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1. Reactive uptake of N ₂ O ₅ by atmospheric aerosol is dominated by interfacial processes

   https://doi.org/10.1126/science.abd7716  

   Galib, Mirza; Limmer, David T. (February 2021, Science)

   Nitrogen oxides are removed from the troposphere through the reactive uptake of N₂O₅into aqueous aerosol. This process is thought to occur within the bulk of an aerosol, through solvation and subsequent hydrolysis. However, this perspective is difficult to reconcile with field measurements and cannot be verified directly because of the fast reaction kinetics of N₂O₅. Here, we use molecular simulations, including reactive potentials and importance sampling, to study the uptake of N₂O₅into an aqueous aerosol. Rather than being mediated by the bulk, uptake is dominated by interfacial processes due to facile hydrolysis at the liquid-vapor interface and competitive reevaporation. With this molecular information, we propose an alternative interfacial reactive uptake model consistent with existing experimental observations. 

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2. Hydrogen peroxide synthesis on porous graphitic carbon nitride using water as a hydrogen source

   https://doi.org/10.1039/C9TA08103H  

   Cao, Yongyong; Zhou, Guobing; Chen, Xianlang; Qiao, Qi; Zhao, Chenxia; Sun, Xiang; Zhong, Xing; Zhuang, Guilin; Deng, Shengwei; Wei, Zhongzhe; et al (January 2020, Journal of Materials Chemistry A)

   null (Ed.)

   Using water as a hydrogen source is a promising strategy for alternative hydrogen peroxide (H 2 O 2 ) synthesis. By a series of ab initio molecular dynamics (AIMD) simulations and reactive molecular dynamics (RxMD) calculations, fundamental details have been revealed regarding how liquid water interacts with oxygen on a metal-free carbon nitride catalyst, and the two-step reaction mechanism of H 2 O 2 synthesis. Metal-free porous graphitic carbon nitride (g-C 5 N 2 ) catalysts are also systematically screened by using a thermodynamics approach through the ab initio density functional theory (DFT) method. Key results include: (a) pristine g-C 5 N 2 is most active to catalyze the H 2 O/O 2 reaction and produce H 2 O 2 ; (b) the adsorption and activation of water at unsaturated carbon sites of g-C 5 N 2 are critical to initiate the H 2 O/O 2 reaction, producing HOO* intermediates; (c) interfacial free water and adsorbed water at g-C 5 N 2 form a synergetic proton transfer cluster to promote HOO* intermediates to form H 2 O 2 . To the best of our knowledge, this work presents long-needed theoretical details of direct H 2 O 2 synthesis via the water/oxygen system, which can guide further optimization of carbon-based catalysts for oxygen reduction reactions. 

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3. On the stratospheric chemistry of midlatitude wildfire smoke

   https://doi.org/10.1073/pnas.2117325119  

   Solomon, Susan; Dube, Kimberlee; Stone, Kane; Yu, Pengfei; Kinnison, Doug; Toon, Owen B.; Strahan, Susan E.; Rosenlof, Karen H.; Portmann, Robert; Davis, Sean; et al (March 2022, Proceedings of the National Academy of Sciences)

   Massive Australian wildfires lofted smoke directly into the stratosphere in the austral summer of 2019/20. The smoke led to increases in optical extinction throughout the midlatitudes of the southern hemisphere that rivalled substantial volcanic perturbations. Previous studies have assumed that the smoke became coated with sulfuric acid and water and would deplete the ozone layer through heterogeneous chemistry on those surfaces, as is routinely observed following volcanic enhancements of the stratospheric sulfate layer. Here, observations of extinction and reactive nitrogen species from multiple independent satellites that sampled the smoke region are compared to one another and to model calculations. The data display a strong decrease in reactive nitrogen concentrations with increased aerosol extinction in the stratosphere, which is a known fingerprint for key heterogeneous chemistry on sulfate/H 2 O particles (specifically the hydrolysis of N 2 O 5 to form HNO 3 ). This chemical shift affects not only reactive nitrogen but also chlorine and reactive hydrogen species and is expected to cause midlatitude ozone layer depletion. Comparison of the model ozone to observations suggests that N 2 O 5 hydrolysis contributed to reduced ozone, but additional chemical and/or dynamical processes are also important. These findings suggest that if wildfire smoke injection into the stratosphere increases sufficiently in frequency and magnitude as the world warms due to climate change, ozone recovery under the Montreal Protocol could be impeded, at least sporadically. Modeled austral midlatitude total ozone loss was about 1% in March 2020, which is significant compared to expected ozone recovery of about 1% per decade. 

