Composition and yields of secondary organic aerosol formed from OH radical-initiated reactions of linear alkenes in the presence of NOx: Modeling and measurements

The products and mechanism of secondary organic aerosol (SOA) formation from the OH radical-initiated reactions of linear alkenes in the presence of NO_x were investigated in an environmental chamber. The SOA consisted primarily of products formed through reactions initiated by OH radical addition to the CC double bond, including β-hydroxynitrates and dihydroxynitrates, as well as cyclic hemiacetals, dihydrofurans, and dimers formed from particle-phase reactions of dihydroxycarbonyls. 1,4-Hydroxynitrates formed through reactions initiated by H-atom abstraction also appeared to contribute. Product yields and OH radical and alkoxy radical rate constants taken from the literature or calculated using structure–reactivity methods were used to develop a quantitative chemical mechanism for these reactions. SOA yields were then calculated using this mechanism with gas-particle partitioning theory and estimated product vapor pressures for comparison with measured values. Calculated and measured SOA yields agreed very well at high carbon numbers when semi-volatile products were primarily in the particle phase, but diverged with decreasing carbon number to a degree that depended on the model treatment of dihydroxycarbonyls, which appeared to undergo reversible reactions in the particle phase. The results indicate that the chemical mechanism developed here provides an accurate representation of the gas-phase chemistry, but the utility of the SOA model depends on the partitioning regime. The results also demonstrate some of the advantages of studying simple aerosol-forming reactions in which the majority of products can be identified and quantified, in this case leading to insights into both gas- and particle-phase chemistry.     

Atmospheric chemistry of 1-methyl-2-pyrrolidinone

Rate constants for the atmospheric reactions of 1-methyl-2-pyrrolidinone with OH radicals, NO₃ radicals and O₃ have been measured at 296±2 K and atmospheric pressure of air, and the products of the OH radical and NO₃ radical reactions investigated. Using relative rate techniques, rate constants for the gas-phase reactions of OH and NO₃ radicals with 1-methyl-2-pyrrolidinone of (2.15±0.36)×10^-11 cm³ molecule^-1 s^-1 and (1.26±0.40)×10^-13 cm³ molecule^-1 s^-1, respectively, were measured, where the indicated errors include the estimated overall uncertainties in the rate constants for the reference compounds. An upper limit to the rate constant for the O₃ reaction of <1×10^-19 cm³ molecule^-1 s^-1 was also determined. These kinetic data lead to a calculated tropospheric lifetime of 1-methyl-2-pyrrolidinone of a few hours, with both the daytime OH radical reaction and the nighttime NO₃ radical reaction being important loss processes. Products of the OH radical and NO₃ radical reactions were analyzed by gas chromatography with flame ionization detection and combined gas chromatography–mass spectrometry. N-methylsuccinimide and (tentatively) 1-formyl-2-pyrrolidinone were identified as products of both of these reactions. The measured formation yields of N-methylsuccinimide and 1-formyl-2-pyrrolidinone were 44±12% and 41±12%, respectively, from the OH radical reaction and 59±16% and ∼4%, respectively, from the NO₃ radical reaction. Reaction mechanisms consistent with formation of these products are presented.     

A new approach to quantify the degradation kinetics of linuron with UV, ozonation and UV/O3 processes

The degradation of linuron, one of phenylurea herbicides, was investigated for its reaction kinetics by different treatment processes including ultraviolet irradiation (UV), ozonation (O₃), and UV/O₃. The decay rate of linuron by UV/O₃ process was found to be around 3.5 times and 2.5 times faster than sole-UV and ozone-alone, respectively. Experimental results also indicate overall rate constants increased exponentially with pH above 9.0 while the increase of rate constants with pH below 9 is insignificant in O₃ system. All dominant parameters involved in the three processes were determined in the assistant of proposed linear models in this study. The approach was found useful in predicting the process performances through the quantification of quantum yield (rate constant for the formation of free radical HOO⁻ from ozone decomposition at high pH), rate constant of linuron with ozone (k_O3,LNR), rate constant of linuron with hydroxyl radical (k_OH,LNR), and α (the ratio of the production rate of OH and the decay rate of ozone in UV/O₃ system).     

