFD with a strong warm minimum temperature bias and low vapor pressure bias most prominent during the growing season. Some of the largest relative biases were found in wind speed, although they were systematic and varied greatly by location. A case study revealed that NDFD consistently underestimates the variability found in observed minimum temperature, solar radiation, wind speed, and ET0. Cloudy days during summer were not well represented in the NDFD estimated solar radiation, which had a cascading impact on temperature, vapor pressure, and ET0 estimates. A monthly ratio-based bias-correction was applied to NDFD ET0, which reduced the root-mean squared error by 5%–30% for most locations. Bias-corrected ET0 forecasts from NDFD or other forecast systems show potential as a guide to develop weekly irrigation schedules for agricultural producers, with the ultimate goal of reducing applications of excess irrigation water.

Estimating Vapor Pressure Data from Gas–Liquid Chromatography Retention Times: Analysis of Multiple Reference Approaches, Review of Prior Applications, and Outlook

Estimating Vapor Pressure Data from Gas–Liquid Chromatography Retention Times: Analysis of Multiple Reference Approaches, Review of Prior Applications, and Outlook

Correction to “Modified Trouton’s Rule for the Estimation, Correlation, and Evaluation of Pure-Component Vapor Pressure”

Correction to “Modified Trouton’s Rule for the Estimation, Correlation, and Evaluation of Pure-Component Vapor Pressure”

Modelling the gas–particle partitioning and water uptake of isoprene-derived secondary organic aerosol at high and low relative humidity

Abstract. This study presents a characterization of the hygroscopic growth behaviour and effects of different inorganic seed particles on the formation of secondary organic aerosols (SOAs) from the dark ozone-initiated oxidation of isoprene at low NOx conditions. We performed simulations of isoprene oxidation using a gas-phase chemical reaction mechanism based on the Master Chemical Mechanism (MCM) in combination with an equilibrium gas–particle partitioning model to predict the SOA concentration. The equilibrium model accounts for non-ideal mixing in liquid phases, including liquid–liquid phase separation (LLPS), and is based on the AIOMFAC (Aerosol Inorganic–Organic Mixtures Functional groups Activity Coefficients) model for mixture non-ideality and the EVAPORATION (Estimation of VApour Pressure of ORganics, Accounting for Temperature, Intramolecular, and Non-additivity effects) model for pure compound vapour pressures. Measurements from the Cosmics Leaving Outdoor Droplets (CLOUD) chamber experiments, conducted at the European Organization for Nuclear Research (CERN) for isoprene ozonolysis cases, were used to aid in parameterizing the SOA yields at different atmospherically relevant temperatures, relative humidity (RH), and reacted isoprene concentrations. To represent the isoprene-ozonolysis-derived SOA, a selection of organic surrogate species is introduced in the coupled modelling system. The model predicts a single, homogeneously mixed particle phase at all relative humidity levels for SOA formation in the absence of any inorganic seed particles. In the presence of aqueous sulfuric acid or ammonium bisulfate seed particles, the model predicts LLPS to occur below ∼ 80 % RH, where the particles consist of an inorganic-rich liquid phase and an organic-rich liquid phase; however, this includes significant amounts of bisulfate and water partitioned to the organic-rich phase. The measurements show an enhancement in the SOA amounts at 85 % RH, compared to 35 % RH, for both the seed-free and seeded cases. The model predictions of RH-dependent SOA yield enhancements at 85 % RH vs. 35 % RH are 1.80 for a seed-free case, 1.52 for the case with ammonium bisulfate seed, and 1.06 for the case with sulfuric acid seed. Predicted SOA yields are enhanced in the presence of an aqueous inorganic seed, regardless of the seed type (ammonium sulfate, ammonium bisulfate, or sulfuric acid) in comparison with seed-free conditions at the same RH level. We discuss the comparison of model-predicted SOA yields with a selection of other laboratory studies on isoprene SOA formation conducted at different temperatures and for a variety of reacted isoprene concentrations. Those studies were conducted at RH levels at or below 40 % with reported SOA mass yields ranging from 0.3 % up to 9.0 %, indicating considerable variations. A robust feature of our associated gas–particle partitioning calculations covering the whole RH range is the predicted enhancement of SOA yield at high RH (> 80 %) compared to low RH (dry) conditions, which is explained by the effect of particle water uptake and its impact on the equilibrium partitioning of all components.

Evaluating In Silico the Potential Health and Environmental Benefits of Houseplant Volatile Organic Compounds for an Emerging 'Indoor Forest Bathing' Approach.

