Nitrate Photolysis Research Articles

Impact of organic and inorganic carbon on the formation of nitrite during the VUV photolysis of nitrate containing water

Nitrite formation during Vacuum UV (VUV) photolysis of nitrate-containing water has been studied in this research. Here the goal was to understand the effect of water matrix, specifically dissolved organic carbon (DOC), dissolved inorganic carbon (DIC) and dissolved oxygen (DO), on the formation of nitrite in nitrate-containing water during treatment with VUV AOP. The results confirmed that the formation of nitrite was considerable during the VUV irradiation, and it increased with increasing the UV fluence. However, the rate of nitrite formation was not linearly correlated with the UV fluence. Also, nitrite formation increased with increasing the DOC in water. Tert-butanol, as OH radical scavenger, was used to elucidate the mechanism behind the impact of DOC. The effect of DIC was only pronounced at very high DIC concentration, which led to lower formation of nitrite. When both DOC and DIC were present in water, DOC had the dominant effect on nitrite formation. DO in water had the impact of reducing the formation of nitrite. All these results can help to understand the mechanisms involved in the transformation of nitrate to nitrite in various water matrices, and guide VUV applications for scientific and industrial proposes.

Heterogeneous Oxidation of SO2 in Sulfate Production during Nitrate Photolysis at 300 nm: Effect of pH, Relative Humidity, Irradiation Intensity, and the Presence of Organic Compounds.

Heterogeneous oxidation of SO2 is one of the promising mechanisms to account for high loading of sulfate during severe haze periods in China. Our earlier work reported on the SO2 oxidation by OH and NO2 produced during 250 nm nitrate photolysis (Environ. Sci. Technol. Lett. 2019, 6, 86-91). Here, we extend that work to examine sulfate production during nitrate photolysis at 300 nm irradiation, which can additionally generate NO2- or HNO2, N(III). Flow cell/in situ Raman experiments showed that the reactive uptake coefficient of SO2, γSO2, can be expressed as γSO2 = 1.64 × pNO3-, where pNO3- is the nitrate photolysis rate in the range of (1.0-8.0) × 10-5 M s-1. Our kinetic model with the pNO3- predicts that N(III) is the main contributor to the SO2 oxidation, followed by NO2 contribution. Furthermore, the addition of OH scavengers (e.g., glyoxal or oxalic acid) does not suppress the sulfate production because of the reduced N(III)-consuming reactions and the high particle pH sustained by their presence. Our calculations illustrate that under characteristic haze conditions, the nitrate photolysis mechanism can produce sulfate at ∼1 μg m-3 h-1 at pH 4-6 and pNO3- = 10-5 M s-1. The present study highlights the importance of in-particle nitrate photolysis in heterogeneous oxidation of SO2 by reactive nitrogen (NO2-/HNO2 and NO2) under atmospherically relevant actinic irradiation. However, the nitrate photolysis rate constant needs to be better constrained for ambient aerosols.

The Role of Reactive Nitrogen Species in Sensitized Photolysis of Wastewater-Derived Trace Organic Contaminants.

Under conditions typically encountered in the aquatic environment, the absorption of sunlight by nitrite and nitrate leads to the transformation of trace organic contaminants. In addition to the well understood mechanism through which hydroxyl radical (·OH) produced by nitrate and nitrite photolysis oxidizes contaminants, absorption of light also results in the formation of reactive nitrogen species that transform organic contaminants. To assess the importance of this process on the fate of trace organic contaminants, radical quenchers and transformation product analysis were used to discriminate among potential reaction pathways. For sulfamethoxazole, an antibiotic that is frequently detected in municipal wastewater effluent, nitrate and nitrite-sensitized photolysis pathways resulted in production of transformation products that were not detected during direct photolysis or reaction with ·OH. The reactivity of sulfamethoxazole with the reactive species produced when nitrite absorbed sunlight was affected by the presence of hydroxyl radical scavengers, indicating the likely involvement of nitrogen dioxide, which forms when nitrite reacts with hydroxyl radical. Reactive nitrogen species also reacted with emtricitabine, propranolol, and other trace organic contaminants commonly detected in wastewater effluent, indicating the potential importance of this process to the fate of other trace organic contaminants. A kinetic model indicated that reactive nitrogen species could be important to the phototransformation of trace organic contaminants when relatively high concentrations of nitrite are present (e.g., in surface waters receiving reverse osmosis concentrate from potable water reuse projects or in agricultural runoff).

