An examination of oxidant amounts on secondary organic aerosol formation and aging

The effect of HO_x radicals (OH and HO₂) and ozone (O₃) on aerosol formation and aging has been studied. Experiments were performed in presence as well as in absence of oxygen in a flow-through chamber at 299 K for three organic precursor gases, isoprene, α-pinene and m-xylene. The HO_x source was the UV photolysis of humidified air or nitrogen and was measured with a GTHOS (Ground-based Tropospheric Hydrogen Oxides Sensor). The precursor gases concentration was monitored with an online GC-FID. The aerosol mass was then quantified by a Tapered Element Oscillating Microbalance (TEOM). Typical oxidant mixing ratios were (0–4.5) ppm for O₃, 200 pptv for OH and 3 ppbv for HO₂. A simple kinetics model is used to infer the aerosol production mechanism. In the present of O₃ (or O₂), the SOA yields were 0.46, 0.036 and 0.12 for α-pinene with an initial concentration of 100 ppbv (RH = 37%), isoprene with an initial concentration of 177 ppbv (RH = 50%) and m-xylene with an initial concentration of 100 ppbv (RH = 37%), respectively. When the chosen precursor gases reacted with HO_x in the absence of O₃, the maximum SOA yields were significantly increased by factors of 1.6 for isoprene 1.1 for α-pinene, and 3 for m-xylene respectively. The comparison of the calculated and measured potential aerosol mass concentrations as function of time shows that presence of ozone or oxygen can influence the aerosol yield and the absence of ozone or oxygen in the system resulted in high concentrations of its organic aerosol products.     

Condensed atmospheric photooxidation mechanisms for isoprene

Two condensed mechanisms for the atmospheric reactions of isoprene, which differ in the number of species used to represent isoprene's reactive products, have been developed for use in ambient air quality modehng. They are based on a detailed isoprene mechanism that has recently been developed and extensively evaluated against environmental chamber data. The new condensed mechanisms give very close predictions to those of the detailed mechanism for ozone, OH radicals, nitric acid, H₂O₂, formaldehyde, total PANS, and for incremental effects of isoprene on ozone formation in one day simulations. The effects of the condensations become somewhat greater in multi-day simulations, particularly in cases where NO₃ reactions are important at nighttime, but the ozone predictions are still very close. On the other hand, the SAPRC-90, RADM-2, and Carbon Bond IV isoprene mechanisms give quite different predictions of these quantities. It is recommended that the new mechanisms replace those currently used in airshed simulations where isoprene emissions are important.     

Isoprene above the Eastern Mediterranean: Seasonal variation and contribution to the oxidation capacity of the atmosphere

Isoprene is one of the most important biogenic volatile organic compounds with large terrestrial emissions and comparatively a small oceanic source on a global scale. This marine source seems to strongly depend on environmental parameters such as phytoplankton abundance, light, temperature, wind speed, and thus, to be highly variable. However, this source can consequently affect the chemistry of the marine boundary layer on a local or mesoscale. The present study investigates the factors that control isoprene levels and estimates the marine source of isoprene and its role in the oxidizing capacity of the atmosphere at a coastal site in the East Mediterranean. More than 2000 measurements of isoprene have been conducted at Finokalia sampling station on the island of Crete over an 8-month period from February to October 2004. Isoprene varies between 5 and 1200 pptv with the highest values observed in summer. The origin of the air masses determines the atmospheric abundance and the prevailing source of isoprene. According to chemical box model calculations, during daytime the isoprene observed under marine conditions is reducing hydroxyl (OH) and hydroperoxy (HO₂) radicals by up to 26% and 13%, respectively, whereas, it can increase the sum of peroxy radicals by a factor of 4. At night, isoprene of marine origin is depressing nitrate radicals by up to 25% and increases the low nighttime levels of OH and HO₂ radicals by up to 25% and 30%, respectively. The seawater emissions of isoprene in the area are estimated between 10⁸ and 6×10⁹ molecules cm⁻² s⁻¹ with a strong seasonal variability.     

