Solid State and Solution Nitrate Photochemistry: Photochemical Evolution of the Solid State Lattice

Phase State Regulates Photochemical HONO Production from NaNO3/Dicarboxylic Acid Mixtures

Field observations of daytime HONO source strengths have not been well explained by laboratory measurements and model predictions up until now. More efforts are urgently needed to fill the knowledge gaps concerning how environmental factors, especially relative humidity (RH), affect particulate nitrate photolysis. In this work, two critical attributes for atmospheric particles, i.e., phase state and bulk-phase acidity, both influenced by ambient RH, were focused to illuminate the key regulators for reactive nitrogen production from typical internally mixed systems, i.e., NaNO3 and dicarboxylic acid (DCA) mixtures. The dissolution of only few oxalic acid (OA) crystals resulted in a remarkable 50-fold increase in HONO production compared to pure nitrate photolysis at 85% RH. Furthermore, the HONO production rates (PHONO) increased by about 1 order of magnitude as RH rose from <5% to 95%, initially exhibiting an almost linear dependence on the amount of surface absorbed water and subsequently showing a substantial increase in PHONO once nitrate deliquescence occurred at approximately 75% RH. NaNO3/malonic acid (MA) and NaNO3/succinic acid (SA) mixtures exhibited similar phase state effects on the photochemical HONO production. These results offer a new perspective on how aerosol physicochemical properties influence particulate nitrate photolysis in the atmosphere.

Evolution of Morphology and Distribution of Salt Crystals on a Photothermal Layer during Solar Interfacial Evaporation

Solar interfacial evaporation (SIE) by leveraging photothermal conversion could be a clean and sustainable solution to the scarcity of fresh water, decontamination of wastewater, and steam sterilization. However, the process of salt crystallization on photothermal materials used in SIE, especially from saltwater evaporation, has not been completely understood. We report the temporal and spatial evolution of salt crystals on the photothermal layer during SIE. By using a typical oil lamp evaporator, we found that salt crystallization always initiates from the edge of the evaporation surface of the photothermal layer due to the local fast flux of the vapor to the surroundings. Interestingly, the salt crystals exhibit either compact or loose morphology, depending on the location and evaporation duration. By employing a suite of complementary analytical techniques of Raman and infrared spectroscopy and temperature mapping, we followed the evolution and spatial distribution of salt crystals, interfacial water, and surface temperature during evaporation. Our results suggested that the compact crystal structure may emerge from the recrystallization of salt in an initially porous structure, driven by continuous water evaporation from the porous and loose crystals. The holistic view provided in this study may lay the foundation for effective strategies for mitigation of the negative impact of salt crystallization in solar evaporation.

Nitrate Photolysis in Mixed Sucrose-Nitrate-Sulfate Particles at Different Relative Humidities

Atmospheric particles can be viscous. The limitation in diffusion impedes the mass transfer of oxidants from the gas phase to the particle phase and hinders multiphase oxidation processes. On the other hand, nitrate photolysis has been found to be effective in producing oxidants such as OH radicals within the particles. Whether nitrate photolysis can effectively proceed in viscous particles and how it may affect the physicochemical properties of the particle have not been much explored. In this study, we investigated particulate nitrate photolysis in mixed sucrose-nitrate-sulfate particles as surrogates of atmospheric viscous particles containing organic and inorganic components as a function of relative humidity (RH) and the molar fraction of sucrose to the total solute (F_SU) with an in situ micro-Raman system. Sucrose suppressed nitrate crystallization, and high photolysis rate constants (∼10^-5 s^-1) were found, irrespective of the RH. For F_SU = 0.5 and 0.33 particles under irradiation at 30% RH, we observed morphological changes from droplets to the formation of inclusions and then likely "hollow" semisolid particles, which did not show Raman signal at central locations. Together with the phase states of inorganics indicated by the full width at half-maxima (FWHM), images with bulged surfaces, and size increase of the particles in optical microscopic imaging, we inferred that the hindered diffusion of gaseous products (i.e., NO_x, NO_y) from nitrate photolysis is a likely reason for the morphological changes. Atmospheric implications of these results are also presented.