Mechanism of OH-initiated atmospheric photooxidation of the organophosphorus insecticide (C2H5O)3PS

O,O,O-triethyl phosphorothioate ((C₂H₅O)₃PS, TEPT) is a widely used organophosphorus insecticide. TEPT may be released into the atmosphere where it can undergo transport and chemical transformations, which include reactions with OH radicals, NO₃ radicals and O₃. The mechanism of the atmospheric reactions of TEPT has not been fully understood due to the short-lifetime of its oxidized radical intermediates, and the extreme difficulty in detection of these species experimentally. In this work, we carried out molecular orbital theory calculations for the OH radical-initiated atmospheric photooxidation of TEPT. The profile of the potential energy surface was constructed, and the possible channels involved in the reaction are discussed. The theoretical study shows that OH addition to the PS bond and H abstractions from the CH₃CH₂O moiety are energetically favorable reaction pathways. The dominant products TEP and SO₂ arise from the secondary reactions, the reactions of OH-TEPT adducts with O₂. The experimentally uncertain dominant product with molecular weight 170 is mostly due to (C₂H₅O)₂P(S)OH and not (C₂H₅O)₂P(O)SH.     

Atmospheric fate of acrylic acid and acrylonitrile: Rate constants with Cl atoms and OH radicals in the gas phase

Rate coefficients for the reactions of hydroxyl radicals and chlorine atoms with acrylic acid and acrylonitrile have been determined at 298 K and atmospheric pressure. The decay of the organics was followed using a gas chromatograph with a flame ionization detector (GC-FID) and the rate constants were determined using a relative rate method with different reference compounds. Room temperature rate constants are found to be (in cm³ molecule⁻¹ s⁻¹): k₁(OH+CH₂CHC(O)OH)=(1.75±0.47)×10⁻¹¹, k₂(Cl+CH₂CHC(O)OH)=(3.99±0.84)×10⁻¹⁰, k₃(OH+CH₂CHCN)=(1.11±0.33)×10⁻¹¹ and k₄(Cl+CH₂CHCN)=(1.11±0.23)×10⁻¹⁰ with uncertainties representing ±2σ. This is the first kinetic study for these reactions under atmospheric pressure. The rate coefficients are compared with previous determinations taking into account the effect of pressure on the rate constants. The effect of substituent atoms or groups on the overall rate constants is analyzed in comparison with other unsaturated compounds in the literature. In addition, atmospheric lifetimes based on the homogeneous sinks of acrylic acid and acrylonitrile are estimated and compared with other tropospheric sinks for these compounds.     

Kinetics and mechanism for omethoate degradation by catalytic ozonation with Fe(III)-loaded activated carbon in water

The kinetics and mechanism for degradation of omethoate (OMT) by catalytic ozonation with Fe(III)-loaded activated carbon (Fe@AC) were investigated in this study with focus on identification of degradation byproducts. The rate constants of OMT reacting with ozone and hydroxyl radical (OH) were determined to be 0.04 and 5.3 × 10⁸ M^?1 s^?1 at pH 7.5 and 20 °C, respectively. OMT was predominantly degraded by OH in the catalytic ozonation with Fe@AC. The high-molecular-weight degradation byproducts identified were O,O,O-trimethyl phosphoric ester (TMP), pyrrolidin-2-one, N-methyl-2-sulfanylacetamide, 2-(methylthio)acetamide, O,O,S-trimethylthiophosphate (STMP), and N-methyl-2-(methylthio)acetamide. Besides, low-molecular-weight organic acids and inorganic anions were also detected and quantified, including formic, acetic and oxalic acids as well as nitrate, sulfate and phosphate ions. In the catalytic ozonation, TMP and phosphate were two major P-containing byproducts resulting from OMT degradation. The toxicity of OMT solution gradually decreased during the catalytic ozonation, indicating that Fe@AC is a safe catalyst for OMT removal by ozone in water.     

pH dependence of persulfate activation by EDTA/Fe(III) for degradation of trichloroethylene

