Photolysis Rate Constants Research Articles

Characterization of a Smog Chamber for Studying Formation and Physicochemical Properties of Secondary Organic Aerosol

An indoor smog chamber facility has been built for carrying out secondary organic aerosol (SOA) formation and for studying physicochemical properties of SOA. This facility comprises of ~7 m3 FEP Teflon reactor placed in temperature controlled room coupled with instruments for gas and particle phase data. Detailed characterization experiments have been presented describing control of reactor temperature, relative humidity (RH), effective mixing time, wall loss rates of gases and particles, light source, and air purification. This chamber showed a wide range of temperature control with acceptable precision (i.e., 18–33 ± 0.5°C). The gas wall loss rates for NO, NO2, and O3 were found to be 3.78 × 10–4 min–1, 4.48 × 10–5 min–1, and 6.47 × 10–5 min–1, respectively. NO2 photolysis rate constant was 0.17 min–1. Particle wall loss constant was found to be 3.96 × 10–3 min–1 at Dp = 100 nm. SOA yields of dark α-pinene ozonolysis ranged from 0.025 to 0.378 for α-pinene concentrations from 10 ppb to 100 ppb.

Anthracene and pyrene photolysis kinetics in aqueous, organic, and mixed aqueous-organic phases

Condensed phases in the atmosphere, such as cloud droplets and aerosols, often contain both water and organic matter (OM). Reactivity can differ significantly between aqueous and organic phases. We have measured photolysis kinetics of the polycyclic aromatic hydrocarbons (PAHs) anthracene and pyrene in several organic solvents and in water, as well as in miscible and phase-separated aqueous-organic mixtures at atmospherically-relevant wavelengths. Photolysis rate constants generally increased with increasing solvent polarity; photolysis of both PAHs was more than ten times faster in water than in octanol. Local polarity had a much greater effect on PAH photolysis kinetics than changes in PAH absorptivity or singlet oxygen concentrations. Photolysis kinetics in homogeneous aqueous-organic mixtures varied monotonically with2 OM volume fraction. Kinetics in immiscible (phase-separated) solutions were more complex, with different dependences on OM content observed in stagnant and turbulent solutions. Our results suggest that OM could greatly affect the photochemical lifetimes of PAHs in atmospheric condensed phases such as aerosols, even if the OM does not itself absorb photons.

Analysis of the photochemical production of ozone using Tropospheric Ultraviolet-Visible (TUV) Radiation Model in an Asian megacity

A study of surface layer ozone (O3) and its primary precursors (NOx = NO + NO2) was carried out at Kolkata (22° 33′ N, 88° 30′ E), an urban site in eastern India from October 2010 to April 2011. A simple NOx cycle-based photochemical model for the net rate of production of tropospheric ozone was studied. Photolytic rate constants for NO2 (\( {j}_{{\mathrm{NO}}_2} \)) were estimated using the Tropospheric Ultraviolet and Visible (TUV) Radiation Model-Version 4.1. Relationships between predicted and measured ozone data were found to be sensitive with respect to time of the day. A correlation between the observed ozone and \( {j}_{{\mathrm{NO}}_2} \) during some case study days in the morning (R2 range, 0.34 to 0.96) and the late afternoon (R2 range, 0.79 to 0.99) implies that the NOx chemistry is predominant due to enhanced automobile emissions during the peak traffic hours. VOCs and some peroxy radicals play a vital role in the chemistry of ozone production (net). Computed air mass backward trajectories using HYSPLIT model established the source and transport pathways of the trace gases. The results obtained on different days indicate the importance of advection of gases from both continental as well as marine air mass on case-specific circumstances. Tropospheric columnar NO2 outputs were also compared to ground-based measurements, and these point towards a good regional contribution of NO2.

On-line sensor monitoring for chemical contaminant attenuation during UV/H2O2 advanced oxidation process

