Steady-state Concentration Of OH Research Articles

Analysis of the steady-state concentrations of reactive species and their role in contaminant degradation by the iron-biochar/persulfate advanced oxidation process: Comparison of probe compound and quenching agent methods

Reactive species, including hydroxyl radicals (OH), sulfate radicals (SO4−), singlet oxygen (1O2), superoxide radicals (O2−), and Fe(IV), are generated by the iron-biochar activated persulfate (Fe-BC/PS) process. These reactive species can be leveraged for treatment of micropollutants, such as the sulfamethoxazole antibiotic. In this study, the steady-state concentrations and contributions of OH, SO4−, 1O2, O2−, and Fe(IV) to sulfamethoxazole degradation were calculated for different operating conditions in the iron-biochar/persulfate (Fe-BC/PS advanced oxidation process. Electron paramagnetic resonance was employed to confirm the production of each reactive species. The nitrobenzene, benzoic acid, furfuryl alcohol, p-chlorobenzoic acid or p-benzoquinone, and phenyl methyl sulfoxide probe compounds were added to experimental solutions in isolation, as mixtures, and at different concentrations to calculate the steady-state concentrations of OH, SO4−, 1O2, O2−, and Fe(IV) and determine their contributions to sulfamethoxazole degradation at variable pH conditions. The results not only informed the primary mechanisms of sulfamethoxazole degradation by the Fe-BC/PS system, but also highlighted best practices for the use of probe compounds and quenching agents in persulfate-based advanced oxidation processes. In particular, the initial concentration of the probe compounds should be as low as possible to avoid impacts on target contaminant degradation and misinterpretation of the role of each reactive species. Furthermore, quenching-based approaches to determination of the key reactive species were less consistent than evaluation by probe compounds. The overall outcomes of this work inform sulfamethoxazole treatment by the Fe-BC/PS system and emphasize the need for internal validation of kinetics results using a multi-pronged approach.

Semiconducting mineral induced photochemical conversion of PAHs in aquatic environment: Mechanism study and fate prediction

Semiconducting minerals (such as iron sulfides) are highly abundant in surface water, but their influences on the natural photochemical process of contaminants are still unknown. By simulating the natural water environment under solar irradiation, this work comprehensively investigated the photochemical processes of anthracene (a typical Polycyclic Aromatic Hydrocarbons) in both freshwater and seawater. The results show that the natural pyrite (NP) significantly promotes the degradation of anthracene under solar illumination via 1) NP induced photocatalytic degradation of anthracene, and 2) Fenton reaction due to the NP induced photocatalytic generation of H2O2. The material characterization and theoretical calculation reveal that the natural impurity in NP enlarges its band gap, which limits the utilization of solar spectra to shorter wavelength. The contribution of generated reactive intermediates on anthracene degradation follows the order of 1O2 >OH > O2− in freshwater and O2− >1O2 >OH in seawater. The photochemically generated H2O2 is a vital source for OH generation (from Fenton reaction). The steady-state concentration of OH, 1O2 and O2− in freshwater were monitored as 3.0 × 10−15 M, 1.1 × 10−13 M, and 4.5 × 10−14 M, respectively. However, the OH concentration in seawater can be negligible due to the quenching effects by halides, and the 1O2 and O2− concentrations are higher than that in freshwater. An anthracene degradation kinetic model was built based on the experimentally determined reactive intermediates concentration and its second order rate constant with anthracene. Moreover, the anthracene degradation pathway was proposed based on intermediates analysis and DFT calculation, and its toxicity evolution during the photochemical process was assessed by quantitative structure-activity relationship (QSAR) based prediction. This finding suggests that the natural semiconducting minerals can affect the fate and environmental risks of contaminants in natural water.

Understanding and modeling the formation and transformation of hydrogen peroxide in water irradiated by 254nm ultraviolet (UV) and 185nm vacuum UV (VUV): Effects of pH and oxygen.