The ability of free ferrous ion activated persulfate (S₂O₈²⁻) to generate sulfate radicals (SO₄⁻) for the oxidation of trichloroethylene (TCE) is limited by the scavenging of SO₄⁻ with excess Fe²⁺ and a quick conversion of Fe²⁺ to Fe³⁺. This study investigated the applicability of ethylene-diamine-tetra-acetic acid (EDTA) chelated Fe³⁺ in activating persulfate for the destruction of TCE in aqueous phase under pH 3, 7 and 10. Fe³⁺ and EDTA alone did not appreciably degrade persulfate. The presence of TCE in the EDTA/Fe³⁺ activated persulfate system can induce faster persulfate and EDTA degradation due to iron recycling to activate persulfate under a higher pH condition. Increasing the pH leads to increases in pseudo-first-order-rate constants for TCE, S₂O₈²⁻ and EDTA degradations, and Cl generation. Accordingly, the experiments at pH 10 with different EDTA/Fe³⁺ molar ratios indicated that a 1/1 ratio resulted in a remarkably higher degradation rate at the early stage of reaction as compared to results by other ratios. Higher persulfate dosage under the EDTA/Fe³⁺ molar ratio of 1/1 resulted in greater TCE degradation rates. However, increases in persulfate concentration may also lead to an increase in the rate of persulfate consumption.     

Degradation of acetic acid with sulfate radical generated by persulfate ions photolysis

The photolysis of was studied for the removal of acetic acid in aqueous solution and compared with the H₂O₂/UV system. The radicals generated from the UV irradiation of ions yield a greater mineralization of acetic acid than the OH radicals. Acetic acid is oxidized by radicals without significant formation of intermediate by-products. Increasing system pH results in the formation of OH radicals from radicals. Maximum acetic acid degradation occurred at pH 5. The results suggest that above this pH, competitive reactions with the carbon mineralized inhibit the reaction of the solute with and also OH radicals. Scavenging effects of two naturally occurring ions were tested; in contrast to ions, the presence of Cl⁻ ions enhances the efficiency of the /UV process towards the acetate removal. It is attributed to the formation of the Cl radical and its great reactivity towards acetate.     

Observations of hydroxyl and the sum of peroxy radicals at Summit,Greenland during summer 2003

The first measurements of peroxy (HO₂+RO₂) and hydroxyl (OH) radicals above the arctic snowpack were collected during the summer 2003 campaign at Summit, Greenland. The median measured number densities for peroxy and hydroxyl radicals were 2.2×10⁸ mol cm⁻³ and 6.4×10⁶ mol cm⁻³, respectively. The observed peroxy radical values are in excellent agreement ($R^2=0.83$, $\mathrm{M}/\mathrm{O}=1.06$) with highly constrained model predictions. However, calculated hydroxyl number densities are consistently more than a factor of 2 lower than the observed values. These results indicate that our current understanding of radical sources and sinks is in accord with our observations in this environment but that there may be a mechanism that is perturbing the (HO₂+RO₂)/OH ratio. This observed ratio was also found to depend on meteorological conditions especially during periods of high winds accompanied by blowing snow. Backward transport model simulations indicate that these periods of high winds were characterized by rapid transport (1–2 days) of marine boundary layer air to Summit. These data suggest that the boundary layer photochemistry at Summit may be periodically impacted by halogens.     

Photosensitized degradation of caffeine: role of fulvic acids and nitrate

The photolysis of caffeine was studied in solutions of fulvic acid isolated from Suwannee River, GA (SRFA) and Old Woman Creek Natural Estuarine Research Reserve, OH (OWCFA) with different chemical amendments (nitrate and iron). Caffeine degrades slowly by direct photolysis (>170 h in artificial sunlight), but we observed enhanced photodegradation in waters containing the fulvic acids. At higher initial concentrations (10 μM) the indirect photolysis of caffeine occurs predominantly through reaction with the hydroxyl radical (OH) generated by irradiated fulvic acids. Both rate constant estimates based upon measured OH steady-state concentrations and quenching studies using isopropanol corroborate the importance of this pathway. Further, OH generated by irradiated nitrate at concentrations present in wastewater effluent plays an important role as a photosensitizer even in the presence of fulvic acids, while the photo-Fenton pathway does not at neutral or higher pH. At lower initial concentrations (0.1 μM) caffeine photolysis reactions proceed even more quickly in fulvic acid solutions and are influenced by both short- and long-lived reactive species. Studies conducted under suboxic conditions suggest that an oxygen dependent long-lived radical e.g., peroxyl radicals plays an important role in the degradation of caffeine at lower initial concentration.     