A combination of surrogate parameters and indicator compounds were measured to predict the removal efficiency of trace organic compounds (TOrCs) using low pressure (LP)-UV/H2O2 advanced oxidation process (AOP), engaged with online sensor-based monitoring system. Thirty-nine TOrCs were evaluated in two distinct secondary wastewater effluents in terms of estimated photochemical reactivity, as a function of the rate constants of UV direct photolysis (kUV) and hydroxyl radical (OH) oxidation (kOH). The selected eighteen TOrCs were classified into three groups that served as indicator compounds: Group 1 for photo-susceptible TOrCs but with minor degradation by OH oxidation (diclofenac, fluoxetine, iohexol, iopamidol, iopromide, simazine and sulfamethoxazole); Group 2 for TOrCs susceptible to both direct photolysis and OH oxidation (benzotriazole, diphenhydramine, ibuprofen, naproxen and sucralose); and Group 3 for photo-resistant TOrCs showing dominant degradation by OH oxidation (atenolol, carbamazepine, DEET, gemfibrozil, primidone and trimethoprim). The results indicate that TOC (optical-based measurement), UVA254 or UVT254 (UV absorbance or transmittance at 254 nm), and total fluorescence can all be used as suitable on-line organic surrogate parameters to predict the attenuation of TOrCs. Furthermore, the automated real-time monitoring via on-line surrogate sensors and equipped with the developed degradation profiles between sensor response and a group of TOrCs removal can provide a diagnostic tool for process control during advanced treatment of reclaimed waters.

UV photolysis kinetics of sulfonamides in aqueous solution based on optimized fluence quantification

The ultraviolet (UV) photolysis kinetics of eight sulfonamide (SA) antibiotics was investigated in a quasi-collimated beam apparatus. By using a micro fluorescent silica detector to monitor online the dynamic irradiance fluctuation, the accuracy in fluence quantification could be increased by up to 15%. Solution pH governed the speciation of selected SAs, thus impacting significantly their molar absorption coefficients (ε), fluence-based photolysis rate constants (k′), and quantum yields. An increasing pH induced a hyperchromic effect and a blue shift of the UV–Vis absorption spectra of selected SAs, thus causing the anionic species to show a relatively higher specific ε value than the neutral species at 254 nm. On UV exposure, the photodegradation of selected SAs all followed pseudo-first order reaction kinetics. The SAs with a penta-heterocycle, because of their higher electron densities, exhibited an obviously higher photodegradation rate than those with a hexa-heterocycle. The specific k′ values of the neutral and anionic species were in the ranges of (0.30–14.49) × 10−3 and (0.61–20.90) × 10−3 cm2 mJ−1, respectively. With the specific k′ values obtained, it is estimated that only part of SAs can be photodegraded during UV disinfection of water and wastewater, so an advanced oxidation process is necessary if a higher removal of selected SAs is to be achieved.

Photochemical transformation of tetrabromobisphenol A under simulated sunlight irradiation: Kinetics, mechanism and influencing factors

A systematic study on photolysis of tetrabromobisphenol A (2,2′,6,6′-tetrabromo-4,4′-isopropylidendiphenol, TBBPA) in water was investigated under simulated sunlight irradiation. The results showed that the photolysis of TBBPA followed apparent pseudo-first-order kinetics. The photolysis rate constants (k) changed from 2.80×10−2 to 0.70×10−2min−1 with the concentrations of TBBPA varying from 0.1 to 10mgL−1. Increasing humic acid (HA) concentration from 0–100mgL−1 led to the decrease of k from 2.53×10−2 to 0.39×10−2min−1, which was due to the competitive adsorption for photons between HA and TBBPA molecules. The photolysis rate was faster at near-neutral conditions (pH=6 and 7) than that in either acidic or basic conditions. Electron spin resonance (ESR) and reactive oxygen species (ROS) scavenging experiments indicated that TBBPA underwent self-sensitized photooxidation via ROS (i.e., OH, 1O2 and O2−), and the process was mainly controlled by O2−. After irradiation of 180min, about 35.0% reduction of TOC occurred accompanied with approximate 99.1% of TBBPA removed. The detection of products (i.e., Br−, bisphenol A, 2,6-dibromophenol, 2-bromophenol and phenol) revealed that the main photolytic pathways of TBBPA were debromination and breakage of C–C bond.

Reactivity of aldehydes at the air–water interface. Insights from molecular dynamics simulations and ab initio calculations

Understanding the influence of solute-solvent interactions on chemical reactivity has been a subject of intense research in the last few decades. Theoretical studies have focused on bulk solvation phenomena and a variety of models and methods have been developed that are now widely used by both theoreticians and experimentalists. Much less attention has been paid, however, to processes that occur at liquid interfaces despite the important role such interfaces play in chemistry and biology. In this study, we have carried out sequential molecular dynamics simulations and quantum mechanical calculations to analyse the influence of the air-water interface on the reactivity of formaldehyde, acetaldehyde and benzaldehyde, three simple aldehydes of atmospheric interest. The calculated free-energy profiles exhibit a minimum at the interface, where the average reactivity indices may display large solvation effects. The study emphasizes the role of solvation dynamics, which are responsible for large fluctuations of some molecular properties. We also show that the photolysis rate constant of benzaldehyde in the range 290-308 nm increases by one order of magnitude at the surface of a water droplet, from 2.7 × 10(-5) s(-1) in the gas phase to 2.8 × 10(-4) s(-1) at the air-water interface, and we discuss the potential impact of this result on the chemistry of the troposphere. Experimental data in this domain are still scarce and computer simulations like those presented in this work may provide some insights that can be useful to design new experiments.