HONO, Particulate Nitrite, and Snow Nitrite at a Midlatitude Urban Site during Wintertime

Nitrous acid (HONO) plays an important role in the oxidative capacity of the atmosphere during wintertime via photolysis to produce hydroxyl radicals (OH). While it is known that HONO is emitted from the Arctic snowpack, sparse observations of HONO in the midlatitude urban wintertime environment have hindered our understanding of cold-season atmospheric chemistry. In this study, measurements of ambient HONO, particulate nitrite (pN(III); N(III) = NO2(aq)– + HONO(aq) + H2ONO(aq)+), and snow nitrite (sN(III)) were conducted in Kalamazoo, Michigan during January–February 2018. Elevated levels of HONO and particulate nitrite were observed over snow-covered ground, likely due to emissions of HONO from the snowpack, as well as weak turbulent mixing in the atmospheric boundary layer. The noontime peak in HONO of 87 ± 60 (1σ) parts per trillion (ppt) over snow-covered ground suggests photochemical snowpack HONO production, likely in part through snowpack nitrate photolysis, with only a minor contribution from par...

Isotope Fractionation of Nitrate During Volatilization in Snow: A Field Investigation in Antarctica

AbstractSeveral postdepositional processes impact snow nitrate; however, only the isotopic effects of nitrate photolysis have been quantified. Here we discuss results from experiments in field Antarctic snow investigating isotopic fractionation of nitrate due to volatilization. At −35 °C, concentration and isotopic composition of nitrate remained constant during the 16‐day experiment. At −24 °C, 7.5% of nitrate was lost, synchronous with 1.5‰ decrease in δ18O and a constant δ15N. At −4 °C, 38% of nitrate was lost, and δ15N and δ18O decreased by 3.1 and 1.8‰, respectively. Results at −4 °C yield calculated fractionation constants close to theoretical estimates including equilibrium isotopic exchange between nitric acid and nitrate and the desorption of nitric acid from water in quasi‐liquid layers. This suggests that isotopic fractionation associated with nitrate volatilization across most of Antarctica, especially at sites with temperatures <−24 °C, should be minor, but the isotopic effects at warmer sites should be considered in interpreting archived nitrate records.

A Comprehensive Model Test of the HONO Sources Constrained to Field Measurements at Rural North China Plain.

As nitrous acid (HONO) photolysis is an important source of hydroxyl radical (OH), apportionment of the ambient HONO sources is necessary to better understand atmospheric oxidation. Based on the data HONO-related species and various parameters measured during the one-month campaign at Wangdu (a rural site in North China plain) in summer 2014, a box model was adopted with input of current literature parametrizations for various HONO sources (nitrogen dioxide heterogeneous conversion, photoenhanced conversion, photolysis of adsorbed nitric acid and particulate nitrate, acid displacement, and soil emission) to reveal the relative importance of each source at the rural site. The simulation results reproduced the observed HONO production rates during noontime in general but with large uncertainty from both the production and destruction terms. NO2 photoenhanced conversion and photolysis of particulate nitrate were found to be the two major mechanisms with large potential of HONO formation but the associated uncertainty may reduce their importance to be nearly negligible. Soil nitrite was found to be an important HONO source during fertilization periods, accounted for (80 ± 6)% of simulation HONO during noontime. For some episodes of the biomass burning, only the NO2 heterogeneous conversion to HONO was promoted significantly. In summary, the study of the HONO budget is still far from closed, which would require a significant effort on both the accurate measurement of HONO and the determination of related kinetic parameters for its production pathways.

Heterogeneous SO2 Oxidation in Sulfate Formation by Photolysis of Particulate Nitrate

Heterogeneous oxidation of sulfur dioxide (SO2) is suggested to be one of the most important pathways for sulfate formation during extreme haze events in China, yet the exact mechanism remains high...

Nitrate Removal via a Formate Radical-Induced Photochemical Process.