Application of OMI observations to a space-based indicator of NOx and VOC controls on surface ozone formation

We investigated variations in the relative sensitivity of surface ozone formation in summer to precursor species concentrations of volatile organic compounds (VOCs) and nitrogen oxides (NO_x) as inferred from the ratio of the tropospheric columns of formaldehyde to nitrogen dioxide (the “Ratio”) from the Aura Ozone Monitoring Instrument (OMI). Our modeling study suggests that ozone formation decreases with reductions in VOCs at Ratios <1 and NO_x at Ratios >2; both NO_x and VOC reductions may decrease ozone formation for Ratios between 1 and 2. Using this criteria, the OMI data indicate that ozone formation became: 1. more sensitive to NO_x over most of the United States from 2005 to 2007 because of the substantial decrease in NO_x emissions, primarily from stationary sources, and the concomitant decrease in the tropospheric column of NO₂, and 2. more sensitive to NO_x with increasing temperature, in part because emissions of highly reactive, biogenic isoprene increase with temperature, thus increasing the total VOC reactivity. In cities with relatively low isoprene emissions (e.g., Chicago), the data clearly indicate that ozone formation became more sensitive to NO_x from 2005 to 2007. In cities with relatively high isoprene emissions (e.g., Atlanta), we found that the increase in the Ratio due to decreasing NO_x emissions was not obvious as this signal was convolved with variations in the Ratio associated with the temperature dependence of isoprene emissions and, consequently, the formaldehyde concentration.     

Development of the SAPRC-07 chemical mechanism

An updated version of the SAPRC-99 gas-phase atmospheric chemical mechanism, designated SAPRC-07, is described. The rate constants and reactions have been updated based on current data and evaluations, the aromatics mechanisms have been reformulated and are less parameterized, chlorine chemistry has been added, the method used to represent peroxy reactions has been reformulated to be more appropriate for modeling gas-phase secondary organic aerosol precursors, and representations for many types of VOCs have been added or improved. This mechanism was evaluated against the result of ～2400 environmental chamber experiments carried out in 11 different environmental chambers, including experiments to test mechanisms for over 110 types of VOCs. The performance in simulating the chamber data was generally satisfactory for most types of VOCs but some biases were seen in simulations of some types of experiments. The mechanism was used to derive updated MIR and other ozone reactivity scales for almost 1100 types of VOCs, though in most cases the changes in MIR values relative to SAPRC-99 were not large. This mechanism update results in somewhat lower predictions of ozone in one-day ambient model scenarios under low VOC/NO_x conditions. The files needed to implement the mechanism and additional documentation is available at the SAPRC mechanism web site at http://www.cert.ucr.edu/～carter/SAPRC.     

A comparison of chemical mechanisms based on TRAMP-2006 field data

A comparison of a model using five widely known mechanisms (RACM, CB05, LaRC, SAPRC-99, SAPRC-07, and MCMv3.1) has been conducted based on the TexAQS II Radical and Aerosol Measurement Project (TRAMP-2006) field data in 2006. The concentrations of hydroxyl (OH) and hydroperoxy (HO₂) radicals were calculated by a zero-dimensional box model with each mechanism and then compared with the OH and HO₂ measurements. The OH and HO₂ calculated by the model with different mechanisms show similarities and differences with each other and with the measurements. First, measured OH and HO₂ are generally greater than modeled for all mechanisms, with the median modeled-to-measured ratios ranging from about 0.8 (CB05) to about 0.6 (SAPRC-99). These differences indicate that either measurement errors, the effects of unmeasured species or chemistry errors in the model or the mechanisms, with some errors being independent of the mechanism used. Second, the modeled and measured ratios of HO₂/OH agree when NO is about 1 ppbv, but the modeled ratio is too high when NO was less and too low when NO is more, as seen in previous studies. Third, mechanism–mechanism HO_x differences are sensitive to the environmental conditions – in more polluted conditions, the mechanism–mechanism differences are less. This result suggests that, in polluted conditions, the mechanistic details are less important than in cleaner conditions, probably because of the dominance of reactive nitrogen chemistry under polluted conditions.     