Determination of Reaction Rate Constants in a Collimated Beam Setup: The Effect of Water Quality and Water Depth

UV/H2O2 processes can be studied using a Collimated Beam (CB) setup. In UV dose calculations, the “water factor” accounts for changes in the irradiation spectrum upon its way through the aqueous solution. Analogous to the “germicidal factor,” the “extinction factor” is introduced, correcting for the fact that resulting compounds will be subjected to an emission spectrum changing over the depth. Using the extinction factor, the fluence-based photolysis and oxidation rate constants of model compounds, as well as the quantum yield for nitrite formation, were found to be independent of the depth of the water layer applied.

Contaminant-mediated photobleaching of wetland chromophoric dissolved organic matter.

Photolytic transformation of organic contaminants in wetlands can be mediated by chromophoric dissolved organic matter (CDOM), which in turn can lose its reactivity from photobleaching. We collected water from a small agricultural wetland (Ohio), Kawai Nui Marsh (Hawaii), the Everglades (Florida), and Okefenokee Swamp (Georgia) to assess the effect of photobleaching on the photofate of two herbicides, acetochlor and isoproturon. Analyte-spiked water samples were irradiated using a solar simulator and monitored for changes in CDOM light absorbance and dissolved oxygen. Photobleaching did not significantly impact the indirect photolysis rates of either herbicide over 24 hours of irradiation. Surprisingly, the opposite effect was observed with isoproturon, which accelerated DOM photobleaching. This phenomenon was more pronounced in higher-CDOM waters, and we believe that the redox pathway between triplet-state CDOM and isoproturon may be responsible for our observations. By contrast, acetochlor indirect photolysis was dependent on reaction with the hydroxyl radical and did not accelerate photobleaching of wetland water as much as isoproturon. Finally, herbicide indirect photolysis rate constants did not correlate strongly to any one chemical or optical property of the sampled waters.

A critical assessment of the photodegradation of pharmaceuticals in aquatic environments: defining our current understanding and identifying knowledge gaps

This work presents a critical assessment of the state and quality of knowledge around the aquatic photochemistry of human- and veterinary-use pharmaceuticals from laboratory experiments and field observations. A standardized scoring rubric was used to assess relevant studies within four categories: experimental design, laboratory-based direct and indirect photolysis, and field/solar photolysis. Specific metrics for each category are defined to evaluate various aspects of experimental design (e.g., higher scores are given for more appropriate characterization of light source wavelength distribution). This weight of evidence-style approach allowed for identification of knowledge strengths and gaps covering three areas: first, the general extent of photochemical data for specific pharmaceuticals and classes; second, the overall quality of existing data (i.e., strong versus weak); and finally, trends in the photochemistry research around these specific compounds, e.g. the observation of specific and consistent oversights in experimental design. In general, those drugs that were most studied also had relatively good quality data. The four pharmaceuticals studied experimentally at least ten times in the literature had average total scores (lab and field combined) of ≥29, considered decent quality; carbamazepine (13 studies; average score of 31), diclofenac (12 studies; average score of 31), sulfamethoxazole (11 studies; average score of 34), and propranolol (11 studies; average score of 29). Major oversights and errors in data reporting and/or experimental design included: lack of measurement and reporting of incident light source intensity, lack of appropriate controls, use of organic co-solvents in irradiation solutions, and failure to consider solution pH. Consequently, a number of these experimental parameters were likely a cause of inconsistent measurements of direct photolysis rate constants and quantum yields, two photochemical properties that were highly variable in the literature. Overall, the assessment rubric provides an objective and scientifically-defensible set of metrics for assessing the quality of a study. A major recommendation is the development of a method guideline, based on this rubric, for conducting and reporting on photochemical studies that would produce consistent and reliable data for quantitative comparison across studies. Furthermore, an emphasis should be placed on conducting more dual-fate studies involving controlled photolysis experiments in natural sunlight, and whole system fate studies in either natural or artificial systems. This would provide accurate data describing the actual contribution of photolysis to the overall fate of pharmaceuticals in the environment.

