Estimation of Rate Constants for Reactions of Organic Compounds under Atmospheric Conditions

Structure–activity (SAR) methods are presented for estimating rate constants at 298 K and approximate temperature dependences for the reactions of organic compounds with OH, NO3, and Cl radicals and O3, and O(3P) in the lower atmosphere. These are needed for detailed mechanisms for the atmospheric reactions of organic compounds. Base rate constants are assigned for the various types of H-abstraction and addition reactions, with correction factors for substituents around the reaction site and in some cases for rings and molecule structure or size. Rate constant estimates are made for hydrocarbons and a wide variety of oxygenates, organic nitrates, amines, and monosubstituted halogen compounds. Rate constants for most hydrocarbons and monofunctional compounds can be estimated to within ±30%, though predictions are not as good for multifunctional compounds, and predictions for ~15% of the rate constants are off by more than a factor of 2. Estimates are more uncertain in the case of NO3 and O3 reactions. The results serve to demonstrate the capabilities and limitations of empirical methods for predicting rate constants for the full variety of organic compounds that may be of interest. Areas where future work is needed are discussed.

Effect of non-uniform passive advection on A+B->C radial reaction-diffusion fronts

<p>The interplay between chemical and transport processes can give rise to complex reaction fronts dynamics, whose understanding is crucial in a wide variety of environmental, hydrological and biological processes, among others. An important class of reactions is A+B->C processes, where A and B are two initially segregated miscible reactants that produce C upon contact. Depending on the nature of the reactants and on the transport processes that they undergo, this class of reaction describes a broad set of phenomena, including combustion, atmospheric reactions, calcium carbonate precipitation and more. Due to the complexity of the coupled chemical-hydrodynamic systems, theoretical studies generally deal with the particular case of reactants undergoing passive advection and molecular diffusion. A restricted number of different geometries have been studied, including uniform rectilinear [1], 2D radial [2] and 3D spherical [3] fronts. By symmetry considerations, these systems are effectively 1D.</p><p>Here, we consider a 3D axis-symmetric confined system in which a reactant A is injected radially into a sea of B and both species are transported by diffusion and passive non-uniform advection. The advective field <em>v<sub>r</sub>(r,z)</em> describes a radial Poiseuille flow. We find that the front dynamics is defined by three distinct temporal regimes, which we characterize analytically and numerically. These are i) an early-time regime where the amount of mixing is small and the dynamics is transport-dominated, ii) a strongly non-linear transient regime and iii) a long-time regime that exhibits Taylor-like dispersion, for which the system dynamics is similar to the 2D radial case.</p><p>                                  <img src="https://contentmanager.copernicus.org/fileStorageProxy.php?f=gnp.ff5ab530bdff57321640161/sdaolpUECMynit/12UGE&app=m&a=0&c=360a1556c809484116c55812c8c06624&ct=x&pn=gnp.elif&d=1" alt="" width="299" height="299">                                                     <img src="https://contentmanager.copernicus.org/fileStorageProxy.php?f=gnp.671a6980bdff51231640161/sdaolpUECMynit/12UGE&app=m&a=0&c=c5a857c3fab835057e3af84001a91d15&ct=x&pn=gnp.elif&d=1" alt="" width="302" height="302"></p><p>                                                   Fig. 1: Concentration profile of the product C in the transient (left) and asymptotic (right) regimes.</p><p> </p><p>References:</p><p>[1] L. Gálfi, Z. Rácz, Phys. Rev. A 38, 3151 (1988);</p><p>[2] F. Brau, G. Schuszter, A. De Wit, Phys. Rev. Lett. 118, 134101 (2017);</p><p>[3] A. Comolli, A. De Wit, F. Brau, Phys. Rev. E, 100 (5), 052213 (2019).</p>