Removal of excess nitrate is critical to balance the nitrogen cycle in aquatic systems. This study investigated a novel denitrification process by tailoring photochemistry of nitrate with formate. Under UV light irradiation, short-lived radicals (i.e., HO•, NO2•, and CO3•-) generated from nitrate photolysis partially oxidized formate to highly reductive formate radical (CO2•-). CO2•- further reduced nitrogen intermediates generated during photochemical denitrification (mainly NO•, HNO, and N2O) to gas-phase nitrogen (i.e., N2O and N2). The degradation kinetics of total dissolved nitrogen was mainly controlled by the photolysis rates of nitrate and nitrite. The distribution of final products was controlled by the reaction between CO2•- and N2O. To achieve a simultaneous and complete removal of dissolved nitrogen (i.e., nitrate, nitrite, and ammonia) and organic carbon, the formate-to-nitrate stoichiometry was determined as 3.1 ± 0.2 at neutral pH in deionized water. Solution pH impacted the removal rates of nitrate and nitrite but not that of total dissolved nitrogen or formate. The presence of dissolved organic matter at levels similar to those ingroundwater had a negligible impact on the photochemical denitrification process. A high denitrification efficiency was also achieved in a synthetic groundwater matrix. Outcome from this study provides a potential denitrification technology for decentralized water treatment and reuse facilities to abate nitrate in local water resources.

Constraints on Aerosol Nitrate Photolysis as a Potential Source of HONO and NO x.

The concentration of nitrogen oxides (NO x) plays a central role in controlling air quality. On a global scale, the primary sink of NO x is oxidation to form HNO3. Gas-phase HNO3 photolyses slowly with a lifetime in the troposphere of 10 days or more. However, several recent studies examining HONO chemistry have proposed that particle-phase HNO3 undergoes photolysis 10-300 times more rapidly than gas-phase HNO3. We present here constraints on the rate of particle-phase HNO3 photolysis based on observations of NO x and HNO3 collected over the Yellow Sea during the KORUS-AQ study in summer 2016. The fastest proposed photolysis rates are inconsistent with the observed NO x to HNO3 ratios. Negligible to moderate enhancements of the HNO3 photolysis rate in particles, 1-30 times faster than in the gas phase, are most consistent with the observations. Small or moderate enhancement of particle-phase HNO3 photolysis would not significantly affect the HNO3 budget but could help explain observations of HONO and NO x in highly aged air.

Characterization of reactive photoinduced species in rainwater.

Rainfall is a highly effective and important carrier that can remove a majority of aerosol mass into land and marine ecosystems. The photochemically formed reactive species in the rainwater are likely dominant oxidants for organic and inorganic substances. Here, we collected rainwater samples from Oct. 2016 to Dec. 2016 in CUG campus (Wuhan, Hubei, China) and measured their formation rates, lifetimes, steady-state concentrations, and apparent quantum yields of reactive photoinduced species, including hydroxyl radical (HO•), H2O2, singlet oxygen (1O2), and chromophoric dissolved organic matter triplet state (3CDOM*) in the laboratory. Results showed that rainwater samples contained photochemical sources, like DOM, nitrate, heavy metals, etc. Quantification of HO• showed that rHO• (the photogeneration rate of HO•) were in the range of 1.05 × 10-10-4.56 × 10-10Ms-1, and [•OH]ss (the steady-state concentrations of OH•) were of 4.06 × 10-18-2.97 × 10-17M for the three samples. Further investigations revealed that 10-24% of r•OH was attributed to nitrate photolysis, suggesting DOM was possibly the prevailing source of HO•. Apparent quantum yields of H2O2 (ΦH2O2) correlated negatively with E2/E3 (the ratio of absorption at 250 and 365nm), while Φ1O2 and Φ3CDOM* increased with elevated E2/E3.

Effects of HCO3- on Degradation of Toxic Contaminants of Emerging Concern by UV/NO3.

This study investigated the significant influence of HCO3- on the degradation of contaminants of emerging concern (CECs) during nitrate photolysis at 254 nm for water reuse applications. The second-order rate constants for the reactions between selected contaminants with carbonate radical (CO3•-) were determined at pH 8.8 and T = 20 °C: estrone ((5.3 ± 1.1) × 108 M-1 s-1), bisphenol A ((2.8 ± 0.2) × 108 M-1 s-1), 17α-ethynylestradiol ((1.6 ± 0.3) × 108 M-1 s-1), triclosan ((4.2 ± 1.4) × 107 M-1 s-1), diclofenac ((2.7 ± 0.7) × 107 M-1 s-1), atrazine ((5.7 ± 0.1) × 106 M-1 s-1), carbamazepine ((4.2 ± 0.01) × 106 M-1 s-1), and ibuprofen ((1.2 ± 1.1) × 106 M-1 s-1). Contributions from UV, reactive nitrogen species (RNS), hydroxyl radical (•OH), and CO3•- to the CEC decomposition in UV/NO3- in the presence and absence of HCO3- were investigated. In addition, possible transformation products and degradation pathways of triclosan, diclofenac, bisphenol A, and estrone in UV/NO3-/HCO3- were proposed based on the mass (MS) and MS2 spectra. Significant reduction in the cytotoxicity of bisphenol A was observed after the treatment with UV/NO3-/HCO3-.

