Evaluation of the UNC toluene-SOA mechanism with respect to other chamber studies and key model parameters

In a companion paper by Hu et al. [2007. A kinetic mechanism for predicting secondary organic aerosol formation from toluene oxidation in the presence of NO_x and natural sunlight. Atmospheric Environment, doi:10.1016/j.atmosenv.2007.04.025], a kinetic mechanism was developed from data generated in the University of North Carolina's (UNC) 270 m³ dual outdoor aerosol smog chamber, to predict secondary organic aerosol (SOA) formation from toluene oxidation in the atmosphere. In this paper, experimental data sets from European Photoreactor (EUPHORE), smog chambers at the California Institute of Technology (Caltech), and the UNC 300 m³ dual-outdoor gas phase chamber were used to evaluate the toluene mechanism. The model simulates SOA formation for the ‘low-NO_x’ and ‘mid-NO_x’ experiments from EUPHORE chambers reasonably well, but over-predicts SOA mass concentrations for the ‘high-NO_x’ run. The model well simulates the SOA mass concentrations observed from the Caltech chambers. Experiments with the three key toluene products, 1,4-butenedial, 4-oxo-2-pentenal and o-cresol in the presence of oxides of nitrogen (NO_x) are also simulated by the developed mechanism. The model well predicts the NO_x time–concentration profiles and the decay of these two carbonyls, but underestimates ozone (O₃) formation for 4-oxo-2-pentenal. It well simulates SOA formation from 1,4-butenedial but overestimates (possibly due to experimental problems) the measured aerosol mass concentrations from 4-oxo-2-pentenal. The model underestimates SOA production from o-cresol, mostly due to its under-prediction of o-cresol decay. The effects of varying temperature, relative humidity, glyoxal uptake, organic nitrate yields, and background seed aerosol concentrations, were also investigated.     

Measurements of aromatic hydrocarbon ratios and NOx concentrations in the rural troposphere: Observation of air mass photochemical aging and NOx removal

Measurements of the aromatic hydrocarbons (benzene, toluene, ethylbenzene and ortho(o)-xylene) at Niwot Ridge, Colorado have shown distinct correlations between the ratios of the concentrations of these compounds and the degree of direct urban influence. The major atmospheric removal mechanism of aromatic hydrocarbons is reaction with the hydroxyl radical, OH. This allows the decrease in the ratios of aromatic hydrocarbon concentrations to be related to the transport time and average OH number density within an air mass, if assumptions are made concerning background sources of aromatic hydrocarbons. Measured ratios of aromatic compounds at this site, along with ratios reported for several cities in the western United States, and estimates of transport times from these cities were used to calculate temporally and spatially averaged OH number densities. Hydroxyl radical number density estimates using toluene-, ethylbenzene-, and o-xylene-to-benzene ratios, were 1.2 ± 0.6 × 10⁶, 1.0 ±0.8 × 10₆ and 0.48 ± 0.8 × 10⁶ molecules cm⁻³, respectively. Considering the uncertainties in the assumptions used in the above estimates, we obtain a diurnal-average upper limit of 2.4 × 10⁶ molecules cm⁻³. The correlations between measured ratios are found to yield slopes consistent with those predicted by experimental OH rate constants for benzene, toluene and ethylbenzene, and approximately a factor of two different in the case of benzene, toluene and o-xylene. The ratio of NO_x: benzene was found to yield no correlation with toluene: benzene ratio for periods of westerly flow, but was well correlated with toluene: benzene ratio during periods of direct urban impact on the site (upslope easterly winds). The correlation of these ratios in urban plume air masses was consistent with NO₂ + OH + M being the major daytime removal mechanism of NO_x in the summertime.     

Gas-phase absorption cross sections of 24 monocyclic aromatic hydrocarbons in the UV and IR spectral ranges