The practice of spending time in green areas to gain the health benefits provided by trees is well known, especially in Asia, as ‘forest bathing’, and the consequent protective and experimentally detectable effects on the human body have been linked to the biogenic volatile organic compounds released by plants. Houseplants are common in houses over the globe and are particularly appreciated for aesthetic reasons as well for their ability to purify air from some environmental volatile pollutants indoors. However, to the best of our knowledge, no attempt has been made to describe the health benefits achievable from houseplants thanks to the biogenic volatile organic compounds released, especially during the day, from some of them. Therefore, we performed the present study, based on both a literature analysis and in silico studies, to investigate whether the volatile compounds and aerosol constituents emitted by some of the most common houseplants (such as peace lily plant, Spathiphyllum wallisii, and iron plant, Aspidistra eliator) could be exploited in ‘indoor forest bathing’ approaches, as proposed here for the first time not only in private houses but also public spaces, such as offices, hospitals, and schools. By using molecular docking (MD) and other in silico methodologies for estimating vapor pressures and chemico-physical/pharmacokinetic properties prediction, we found that β-costol is an organic compound, emitted in appreciable amounts by the houseplant Spathiphyllum wallisii, endowed with potential antiviral properties as emerged by our MD calculations in a SARS-CoV-2 Mpro (main protease) inhibition study, together with sesquirosefuran. Our studies suggest that the anti-COVID-19 potential of these houseplant-emitted compounds is comparable or even higher than known Mpro inhibitors, such as eugenol, and sustain the utility of houseplants as indoor biogenic volatile organic compound emitters for immunity boosting and health protection.

Quantifying contributions of natural variability and anthropogenic forcings on increased fire weather risk over the western United States

Previous studies have identified a recent increase in wildfire activity in the western United States (WUS). However, the extent to which this trend is due to weather pattern changes dominated by natural variability versus anthropogenic warming has been unclear. Using an ensemble constructed flow analogue approach, we have employed observations to estimate vapor pressure deficit (VPD), the leading meteorological variable that controls wildfires, associated with different atmospheric circulation patterns. Our results show that for the period 1979 to 2020, variation in the atmospheric circulation explains, on average, only 32% of the observed VPD trend of 0.48 ± 0.25 hPa/decade (95% CI) over the WUS during the warm season (May to September). The remaining 68% of the upward VPD trend is likely due to anthropogenic warming. The ensemble simulations of climate models participating in the sixth phase of the Coupled Model Intercomparison Project suggest that anthropogenic forcing explains an even larger fraction of the observed VPD trend (88%) for the same period and region. These models and observational estimates likely provide a lower and an upper bound on the true impact of anthropogenic warming on the VPD trend over the WUS. During August 2020, when the August Complex "Gigafire" occurred in the WUS, anthropogenic warming likely explains 50% of the unprecedented high VPD anomalies.

Estimation of Aromatic Secondary Organic Aerosol Using a Molecular Tracer-A Chemical Transport Model Assessment.

A modified community multiscale air quality model, which can simulate the regional distributions of 2,3-dihydroxy-4-oxopentanoic acid (DHOPA), a marker species for monoaromatic secondary organic aerosol (SOA), was applied to assess the applicability of using the DHOPA to aromatic SOA mass ratio (fSOA) from smog chamber experiments to estimate aromatic SOA during a three-week wintertime air quality campaign in urban Shanghai. The modeled daily DHOPA concentrations based on the chamber-derived mass yields agree well with the organic marker field measurements (R = 0.79; MFB = 0.152; and MFE = 0.440). Two-thirds of the DHOPA are from the oxidation of ARO1 (lumped less-reactive aromatic species; mostly toluene), with the rest from ARO2 (lumped more-reactive aromatic species; mostly xylenes). Modeled DHOPA is mainly in the particle phase under ambient organic aerosol (OA) loading but could exhibit significant gas-particle partitioning when a higher estimation of the DHOPA vapor pressure is used. The modeled fSOA shows a strong dependence on the OA loading when only semivolatile aromatic SOA components are included in the fSOA calculations. However, this OA dependence becomes weaker when non-volatile oligomers and dicarbonyl SOA products are considered. A constant fSOA value of ∼0.002 is determined when all aromatic SOA components are included, which is a factor of 2 smaller than the commonly applied chamber-based fSOA value of 0.004 for toluene. This model-derived fSOA value does not show much spatial variation and is not sensitive to alternative estimates of DHOPA vapor pressures and SOA yields, and thus provides an appropriate scaling factor to assess aromatic SOA from DHOPA measurements. This result helps refine the quantification of SOA attributable to monoaromatic hydrocarbons in urban environments and thereby facilitates the evaluation of control measures targeting these specific precursors.

Atmospheric Chemistry of 2-Amino-2-methyl-1-propanol: A Theoretical and Experimental Study of the OH-Initiated Degradation under Simulated Atmospheric Conditions.

