The Essential Role for Laboratory Studies in Atmospheric Chemistry

Laboratory studies of atmospheric chemistry characterize the nature of atmospherically relevant processes down to the molecular level, providing fundamental information used to assess how human activities drive environmental phenomena such as climate change, urban air pollution, ecosystem health, indoor air quality, and stratospheric ozone depletion. Laboratory studies have a central role in addressing the incomplete fundamental knowledge of atmospheric chemistry. This article highlights the evolving science needs for this community and emphasizes how our knowledge is far from complete, hindering our ability to predict the future state of our atmosphere and to respond to emerging global environmental change issues. Laboratory studies provide rich opportunities to expand our understanding of the atmosphere via collaborative research with the modeling and field measurement communities, and with neighboring disciplines

The Essential Role for Laboratory Studies in Atmospheric Chemistry

Laboratory studies of atmospheric chemistry characterize the nature of atmospherically relevant processes down to the molecular level, providing fundamental information used to assess how human activities drive environmental phenomena such as climate change, urban air pollution, ecosystem health, indoor air quality, and stratospheric ozone depletion. Laboratory studies have a central role in addressing the incomplete fundamental knowledge of atmospheric chemistry. This article highlights the evolving science needs for this community and emphasizes how our knowledge is far from complete, hindering our ability to predict the future state of our atmosphere and to respond to emerging global environmental change issues. Laboratory studies provide rich opportunities to expand our understanding of the atmosphere via collaborative research with the modeling and field measurement communities, and with neighboring disciplines

Photochemical transformations in ice: implications for the fate of chemical species

Post-depositional photochemical alterations in snowpacks and sea ice may affect the chemical records in polar caps and the chemistry of the polar atmospheric boundary layer. Although it is known that UV-induced photochemistry actually occurs in ice matrices, quantitative information on such processes is still lacking. With new methods for determining the light absorption by chromophores embedded in packed ice, this study investigates the rates and products of the photodegradation of 4-nitrophenol and nitrate in ice. A quantum yield (Φ_(ice)) of (2.3 ± 0.4) x 10^(-4) was obtained for the photochemical degradation of 4-nitrophenol over the wavelength range of 300 to 370 nm in ice pellets (pH 5.6). Five reaction products were positively identified: hydroquinone, benzoquinone, 4-nitrosophenol, nitrate, and nitrite. Indirect evidence suggests the formation of organic polymers. These results are similar to those found for 4-nitrophenol photolysis in aqueous solutions, indicating that comparable mechanisms operate in both phases. Upon irradiation (λ = 313 ± 15 nm) of NO_3^- doped ice layers, the formation of NO_2(g) and NO_2^- was observed. The yield for both products increased with temperature over the range 248 - 268 K; with values of Φ_(NO_2-) ~ (4.8 ± 1.5) x 10^(-3) and Φ'(NO_2) (1.2 ± 0.9) x 10^(-3) at 263 K, 10 mM KNO_3. The formation of NO_2^- during the photolysis of NO_3^- in ice pellets has apparent activation energy, E_a, of 5.8 kcal mole^(-1). This E_a is similar to the water cage-effect for supercooled water. Φ_(NO_2) showed a much stronger temperature dependence (E_a ~ 10 kcal mole^(-1)); This can be interpreted as the probability of the product NO_2 escaping into the gas-phase, before it is photolyzed into NO. These results suggest that, under our experimental conditions, the photochemical transformations occur within the quasi-liquid layer, which behaves as a supercooled solution. The experimental data for Φ'_(NO_2), coupled with snow absorptivity data, lead to a predicted NO_2 fluxes in reasonable agreement with recent measurements in Antarctic snow under solar illumination. NO_3^- photolysis within snowpacks may also be a significant source for OH radicals, which may further react and cause chemical changes in important species, such as H_2O_2 and H_2CO and CH_3CHO

H2S Removal from Groundwater by Chemical Free Advanced Oxidation Process Using UV-C/VUV Radiation

Sulfide species may be present in groundwater due to natural processes or due to anthropogenic activity. H2S contamination poses odor nuisance and may also lead to adverse health effects. Advanced oxidation processes (AOPs) are considered promising treatments for hydrogen-sulfide removal from water, but conventional AOPs usually require continuous chemical dosing, as well as post-treatment, when solid catalysts are applied. Vacuum-UV (VUV) radiation can generate ·OH in situ via water photolysis, initiating chemical-free AOP. The present study investigated the applicability of VUV-based AOP for removal of H2S both in synthetic solutions and in real groundwater, comparing combined UV-C/VUV and UV-C only radiation in a continuous-flow reactor. In deionized water, H2S degradation was much faster under the combined radiation, dominated by indirect photolysis, and indicated the formation of sulfite intermediates that convert to sulfate at high radiation doses. Sulfide was efficiently removed from natural groundwater by the two examined lamps, with no clear preference between them. However, in anoxic conditions, common in sulfide-containing groundwater, a small advantage for the combined lamp was observed. These results demonstrate the potential of utilizing VUV-based AOP for treating H2S contamination in groundwater as a chemical-free treatment, which can be especially attractive to remote small treatment facilities

Removal of Trihalomethane Precursors by Nanofiltration in Low-SUVA Drinking Water