VUV and mid-UV photoabsorption cross sections of thin films of adenine: Application on its photochemistry in the solar system

The photochemistry of an organic compound in extraterrestrial environments is related to its photoabsorption cross sections in the VUV/UV range (<300nm). Such data, especially in the VUV range (<200nm) are quite scarce in the literature. This paper presents an experimental setup and associated methodology to measure VUV/UV spectra of thin films of organic molecules. The case of adenine is extensively discussed as an example study. The absorption cross sections spectrum of adenine between 115 and 300nm is measured from transmission measurements and accurate calculation of the thickness of our samples set thanks to interferometry techniques and infrared spectrometry. From these data, the infrared integrated band strength of solid adenine between 3600 and 1970cm−1 has also been measured and is equal to 7.9×10–16±4%cmmolecule−1. The use of the VUV/UV spectrum to estimate the photolysis rate constant is discussed and compared to direct kinetic measurements available in the literature for low Earth orbit experiments. However the lack of measurements of the photodissociation quantum yield as a function of the wavelength prevents a precise calculation. VUV/UV cross section spectrum is yet a necessity to conduct either proper kinetic studies on optically thin samples (less than 2nm in the VUV for adenine) or an appropriate modeling for optically thick samples.

Photolytic and photocatalytic degradation of micro pollutants in a tubular reactor and the reaction kinetic models

The photocatalytic decomposition and mineralization of p-chlorobenzoic acid (p-CBA), 4-chlorophenol (4-CP), hydroquinone and 4-nitrophenol (4-NP) in aqueous solution were investigated using two kinds of low-pressure mercury lamps: one was UV (ultraviolet) lamp emitting at 254nm and the other was VUV (vacuum ultraviolet) lamp emitting at both 254 and 185nm. VUV irradiation led to the most efficient degradation of the organics. The validation of calculation shows the effect of bubble on radiation scattering was little. The evolution of the radiation absorbance of the reaction solutions suggests the radiation adsorption of the intermediates was negligible. Based on these results simple reaction kinetic models were established with the pseudo-first order reaction rate constants relating to incident radiation intensity. The reaction rate constants of photolysis and photocatalysis (kPD and KPCD) were calculated through the numerical simulation of the models. Using the calculated kPD and KPCD of p-CBA (0.1 and 11.0min−1, respectively), the removals of p-CBA in TiO2/UV photocatalysis were predicted, which were almost the same as the measured removals. With the calculated kPD and KPCD of 4-NP (0.011 and 4.8min−1, respectively), the removals of 4-NP in TiO2/UV photocatalysis process in a big reactor under the different gas flow rates were also predicated, which agreed well with the measured results.

Study of the Atmospheric Chemistry of 2-Formylcinnamaldehyde

2-Formylcinnamaldehyde is a significant product of the reaction of naphthalene with OH radicals, and its photolysis and gas-phase reactions with O3, NO3 radicals, and OH radicals have been investigated in this work. 2-Formylcinnamaldehyde was observed to undergo photolysis by black lamps, with a photolysis rate of 0.14 × J(NO2), where J(NO2) is the NO2 photolysis rate. The measured rate constants for the reactions of 2-formylcinnamaldehyde with O3, NO3 radicals, and OH radicals (in units of cm(3) molecule(-1) s(-1)) were 1.8 × 10(-18), 4.3 × 10(-14), and 2.1 × 10(-11), respectively, with those for the O3 and NO3 reactions being for the E-isomer. 2-Formylcinnamaldehyde was observed to undergo significant adsorption and desorption from the reaction chamber Teflon film walls, and the photolysis rate and rate constants are subject to significant uncertainties. In the atmosphere, the dominant chemical loss processes for 2-formylcinnamaldehyde will be photolysis during daylight hours and reaction with NO3 radicals during nighttime. Phthaldialdehyde and glyoxal were observed as products of the OH radical and O3 reactions, and photolysis of E-2-formylcinnamaldehyde led to formation of Z-2-formylcinnamaldehyde plus two other molecular weight 160 isomers. The present results are compared with previous literature data, and reaction mechanisms are discussed.

