Role of nitrite in the photochemical formation of radicals in the snow

Photodegradation of bisphenol A and identification of photoproducts in artificial snow under UVA radiation

Bisphenol A (BPA) is an organic micropollutant detected in various environments, from urban to remote areas, including Arctic snow. As a known endocrine disruptor, it is essential to investigate its environmental fate and potential impact on ecosystems. Previous studies have explored BPA photodegradation and its transformation products in different aqueous environments (freshwater, seawater, and ice), by using photosensitizers to trigger specific reactions. However, there is still a significant gap in understanding the photodegradation processes in snow, which, although similar to ice, has distinct chemical and physical characteristics. In this work, we investigated the direct and indirect photodegradation of BPA in artificial snow and identified its degradation products through HPLC-HRMS. Nitrite and benzophenone-4-carboxylate, the latter used as a surrogate of chromophoric dissolved organic matter, induced significant BPA photodegradation under UVA irradiation. The photoproducts found in snow were partly similar to those previously observed in liquid water and ice. Their toxicity towards aquatic organisms was predicted with ECOSAR software as well. Finally, BPA photolysis and formation of photoproducts were investigated in two Alpine snow samples collected above and below the tree line, with a different organic matter content. Oxidation and nitration products of BPA were detected in these samples, suggesting that BPA photodegradation may indeed occur in natural snow. It was also noted that the aquatic toxicity of several identified photoproducts would be similar to that of BPA, but others may be even more toxic than the parent contaminant.

Phototransformation of Vanillin in Artificial Snow by Direct Photolysis and Mediated by Nitrite

The photodegradation of vanillin, as a proxy of methoxyphenols emitted by biomass burning, was investigated in artificial snow at 243 K and in liquid water at room temperature. Nitrite (NO₂^-) was used as a photosensitizer of reactive oxygen and nitrogen species under UVA light, because of its key photochemical role in snowpacks and atmospheric ice/waters. In snow and in the absence of NO₂^-, slow direct photolysis of vanillin was observed due to back-reactions taking place in the quasi-liquid layer at the ice-grain surface. The addition of NO₂^- made the photodegradation of vanillin faster, because of the important contribution of photoproduced reactive nitrogen species in vanillin phototransformation. These species triggered both nitration and oligomerization of vanillin in irradiated snow, as the identified vanillin by-products showed. Conversely, in liquid water, direct photolysis was the main photodegradation pathway of vanillin, even in the presence of NO₂^-, which had negligible effects on vanillin photodegradation. The results outline the different role of iced and liquid water in the photochemical fate of vanillin in different environmental compartments.

Catalytic behavior of nitrous acid for acetaminophen transformation during the freezing process

This study demonstrates the transformation of acetaminophen by reactive nitrous acid in a frozen solution and its abnormal stoichiometry. The chemical reaction between acetaminophen and nitrous acid (AAP/NO₂^- system) was negligible in the aqueous solution; however, the reaction rapidly progressed if the solution started to freeze. The ultrahigh performance liquid chromatography-electrospray ionization tandem mass spectrometry measurements showed that polymerized acetaminophen and nitrated acetaminophen were formed in the proceeding reaction. Electron paramagnetic resonance spectroscopy measurements showed that nitrous acid oxidized acetaminophen via a one-electron transfer reaction producing acetaminophen-derived radical species, which is the cause of acetaminophen polymerization. We demonstrated that a relatively smaller dose of nitrite than acetaminophen caused significant acetaminophen degradation in the frozen AAP/NO₂^- system and revealed that the dissolved oxygen content notably affected acetaminophen degradation. We showed that the reaction occurs in a natural Arctic lake matrix (nitrite and acetaminophen spiked). Considering that the freezing phenomenon is common in the natural environment, our research provides a possible scenario for the freezing chemistry of nitrite and pharmaceuticals in environmental chemistry.

Frozen Hydrogen Peroxide and Nitrite Solution: The Acceleration of Benzoic Acid Oxidation via the Decreased pH in Ice

We investigated benzoic acid oxidation via the reaction of hydrogen peroxide (H₂O₂) and nitrite (NO₂^-). The oxidation of benzoic acid by reactive nitrous acid (HONO) was negligible, and the reactivity of the H₂O₂/NO₂^- system decreased with a decrease in temperature under aqueous conditions. However, freezing markedly accelerated the chemical reaction. Based on Raman microscope measurements, concentrated species were confirmed in certain regions of the ice. We proposed that the change in nitrite speciation (accordingly, a decrease in the pH below pK_a), derived from the freezing concentration effect, was the reason for the accelerated reactions. The oxidation characteristics of the system were monitored under varying conditions, such as initial pH, dosage ratio, benzoic acid concentration, and reaction with various benzene derivatives. The ultrahigh-performance liquid chromatography/electrospray ionization/mass spectrometry (UHPLC/ESI/MS) measurement showed that peroxynitrous acid (HOONO)-mediated oxidation generated hydroxylated and nitrated byproducts. Additionally, decarboxylated products were detected, indicating direct electron transfer from the organic compounds to HOONO. As freezing is a global phenomenon, and H₂O₂ and NO₂^- are ubiquitous in the environment, the transformation of aromatic compounds with H₂O₂/NO₂^- in cold environments must be considered in environmental chemistry.