Significant HONO formation by the photolysis of nitrates in the presence of humic acids

The generation of HONO and NO2 by the photolysis of nitrates in the presence of humic acids (HA) was measured under various conditions. The photolysis experiments of HA, KNO3 and KNO3/HA under simulated sunlight was carried out by a flow tube reactor at ambient temperature and pressure. HONO and NO2 were major products by the photolysis of KNO3. By contrast, the photolysis of HA and KNO3/HA mainly generated HONO. HA significantly enhanced the formation of HONO during the photolysis process of KNO3. With increasing the KNO3 mass, the HONO formation rate (RHONO) on KNO3/HA increased while the photolysis rate normalized by the KNO3 mass exhibited an opposite trend. RHONO on KNO3/HA linearly increased with irradiation intensity (88–262 W/m2) and relative humidity (7–70%), whereas it linearly decreased with the pH (pH = 2–12). In addition, the reaction paths of the HONO formation by the photolysis of nitrates in the presence of HA were proposed according to experimental results. Finally, atmospheric implications of the enhanced HONO formation by the photolysis of nitrates in the presence of HA were discussed.

Impact of adsorbed nitrate on the heterogeneous conversion of SO2 on α-Fe2O3 in the absence and presence of simulated solar irradiation

Adsorbed nitrate is ubiquitous in the atmosphere, and it can undergo photolysis to produce oxidizing active radicals. Nitrate photolysis may be coupled with the oxidation conversions of atmospheric gaseous pollutants. However, the processes involved remain poorly understood. In this study, the impact of adsorbed nitrate on the heterogeneous oxidation of SO2 on α-Fe2O3 was investigated in the absence and presence of simulated solar irradiation by using in situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS). The results indicate that for α-Fe2O3 particles with no adsorbed nitrate, the formation of adsorbed sulfate on humid particles is stronger than that on dry particles. Meanwhile, light can also promote the heterogeneous conversion of SO2 and the formation of sulfate on dry particles because α-Fe2O3 is a typical photocatalyst. However, the heterogeneous conversion of SO2 on humid α-Fe2O3 particles is somewhat suppressed under light, suggesting the occurrence of photoinduced reductive dissolution. For the heterogeneous conversion of SO2 on α-Fe2O3 particles with adsorbed nitrate, the formation of sulfate on humid particles is still higher than that on dry particles. For the dry α-Fe2O3 particles with adsorbed nitrate, light promotes the formation of adsorbed sulfate. For the humid α-Fe2O3 particles with adsorbed nitrate, the heterogeneous conversion of SO2 under light is stronger than that under no light, indicating that the photolysis of adsorbed nitrate is coupled with the oxidation of SO2 and the formation of sulfate. The consumption of adsorbed nitrate and the formation of adsorbed N2O4 are observed during the introduction of SO2. A possible mechanism for the impact of adsorbed nitrate on the heterogeneous conversion of SO2 on α-Fe2O3 particles is proposed, and atmospheric implications based on these results are discussed.

Global impact of nitrate photolysis in sea-salt aerosol on NO<sub><i>x</i></sub>, OH, and O<sub>3</sub> in the marine boundary layer