The OH-initiated degradation of 2-amino-2-methyl-1-propanol [CH3C(NH2)(CH3)CH2OH, AMP] was investigated in a large atmospheric simulation chamber, employing time-resolved online high-resolution proton-transfer reaction-time-of-flight mass spectrometry (PTR-ToF-MS) and chemical analysis of aerosol online PTR-ToF-MS (CHARON-PTR-ToF-MS) instrumentation, and by theoretical calculations based on M06-2X/aug-cc-pVTZ quantum chemistry results and master equation modeling of the pivotal reaction steps. The quantum chemistry calculations reproduce the experimental rate coefficient of the AMP + OH reaction, aligning k(T) = 5.2 × 10–12 × exp (505/T) cm3 molecule–1 s–1 to the experimental value kexp,300K = 2.8 × 10–11 cm3 molecule–1 s–1. The theoretical calculations predict that the AMP + OH reaction proceeds via hydrogen abstraction from the −CH3 groups (5–10%), −CH2– group, (>70%) and −NH2 group (5–20%), whereas hydrogen abstraction from the −OH group can be disregarded under atmospheric conditions. A detailed mechanism for atmospheric AMP degradation was obtained as part of the theoretical study. The photo-oxidation experiments show 2-amino-2-methylpropanal [CH3C(NH2)(CH3)CHO] as the major gas-phase product and propan-2-imine [(CH3)2C=NH], 2-iminopropanol [(CH3)(CH2OH)C=NH], acetamide [CH3C(O)NH2], formaldehyde (CH2O), and nitramine 2-methyl-2-(nitroamino)-1-propanol [AMPNO2, CH3C(CH3)(NHNO2)CH2OH] as minor primary products; there is no experimental evidence of nitrosamine formation. The branching in the initial H abstraction by OH radicals was derived in analyses of the temporal gas-phase product profiles to be BCH3/BCH2/BNH2 = 6:70:24. Secondary photo-oxidation products and products resulting from particle and surface processing of the primary gas-phase products were also observed and quantified. All the photo-oxidation experiments were accompanied by extensive particle formation that was initiated by the reaction of AMP with nitric acid and that mainly consisted of this salt. Minor amounts of the gas-phase photo-oxidation products, including AMPNO2, were detected in the particles by CHARON-PTR-ToF-MS and GC×GC-NCD. Volatility measurements of laboratory-generated AMP nitrate nanoparticles gave ΔvapH = 80 ± 16 kJ mol–1 and an estimated vapor pressure of (1.3 ± 0.3) × 10–5 Pa at 298 K. The atmospheric chemistry of AMP is evaluated and a validated chemistry model for implementation in dispersion models is presented.

Impact of organic molecular structure on the estimation of atmospherically relevant physicochemical parameters

Abstract. Many methods are currently available for estimating physicochemical properties of atmospherically relevant compounds. Though a substantial body of literature has focused on the development and intercomparison of methods based on molecular structure, there has been an increasing focus on methods based only on molecular formula. However, prior work has not quantified the extent to which isomers of the same formula may differ in their properties or, relatedly, the extent to which lacking or ignoring molecular structure degrades estimates of parameters. Such an evaluation is complicated by the fact that structure-based methods bear significant uncertainty and are typically not well constrained for atmospherically relevant molecules. Using species produced in the modeled atmospheric oxidation of three representative atmospheric hydrocarbons, we demonstrate here that estimated differences between isomers are greater than differences between three widely used estimation methods. Specifically, isomers tend to differ in their estimated vapor pressures and Henry's law constants by a half to a full order of magnitude greater than differences between estimation methods, and they differ in their rate constant for reaction with OH radicals (kOH) by a factor of 2. Formula-based estimation of these parameters, using certain methods, is shown to agree with structure-based estimates with little bias and approximately normally distributed error. Specifically, vapor pressure can be estimated using a combination of two existing methods, Henry's law constants can be estimated based on vapor pressure, and kOH can be approximated as a constant for all formulas containing a given set of elements. Formula-based estimation is, therefore, reasonable when applied to a mixture of isomers but creates uncertainty commensurate with the lack of structural information.

Estimate of the Vapor Pressure of Squalane at Approximately 293 K Using a Knudsen Cell Method

I report the vapor pressure of squalane hydrocarbon (2,6,10,15,19,23-hexamethyltetracosane, C30H62) estimated from a statistical analysis of multiple weight loss measurements of Knudsen cells with orifice diameters of 4 and 8 mm exposed in vacuum for aggregated durations 5400 and 2400 h, respectively, at 293.61 ± 1.47 K. The estimated vapor pressure pv = (4.15 ± 0.17) × 10–6 Pa is more than a factor of 6 larger than the value of ∼6.5 × 10–7 Pa derived from estimates reported in the literature. Plausible systematic errors cannot account for this discrepancy, which suggests that the vapor pressure of squalane at temperatures near 293.61 K may be substantially higher than previously estimated. Empirical equations for the vapor pressure of squalane are proposed for the temperature range 293.61 K ≤ T ≤ 795.9 K.