Factors influencing ozone chemistry in subsonic aircraft plumes

This study investigates several factors that could influence ozone chemistry occurring in subsonic aircraft plumes in the upper troposphere. The study focuses on uncertainties in gas-phase rate parameters, but also examines the influence of selected heterogeneous reactions, the rate of expansion of the plume, ambient and initial plume concentrations, and the time of emissions. Monte Carlo analysis with Latin hypercube sampling was applied to an expanding box model of an aircraft plume, in order to estimate the sensitivities of O₃ perturbations (ΔO₃) to uncertainties in rate constants in the RADM2 chemical mechanism. The resulting coefficient of variation in ΔO₃ at the end of a 36 h simulation was about 50%. Influential uncertainties in gas-phase rate parameters include those for photolysis of NO₂ and HCHO, O₃+NO, HO₂+NO, and formation of PAN and HNO₃. With high background concentrations of non-methane hydrocarbons, uncertainties in rate parameters of reactions involving peroxy radicals from ethene and propene oxidation were also influential. The coefficient of variation for ΔO₃ due to uncertainties in emission indices of NO_x, CO, and organic compounds was less than 15%. The effects of the heterogeneous reaction of N₂O₅ leading to HNO₃ formation, and hypothesized reactions of HNO₃ and NO₂ on soot, were also investigated. The results suggest that the latter two reactions could be influential for ΔO₃ if published estimates of reaction probabilities and high estimates of soot concentrations in plumes are realistic.     

Measurement and analysis of atmospheric concentrations of isoprene and its reaction products in central Texas

A field experiment was conducted in August 1998 to investigate the concentrations of isoprene and isoprene reaction products in the surface and mixed layers of the atmosphere in Central Texas. Measured near ground-level concentrations of isoprene ranged from 0.3 (lower limit of detection – LLD) to 10.2 ppbv in rural regions and from 0.3 to 6.0 ppbv in the Austin urban area. Rural ambient formaldehyde levels ranged from 0.4 ppbv (LLD) to 20.0 ppbv for 160 rural samples collected, while the observed range was smaller at Austin (0.4–3.4 ppbv) for a smaller set of samples (37 urban samples collected). Methacrolein levels did not vary as widely, with rural measurements from 0.1 ppbv (LLD) to 3.7 ppbv and urban concentrations varying between 0.2 and 5.7 ppbv. Isoprene flux measurements, calculated using a simple box model and measured mixed-layer isoprene concentrations, were in reasonable agreement with emission estimates based on local ground cover data. Ozone formation attributable to biogenic hydrocarbon oxidation was also calculated. The calculations indicated that if the ozone formation occurred at low VOC/NO_x ratios, up to 20 ppbv of ozone formed could be attributable to biogenic photooxidation. In contrast, if the biogenic hydrocarbon reaction products were formed under low NO_x conditions, ozone production attributable to biogenics oxidation would be as low as 1 ppbv. This variability in ozone formation potentials implies that biogenic emissions in rural areas will not lead to peak ozone levels in the absence of transport of NO_x from urban centers or large rural NO_x sources.     

Chemistry of HOx radicals in the upper troposphere

Aircraft observations from three recent missions (STRAT, SUCCESS, SONEX) are synthesized into a theoretical analysis of the factors controlling the concentrations of HO_x radicals (HO_x=OH+peroxy) and the larger reservoir family HO_y (HO_y=HO_x+2H₂O₂+2CH₃OOH+HNO₂+HNO₄) in the upper troposphere. Photochemical model calculations capture 66% of the variance of observed HO_x concentrations. Two master variables are found to determine the variance of the 24 h average HO_x concentrations: the primary HO_x production rate, P(HO_x), and the concentration of nitrogen oxide radicals (NO_x=NO+NO₂). We use these two variables as a coordinate system to diagnose the photochemistry of the upper troposphere and map the different chemical regimes. Primary HO_x production is dominated by the O(¹D)+H₂O reaction when [H₂O]>100 ppmv, and by photolysis of acetone (and possibly other convected HO_x precursors) under drier conditions. For the principally northern midlatitude conditions sampled by the aircraft missions, the HO_x yield from acetone photolysis ranges from 2 to 3. Methane oxidation amplifies the primary HO_x source by a factor of 1.1–1.9. Chemical cycling within the HO_x family has a chain length of 2.5–7, while cycling between the HO_x family and its HO_y reservoirs has a chain length of 1.6–2.2. The number of ozone molecules produced per HO_y molecule consumed ranges from 4 to 12, such that ozone production rates vary between 0.3 and 5 ppbv d⁻¹ in the upper troposphere. Three chemical regimes (NO_x-limited, transition, NO_x-saturated) are identified to describe the dependence of HO_x concentrations and ozone production rates on the two master variables P(HO_x) and [NO_x]. Simplified analytical expressions are derived to express these dependences as power laws for each regime. By applying an eigenlifetime analysis to the HO_x–NO_x–O₃ chemical system, we find that the decay of a perturbation to HO_y in the upper troposphere (as from deep convection) is represented by four dominant modes with the longest time scale being factors of 2–3 times longer than the steady-state lifetime of HO_y.     