Deep Ultraviolet Standoff Photoacoustic Spectroscopy of Trace Explosives

We demonstrate deep ultraviolet (UV) photoacoustic spectroscopy (PAS) of trace explosives using a sensitive microphone at meter standoff distances. We directly detect 10 µg/cm² of pentaerythritol tetranitrate (PETN), 2,4,6-trinitrotoluene (TNT), and ammonium nitrate (AN) with 1 s accumulations from a 3 m standoff distance. Large PAS signals for standoff detection are achieved by exciting into the absorption bands of the explosives with a 213 nm laser. We also investigate the impact of the deep UV photochemistry of AN on the PAS signal strength and stability. We find that production of gaseous species during photolysis of AN enhances the PAS signal strength. This deep UV photochemistry can, however, limit the PAS signal lifetimes when detecting trace quantities.

Manganese-doped carbon quantum dots-based fluorescent probe for selective and sensitive sensing of 2,4,6-trinitrophenol via an inner filtering effect

In the present work, a selective and sensitive method for detecting TNP using manganese doped carbon quantum dots (Mn-CDs) was developed. The Mn-CDs were prepared via a simple hydrothermal method using 1-(2-pyridinylazo)-2-naohthalenol naohthalenol (PAN) and MnCl₂ as precursors. The as-prepared Mn-CDs have UV emission with high quantum yield (83.2%). Because of the strong characteristic absorption of TNP at 356 nm, which has good spectral overlap with the emission peak of Mn-CDs, the fluorescence intensity of Mn-CDs at 360 nm is linearly quenched in the presence of TNP in the concentration range of 0.1-200 μM. The developing assay based on an inner filter effect (IFE) mechanism for detecting TNP is selective, convenient, and shows that the as-prepared Mn-CDs have application prospects for simple and specific analytical chemistry.

Synthesis and characterization of the alkali borate-nitrates Na3–x K x [B6O10]NO3 (x=0.5, 0.6, 0.7)

Three novel mixed alkali borate-nitrates Na3−x K x [B6O10]NO3 (x=0.5, 0.6, 0.7) were synthesized hydrothermally; their crystal structures were determined through Rietveld analyses, and supported through EDX as well as vibrational spectroscopy. The phases represent solid solutions of the alkali borate-nitrate Na3(NO3)[B6O10], which was reported in 2002 as a “New type of boron-oxygen framework in the Na3(NO3)[B6O10] crystal structure” (O. V. Yakubovich, I. V. Perevoznikova, O. V. Dimitrova, V. S. Urusov, Dokl. Phys. 2002, 47, 791). Only two of the three crystallographically independent Na+ positions in the new structures are partially substituted by K+; a pure potassium borate-nitrate was not formed until now. The cell parameters of the novel phases vary from a=1261.72(5)–1267.12(5), b=1004.19(5)–1007.96(4), c=770.55(3)–774.38(3) pm, and V=0.97630(6)–0.98905(6) nm3 in the orthorhombic space group Pnma (no. 62), in alignment with increasing K+ content.

DFT study of the formation mechanism of anthraquinone from the reaction of NO2 and anthracene on NaCl clusters: the role of NaNO3

Polycyclic aromatic hydrocarbons (PAHs) and oxygenated-PAHs are globally worrisome air pollutants because of their highly direct-acting mutagenicity and carcinogenicity. The formation of oxygenated-PAHs is of crucial importance for the prevention of their atmospheric pollution successfully. In this paper, the formation mechanism of oxygenated-PAHs from the heterogeneous reaction of NO₂ with anthracene on the surface of NaCl was studied by density functional theory (DFT) calculations. At first, the various adsorption configurations of NO₂ and N₂O₄ on NaCl were investigated. The chemical conversion mechanisms among these configurations were also investigated. It is found that these structures can easily interconvert due to their low energy barriers. NaNO₃ was found to be the main product of the reaction of NO₂/N₂O₄ on NaCl. Then the oxidation mechanism of anthracene by NO₂ on the NaCl surface showed that NaNO₃ is able to oxidize anthracene and plays a catalytic role in the reaction process. This means that the formation of NaNO₃ is very important to promote the formation of 9,10-anthraquinone from the heterogeneous reaction of NO₂ with anthracene. Our calculations also showed that the introduction of water can greatly accelerate this reaction process.