Absorption cross sections of 24 volatile and non-volatile derivatives of benzene in the ultraviolet (UV) and the infrared (IR) regions of the electromagnetic spectrum have been determined using a 1080 l quartz cell. For the UV a 0.5 m Czerny-Turner spectrometer coupled with a photodiode array detector (spectral resolution 0.15 nm) was used. IR spectra were recorded with an FT-IR spectrometer (Bruker IFS-88, spectral resolution 1 cm^-1). Absolute absorption cross sections and the instrument function are given for the UV, while for the IR, absorption cross sections and integrated band intensities are reported.The study focused primarily on the atmospherically relevant methylated benzenes (benzene, toluene, o-xylene, m-xylene, p-xylene, 1,2,4-trimethylbenzene, 1,3,5-trimethylbenzene, ethylbenzene, styrene) and their ring retaining oxidation products (benzaldehyde, o-tolualdehyde, m-tolualdehyde, p-tolualdehyde, phenol, o-cresol, m-cresol, p-cresol, 2,3-dimethylphenol, 2,4-dimethylphenol, 2,5-dimethylphenol, 2,6-dimethylphenol, 3,4-dimethylphenol, 3,5-dimethylphenol, 2,4,6-trimethylphenol and (E,Z)- and (E,E)-2,4-hexadienedial).The UV absorption cross sections reported here can be used for the evaluation of DOAS spectra (Differential Optical Absorption Spectroscopy) for measurements of the above compounds in the atmosphere and in reaction chambers, while the IR absorption cross sections will primarily be useful in laboratory studies on atmospheric chemistry, where FT-IR spectrometry is an important tool.     

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.     

Roadside BTEX and other gaseous air pollutants in relation to emission sources

Hourly concentrations of benzene, toluene, ethylbenzene, m,p-xylenes, and o-xylene (BTEX) plus CO, NO_x, SO₂ were monitored at roadsides simultaneously with the traffic volume during the dry season of 2004, in Hanoi, Vietnam. The selected three streets included Truong Chinh (TC) with high traffic volume, Dien Bien Phu (DBP) with low traffic volume, and Nguyen Trai (NT) with high traffic volume running through an industrial estate. BTEX were sampled by SKC charcoal tubes and analyzed by GC–FID. Geometric means of hourly benzene, toluene, ethylbenzene, m,p-xylenes and o-xylene are, respectively, 65, 62, 15, 43, and 22 μg m⁻³ in TC street; 30, 38, 9, 26, and 13 μg m⁻³ in DBP street; and 123, 87, 24, 56, and 30 μg m⁻³ in NT street. Levels of other gaseous pollutants including CO, NO_x, and SO₂, measured by automatic instruments, were low and not exceeding the Vietnam national ambient air quality standards. BTEX levels were comparatively analyzed for different downwind distances (3–50 m) from the street, between peak hours and off-peak hours, as well as between weekdays and weekend. Results of principal component analysis suggest that the gaseous pollutants are associated with different vehicle types.     

Hydroxyl (OH) radical production rates in snowpacks from photolysis of hydrogen peroxide (H2O2) and nitrate (NO3−)

Atmospheric chemistry directly above snowpacks is strongly influenced by ultraviolet (UV) radiation initiated emissions of chemicals from the snowpack. The emission of gases from the snowpack to the atmosphere is in part due to chemical reactions between hydroxyl radical, OH (produced from photolysis of hydrogen peroxide (H₂O₂) or nitrate (NO₃⁻)) and impurities in the snowpack. The work presented here is a radiative-transfer modelling study to calculate the depth-integrated production rates of hydroxyl radical from the photolysis of hydrogen peroxide and nitrate anion in snow for four different snowpacks and for solar zenith angles 30°–90°. This work also demonstrates the importance of hydrogen peroxide photolysis to produce hydroxyl radical relative to nitrate photolysis with (a) different snowpacks, (b) different ozone column depths, and (c) snowpack depths. The importance of hydrogen peroxide photolysis over nitrate photolysis for hydroxyl radical production increases with increasing depth in snowpack, column ozone depth, and solar zenith angle. With a solar zenith angle of 60° the production of hydroxyl radical from hydrogen peroxide photolysis accounts for 91–99% of all hydroxyl radical production from hydrogen peroxide and nitrate photolysis.     

Estimation of night-time N2O5 concentrations from ambient NO2 and NO3 radical concentrations and the role of N2O5 in night-time chemistry

Dinitrogen pentoxide (N₂O₅), which is present in equilibrium with NO₃ radicals and NO₂, has been recognized for some time as an intermediate in the NO_x chemistry of night-time atmospheres. However, until the advent of long pathlength spectroscopic techniques for the measurement of atmospheric NO₃ radical concentrations, no reliable method for estimating N₂O₅ concentrations has been available. We have calculated maximum night-time N₂O₅ concentrations from the available experimentally determined concentrations of the NO₃ radical and NO₂ in the U.S. and Germany, and find that N₂O₅ concentrations as high as ~ 15 ppb can occur. We have also estimated removal rates for N₂O₅ and for NO₃ radicals during these nights. From data obtained under conditions devoid of point sources of NO_x, upper limit estimates of the homogeneous rate constant for the reaction of N₂O₅ with water vapor are obtained, leading to the conclusion that the homogeneous gas phase rate constant for this reaction is ⩽ 1 × 10⁻²¹ cm³ molecule⁻¹ s⁻¹ at 298 K, consistent with recent environmental chamber data.     