Understanding ultraviolet photolysis induced by low pressure mercury lamp that emits both 254nm ultraviolet (UV254) and 185nm vacuum UV (VUV185) is currently challenging due to the copresence of multiple direct and indirect photochemical processes involving a series of highly-reactive radicals. Herein we examined the formation and transformation of H2O2 in water, which is both a precursor and a product of radicals, under various pH and dissolved oxygen (DO) conditions. The trends show that H2O2 increased rapidly at early stage and then remained steady in DO-rich water or declined somewhat in DO-poor water, ultimately leading to higher steady-state H2O2 in DO-rich water. The maximum H2O2 contents nonetheless were similar among waters with different DO, suggesting that H2O2 in this system was mostly generated by hydroxyl radical (OH) recombination, which is an oxygen-independent H2O2 formation pathway, rather than by reduced oxygen via hydrogen atom (H) or hydrated electron (eaq-), which is an oxygen-dependent pathway. Increasing pH (from 6.3 to 10.0) or bicarbonate dosage dramatically decreased H2O2 formation too. Mathematically, the fates of H2O2 as a function of pH, DO, and time were well modeled (R2≥0.92), in which the rates of H2O2 formation and destruction were greater in DO-poor water than those in DO-rich water. In addition, we found that the steady-state concentrations of OH used for degradation of p-chlorobenzoic acid, an OH probe, correlated well with the OH levels used for H2O2 formation (R2=0.98). These results hence may help better understand the UV/VUV process via H2O2 evolutions.

A study of synergistic oxidation between ozone and chlorine on benzalkonium chloride degradation: Reactive species and degradation pathway

Ozonation and chlorination are widely used for disinfection or micropollutant elimination in water treatment or reclamation fields. It was found in this study that the presence of free chlorine increased the production of reactive species significantly during ozonation of dodecylbenzyldimethylammonium chloride (DDBAC, quaternary ammonium compound). DDBAC hardly underwent degradation by the chlorination process. However, the removal of DDBAC by ozonation after 10 min increased from 22% to 92%, when the dosage of free chlorine increased from 0 to 112 mg/L. pH affected the synergistic effect of ozone/chlorine oxidation of DDBAC. The promotion effect of free chlorine was greater at acidic conditions than that at basic conditions. The radical quenching experiments suggested that various reactive species formed during ozone/chlorine oxidation of DDBAC. The contributions of ozone, reactive species that could or could not be quenched by tBuOH were 8%, 78% and 13%, respectively. In the mixed system of DDBAC and pCBA, the degradation percentage of DDBAC and pCBA after 15 min increased from 31.0% and 13.1% to 53.8% and 28.3% with the presence of chlorine, respectively. Based on the steady-state concentration assumption, the steady-state concentration of OH during ozone/chlorine oxidation of DDBAC was 2.3 times higher than that during the sole ozonation process, being 7.5 × 10−14 M and 3.3 × 10−14 M, respectively. Ten oxidation products were identified by HPLC-MS/MS during ozone/chlorine oxidation of DDBAC. Three degradation mechanisms were observed: the hydrogen abstraction of the alkyl chain by hydroxyl radical; the cleavage of the benzyl-nitrogen bond of DDBAC; and the addition of hydroxyl radical on the aromatic ring or the alkyl chain.

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.

Fe(III)-oxalate complex mediated phosphate released from diazinon photodegradation: Pathway signatures based on oxygen isotopes

The photodegradation of organophosphorus pesticides has an important influence on their fate and bioavailability in the water environment. In this study, the kinetics and mechanisms of diazinon photodegradation by Fe(III)–oxalate complex have been determined. Special attention was given to the pathway by which phosphate is released following diazinon photodegradation, as assessed by HPLC-ESI-Q/TOF-MS coupled with oxygen isotope. The results showed that diazinon was stable under dark treatment. However, the degradation of diazinon was observed in the UV-only, UV-Fe(III), and UV-Fe(III)-oxalate treatments. The degradation rate constant is the largest in the UV-Fe(III)-oxalate treatment and clearly influenced by the pH and Fe(III) / oxalate ratio. The hydroxyl radical (OH) was the main reactive oxygen species (ROS) in the UV-Fe(III)-oxalate complex treatment and the steady-state concentration of OH was 5.75 × 10−14 M. The products analysis revealed that phosphate could be released during the photodegradation of diazinon; the intermediate products were diazonon, 2-hydroxydiazonon, hydroxydiazonon, hydrogen phosphorothioate, O,O-diethyl thiophosphate (DETP), diethyl phosphate (DEP) and pyrimidinol (IMP). Compound stable oxygen isotope analysis coupled to Q-TOF/MS revealed that the degradation of diazinon initiated by the P-O bond cleavage.

