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Backward models for super-diffusion have been developed to identify pollutant source locations, but they are limited to a single observation and disregard field-measured concentrations. To overcome these limitations, this study derives the adjoint of the space-fractional advection–dispersion equation, incorporating measured concentrations from multiple observation data. Backward probabilities, such as the backward location probability density function (PDF), describe the likely source location(s) at a fixed time before sampling, offering a comprehensive modeling approach for source identification. By applying Bayes’ theorem, the individual PDFs from each observation and its corresponding concentration are combined into a joint PDF, enhancing both the information and reliability compared to the previous single PDF. Field applications show that the improved model enhances accuracy (with PDF peak locations closer to the actual source) and precision (with reduced variance) of backward PDFs for identifying point sources in a natural river and aquifer. The model’s performance is affected by observation count and measurement errors, with double peaks in the backward location PDF possible due to source mass uncertainty. Future refinements, such as incorporating backward travel time analysis and extending applications to reactive pollutants, could further enhance the utility of the conditioned backward fractional-derivative model developed in this study. 

This article highlights recent computational research on heme-based carbene transfer reactions. Mechanistic insights reveal how cofactor components, coordination modes, substrates, and protein environments influence reactivity and selectivity. 

Calculations provide the first comprehensive mechanistic study of carbene N–H insertion for His-ligated heme biocatalysts, as validatedvianew experiments. 

Nitric oxide (NO) is an important molecule that regulates many physiological processes in humans and plants and contributes to the formation of greenhouse gases. Bacterial NO reductases utilize a di-Fe heme/nonheme active site to couple two NOs to generate nitrous oxide (N2O) via a two-electron mechanism. Here, we report a previously unexplored Cr porphyrin NO complex with a Lewis acid (LA) BF3 for the NO reduction reaction. Density functional theory calculations were first employed to reveal its reaction mechanism with a reasonable barrier for experimental realization. Subsequent experimental synthesis work confirms this reactivity and reports the first nitrosyl Cr porphyrin X-ray crystal structure. Theoretical analysis uncovered a distinctive reaction feature for the Cr system compared to Fe and Co porphyrins: the electron transfer from the metal to the bound NO occurs before LA binding. A comparative study of the NO coupling mechanisms with the three representative metals suggests that the metal reduction potential should be finely tuned, as found in previous studies of NOR enzymatic systems. Overall, this study offers new theoretical and experimental insights to further facilitate the development of alternative NO reduction compounds with biological, environmental, and industrial applications. 

Abstract Specific polar cap auroras, such as 15MLT‐PCA, linked to lobe reconnection due to the influence of the interplanetary magnetic field (IMF) B_ycomponent, were only observed in the summer. Although the variance in ionospheric conductivity between winter and summer has been proposed as a potential explanation for this seasonal dependency, it has also been argued that the differences in lobe reconnection between the winter and summer hemispheres could be the cause. To address this debate, we examined two data periods with similar IMF conditions when the northern hemisphere was in summer and winter, respectively. Using DMSP/SSUSI and AMPERE observations, we detected clear 15MLT‐PCA and associated field‐aligned currents in the summer, but not in the winter. These observations were compared with global MHD simulations from OpenGGCM. Lobe reconnection signatures were identified for both winter and summer in the simulation results. However, a detailed analysis showed that the pattern of lobe reconnection in the winter hemisphere was different from that in the summer. Based on the combined observation and simulation results, we suggest that particular lobe reconnection in summer is critical for generating 15MLT‐PCA, while the winter's reconnection may lead to transient or small‐scale auroral responses that were not easily identified by DMSP/SSUSI observations as a 15MLT‐PCA event.