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4. N ₂ O ₅ reactive uptake kinetics and chlorine activation on authentic biomass-burning aerosol

   https://doi.org/10.1039/c9em00330d  

   Goldberger, Lexie A.; Jahl, Lydia G.; Thornton, Joel A.; Sullivan, Ryan C. (October 2019, Environmental Science: Processes & Impacts)

   We examined the reactive uptake of dinitrogen pentoxide (N 2 O 5 ) to authentic biomass-burning aerosol (BBA) using a small chamber reservoir in combination with an entrained aerosol flow tube. BBA was generated from four different fuel types and the reactivity of N 2 O 5 was probed from 30 to 70% relative humidity (RH). The N 2 O 5 reactive uptake coefficient, γ (N 2 O 5 ), depended upon RH, fuel type, and to a lesser degree on aerosol chloride mass fractions. The γ (N 2 O 5 ) ranged from 2.0 (±0.4) ×10 −3 on black needlerush derived BBA at 30% RH to 6.0 (±0.6) ×10 −3 on wiregrass derived BBA at 65% RH. Major N 2 O 5 reaction products were observed including gaseous ClNO 2 and HNO 3 and particulate nitrate, and used to create a reactive nitrogen budget. Black needlerush BBA had the most particulate chloride, and the only measured ClNO 2 yield > 1%. The ClNO 2 yield on black needlerush decayed from an initial value of ∼100% to ∼30% over the course of the burn experiment, suggesting a depletion of BBA chloride over time. Black needlerush was also the only fuel for which the reactive nitrogen budget indicated other N-containing products were generated. Generally, the results suggest limited chloride availability for heterogeneous reaction for BBA in the RH range probed here, including BBA with chloride mass fractions on the higher end of previously reported values (∼17–34%). Though less than fresh sea spray aerosol, ∼50%. We use these measured quantities to discuss the implications for nocturnal aerosol nitrate formation, the chemical fate of N 2 O 5 (g), and the availability of particulate chloride for activation in biomass burning plumes. 

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5. Mechanisms and competition of halide substitution and hydrolysis in reactions of N ₂ O ₅ with seawater

   https://doi.org/10.1126/sciadv.aav6503  

   McCaslin, Laura M.; Johnson, Mark A.; Gerber, R. Benny (June 2019, Science Advances)

   S N 2-type halide substitution and hydrolysis are two of the most ubiquitous reactions in chemistry. The interplay between these processes is fundamental in atmospheric chemistry through reactions of N 2 O 5 and seawater. N 2 O 5 plays a major role in regulating levels of O 3 , OH, NO x , and CH 4 . While the reactions of N 2 O 5 and seawater are of central importance, little is known about their mechanisms. Of interest is the activation of Cl in seawater by the formation of gaseous ClNO 2 , which occurs despite the fact that hydrolysis (to HNO 3 ) is energetically more favorable. We determine key features of the reaction landscape that account for this behavior in a theoretical study of the cluster N 2 O 5 /Cl − /H 2 O. This was carried out using ab initio molecular dynamics to determine reaction pathways, structures, and time scales. While hydrolysis of N 2 O 5 occurs in the absence of Cl − , results here reveal that a low-lying pathway featuring halide substitution intermediates enhances hydrolysis. 

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