Atmospheric oxidation capacity in the summer of Houston 2006: Comparison with summer measurements in other metropolitan studies

Both similarities and differences in summertime atmospheric photochemical oxidation appear in the comparison of four field studies: TEXAQS2000 (Houston, 2000), NYC2001 (New York City, 2001), MCMA2003 (Mexico City, 2003), and TRAMP2006 (Houston, 2006). The compared photochemical indicators are OH and HO₂ abundances, OH reactivity (the inverse of the OH lifetime), HO_x budget, OH chain length (ratio of OH cycling to OH loss), calculated ozone production, and ozone sensitivity. In terms of photochemical activity, Houston is much more like Mexico City than New York City. These relationships result from the ratio of volatile organic compounds (VOCs) to nitrogen oxides (NO_x), which are comparable in Houston and Mexico City, but much lower in New York City. Compared to New York City, Houston and Mexico City also have higher levels of OH and HO₂, longer OH chain lengths, a smaller contribution of reactions with NO_x to the OH reactivity, and NO_x-sensitivity for ozone production during the day. In all four studies, the photolysis of nitrous acid (HONO) and formaldehyde (HCHO) are significant, if not dominant, HO_x sources. A problematic result in all four studies is the greater OH production than OH loss during morning rush hour, even though OH production and loss are expected to always be in balance because of the short OH lifetime. The cause of this discrepancy is not understood, but may be related to the under-predicted HO₂ in high NO_x conditions, which could have implications for ozone production. Three photochemical indicators show particularly high photochemical activity in Houston during the TRAMP2006 study: the long portion of the day for which ozone production was NO_x-sensitive, the calculated ozone production rate that was second only to Mexico City's, and the OH chain length that was twice that of any other location. These results on photochemical activity provide additional support for regulatory actions to reduce reactive VOCs in Houston in order to reduce ozone and other pollutants.     

Intercomparison of chemical mechanisms for air quality policy formulation and assessment under North American conditions

The intercomparison of seven chemical mechanisms for their suitability for air quality policy formulation and assessment is described. Box modeling techniques were employed using 44 sets of background environmental conditions covering North America to constrain the chemical development of the longer lived species. The selected mechanisms were modified to enable an unbiased assessment of the adequacy of the parameterizations of photochemical ozone production from volatile organic compound (VOC) oxidation in the presence of NO_x. Photochemical ozone production rates responded differently to 30% NO_x and VOC reductions with the different mechanisms, despite the striking similarities between the base-case ozone production rates. The 30% reductions in NO_x and VOCs also produced changes in OH. The responses in OH to 30% reductions in NO_x and VOCs appeared to be more sensitive to mechanism choice, compared with the responses in the photochemical ozone production rates. Although 30% NO_x reductions generally led to decreases in OH, 30% reductions in VOCs led to increases in OH, irrespective of mechanism choice and background environmental conditions. The different mechanisms therefore gave different OH responses to NO_x and VOC reductions and so would give different responses in terms of changes in the fate and behavior of air toxics, acidification and eutrophication, and fine particle formation compared with others, in response to ozone control strategies. Policymakers need to understand that there are likely to be inherent differences in the responses to ozone control strategies between different mechanisms, depending on background environmental conditions and the extents of NO_x and VOC reductions under consideration.

Implications: The purpose of this paper is to compare predicted ozone responses to NO_x and VOC reductions with seven chemical mechanisms under North American conditions. The good agreement found between the tested mechanisms should provide some support for their application in the air quality models used for policymaking.     

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.     

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.     

Summertime photochemical ozone formation in Santiago,Chile

The city of Santiago, Chile experiences frequent high pollution episodes and as a consequence very high ozone concentrations, which are associated with health problems including increasing daily mortality and hospital admissions for respiratory illnesses. The development of ozone abatement strategies requires the determination of the potential of each pollutant to produce ozone, taking into account known mechanisms and chemical kinetics in addition to ambient atmospheric conditions. In this study, the photochemical formation of ozone during a summer campaign carried out from March 8–20, 2005 has been investigated using an urban photochemical box model based on the Master Chemical Mechanism (MCMv3.1). The MCM box model has been constrained with 10 min averages of simultaneous measurements of HONO, HCHO, CO, NO, j(O¹D), j(NO₂), 31 volatile organic compounds (VOCs) and meteorological parameters. The O₃–NO_x–VOC sensitivities have been determined by simulating ozone formation at different VOC and NO_x concentrations. Ozone sensitivity analyses showed that photochemical ozone formation is VOC-limited under average summertime conditions in Santiago. The results of the model simulations have been compared with a set of potential empirical indicator relationships including H₂O₂/HNO₃, HCHO/NO_y and O₃/NO_z. The ozone forming potential of each measured VOC has been determined using the MCM box model. The impacts of the above study on possible summertime ozone control strategies in Santiago are discussed.     