Formation of 9,10-phenanthrenequinone by atmospheric gas-phase reactions of phenanthrene

Phenanthrene is a 3-ring polycyclic aromatic hydrocarbon which exists mainly in the gas-phase in the atmosphere. Recent concern over the presence of 9,10-phenanthrenequinone in ambient particles led us to study the products of the gas-phase reactions of phenanthrene with hydroxyl radicals, nitrate radicals and ozone. The formation yields of 9,10-phenanthrenequinone were measured to be ∼3%, 33±9%, and ∼2% from the OH radical, NO₃ radical and O₃ reactions, respectively. Calculations suggest that daytime OH radical-initiated and nighttime NO₃ radical-initiated reactions of gas-phase phenanthrene may be significant sources of 9,10-phenanthrenequinone in ambient atmospheres. In contrast, the ozone reaction with phenanthrene is unlikely to contribute significantly to ambient 9,10-phenanthrenequinone.     

The wall as a source of hydroxyl radicals in smog chambers

The role of the wall of a smog chamber as a radical source has been investigated in several ways. From data in the literature evidence is obtained that HNO₃, present on the reactor wall, may react with NO in the gas phase according to the reaction HNO₃(wall) + 2NO + H₂O → 3HNO₂ to give nitrous acid. Nitrous acid may subsequently photolyze to give hydroxyl radicals.Experimental evidence about the occurrence of this reaction was obtained by u.v.-irradiation of propane-NO_x mixtures with and without NH₃. The presence of NH₃ resulted in a drastically reduced photochemical reactivity, suggesting that neutralization of nitric acid prevented the reaction of HNO₃ with NO. Inclusion of the reaction mentioned above into a computer-model gave a good agreement between experimental and calculated concentration profiles of NO, NO₂ and O₃ in experiments with CO-NO_x and propane-NO_x, mixtures. The results of our findings and those from others are discussed.     

Evidence for large average concentrations of the nitrate radical (NO3) in Western Europe from the HANSA hydrocarbon database

The nitrate radical (NO₃) was first measured in the atmosphere in the 1970s and suggestions were made that it could play a major role in oxidising many unsaturated hydrocarbons, such as those emitted from the biosphere. Analysis of the hydrocarbon mix over the North Atlantic Ocean suggested subsequently that the influence of NO₃ radical chemistry at night was even more extensive, being on a par with hydroxyl radical chemistry at some times of the year.The paper presents a detailed analysis of an extensive database of many nonmethane hydrocarbons collected at various sites around the North Sea in the mid 1990s during the HANSA project. By comparing the relative rates of oxidation of iso and normal pentane with that of toluene and benzene it clearly shows that the efficiency of NO₃ radical chemistry and hydroxyl radical chemistry over northwest Europe are similar in springtime and predicts an average nighttime NO₃ concentration of the order of 350 pptv, assuming an annual average OH concentration of 0.6×10⁶ cm⁻³. This value is very dependant on the average emission ratios of the different hydrocarbons and values between 200 and 600 pptv are possible. It is much larger than direct measurements made in Europe at the surface, but is of the same magnitude as concentrations measured recently from aircraft in the boundary layer over the northeast USA, and previously in vertical profiles by remote sounding over Europe.A simple analytical expression can be derived to calculate the NO₃ concentration at night with the only variables being ozone and the loss rate of N₂O₅, either to the ground or to aerosol surfaces. The concentrations of NO₃ calculated in this manner are similar to those derived from the analysis of the HANSA hydrocarbon database for typical conditions expected over Europe, but they are very dependant on the efficiency of the aerosol sink for N₂O₅.It is shown that NO₃ oxidation of many unsaturated hydrocarbons can indeed be more efficient than OH oxidation, especially at times of the year outside the summer season. Direct evidence for hydrocarbon oxidation by NO₃ radicals is shown by a series of peroxy radical measurements where the nighttime concentrations can be significantly higher than daytime concentrations in polluted air on occasion. Also the winter/summer (W/S) ratios of many unsaturated hydrocarbons are much lower than those expected from their removal purely by hydroxyl radical chemistry.The consequences of these findings are profound especially as satellite measurements of NO₂, a major precursor to NO₃, suggest that these high average concentrations of several hundred pptv could be widespread over most of the continents. This needs to be confirmed by direct in-situ measurement of nitrate radicals but it suggests a much larger role for NO₃ chemistry in the oxidation capacity of the atmosphere than realised hitherto.     