Abstract. Recent field studies have suggested that sea-salt particulate nitrate (NITs) photolysis may act as a significant local source of nitrogen oxides (NOx) over oceans. We present a study of the global impact of this process on oxidant concentrations in the marine boundary layer (MBL) using the GEOS-Chem model, after first updating the model to better simulate observed gas–particle phase partitioning of nitrate in the marine boundary layer. Model comparisons with long-term measurements of NOx from the Cape Verde Atmospheric Observatory (CVAO) in the eastern tropical North Atlantic provide support for an in situ source of NOx from NITs photolysis, with NITs photolysis coefficients about 25–50 times larger than corresponding HNO3 photolysis coefficients. Short-term measurements of nitrous acid (HONO) at this location show a clear daytime peak, with average peak mixing ratios ranging from 3 to 6 pptv. The model reproduces the general shape of the diurnal HONO profile only when NITs photolysis is included, but the magnitude of the daytime peak mixing ratio is under-predicted. This under-prediction is somewhat reduced if HONO yields from NITs photolysis are assumed to be close to unity. The combined NOx and HONO analysis suggests that the upper limit of the ratio of NITs : HNO3 photolysis coefficients is about 100. The largest simulated relative impact of NITs photolysis is in the tropical and subtropical marine boundary layer, with peak local enhancements ranging from factors of 5 to 20 for NOx, 1.2 to 1.6 for OH, and 1.1 to 1.3 for ozone. Since the spatial extent of the sea-salt aerosol (SSA) impact is limited, global impacts on NOx, ozone, and OH mass burdens are small ( ∼ 1–3 %). We also present preliminary analysis showing that particulate nitrate photolysis in accumulation-mode aerosols (predominantly over continental regions) could lead to ppbv-level increases in ozone in the continental boundary layer. Our results highlight the need for more comprehensive long-term measurements of NOx, and related species like HONO and sea-salt particulate nitrate, to better constrain the impact of particulate nitrate photolysis on marine boundary layer oxidant chemistry. Further field and laboratory studies on particulate nitrate photolysis in other aerosol types are also needed to better understand the impact of this process on continental boundary layer oxidant chemistry.

Tropospheric HONO distribution and chemistry in the southeastern US

Abstract. Here we report the measurement results of nitrous acid (HONO) and a suite of relevant parameters on the NCAR C-130 research aircraft in the southeastern US during the NOMADSS 2013 summer field study. The daytime HONO concentration ranged from low parts per trillion by volume (pptv) in the free troposphere (FT) to mostly within 5–15 pptv in the background planetary boundary layer (PBL). There was no discernible vertical HONO gradient above the lower flight altitude of 300 m in the PBL, and the transport of ground surface HONO was not found to be a significant contributor to the tropospheric HONO budget. The total in situ HONO source mean (±1 SD) was calculated as 53 (±21) pptv h−1 during the day. The upper-limit contribution from NOx-related reactions was 10 (±5) pptv h−1, and the contribution from photolysis of particulate nitrate (pNO3) was 38 (±23) pptv h−1, based on the measured pNO3 concentrations and the median pNO3 photolysis rate constant of 2.0 × 10−4 s−1 determined in the laboratory using ambient aerosol samples. The photolysis of HONO contributed to less than 10 % of the primary OH source. However, a recycling NOx source via pNO3 photolysis was equivalent to ∼ 2.3 × 10−6 mol m−2 h−1 in the air column within the PBL, a considerable supplementary NOx source in the low-NOx background area. Up to several tens of parts per trillion by volume of HONO were observed in power plant and urban plumes during the day, mostly produced in situ from precursors including NOx and pNO3. Finally, there was no observable accumulation of HONO in the nocturnal residual layer and the nocturnal FT in the background southeastern US, with an increase in the HONO ∕ NOx ratio of ≤ 3 × 10−4 h−1 after sunset.

Nitrate Photochemistry at the Air-Ice Interface and in Other Ice Reservoirs.

The photolysis of snowpack nitrate (NO3-) is an important source of gaseous reactive nitrogen species that affect atmospheric oxidants, particularly in remote regions. However, it is unclear whether nitrate photochemistry differs between the three solute reservoirs in/on ice: in liquid-like regions (LLRs) in the ice; within the solid ice matrix; and in a quasi-liquid layer (QLL) at the air-ice interface, where past work indicates photolysis is enhanced. In this work, we explore the photoformation of nitrite in these reservoirs using laboratory ices. Nitrite quantum yields, Φ(NO2-), at 313 nm for aqueous and LLR ice samples agree with previous values, e.g., 0.65 ± 0.07% at -10 °C. For ice samples made via flash-freezing solution in liquid nitrogen, where nitrate is possibly present as a solid solution, the nitrite quantum yield is 0.57 ± 0.05% at -10 °C, similar to the LLR results. In contrast, the quantum yield at the air-ice interface is enhanced by a factor of 3.7 relative to LLRs, with a value of 2.39 ± 0.24%. These results indicate nitrate photolysis is enhanced at the air-ice interface, although the importance of this enhancement in the environment depends on the amount of nitrate present at the interface.