Photo-oxidant chemistry in the polluted boundary layer under changing UV-B radiation

UV-B radiation is a driving factor for the chemistry of the polluted boundary layer. It is involved in the formation of radicals and consequently influences the formation and concentration of photo-oxidants. The 3-D mesoscale photochemical Metphomod model was employed to study the effect of changes in UV-B radiation on the concentration of photo-oxidants in the boundary layer over the Swiss Plateau. The model chemistry is based on the RACM mechanism and a two-stream approximation of radiative transfer. A summer (July) and a late winter (February) episode were simulated. All simulations were replicated with relatively large changes in the prescribed total ozone. The results for an increase in UV-B radiation show increases in PAN, HNO₃, and ozone at noon in NO_x-rich areas and a decrease in NO_x. In NO_x-poor areas in summer the effect on ozone is weak and has a negative sign, the main effect being an increase in H₂O₂. The spatial variability of NO_x concentrations in the Swiss Plateau in the summer case is such that the effect of increased UV-B radiation on ozone is spatially variable. The effect on the ozone production rate in summer is strongest positive at the surface in the NO_x-rich regions in the morning and strongest negative at some altitude above ground in NO_x-poor regions in the early afternoon. In the winter episode, NO_x-rich conditions are found almost everywhere on the Swiss Plateau, the effect of increased UV-B radiation on the ozone production rate is positive all day long and is largest at 300 m above ground at noon. In this case, in contrast to the summer case, the increase in ozone is carried over to the next day. The model results for ozone are in good agreement with results from a case study and a time series analysis of surface ozone measurements. We estimate the effect of day-to-day changes in total ozone on surface ozone peaks to range from 4 to 6 ppb at most.     

Nocturnal NO3 radical chemistry in Houston,TX

Radical chemistry in the nocturnal urban boundary layer is dominated by the nitrate radical, NO₃, which oxidizes hydrocarbons and, through the aerosol uptake of N₂O₅, indirectly influences the nitrogen budget. The impact of NO₃ chemistry on polluted atmospheres and urban air quality is, however, not well understood, due to a lack of observations and the strong impact of vertical stability of the boundary layer, which makes nocturnal chemistry highly altitude dependent.Here we present long-path DOAS observations of the vertical distribution of the key nocturnal species O₃, NO₂, and NO₃ during the TRAMP experiment in Summer 2006 in Houston, TX. Our observations confirm the altitude dependence of nocturnal chemistry, which is reflected in the concentration profiles of all trace gases at night. In contrast to other study locations, NO₃ chemistry in Houston is dominated by industrial emissions of alkenes, in particular of isoprene, isobutene, and sporadically 1,3-butadiene, which are responsible for more than 70% of the nocturnal NO₃ loss. The nocturnally averaged loss of NO_x in the lowest 300 m of the Houston atmosphere is ～0.9 ppb h^?1, with little day-to-day variability. A comparison with the daytime NO_x loss shows that NO₃ chemistry is responsible for 16–50% of the NO_x loss in a 24-h period in the lowest 300 m of the atmosphere. The importance of the NO₃ + isoprene/1,3-butadiene reactions implies the efficient formation of organic nitrates and secondary organic aerosol at night in Houston.     

Nighttime variations in HO2 radical mixing ratios at Rishiri Island observed with elevated monoterpene mixing ratios

HO₂ radical concentrations were measured by a laser-induced fluorescence instrument for three nighttime periods during the intensive field campaign at Rishiri Island, Japan, in June 2000. The HO₂ mixing ratio had temporal variations around its average of 4.2±1.2 (1σ) pptv and showed a positive correlation with the summed mixing ratio of four monoterpene species, α-pinene, β-pinene, camphene, and limonene, that sometimes reached 1 ppbv. Our model calculations suggested that ozonolysis reactions of monoterpenes were the main source of nighttime radicals and they explained 58% of measured HO₂ concentration levels. The model roughly reproduced the dependence of the HO₂ mixing ratio on the square root of the radical production rate due to the ozonolysis reactions of the monoterpenes. However, the absolute HO₂ mixing ratio was significantly underpredicted by the model. We discuss possible reasons in terms of misunderstood RO₂ chemistry, RO₂ interference with HO₂ observations, unknown radical production process associated by high NO₂ mixing ratio, and the contribution of unmeasured olefinic species to radical production via their reactions with ozone.     