Gas-phase rate coefficients for the reactions of NO3, OH and O3 with α,β-unsaturated esters and ketones: Structure−activity relations (SARs)

Gas-phase rate coefficients for the atmospherically important reactions of NO₃, OH and O₃ are predicted for 55 α,β-unsaturated esters and ketones. The rate coefficients were calculated using a correlation described previously [Pfrang, C., King, M.D., C. E. Canosa-Mas, C.E., Wayne, R.P., 2006. Atmospheric Environment 40, 1170–1179]. These rate coefficients were used to extend structure–activity relations for predicting the rate coefficients for the reactions of NO₃, OH or O₃ with alkenes to include α,β-unsaturated esters and ketones. Conjugation of an alkene with an α,β-keto or α,β-ester group will reduce the value of a rate coefficient by a factor of ∼110, ∼2.5 and ∼12 for reaction with NO₃, OH or O₃, respectively. The actual identity of the alkyl group, R, in −C(O)R or −C(O)OR has only a small influence. An assessment of the reliability of the SAR is given that demonstrates that it is useful for reactions involving NO₃ and OH, but less valuable for those of O₃ or peroxy nitrate esters.     

Evidence of singlet oxygen and hydroxyl radical formation in aqueous goethite suspension using spin-trapping electron paramagnetic resonance (EPR)

The generation of reactive species in an aqueous goethite suspension, under room light and aeration conditions, was investigated using the electron paramagnetic resonance (EPR) technique employing spin trap agents. The trap reagents, including 5,5-dimethylpyrroline N-oxide (DMPO) and 2,2,6,6-tetramethylpiperidine (TEMP), were used for the detection of OH radicals (OH) and singlet oxygen (¹O₂), respectively. On the addition of DMPO to the goethite suspended solution, a DMPO-OH adduct was formed, which was not decreased, even in the presence of the OH scavenger, mannitol. This result implied a false positive interpretation from the DMPO-OH EPR signal. In the presence of TEMP reagent, a TEMP-O signal was detected, which was completely inhibited in the presence of the singlet oxygen scavenger, sodium azide. With both DMPO-OH and TEMP-O radicals in the presence and absence of radical scavengers, singlet oxygen was observed to be the key species formed in the room-light sensitized goethite suspension. In the goethite/H₂O₂ system; however, both OH and singlet oxygen were generated, with significant portions of DMPO-OH resulting from both OH and singlet oxygen. In fact, the DMPO-OH resulting from OH should be carefully calculated by correcting for the amount of DMPO-OH due to singlet oxygen. This study reports, for the first time, that the goethite suspensions may also act as a natural sensitizer, such as fulvic acids, to form singlet oxygen.     

Determination of direct photolysis rate constants and OH radical reactivity of representative odour compounds in brewery broth using a continuous flow-stirred photoreactor

A method based on photolysis was developed for the appropriate treatment of organic pollutants in air exhausting from breweries upon wort decoction, and thereby causing smell nuisance. A continuous flow stirred photoreactor was built-up exclusively, allowing OH radicals to react with selected odorous compounds contained in exhaust vapours, such as: 2-methylpropanal, 3-methylbutanal, 2-methylbutanal, 3-methyl-1-butanol, n-hexanal, 2-methylbutyl isobutyrate, 2-undecanone, phenyl acetaldehyde, myrcene, limonene, linalool, humulene, dimethylsulphide, and dimethyltrisulphide. These substances were quantified in brewery broth before and after UV irradiation using high-resolution gas chromatography–mass spectrometry (HRGC–MS). For odour analysis, high-resolution gas chromatography-flame ionisation detection (HRGC-FID) coupled with sensory methods was used. Determined quantum yields of about 10⁻³ for phenyl acetaldehyde, myrcene, and humulene pointed out that direct photolysis contributed to their decay. Quantum yields of below 10⁻⁴ for the other substances indicated that UV irradiation did not contribute significantly to their degradation processes. Hydroxyl radical reaction rate constants and Henry constants of organic compounds were also measured. Substances accompanied with low Henry constants converted rapidly, whereas those with higher ones, relatively slowly. Determined aroma values concluded that after UV–H₂O₂ treatment, only dimethylsulphide and myrcene remained as important odorous compounds, but in significantly reduced concentrations. The UV–H₂O₂ treatment of brewery broth has been proved effective to reduce smell-irritating substances formed upon wort decoction.     

Do aerosols act as catalysts in the OH radical initiated atmospheric oxidation of volatile organic compounds?