Distinguishing summertime atmospheric production of nitrate across the East Antarctic Ice Sheet

Surface snow and atmospheric samples collected along a traverse from the coast to the ice sheet summit (Dome A) are used to investigate summertime atmospheric production of nitrate (NO3–) across East Antarctica. The strong relationship observed between δ15N and δ18O of nitrate in the surface snow suggests a large (lesser) extent of nitrate photolysis in the interior (coastal) region. A linear correlation between the oxygen isotopes of nitrate (δ18O and Δ17O) indicates mixing of various oxidants that react with NOx (NOx = NO + NO2) to produce atmospheric nitrate. On the plateau, the isotopes of snow nitrate are best explained by local reoxidation chemistry of NOx, possibly occurring in both condensed and gas phases. Nitrate photolysis results in redistribution of snow nitrate, and the plateau snow is a net exporter of nitrate and its precursors. Our results suggest that while snow-sourced NOx from the plateau due to photolysis is a significant input to the nitrate budget in coastal snow (up to ∼35%), tropospheric transport from mid-low latitudes dominates (∼65%) coastal snow nitrate. The linear relationship of δ18O vs. Δ17O of the snow nitrate suggests a predominant role of hydroxyl radical (OH) and ozone (O3) in nitrate production, although a high Δ17O(O3) is required to explain the observations. Across Antarctica the oxygen isotope composition of OH appears to be dominated by exchange with water vapor, despite the very dry environment. One of the largest uncertainties in quantifying nitrate production pathways is the limited knowledge of atmospheric oxidant isotopic compositions.

The meteorology and chemistry of high nitrogen oxide concentrations in the stable boundary layer at the South Pole

Abstract. Four summer seasons of nitrogen oxide (NO) concentrations were obtained at the South Pole (SP) during the Sulfur Chemistry in the Antarctic Troposphere (ISCAT) program (1998 and 2000) and the Antarctic Tropospheric Chemistry Investigation (ANTCI) in (2003, 2005, 2006–2007). Together, analyses of the data collected from these studies provide insight into the large- to small-scale meteorology that sets the stage for extremes in NO and the significant variability that occurs day to day, within seasons, and year to year. In addition, these observations reveal the interplay between physical and chemical processes at work in the stable boundary layer of the high Antarctic plateau. We found a systematic evolution of the large-scale wind system over the ice sheet from winter to summer that controls the surface boundary layer and its effect on NO: initially in early spring (Days 280–310) the transport of warm air and clouds over West Antarctica dominates the environment over the SP; in late spring (Days 310–340), the winds at 300 hPa exhibit a bimodal behavior alternating between northwest and southeast quadrants, which is of significance to NO; in early summer (Days 340–375), the flow aloft is dominated by winds from the Weddell Sea; and finally, during late spring, winds aloft from the southeast are strongly associated with clear skies, shallow stable boundary layers, and light surface winds from the east – it is under these conditions that the highest NO occurs. Examination of the winds at 300 hPa from 1961 to 2013 shows that this seasonal pattern has not changed significantly, although the last twenty years have seen an increasing trend in easterly surface winds at the SP. What has also changed is the persistence of the ozone hole, often into early summer. With lower total ozone column density and higher sun elevation, the highest actinic flux responsible for the photolysis of snow nitrate now occurs in late spring under the shallow boundary layer conditions optimum for high accumulation of NO. This may occur via the non-linear HOX–NOx chemistry proposed after the first ISCAT field programs and NOx recycling to the surface where quantum yields may be large under the low-snow-accumulation regime of the Antarctic plateau. During the 2003 field program a sodar made direct measurements of the stable boundary layer depth (BLD), a key factor in explaining the chemistry of the high NO concentrations. Because direct measurements were not available in the other years, we developed an estimator for BLD using direct observations obtained in 2003 and step-wise linear regression with meteorological data from a 22 m tower (that was tested against independent data obtained in 1993). These data were then used with assumptions about the column abundance of NO to estimate surface fluxes of NOx. These results agreed in magnitude with results at Concordia Station and confirmed significant daily, intraseasonal and interannual variability in NO and its flux from the snow surface. Finally, we found that synoptic to mesoscale eddies governed the boundary layer circulation and accumulation pathways for NO at the SP rather than katabatic forcing. It was the small-scale features of the circulation including the transition from cloudy to clear conditions that set the stage for short-term extremes in NO, whereas larger-scale features were associated with more moderate concentrations.