Compact Solid-State 213 nm Laser Enables Standoff Deep Ultraviolet Raman Spectrometer: Measurements of Nitrate Photochemistry

We describe a new compact acousto-optically Q-switched diode-pumped solid-state (DPSS) intracavity frequency-tripled neodymium-doped yttrium vanadate laser capable of producing ~100 mW of 213 nm power quasi-continuous wave as 15 ns pulses at a 30 kHz repetition rate. We use this new laser in a prototype of a deep ultraviolet (UV) Raman standoff spectrometer. We use a novel high-throughput, high-resolution Echelle Raman spectrograph. We measure the deep UV resonance Raman (UVRR) spectra of solid and solution sodium nitrate (NaNO3) and ammonium nitrate (NH4NO3) at a standoff distance of ~2.2 m. For this 2.2 m standoff distance and a 1 min spectral accumulation time, where we only monitor the symmetric stretching band, we find a solid state NaNO3 detection limit of ~100 μg/cm(2). We easily detect ~20 μM nitrate water solutions in 1 cm path length cells. As expected, the aqueous solutions UVRR spectra of NaNO3 and NH4NO3 are similar, showing selective resonance enhancement of the nitrate (NO3(-)) vibrations. The aqueous solution photochemistry is also similar, showing facile conversion of NO3(-) to nitrite (NO2(-)). In contrast, the observed UVRR spectra of NaNO3 and NH4NO3 powders significantly differ, because their solid-state photochemistries differ. Whereas solid NaNO3 photoconverts with a very low quantum yield to NaNO2, the NH4NO3 degrades with an apparent quantum yield of ~0.2 to gaseous species.

Solution and Solid Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) Ultraviolet (UV) 229 nm Photochemistry

We measured the 229 nm deep-ultraviolet resonance Raman (DUVRR) spectra of solution and solid-state hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX). We also examined the photochemistry of RDX both in solution and solid states. RDX quickly photodegrades with a solution quantum yield of φ ~ 0.35 as measured by high-performance liquid chromatography (HPLC). New spectral features form over time during the photolysis of RDX, indicating photoproduct formation. The photoproduct(s) show stable DUVRR spectra at later irradiation times that allow standoff detection. In the solution-state photolysis, nitrate is a photoproduct that can be used as a signature for detection of RDX even after photolysis. We used high-performance liquid chromatography-high-resolution mass spectrometry (HPLC-HRMS) and gas chromatography mass spectrometry (GCMS) to determine some of the major solution-state photoproducts. X-ray photoelectron spectroscopy (XPS) was also used to determine photoproducts formed during solid-state RDX photolysis.

Recent advances and remaining challenges for the spectroscopic detection of explosive threats

In 2010, the U.S. Army initiated a program through the Edgewood Chemical Biological Center to identify viable spectroscopic signatures of explosives and initiate environmental persistence, fate, and transport studies for trace residues. These studies were ultimately designed to integrate these signatures into algorithms and experimentally evaluate sensor performance for explosives and precursor materials in existing chemical point and standoff detection systems. Accurate and validated optical cross sections and signatures are critical in benchmarking spectroscopic-based sensors. This program has provided important information for the scientists and engineers currently developing trace-detection solutions to the homemade explosive problem. With this information, the sensitivity of spectroscopic methods for explosives detection can now be quantitatively evaluated before the sensor is deployed and tested.