Micropollutant Dynamics in Vidy Bay—A Coupled Hydrodynamic-Photolysis Model to Assess the Spatial Extent of Ecotoxicological Risk

The direct discharge of effluent wastewater into Vidy Bay (Lake Geneva) results in the formation of an effluent plume with locally high concentrations of wastewater-derived micropollutants. The micropollutant hotspots above the wastewater outfall present a potential ecotoxicological risk, yet the spatial extent of the plume and the associated ecotoxicological risk zone remain unclear. This work combines the two main processes affecting the spreading of the plume, namely dilution of micropollutants due to mixing and degradation by photolysis, into a coupled hydrodynamic-photolysis model, with which we estimated the spatial extent of the risk zone in Vidy Bay. The concentration of micropollutants around the wastewater outfall was simulated for typical wind scenarios and seasons relevant to Vidy Bay, and the resulting ecotoxicological risk was evaluated. Specifically, we determined the direct and indirect photolysis rate constants for 24 wastewater-derived micropollutants and implemented these in a hydrodynamic particle tracking model, which tracked the movement of water parcels from the wastewater outfall. Simulations showed that owing to thermal stratification, the zone of ecotoxicological risk is largest in summer and extends horizontally over 300 m from the outfall. Photolysis processes contribute to reducing the plume extent mainly under unstratified conditions when the plume surfaces. Moreover, it was shown that only a few compounds, mainly antibiotics, dominate the total ecotoxicological risk.

Towards Universal Wavelength-Specific Photodegradation Rate Constants for Methyl Mercury in Humic Waters, Exemplified by a Boreal Lake-Wetland Gradient

We report experimentally determined first-order rate constants of MeHg photolysis in three waters along a Boreal lake-wetland gradient covering a range of pH (3.8-6.6), concentrations of total organic carbon (TOC 17.5-81 mg L(-1)), total Fe (0.8-2.1 mg L(-1)), specific UV254 nm absorption (3.3-4.2 L mg(-1) m(-1)) and TOC/TON ratios (24-67 g g(-1)). Rate constants determined as a function of incident sunlight (measured as cumulative photon flux of photosynthetically active radiation, PAR) decreased in the order dystrophic lake > dystrophic lake/wetland > riparian wetland. After correction for light attenuation by dissolved natural organic matter (DOM), wavelength-specific (PAR: 400-700 nm, UVA: 320-400 nm and UVB: 280-320 nm) first-order photodegradation rate constants (kpd) determined at the three sites were indistinguishable, with average values (± SE) of 0.0023 ± 0.0002, 0.10 ± 0.024 and 7.2 ± 1.3 m(2) E(-1) for kpdPAR, kpdUVA, and kpdUVB, respectively. The relative ratio of kpdPAR, kpdUVA, and kpdUVB was 1:43:3100. Experiments conducted at varying MeHg/TOC ratios confirm previous suggestions that complex formation with organic thiol groups enhances the rate of MeHg photodegradation, as compared to when O and N functional groups are involved in the speciation of MeHg. We suggest that if the photon fluxes of PAR, UVA, and UVB radiation are separately determined and the wavelength-specific light attenuation is corrected for, the first-order rate constants kpdPAR, kpdUVA, and kpdUVB will be universal to waters in which DOM (possibly in concert with Fe) controls the formation of ROS, and the chemical speciation of MeHg is controlled by the complexation with DOM associated thiols.

Aqueous photochemical behavior of oxazolidinone antibiotic linezolid

Photochemical degradation is a central factor in determining the fate of antibiotic micropollutants in surface waters. In this study, the photodegradation kinetics, influencing factors and transformation products of linezolid, a representative oxazolidinone antibiotic, were investigated. Under simulated sunlight irradiation ( λ >290 nm), the photodegradation followed the pseudo-first-order kinetics with an apparent quantum yield of 0.834±0.054 in pure water. The rates of linezolid photodegradation were slower in freshwater or seawater than in pure water, which was attributed to the effects of the ubiquitous aqueous dissolved matter on the photodegradation. pH, Cl– and seawater salinity did not affect the photodegradation kinetics ( P >0.05), while humic acid, NO3− and Fe(III) acted mainly as radiation filters and inhibited the photodegradation ( P ≤0.05). Under simulated sunlight as well as UV-vis ( λ >200 nm) irradiation, scavenging experiments revealed that linezolid underwent direct photolysis and self-sensitized photodegradation via 1O2, which resulted in a negative correlation between the photolysis rate constants and the initial concentrations. The main photodegradation products and pathways varied under irradiation of different light sources. Linezolid mainly underwent defluorination and photoinduced hydrolysis under UV-vis irradiation, whereas morpholine-dehydrogen was primary under simulated sunlight.