Modelling the physical multiphase interactions of HNO<sub>3</sub> between snow and air on the Antarctic Plateau (Dome C) and coast (Halley)

Abstract. Emissions of nitrogen oxide (NOx = NO + NO2) from the photolysis of nitrate (NO3−) in snow affect the oxidising capacity of the lower troposphere especially in remote regions of high latitudes with little pollution. Current air–snow exchange models are limited by poor understanding of processes and often require unphysical tuning parameters. Here, two multiphase models were developed from physically based parameterisations to describe the interaction of nitrate between the surface layer of the snowpack and the overlying atmosphere. The first model is similar to previous approaches and assumes that below a threshold temperature, To, the air–snow grain interface is pure ice and above To a disordered interface (DI) emerges covering the entire grain surface. The second model assumes that air–ice interactions dominate over all temperatures below melting of ice and that any liquid present above the eutectic temperature is concentrated in micropockets. The models are used to predict the nitrate in surface snow constrained by year-round observations of mixing ratios of nitric acid in air at a cold site on the Antarctic Plateau (Dome C; 75°06′ S, 123°33′ E; 3233 m a.s.l.) and at a relatively warm site on the Antarctic coast (Halley; 75°35′ S, 26°39′ E; 35 m a.s.l). The first model agrees reasonably well with observations at Dome C (Cv(RMSE) = 1.34) but performs poorly at Halley (Cv(RMSE) = 89.28) while the second model reproduces with good agreement observations at both sites (Cv(RMSE) = 0.84 at both sites). It is therefore suggested that in winter air–snow interactions of nitrate are determined by non-equilibrium surface adsorption and co-condensation on ice coupled with solid-state diffusion inside the grain, similar to Bock et al. (2016). In summer, however, the air–snow exchange of nitrate is mainly driven by solvation into liquid micropockets following Henry's law with contributions to total surface snow NO3− concentrations of 75 and 80 % at Dome C and Halley, respectively. It is also found that the liquid volume of the snow grain and air–micropocket partitioning of HNO3 are sensitive to both the total solute concentration of mineral ions within the snow and pH of the snow. The second model provides an alternative method to predict nitrate concentration in the surface snow layer which is applicable over the entire range of environmental conditions typical for Antarctica and forms a basis for a future full 1-D snowpack model as well as parameterisations in regional or global atmospheric chemistry models.

The persistence and photostabilizing characteristics of benzotriazole and 5-methyl-1H-benzotriazole reduce the photochemical behavior of common photosensitizers and organic compounds in aqueous environments.

Benzotriazole (BT) and 5-methyl-1H-benzotriazole (5-MeBT) are broadly used in industrial applications, such as anti-icing fluids and dishwashing detergent, and act as the primary building blocks for UV absorbers and photostabilizers. This study examined the occurrence of these two compounds in the environment and their unique photochemical behavior affecting photosensitizers and other micro-organic pollutants in aqueous environments. BT and 5-MeBT were detected in all river water samples from the major rivers in Taipei City in the concentration ranges of 147 to 1560ng/L and 22 to 235ng/L, respectively, and both compounds persisted through a conventional wastewater treatment plant. The direct photolysis half-lives of BT and 5-MeBT were 56.9 and 14.0h, respectively. The half-life of photolysis in river water for BT was 44.2h, whereas the half-life of 5-MeBT was 24.7h. The long half-lives in real-water matrices resulted in their prevalence in water bodies, and these compounds were also found to minimize the photosensitizing ability of nitrate and dissolved organic matter (DOM) and increase the persistence of other micro-organic pollutant. With BT present, the production of ·OH in nitrate photolysis was reduced, the degradation of DOM under sunlight was hindered, and the photodegradation of pharmaceutical residues in surface water, such as methotrexate, was completely impeded. This study suggests that in cases in which BT and 5-MeBT are highly concentrated, the effectiveness of natural attenuation process, i.e., photodegradation, in the aqueous environment is diminished, which increases the persistence of the pollutants as well as the risk of exposure.