活性炭吸附和脱附-等离子体氧化净化有机废气的研究

根据滑动弧放电等离子体适于降解高浓度有机物废气的特性,结合活性炭吸附法,提出了吸附器的吸附浓缩和热脱附-等离子体氧化净化有机废气的方法。在活性炭吸附过程中,最初2 h内甲苯净化率达到100%,随着时间的增加净化率下降;在热脱附滑动弧放电等离子体净化过程中,甲苯降解效率最高为97.3%。将滑动弧放电等离子体反应器出口气相产物收集进行FT-IR检测,发现放电后有CO2、CO、H2O和NO2产生,并分析了甲苯的降解机理。     

Extent of H-atom abstraction from OH + p-cymene and upper limits to the formation of cresols from OH + m-xylene and OH + p-cymene

Aromatic hydrocarbons are important constituents of vehicle exhaust and of non-methane volatile organic compounds in ambient air in urban areas. It has recently been proposed that dealkylation is a significant pathway for the OH radical-initiated reactions, leading to the formation of phenolic compounds and/or oxepins (Noda, J., Volkamer, R., Molina, M.J., 2009. Dealkylation of alkylbenzenes: a significant pathway in the toluene, o-, m-, and p-xylene + OH reaction. Journal of Physical Chemistry A 113, 9658–9666.). We have investigated the formation of cresols from the reactions of OH radicals with m-xylene and p-cymene, and obtain upper limits of <1% for formation of each cresol isomer from OH + m-xylene and <2% for formation of each cresol isomer from OH + p-cymene. In addition, we have measured the formation yield of 4-methylacetophenone (the major product formed subsequent to H-atom abstraction from the CH(CH₃)₂ group) in the OH + p-cymene reaction to be 14.8 ± 3.2%, and estimate that H-atom abstraction from the CH₃ and CH(CH₃)₂ groups in p-cymene accounts for 20 ± 4% of the overall OH radical reaction. We also used a relative rate technique to measure the rate constant for the reaction of OH radicals with 4-methylacetophenone to be (4.50 ± 0.43) × 10^?12 cm³ molecule^?1 s^?1 at 297 ± 2 K.     

FT-IR product study on the photo-oxidation of dimethyl sulphide in the presence of NOx—temperature dependence

The products of the OH radical-initiated oxidation of dimethyl sulphide (DMS) have been investigated as a function of temperature (284, 295, and 306 K) and different initial NO_x (NO+NO₂) concentrations: initial NO was varied between 434 and 2821 ppb and NO₂ between 135 and 739 ppb. The experiments were performed at 1000 mbar total pressure in synthetic air using the photolysis of H₂O₂ as the OH-radical source and FT-IR spectroscopy to monitor reactants and products. The major sulphur-containing products identified were SO₂, dimethyl sulphoxide (DMSO), dimethyl sulphone (DMSO₂), methane sulphonic acid (MSA), methane sulphonyl peroxynitrate (MSPN) and OCS. The variation of the product yields with temperature and NO_x concentration are consistent with the occurrence of both addition and abstraction channels in OH radical-initiated oxidation of DMS. Distinct trends in the yields of the various products have been observed as a function of temperature, initial NO_x conditions and also reaction time as NO is consumed in the system. Increasing the initial NO concentration was found to depress the DMSO, SO₂ and OCS formation yields and enhance those of DMSO₂, MSA and MSPN. The yield–time behaviour of DMSO₂ is supportive of a formation mechanism involving addition of O₂ to a (CH₃)₂SOH adduct, formed via the addition channel, followed by sequential reactions with NO and O₂. The mechanisms controlling the concentration–time profiles of the individual products under the present experimental conditions are discussed in detail and consideration is given to possible implications for the photo-oxidation of DMS under ambient conditions.     