Smog chamber/FTIR techniques were used to study the relative reactivity of OH radicals with methanol, ethanol, phenol, C₂H₄, C₂H₂, and p-xylene in 750 Torr of air diluent at 296±2 K. Experiments were performed with, and without, 500–8000 μg m⁻³ (4000–50 000 μm² cm⁻³ surface area per volume) of NaCl, (NH₄)₂SO₄ or NH₄NO₃ aerosol. In contrast to the recent findings of Oh and Andino (Atmospheric Environment 34 (2000) 2901, 36 (2002) 149; International Journal of Chemical Kinetics 33 (2001) 422) there was no discernable effect of aerosol on the rate of loss of the organic compounds via reaction with OH radicals. Gas kinetic theory arguments cast doubt upon the findings of Oh and Andino. The available data suggest that the answer to the title question is “No”. As part of this work the rate constants for reactions of OH radicals with methanol, ethanol, and phenol in 750 Torr of air at 296 K were determined to be: k_OH+CH3OH=(8.12±0.54)×10⁻¹³, k_OH+C2H5OH=(3.47±0.32)×10⁻¹² and k_OH+phenol=(3.27±0.31)×10⁻¹¹ cm³ molecule⁻¹ s⁻¹.     

Kinetics of the gas-phase reactions of alkylnaphthalenes with O3, N2O5 and OH radicals at 298 ± 2 K

Rate constants for the gas-phase reactions of the OH radical with 1-methylnaphthalene and of N₂O₅ with 1- and 2-methylnaphthalene and 2,3-dimethylnaphthalene have been determined at 298 ± 2 K by use of relative rate techniques. The rate constants determined were: for the reaction of OH radicals with 1-methylnaphthalene, (5.30 ± 0.48) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹; for the reaction of N₂O₅ with 1-methylnaphthalene, 2-methylnaphthalene and 2,3-dimethylnaphthalene, (3.3 ± 0.7) × 10⁻¹⁷, (4.2 ± 0.9) × 10⁻¹⁷ and (5.7 ± 1.9) × 10⁻¹⁷ cm³ molecule⁻¹ s⁻¹, respectively. In addition, an upper limit to the rate constant of 1.3 × 10⁻¹⁹ cm³ molecule⁻¹ s⁻¹ was measured for the reaction of O₃ with 1-methylnaphthalene at 298 ± 2 K. These data, when combined with data from previous literature, allow the atmospheric gas-phase removal processes of these alkylnaphthalenes to be quantified.     

A new detailed chemical model for indoor air pollution

A detailed chemical box model has been constructed based on a comprehensive chemical mechanism (the Master Chemical Mechanism) to investigate indoor air chemistry in a typical urban residence in the UK. Unlike previous modelling studies of indoor air chemistry, the mechanism adopted contains no simplifications such as lumping or the use of surrogate species, allowing more insight into indoor air chemistry than previously possible. The chemical mechanism, which has been modified to include the degradation reactions of key indoor air pollutants, contains around 15,400 reactions and 4700 species. The results show a predicted indoor OH radical concentration up to 4.0×10⁵ molecule cm⁻³, only a factor of 10–20 less than typically observed outdoors and sufficient for significant chemical cycling to take place. Concentrations of PAN-type species and organic nitrates are found to be important indoors, reaching concentrations of a few ppb. Sensitivity tests highlight that the most crucial parameters for modelling the concentration of OH are the light-intensity levels and the air exchange rate. Outdoor concentrations of O₃ and NO_X are also important in determining radical concentrations indoors. The reactions of ozone with alkenes and monoterpenes play a major role in producing new radicals, unlike outdoors where photolysis reactions are pivotal radical initiators. In terms of radical propagation, the reaction of HO₂ with NO has the most profound influence on OH concentrations indoors. Cycling between OH and RO₂ is dominated by reaction with the monoterpene species, whilst alcohols play a major role in converting OH to HO₂. Surprisingly, the absolute reaction rates are similar to those observed outdoors in a suburban environment in the UK during the summer. The results from this study highlight the importance of tailoring a model for its particular location and the need for future indoor air measurements of radical species, nitrated species such as PANs and organic nitrates, photolysis rates of key species over the range of wavelengths observed indoors and concurrent measurements of outdoor air pollutant concentrations.