Development of a reduced speciated VOC degradation mechanism for use in ozone models

A reduced mechanism to describe the formation of ozone from VOC oxidation has been developed, using the master chemical mechanism (MCM v2) as a reference benchmark. The ‘common representative intermediates’ (CRI) mechanism treats the degradation of methane and 120 VOC using ca. 570 reactions of ca. 250 species (i.e. the emitted VOC plus an average of about one additional species per VOC). It thus contains only ca. 5% of the number of reactions and ca. 7% of the number of chemical species in MCM v2, providing a computationally economical alternative. The CRI mechanism contains a series of generic intermediate radicals and products, which mediate the breakdown of larger VOC into smaller fragments (e.g., formaldehyde), the chemistry of which is treated explicitly. A key assumption in the mechanism construction methodology is that the potential for ozone formation from a given VOC is related to the number of reactive (i.e., C–C and C–H) bonds it contains, and it is this quantity which forms the basis of the generic intermediate groupings. Following a small degree of optimisation, the CRI mechanism is shown to generate levels of ozone, OH, peroxy radicals, NO and NO₂ which are in excellent agreement with those calculated using MCM v2, in simulations using a photochemical trajectory model applied previously to simulation of episodic ozone formation. The same model is used to calculate photochemical ozone creation potentials for 63 alkanes, alkenes, carbonyls and alcohols using both mechanisms. Those determined with the CRI mechanism show a variation from compound to compound which is remarkably consistent with that calculated with the detailed chemistry in MCM v2. This suggests that the CRI mechanism construction methodology is able to capture both the salient features of the ozone formation process in general, and how this varies from one VOC to another.     

A new environmental chamber for evaluation of gas-phase chemical mechanisms and secondary aerosol formation

A new state-of-the-art indoor environmental chamber facility for the study of atmospheric processes leading to the formation of ozone and secondary organic aerosol (SOA) has been constructed and characterized. The chamber is designed for atmospheric chemical mechanism evaluation at low reactant concentrations under well-controlled environmental conditions. It consists of two collapsible 90 m³ FEP Teflon film reactors on pressure-controlled moveable frameworks inside a temperature-controlled enclosure flushed with purified air. Solar radiation is simulated with either a 200 kW Argon arc lamp or multiple blacklamps. Results of initial characterization experiments, all carried out at 300–305 K under dry conditions, concerning NO_x and formaldehyde offgasing, radical sources, particle loss rates, and background PM formation are described. Results of initial single organic–NO_x and simplified ambient surrogate–NO_x experiments to demonstrate the utility of the facility for mechanism evaluation under low NO_x conditions are summarized and compared with the predictions of the SAPRC-99 chemical mechanism. Overall, the results of the initial characterization and evaluation indicate that this new environmental chamber can provide high quality mechanism evaluation data for experiments with NO_x levels as low as 2 ppb, though the results indicate some problems with the gas-phase mechanism that need further study. Initial evaluation experiments for SOA formation, also carried out under dry conditions, indicate that the chamber can provide high quality secondary aerosol formation data at relatively low hydrocarbon concentrations.     

A diagnostic comparison of measured and model-predicted speciated VOC concentrations

This study compares speciated model-predicted concentrations (i.e., mixing ratios) of volatile organic compounds (VOCs) with measurements from the Photochemical Assessment Monitoring Stations (PAMS) network at sites within the northeastern US during June–August of 2006. Measurements of total non-methane organic compounds (NMOC), ozone (O₃), oxides of nitrogen (NO_x) and reactive nitrogen species (NO_y) are used for supporting analysis. The measured VOC species were grouped into the surrogate classes used by the Carbon Bond IV (CB4) chemical mechanism. It was found that the model typically over-predicted all the CB4 VOC species, except isoprene, which might be linked to overestimated emissions. Even with over-predictions in the CB4 VOC species, model performance for daily maximum O₃ was typically within ±15%. Analysis at an urban site in NY, where both NMOC and NO_x data were available, suggested that the reasonable ozone performance may be possibly due to compensating overestimated NO_x concentrations, thus modulating the NMOC/NO_x ratio to be in similar ranges as that of observations.