A kinetic mechanism for predicting secondary organic aerosol formation from toluene oxidation in the presence of NOx and natural sunlight

A kinetic mechanism to predict secondary organic aerosol (SOA) formation from the photo-oxidation of toluene was developed. Aerosol phase chemistry that includes nucleation, gas–particle partitioning and particle-phase reactions as well as the gas-phase chemistry of toluene and its degradation products were represented. The mechanism was evaluated against experimental data obtained from the University of North Carolina (UNC) 270 m³ dual outdoor aerosol smog chamber facility. The model adequately simulates the decay of toluene, the nitric oxide (NO) to nitrogen dioxide (NO₂) conversion and ozone formation. It also provides a reasonable prediction of SOA production under different conditions that range from 15 to 300 μg m⁻³. Speciation of simulated aerosol material shows that up to 70% of the aerosol mass comes from oligomers and polymers depending on initial reactant concentrations. The dominant particle-phase species predicted by the mechanism are glyoxal oligomers, ketene oligomers from the photolysis of the toluene OH reaction product 2-methyl-2,4-hexadienedial, organic nitrates, methyl nitro-phenol analogues, C7 organic peroxides, acylperoxy nitrates and for the low-concentration experiments, unsaturated hydroxy nitro acids.     

A laboratory study of the effect of NO2 on the atmospheric corrosion of zinc

Studies on the effect of NO_x on zinc corrosion are scarce and their results are variable and at times seemingly contradictory. This paper reports laboratory tests involving the dry deposition on zinc surfaces of 800 μg m⁻³ NO₂, alone and in combination with 800 μg m⁻³ SO₂, at temperatures of 35 and 25 °C and relative humidities of 90% and 70%. From the gravimetric results obtained and from the characterisation of the corrosion products by optical microscopy, scanning electron microscopy (SEM/EDX), grazing incidence X-ray diffraction (GIXD) and X-ray photoelectron spectroscopy (XPS), it has been verified that the corrosive action of NO₂ alone is negligible compared with SO₂. However, an accelerating effect has been observed when NO₂ acts in conjunction with SO₂ at 25 °C and 90% RH. At 35 °C and 90% RH, the accelerating effect is smaller, and at low relative humidities (70%), the synergistic effect is only slight, which suggests it may be favoured by the presence of moisture. In those cases where an accelerating effect has been observed, a greater proportion of sulphate ions has been found in the corrosion products, and nitrogen compounds have not been detected, indicating that NO₂ participates indirectly as a catalyst of the oxidation of SO₂ to sulphate.     

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.     

C2–C10 nonmethane hydrocarbons measured in Dallas,USA—Seasonal trends and diurnal characteristics

Nonmethane hydrocarbons (NMHCs) are important precursors of ozone and other photo oxidants. We presented continuous hourly average concentrations of 45 C₂–C₁₀ NMHCs measured in urban area of Dallas, USA from 1996 to 2004. Most of the selected compounds are good variables with less noise. The top 10 species with high ozone-generating potential were identified according to their concentrations and reactivities. The ambient concentrations of abundant anthropogenic emission hydrocarbons measured in Dallas were about 2–4 times of the background values measured in the remote areas with adjacent latitude. The time series for anthropogenic emission hydrocarbons showed an obvious seasonal cycle with relatively high concentration in winter and low concentration in summer. The sinusoidal function with a linearly decreasing factor could well fit the time series of NMHCs. The phase of seasonal cycle for the aromatic hydrocarbons of toluene, m/p xylene and o-xylene that might come from both vehicle emission and solvent utilities evaporation was about 1 month earlier than that for alkanes and alkenes that mainly came from vehicle emission. Ambient NMHCs in Dallas decreased with a stable rate during 1996–2004. For most of compounds with high ozone-generating potential, the rate of ambient concentration decrease was higher or much higher than the rate of volatile organic compounds (VOCs) source emission reduction estimated by EPA's National Emission Inventory. On weekdays, the morning hydrocarbon concentration peak was coincident with morning traffic rush time in Dallas. Another concentration peak was delayed to afternoon traffic rush time. The characteristics of VOCs sources, photochemical removal processes and atmospheric dilution could be interpreted by the diurnal variations of benzene/ethylbenzene (B/E), toluene/ethylbenzene (T/E) and xylene/ethylbenzene (X/E). The ratio of VOC/NO_x measured in Dallas was substantially smaller than that calculated for USA cities. Ozone formation in Dallas was VOC sensitive.     