Gas-phase photolytic production of hydroxyl radicals in an ultraviolet purifier for air and surfaces

ABSTRACTWe have measured the concentration of hydroxyl radicals (OH) produced in the gas phase by a commercially available purifier for air and surfaces, using the time rate of decay of n-heptane added to an environmental chamber. The hydroxyl generator, an Odorox® BOSS™ model, produces the OH through 185-nm photolysis of ambient water vapor. The steady-state concentration of OH produced in the 120 m3 chamber is, with 2σ error bars, (3.25 ± 0.80) × 106 cm−3. The properties of the hydroxyl generator, in particular the output of the ultraviolet lamps and the air throughput, together with an estimation of the water concentration, were used to predict the amount of OH produced by the device, with no fitted parameters. To relate this calculation to a steady-state concentration, we must estimate the OH loss rate within the chamber owing to reaction with the n-heptane and the 7 ppb of background hydrocarbons that are present. The result is a predicted steady-state concentration in excellent agreement with the measured value. This shows we understand well the processes occurring in the gas phase during operation of this hydroxyl radical purifier.Implications: Hydroxyl radical air purifiers are used for cleaning both gaseous contaminants, such as volatile organic compounds (VOCs) or hazardous gases, and biological pathogens, both airborne and on surfaces. This is the first chemical kinetic study of such a purifier that creates gas-phase OH by ultraviolet light photolysis of H2O. It shows that the amount of hydroxyls produced agrees well with nonparameterized calculations using the purifier lamp output and device airflow. These results can be used for designing appropriate remediation strategies.

Bicarbonate enhanced removal of triclosan by copper(II) catalyzed Fenton-like reaction in aqueous solution

The antimicrobial and preservative agent triclosan (TCS) is an emerging and persistent pollutant, and is ubiquitous in the environment. Low cost and efficient removing technology for TCS in water has been the target of many researches. In this study, effective removal of TCS from water was achieved by Cu2+/H2O2 system in the presence of environmental relevant concentration of bicarbonate (HCO3−). The removal efficiency of TCS increased with the increase of HCO3− concentration from 0 to 2.0mM, while further increase of the HCO3− concentration caused the decrease of removal efficiency. TCS (0.01mM) could be totally removed over 120min when HCO3− was 2.0mM (pH 8.72) in the presence of 0.01mM Cu2+ ions at 25.0°C. In order to explore the reaction mechanism, radical scavenging technologies, molecular probe compound p-Chlorobenzoic acid (PCBA) and cyclovoltammetric (CV) experiments were employed to determine the reactive oxygen species (ROS), the steady-state concentrations of OH and Cu2+-complex, respectively. The results suggested that the Cu2+-complex formed between Cu2+ and HCO3− could catalyze H2O2 to produce O2−· and OH, which were responsible for the removal of TCS. Based on the reaction mechanism and identified intermediates of the oxidation process using TOF–LC–MS techniques, the reaction pathways for the oxidation of TCS were proposed including the cleavage of the ether bond, hydroxylation and polymerization. Finally, evaluation on the toxicity change of TCS reaction solution mediated by Cu2+/H2O2-HCO3− system with the freshwater green alga Scenedesmus obliquus showed significant detoxification of TCS.