Different photolysis kinetics and photooxidation reactivities of neutral and anionic hydroxylated polybrominated diphenyl ethers

Hydroxylated polybrominated diphenyl ethers (HO-PBDEs) pose potential ecological risks due to their endocrine disrupting effects and extensive sources. It is of great importance to know their environmental transformation for the purpose of ecological risk assessment. Photodegradation is an important transformation pathway of HO-PBDEs. As HO-PBDEs ionize in natural waters, the photochemical reactivities of both neutral and anionic HO-PBDEs need to be unveiled. In this study, six HO-PBDEs were selected as model compounds. Their direct photolysis rate constants (kd), quantum yields (Φ), light absorptions (A), second-order reaction rate constants with 1O2 (k1O2) and OH (kOH) at different pH were determined, as well as these parameters for the neutral and anionic HO-PBDEs. The kd, Φ, A, k1O2 and kOH for the anions are much higher than those for the neutral molecules and vary with the bromination degree. Molecular parameters computed with the density functional theory (DFT) were employed to construct structure–reactivity equations. The ether bond strength, the frontier molecular orbital energy and the charge distribution were found to be the intrinsic structural characters governing the photochemical reactivities. The half-lives range from 0.7 to 60.1h for the photodegradation including the direct photolysis and the reactions with 1O2 and OH in surface waters at 45°N latitude under the continuous solar irradiation of sunny noon on 15 July. Direct photolysis is the dominant pathway. The photochemical reactivities of other HO-PBDEs at a given pH can also be estimated based on the structure–reactivity equations, which is important for the ecological risk assessment of HO-PBDEs.

Degradation of Trichloroethylene and Tetrachloroethylene in Simulated Groundwater in a Flow-Through Photosono Reactor

AbstractSimulated groundwater solutions contaminated with trichloroethylene (TCE) and tetrachloroethylene (PCE) were treated in a flow-through reactor equipped with ultrasound (US) and ultraviolet light (UV) sources. The treatment process was operated using UV or US individually, and UV and US concurrently (UVUS). The solutions contaminated with TCE or PCE were exposed to UV light emitted from Hg-arc lamps surrounding the quartz reactor vessel, and to acoustic waves from a US horn located at the bottom of the reactor vessel. The rate constants (k) of photolysis by UV, sonolysis by US, and photosonolysis by UVUS were evaluated using a mathematical model in conjunction with a nonlinear regression analysis. The removal efficiencies (E) for the UV, US, and UVUS treatments were also determined. For TCE, the US rate constant (kus=0.016±0.010 min−1) and the UV rate constant (kuv=0.076±0.004 min−1) yield the theoretical additive rate constant (kad) of 0.092 min−1, whereas the observed UVUS rate constant (kuvus...

Null Variance Altitudes for the photolysis rate constants of species with barometric distribution: Illustration on Titan upper atmosphere modeling

The precision of the rates of the photolysis processes initiating the complex chemistry of Titan’s upper atmosphere conditions strongly the predictivity of photochemical models. Recent studies in sensitivity analysis of such models point out photolysis rate constants as key parameters. However, they have been treated approximately so far. We deal here directly with uncertainty in the absorption cross-sections to derive the uncertain altitude-dependent photolysis rate constants. We observe that the uncertainty on the photolysis rate constants of the major species, N2 and CH4, varies strongly with altitude and rather surprisingly vanishes at specific altitudes. We propose a simple model to interpret these features and we demonstrate that they are transferable to any major absorber distributed barometrically in an atmosphere.

Cavity-Enhanced Measurements of Hydrogen Peroxide Absorption Cross Sections from 353 to 410 nm

We report near-ultraviolet and visible absorption cross sections of hydrogen peroxide (H(2)O(2)) using incoherent broad-band cavity-enhanced absorption spectroscopy (IBBCEAS), a recently developed, high-sensitivity technique. The measurements reported here span the range of 353-410 nm and extend published electronic absorption cross sections by 60 nm to absorption cross sections below 1 × 10(-23) cm(2) molecule(-1). We have calculated photolysis rate constants for H(2)O(2) in the lower troposphere at a range of solar zenith angles by combining the new measurements with previously reported data at wavelengths shorter than 350 nm. We predict that photolysis at wavelengths longer than those included in the current JPL recommendation may account for up to 28% of the total hydroxyl radical (OH) production from H(2)O(2) photolysis under some conditions. Loss of H(2)O(2) via photolysis may be of the same order of magnitude as reaction with OH and dry deposition in the lower atmosphere; these processes have very different impacts on HO(x) loss and regeneration.