Polarizability of Pyruvate Anion in Small Water Clusters

Pyruvate anion is important in a broad array of physicochemical processes ranging from glucose homeostasis to atmospheric reactions. However, the electronic polarizability of the moiety has not been investigated extensively. We present theoretical results from electronic structure calculations on the static and dynamic polarizability of microhydrated pyruvate anions. These investigations were carried out with the CH3COCOO[Formula: see text]n H2O clusters ([Formula: see text]–9) to mimic microhydration conditions. These density functional theory calculations were carried out at the B3LYP/aug-cc-PVTZ level to uncover the optimal geometry of the anion water cluster. Sadlej basis set functions with the B3LYP functional were used to calculate the static and dynamic polarizabilities. Multiple simulated annealing simulations were carried out to discover additional low-lying minima. It is observed that the electronic polarizability varies linearly with the size of the hydrated cluster. Expressions that describe the relationship between the dynamic polarizability and the field frequency are provided for each cluster.

Is reducing new particle formation a plausible solution to mitigate particulate air pollution in Beijing and other Chinese megacities?

Based on our comprehensive observations in Beijing, we show that 80–90% of PM2.5 was formed via atmospheric reactions during haze days and over 65% of the number concentration of haze particles resulted from new particle formation.

Photochemistry of oxidized Hg(I) and Hg(II) species suggests missing mercury oxidation in the troposphere

Mercury (Hg), a global contaminant, is emitted mainly in its elemental form Hg0to the atmosphere where it is oxidized to reactive HgIIcompounds, which efficiently deposit to surface ecosystems. Therefore, the chemical cycling between the elemental and oxidized Hg forms in the atmosphere determines the scale and geographical pattern of global Hg deposition. Recent advances in the photochemistry of gas-phase oxidized HgIand HgIIspecies postulate their photodissociation back to Hg0as a crucial step in the atmospheric Hg redox cycle. However, the significance of these photodissociation mechanisms on atmospheric Hg chemistry, lifetime, and surface deposition remains uncertain. Here we implement a comprehensive and quantitative mechanism of the photochemical and thermal atmospheric reactions between Hg0, HgI, and HgIIspecies in a global model and evaluate the results against atmospheric Hg observations. We find that the photochemistry of HgIand HgIIleads to insufficient Hg oxidation globally. The combined efficient photoreduction of HgIand HgIIto Hg0competes with thermal oxidation of Hg0, resulting in a large model overestimation of 99% of measured Hg0and underestimation of 51% of oxidized Hg and ∼66% of HgIIwet deposition. This in turn leads to a significant increase in the calculated global atmospheric Hg lifetime of 20 mo, which is unrealistically longer than the 3–6-mo range based on observed atmospheric Hg variability. These results show that the HgIand HgIIphotoreduction processes largely offset the efficiency of bromine-initiated Hg0oxidation and reveal missing Hg oxidation processes in the troposphere.

Database for the kinetics of the gas-phase atmospheric reactions of organic compounds

We present a digital, freely available, searchable, and evaluated compilation of rate coefficients for the gas-phase reactions of organic compounds with OH, Cl, and NO3 radicals and with O3. Although other compilations of many of these data exist, many are out of date, most have limited scope, and all are difficult to search and to load completely into a digitized form. This compilation uses results of previous reviews, though many recommendations are updated to incorporate new or omitted data or address errors, and includes recommendations on many reactions that have not been reviewed previously. The database, which incorporates over 50 years of measurements, consists of a total of 2765 recommended bimolecular rate coefficients for the reactions of 1357 organic substances with OH, 709 with Cl, 310 with O3, and 389 with NO3, and is much larger than previous compilations. Many compound types are present in this database, including naturally occurring chemicals formed in or emitted to the atmosphere and anthropogenic compounds such as halocarbons and their degradation products. Recommendations are made for rate coefficients at 298 K and, where possible, the temperature dependences over the entire range of the available data. The primary motivation behind this project has been to provide a large and thoroughly evaluated training dataset for the development of structure–activity relationships (SARs), whose reliability depends fundamentally upon the availability of high-quality experimental data. However, there are other potential applications of this work, such as research related to atmospheric lifetimes and fates of organic compounds, or modelling gas-phase reactions of organics in various environments. This database is freely accessible at https://doi.org/10.25326/36 (McGillen et al., 2019).