Photolytic and photocatalytic degradation of tetracycline: Effect of humic acid on degradation kinetics and mechanisms

The widespread occurrence of tetracycline (TC) in the aquatic environment poses a potential risk to aquatic ecosystem and human health. In this study, elimination of TC by photolysis and TiO2 photocatalysis were investigated by using mercury-free UVA-LED as an alternative light source. Particular emphasis was given to the effect of humic acid (HA) on the reaction kinetics and mechanisms of TC removal. Photolytic degradation of TC was slightly enhanced by HA due to its photosensitization effect, as evidenced by the increased steady-state concentrations of OH. The most abundant transformation product of TC, which was formed by the attack of OH radical, was enhanced during photolytic degradation. During photocatalytic experiments, HA dramatically inhibited TC loss due to the surface deactivation of TiO2 and OH quenching. The steady-state concentration of OH was dramatically decreased in the presence of HA. Identification of transformation products showed that HA could inhibit the oxidation pathways initiated by OH during photocatalysis of TC. These findings provide further insights into the assessment of photolysis and photocatalysis for antibiotics elimination in natural waters where HA exists ubiquitously.

Kinetics and mechanism of carbamazepine degradation by a modified Fenton-like reaction with ferric-nitrilotriacetate complexes

This study investigated the kinetics and mechanism of carbamazepine (CBZ) degradation over an initial pH range of 5.0–9.0 by a modified Fenton-like reaction using ferric-nitrilotriacetate (FeIII-NTA) complexes. The results indicate that CBZ degradation by FeIII-NTA/H2O2 can be described by pseudo first-order kinetics and mainly attributed to hydroxyl radical (OH) attack. Ten intermediates were indentified during the degradation process, including hydroxy-CBZs, 10,11-epoxy-CBZ, quinonid CBZ derivatives, dihydroxy-CBZs, and hydroxy-CBZ-10,11-diols. The steady-state concentration of OH, ranging from 3.8×10−16 to 2.1×10−13M, was strongly dependent on the concentration of FeIII, the initial pH, and H2O2:FeIII and NTA:FeIII molar ratios. Optimal conditions of [FeIII]=1×10−4M, [H2O2:FeIII]=155:1 and [NTA:FeIII]=3:1 were obtained for the degradation of CBZ at neutral pH (7.0) and ambient temperature (25°C); the corresponding degradation rate constant of CBZ, kapp, was 0.0419 (±0.002) min−1. The value of kapp increased with increasing pH from 5.0 to 9.0 due to the strong pH-dependence of FeIII-NTA complexes; FeIII(NTA)(OH)22− was the most likely active iron species to activate H2O2 to produce OH. The temperature dependence of CBZ degradation by FeIII-NTA/H2O2 was characterized by an activation energy of 76.16kJmol−1. A potential mechanism for the formation of OH by FeIII-NTA/H2O2 and possible degradation pathways of CBZ are proposed.

Evaluation of HO<sub>x</sub> sources and cycling using measurement-constrained model calculations in a 2-methyl-3-butene-2-ol (MBO) and monoterpene (MT) dominated ecosystem

Abstract. We present a detailed analysis of OH observations from the BEACHON (Bio-hydro-atmosphere interactions of Energy, Aerosols, Carbon, H2O, Organics and Nitrogen)-ROCS (Rocky Mountain Organic Carbon Study) 2010 field campaign at the Manitou Forest Observatory (MFO), which is a 2-methyl-3-butene-2-ol (MBO) and monoterpene (MT) dominated forest environment. A comprehensive suite of measurements was used to constrain primary production of OH via ozone photolysis, OH recycling from HO2, and OH chemical loss rates, in order to estimate the steady-state concentration of OH. In addition, the University of Washington Chemical Model (UWCM) was used to evaluate the performance of a near-explicit chemical mechanism. The diurnal cycle in OH from the steady-state calculations is in good agreement with measurement. A comparison between the photolytic production rates and the recycling rates from the HO2 + NO reaction shows that recycling rates are ~20 times faster than the photolytic OH production rates from ozone. Thus, we find that direct measurement of the recycling rates and the OH loss rates can provide accurate predictions of OH concentrations. More importantly, we also conclude that a conventional OH recycling pathway (HO2 + NO) can explain the observed OH levels in this non-isoprene environment. This is in contrast to observations in isoprene-dominated regions, where investigators have observed significant underestimation of OH and have speculated that unknown sources of OH are responsible. The highly-constrained UWCM calculation under-predicts observed HO2 by as much as a factor of 8. As HO2 maintains oxidation capacity by recycling to OH, UWCM underestimates observed OH by as much as a factor of 4. When the UWCM calculation is constrained by measured HO2, model calculated OH is in better agreement with the observed OH levels. Conversely, constraining the model to observed OH only slightly reduces the model-measurement HO2 discrepancy, implying unknown HO2 sources. These findings demonstrate the importance of constraining the inputs to, and recycling within, the ROx radical pool (OH + HO2 + RO2).