Trihalomethanes (THMs) are prevalent disinfection by-products. High THM formation is usually associated with natural organic matter with high molecular weight and aromatic characteristics, which is efficiently removed by nanofiltration (NF). In the Sea of Galilee and the Israeli National Water Carrier (NWC), water shows high THM formation potential, although it mainly contains low molecular weight and hydrophilic organic matter with low aromaticity. In the present study, NF removal abilities were tested on treated NWC water using three different spiral wound membranes (NF90, NF270, and DL). Rejections and fluxes were tested as a function of pressure, water recovery, and membrane type. Feed and permeate dissolved organic carbon (DOC), UVA254, total THM formation (THMF), and total THM formation potential (THMFP), as well as alkalinity, conductivity, hardness, Ca2+, Mg2+, and Cl− were measured to evaluate rejection and THM formation reduction. The results demonstrated that NF can efficiently remove natural organic matter (NOM) and reduce THM formation, even in this challenging type of water. At low water recovery, membranes showed average rejection of about 70–85% for THMFP and THM. Upon elevating recovery, average THM and THMFP rejection decreased to 55–70%, with THM content still well below regulation limits. Of the membranes tested, the higher permeability of NF270 appears to make it economically favorable for the applications tested in this work

Nitrogen Dioxide Release in the 302 nm Band Photolysis of Spray-Frozen Aqueous Nitrate Solutions. Atmospheric Implications

We quantify the NO_2 fluxes released into the gas phase during the continuous λ ∼ 300 nm photolysis of NO_3^- in submillimeter ice layers produced by freezing aqueous KNO_3 sprays on cold surfaces. Fluxes, F_(NO_2), increase weakly with [NO_3^-] between 5 ≤ [NO_3^-]/mM ≤ 50 and increase markedly with temperature in the range of 268 ≥ T/K ≥ 248. We found that F_(NO_2), the photostationary concentration of NO_2^- (another primary photoproduct), and the quantum yield of 2-nitrobenzaldehyde in situ photoisomerization are nearly independent of ice layer thickness d within 80 ≤ d/μm ≤ 400. We infer that radiation is uniformly absorbed over the depth of the ice layers, where NO_3^- is photodecomposed into NO_2 (+ OH) and NO_2^- (+ O), but that only the NO_2 produced on the uppermost region is able to escape into the gas phase. The remainder is trapped and further photolyzed into NO. We obtain φ_(NO_2^−) ∼ 4.8 × 10^(-3) at 263 K, i.e., about the quantum yield of nitrite formation in neutral NO_3^- aqueous solutions, and an apparent quantum yield of NO2 release φ‘_(NO_2) ∼ 1.3 × 10^(-3) that is about a factor of 5 smaller than solution φ_(OH) data extrapolated to 263 K. These results suggest that NO_3^- photolysis in ice takes place in a liquidlike environment and that actual φ‘_(NO_2) values may depend on the morphology of ice deposits. Present φ‘_(NO_2) data, in conjunction with snow albedo and absorptivity data, lead to F_(NO_2) values in essential agreement with recent measurements in Antarctic snow under solar illumination

Monotonic Increase of Nitrite Yields in the Photolysis of Nitrate in Ice and Water between 238 and 294 K

The quantum yield, φ, of nitrite formation in the 302 nm band photolysis of fluid or frozen aqueous nitrate solutions increases monotonically with temperature over the range 238−294 K. The presence of formate increases φ 5-fold but does not modify its temperature dependence. Considering that the detection of nitrite as a product is only possible after the initial photofragments (NO_2^- + O) escape the solvent cage and that the diffusivity of ice, D_(ice), is about 6 orders of magnitude smaller than that of supercooled water, D_(aq), at the same temperature, we infer that nitrate photodecomposition takes place in similar liquidlike media at all temperatures. We found that the nitrite dispersed into the bulk is subsequently degraded by OH radicals, another primary photoproduct that can be scavenged by formate. The fact that experimental φ values in ice are actually larger than those derived from linear φ vs D_(aq)T^(1/2) extrapolation of aqueous phase data, as expected for cage processes in homogeneous media, suggests that the photochemically relevant properties of the quasi-liquid layer covering ice below the normal melting point resemble those of bulk supercooled water, but other effects, such as the dissipation of excess photon energy into the medium, may also play a role

Diurnal patterns of micropollutants concentrations in domestic greywater

<p>In recent years, much interest has been given to presence of micropollutants in municipal wastewater, some of which are suspected to be endocrine disruptors, toxic or carcinogenic. Much less attention has been paid to their presence in greywater. The present research studies the diurnal patterns of micropollutants in greywater and computes their daily loads. Monitoring was carried-out using auto-controlled sampling system, designed to overcome the erratic greywater generation. Two main generation periods were identified: morning (5:00–11:00) and evening-night (18:00–2:00), contributing about 20% and >50% of daily greywater discharge, respectively. Average specific daily greywater discharge was 57 L p⁻¹d⁻¹, which matches reported value for greywater generated by showers and washbasins in Israel. The most frequently detected micropollutants in this study were methylparben (preservative), galaxolide (fragrance) and oxybenzone (UV-filter), which are common ingredients in many personal care-products. Their daily loads were 2, 840, 1, 887 and 728 µg p⁻¹d⁻¹, respectively.</p