Photochemical degradation of β-hexachlorocyclohexane in snow and ice

Hexachlorocyclohexane (HCH), a typical organochloride pesticide, is one of the persistent organic pollutants. Despite the ban on technical grade HCH, it has been continuously observed at a steady level in the environment. The photochemical degradation of β-HCH in snow and ice under ultraviolet (UV) irradiation was investigated in this study. The effects of pH as well as common chemical components in snow on the degradation kinetics were investigated. In addition, the photodegradation products were determined and the reaction mechanism was hypothesized. The results showed that under UV irradiation, β-HCH can be photolyzed in snow and ice, with the photochemical degradation process conforming to the first-order kinetic equation. Changing the pH and adding Fe²⁺ had minimal effect on the photochemical degradation kinetics, while the presence of acetone, NO₂^-, NO₃^- and Fe³⁺ significantly inhibited the process. The addition of hydrogen peroxide slightly inhibited the photochemical degradation of β-HCH. Finally, the reaction rate, products and degradation mechanism of β-HCH in snow were compared with those in the ice phase. The photochemical degradation rate of β-HCH in snow was approximately 24 times faster than that in the ice phase. The photolysis product of β-HCH in snow was α-HCH, produced by the isomerization of β-HCH. However, in ice, in addition to α-HCH, pentachlorocyclohexene was produced by dechlorination. The results of this study are helpful in understanding the transformation of organochlorine pesticides in snow and ice, as well as in providing a theoretical basis for snow and ice pollution prevention and control.

Snow pollution by nitrogen-containing substances as a consequence of rocket launches from the Baikonur Cosmodrome

In this paper, we assessed snow pollution by nitrogen-containing substances including rocket propellants - UDMH (unsymmetrical dimethylhydrazine, (СН₃)₂NNH₂) and NT (nitrogen tetroxide, N₂O₄) - and their transformation products (NDMA (nitrosodimethylamine, (CH₃)₂NNO), NO₃^-, NO₂^- and NH₄⁺) within the falling regions (FRs) of the first and second stages of Proton-M rockets launched from the Baikonur Cosmodrome. At the first stage FR in Central Kazakhstan, snow with a pH range from 1.7 to 9.0 was contaminated by N-containing substances (maximal value in g/L): UDMH - 0.27, NDMA - 0.04, NO₃^- - 19, NH₄⁺ - 0.04 and NO₂^- - 0.13. The first stage landing resulted in snow contamination by soil dust particles and N-containing substances at a rate of 13 g/m² and 82 mg/m²/day, respectively. The maximal permissible addition (MPA) for UDMH, NDMA and NO₃^- to the 0-5 cm layer of soil was estimated at 0.06, 0.006 and 70.2 mg/m², respectively. At the second stage FR in the NE Altai, substances released by space transportation were absent and the concentration of NO₃^- and NH₄⁺ corresponded to the natural background level. The index of contamination (IC) was used for characterizing the degree of snow contamination by N-containing substances. A simulation model was developed for analysing the dependence of snow contamination by rocket propellant components on the weather parameters.

Nitrite-Induced Activation of Iodate into Molecular Iodine in Frozen Solution

A new mechanism for the abiotic production of molecular iodine (I₂) from iodate (IO₃^-), which is the most abundant iodine species, in dark conditions was identified and investigated. The production of I₂ in aqueous solution containing IO₃^- and nitrite (NO₂^-) at 25 °C was negligible. However, the redox chemical reaction between IO₃^- and NO₂^- rapidly proceeded in frozen solution at -20 °C, which resulted in the production of I₂, I^-, and NO₃^-. The rapid redox chemical reaction between IO₃^- and NO₂^- in frozen solution is ascribed to the accumulation of IO₃^-, NO₂^-, and protons in the liquid regions between ice crystals during freezing (freeze concentration effect). This freeze concentration effect was verified by confocal Raman microscopy for the solute concentration and UV-visible absorption spectroscopy with cresol red (acid-base indicator) for the proton concentration. The freezing-induced production of I₂ in the presence of IO₃^- and NO₂^- was observed under various conditions, which suggests this abiotic process for I₂ production is not restricted to a specific region and occurs in many cold regions. NO₂^--induced activation of IO₃^- to I₂ in frozen solution may help explain why the measured values of iodine are larger than the modeled values in some polar areas.

Heterogeneous oxidation of nitrite anion by gas-phase ozone in an aqueous droplet levitated by laser tweezers (optical trap): is there any evidence for enhanced surface reaction?

Optical trapping of micron-sized droplet morphology and heterogeneous kinetics with gas-phase ozone with nitrite in a wall-less apparatus.