Advanced oxidation treatment and photochemical fate of selected antidepressant pharmaceuticals in solutions of Suwannee River humic acid

Antidepressant pharmaceuticals have recently been detected at low concentrations in wastewater and surface water. This work reports studies of the direct and indirect photochemical fate and treatment by advanced oxidation of three antidepressant compounds (duloxetine, venlafaxine and bupropion) in solutions of humic acid in order to elucidate their behavior in the natural environment prior to reaching a water treatment facility and potentially entering a potable water supply. Humic acid solution was prepared by adding to distilled water a known amount of organic matter as a photosensitizer. All three antidepressants react very rapidly with hydroxyl radicals (OH) and hydrated electrons (e−aq) with rate constants of ∼108 to 1010M−1s−1, but significantly slower with singlet oxygen (1ΔO2) (∼103 to 105M−1s−1). The steady-state concentrations of OH and 1ΔO2, in a sample of humic acid solution were measured and used with the second order rate constants to show that the hydroxyl radical was an order of magnitude more effective than the singlet oxygen in the solar-induced photochemical degradation of the antidepressants. Excited state dissolved organic matter also accounted for a substantial portion of degradation of duloxetine, decreasing its half-life by 27% under solar irradiation. Several reaction pathways and by-products arising from the photodegradation were identified using gamma-irradiation followed by LC–MS analysis.

Isoprene in poplar emissions: effects on new particle formation and OH concentrations

Abstract. Stress-induced volatile organic compound (VOC) emissions from transgenic Grey poplar modified in isoprene emission potential were used for the investigation of photochemical secondary organic aerosol (SOA) formation. In poplar, acute ozone stress induces the emission of a wide array of VOCs dominated by sesquiterpenes and aromatic VOCs. Constitutive light-dependent emission of isoprene ranged between 66 nmol m−2 s−1 in non-transgenic controls (wild type WT) and nearly zero (<0.5 nmol m−2 s−1) in isoprene emission-repressed plants (line RA22), respectively. Nucleation rates of up to 3600 cm−3 s−1 were observed in our experiments. In the presence of isoprene new particle formation was suppressed compared to non-isoprene containing VOC mixtures. Compared to isoprene/monoterpene systems emitted from other plants the suppression of nucleation by isoprene was less effective for the VOC mixture emitted from stressed poplar. This is explained by the observed high efficiency of new particle formation for emissions from stressed poplar. Direct measurements of OH in the reaction chamber revealed that the steady state concentration of OH is lower in the presence of isoprene than in the absence of isoprene, supporting the hypothesis that isoprenes' suppressing effect on nucleation is related to radical chemistry. In order to test whether isoprene contributes to SOA mass formation, fully deuterated isoprene (C5D8) was added to the stress-induced emission profile of an isoprene free poplar mutant. Mass spectral analysis showed that, despite the isoprene-induced suppression of particle formation, fractions of deuterated isoprene were incorporated into the SOA. A fractional mass yield of 2.3% of isoprene was observed. Future emission changes due to land use and climate change may therefore affect both gas phase oxidation capacity and new particle number formation.

Determination of Hydroxyl Radical in Seto Inland Sea and its Potential to Degrade Irgarol

Photo-production rates and overall scavenging rate constants of the highly reactive hydroxyl radicals (•OH) were measured by irradiation of surface seawater samples collected from the Seto Inland Sea, Japan. Photo-production rates ofOH ranged from 7.2 × 10 -12 M s -1 to 424 × 10 12 M s -1 and scavenging rate constants were 1.3 - 4.1×10 6 s -1 . The steady state concentrations ofOH in seawater, which were calculated from the photo-production rates and scavenging rate constants were in the range 2.6 -189 × 10 -18 M. Estimation from the scavenging rate constant showed that the lifetime ofOH in seawater ranged from 0.29 ×10 -6 s to 0.55 ×10 -6 s. Nitrates and hydrogen peroxide contributions to OH radical formation rates were negligible in the samples studied. However, direct photolysis of NO2 - accounted for up to 73% of the observed •OH formation rates. The rate constant for the reactionOH with irgarol was found to be (9.7 ± 0.52) × 10 9 M - 1 s -1. Based on the steady state concentration ofOH, the calculated half-lives for Irgarol due to the reaction with hydroxyl radicals in the Seto Inland Sea were in the range 4.3-318 days.