Laboratory studies of the impact of aerosol composition on the heterogeneous oxidation of 1-propanol

The effects of (NH₄)₂SO₄, NH₄NO₃, NaCl, NH₄Cl, and Na₂SO₄ aerosols on the kinetics of 1-propanol oxidation in the presence of the hydroxyl radical have been investigated using the relative rate technique. p-Xylene was used as a reference compound. Two different aerosol concentrations that are typical of polluted urban conditions were tested. The total surface areas of aerosols were 1400 (condition I) and 3400 μm² cm⁻³ (condition II). Results indicate that aerosols promote the oxidation of 1-propanol, and the extents of the promoting effects depend on the aerosol composition and concentration. Increases in the relative rates of the 1-propanol/OH reaction vs. the p-xylene/OH reaction were only observed for (NH₄)₂SO₄ aerosol conditions I and II, NH₄NO₃ aerosol condition II, and NH₄Cl aerosol condition II. These results indicate that NH₄⁺ is the species promoting the oxidation of 1-propanol, and suggests the possibility of a strong interaction between NH₄⁺ and 1-propanol that can change the activation energy of the initial OH attack. These results have profound implications on the use of air quality models for the assessment of air pollution control strategies.     

Evaluation of the SAPRC-07 mechanism against CSIRO smog chamber data

The updated SAPRC-07 mechanism was evaluated against data from experiments performed in the CSIRO smog chamber. The mechanism predictions have been compared to experimental results as well as predictions by SAPRC-99.Experiments were performed using either toluene or m-xylene in the presence of NO_x at sub-0.1 ppmv concentrations. For the majority of m-xylene experiments, the modelled Δ(O₃–NO) concentration was within 20% of observed values for both SAPRC mechanisms. However during the oxidation of toluene the production of radicals was poorly predicted, with final Δ(O₃–NO) concentration under-predicted by up to 60%. The predictions of major oxidants from isoprene oxidation were in good agreement with observed values. For the NO_x-limited conditions however, the ozone concentration predicted by both mechanisms were under-predicted by approximately 20% in the five experiments tested.The performance of the SAPRC-07 mechanism was also evaluated against twelve evaporated fuel experiments. Two types of evaporative mode experiments were performed: headspace evaporated fuel and wholly evaporated fuel. The major difference was a significantly higher concentration of aromatic hydrocarbons and larger alkane products in wholly evaporated fuels. For headspace evaporated fuel experiments both SAPRC mechanisms were in good agreement with experimental results. For wholly evaporated experiments the average Δ(O₃–NO) model error was ?25% with SAPRC-07 compared to less than ?5% for SAPRC-99. Updates to the photolysis data for dicarbonyls, the light source used and the experimental conditions under which these experiments were performed are possible causes for the discrepancy between SAPRC-99 and -07 predictions for wholly evaporated experiments.     

Effects of uncertainty in the reaction of the hydroxyl radical with nitrogen dioxide on model-simulated ozone control strategies

We evaluated the effect of a 20% reduction in the rate constant of the reaction of the hydroxyl radical with nitrogen dioxide to produce nitric acid (OH+NO₂→HNO₃) on model predictions of ozone mixing ratios ([O₃]) and the effectiveness of reductions in emissions of volatile organic compounds (VOC) and nitrogen oxides (NO_x) for reducing [O₃]. By comparing a model simulation with the new rate constant to a base case scenario, we found that the [O₃] increase was between 2 and 6% for typical rural conditions and between 6 and 16% for typical urban conditions. The increases in [O₃] were less than proportional to the reduction in the OH+NO₂ rate constant because of negative feedbacks in the photochemical mechanism. Next, we used two different approaches to evaluate how the new OH+NO₂ rate constant changed the effectiveness of reductions in emissions of VOC and NO_x: first, we evaluated the effect on [O₃] sensitivity to small changes in emissions of VOC (d[O₃]/dE_VOC) and NO_x (d[O₃]/dE_NOx); and secondly, we used the empirical kinetic modeling approach to evaluate the effect on the level of emissions reduction necessary to reduce [O₃] to a specified level. Both methods showed that reducing the OH+NO₂ rate constant caused control strategies for VOC to become less effective relative to NO_x control strategies. We found, however, that d[O₃]/dE_VOC and d[O₃]/dE_NOx did not quantitatively predict the magnitude of the change in the control strategy because the [O₃] response was nonlinear with respect to the size of the emissions reduction. We conclude that model sensitivity analyses calculated using small emissions changes do not accurately characterize the effect of uncertainty in model inputs (in this case, the OH+NO₂ rate constant) on O₃ attainment strategies. Instead, the effects of changes in model inputs should be studied using large changes in precursor emissions to approximate realistic attainment scenarios.