Roles of hydroxyl radical generating/scavenging mechanisms in pseudo polluted dew in reducing the foliar CO 2 assimilation rate and biomass production of Japanese red pine ( Pinus densiflora Sieb. et Zucc.) seedlings

Hydroxyl radical ( OH) is one of the most highly reactive of all active oxygen species. Aqueous-phase OH is photochemically generated in polluted dew droplets on needle surfaces of Japanese red pine ( Pinus densiflora Sieb. et Zucc., an evergreen coniferous tree) in declining pine forests. In former studies, we examined the effects of OH-generating mist solutions, which simulate polluted dew droplets, on needle ecophysiological traits of pine seedlings. Two types of solution with different OH-generating sources, i.e. the photo-Fenton reaction (H 2O 2-Fe-oxalate) and photolysis of HONO and NO 2 − [N(III)], had contrasting functional effects on needle gas exchange characteristics even though they had similar OH photoformation rates. In the present study, we investigated the effects of OH-generating mist solutions containing mixed-sources of OH (photo-Fenton and N(III)) on needle gas exchange and the biomass production of potted Japanese red pine seedlings. The N(III) and H 2O 2 concentrations of the OH-generating mist solutions were regulated to the same concentration (0, 25, 50, 75 or 100 μM, pH 5.2–5.3). Treated needles with the lowest photoformation rate and scavenging rate constant of OH had the smallest CO 2 assimilation rate ( A max) and needle conductance ( g n) of all treated pine seedlings. This suggests that the photoformation rate and scavenging rate constant do not always explain the ecophysiological disorders of pine needles subjected to OH-generating wet deposition. On the other hand, the calculated steady-state concentration of OH in the mist solutions was significantly negatively correlated with A max. In addition, pine seedlings with reduced A max showed suppressed biomass (dry weight) production. These results suggest that the OH concentration in the mist solutions is the decisive factor in explaining the negative effects of pseudo dew droplets not only on needle but also plant productivity. We propose that the deposition of polluted dew with chemical components and systems that result in a high OH concentration is one of the causes of growth decline of Japanese red pine.

A smog chamber for studies of the photochemical degradation of chemicals in the presence of aerosols

A smog chamber, especially designed for the simulation of the tropospheric photochemical degradation of chemicals adsorbed on aerosol surfaces, is described. With optimum aerodynamic diameters of the aerosols (0.5 μm) and by special thermostatization of the chamber residence times of the aerosols up to 2 days in the dark and up to 1 day in the presence of simulated sun irradiation are achieved. Experiments on the degradation of simple alkanes (up to n-nonane) and aromatics in the absence and presence of aerosols (SiO 2 and TiO 2) are reported. Rate constants for the reactions of OH with n-alkanes from n-butane to n-tridecane are determined to be ( k ± 2 σ)/10 −12 cm 3 s −3 = 2.54 ± 0.04, 4.13 ± 0.05, 5.68 ± 0.04, 7.31 ± 0.08, 8.79 (reference value), 10.3 ± 0.2, 12.4 ± 0.2, 13.3 ± 0.2, 13.9 ± 0.2, 15.5 ± 0.2), respectively, at 300 K. These rate constant measurements and the observed concentrations of OH remain unaffected in the presence of SiO 2 aerosol at mass concentrations of l mg m −3. In the presence of TiO 2 at 2 mg m −3 mass concentration, the steady-state concentration of OH is enhanced by an order of magnitude. In addition, a heterogeneous photodegradation process of the hydrocarbons occurs on the surface of the TiO 2 aerosol.