Comparative analysis of chemical reaction mechanisms for photochemical smog—II. Sensitivity of EKMA to chemical mechanism and input parameters

The six chemical reaction mechanisms for photochemical smog described in Pan I (Leone and Seinfeld, 1985, Atmospheric Environment 19,437–464) were used to study the effect of input parameters on volatile organic compound (VOC) control requirements needed to meet the National Ambient Air Quality Standard for ozone. The parameters studied were initial VOC composition, dilution rate, post 8-a.m. emissions, base case (present day) O₃ levels, entrainment from aloft of VOC and ozone and initial VOC/NO_x ratio. The Empirical Kinetic Modeling Approach (EKMA) was used to generate ozone isopleths for each chemical mechanism. The VOC control needed to reduce the maximum ozone concentration from some present day value to 0.12 ppm, assuming no NO_x control and a specified initial VOC/NO_x ratio, was calculated using the six chemical reaction mechanisms. The initial VOC/NO_x ratio was found to have the largest effect of all the parameters studied on VOC control requirements. Choice of chemical mechanism, ozone and VOC entrainmem from aloft, base-case ozone and the composition of the initial VOC mixture also had a large effect on predicted control requirements. To reduce the degree of uncertainty in control predictions using EKMA it is necessary to establish as accurately as possible the composition of urban air in early morning. Also, because of the substantial effect the choice of chemical mechanism has on the predicted control requirements using EKMA, it is important that future work continues to be directed toward evaluating candidate chemical mechanisms with respect to their ability to simulate atmospheric smog chemistry.     

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.     

The relation between ozone,NOx and hydrocarbons in urban and polluted rural environments

Research over the past ten years has created a more detailed and coherent view of the relation between O₃ and its major anthropogenic precursors, volatile organic compounds (VOC) and oxides of nitrogen (NO_x). This article presents a review of insights derived from photochemical models and field measurements. The ozone–precursor relationship can be understood in terms of a fundamental split into a NO_x-senstive and VOC-sensitive (or NO_x-saturated) chemical regimes. These regimes are associated with the chemistry of odd hydrogen radicals and appear in different forms in studies of urbanized regions, power plant plumes and the remote troposphere. Factors that affect the split into NO_x-sensitive and VOC-sensitive chemistry include: VOC/NO_x ratios, VOC reactivity, biogenic hydrocarbons, photochemical aging, and rates of meteorological dispersion. Analyses of ozone–NO_x–VOC sensitivity from 3D photochemical models show a consistent pattern, but predictions for the impact of reduced NO_x and VOC in indivdual locations are often very uncertain. This uncertainty can be identified by comparing predictions from different model scenarios that reflect uncertainties in meteorology, anthropogenic and biogenic emissions. Several observation-based approaches have been proposed that seek to evaluate ozone–NO_x–VOC sensitivity directly from ambient measurements (including ambient VOC, reactive nitrogen, and peroxides). Observation-based approaches have also been used to evaluate emission rates, ozone production efficiency, and removal rates of chemically active species. Use of these methods in combination with models can significantly reduce the uncertainty associated with model predictions.     

Modeling alkene chemistry using condensed mechanisms for conditions relevant to southeast Texas,USA

Alkenes are important in photochemical smog formation in southeast Texas due to their high emissions, especially from industrial sources in and around Houston, and their high reactivities. Therefore, properly characterizing the chemistry of alkenes in condensed mechanisms used in regional photochemical models is important in understanding the formation of ozone and other photochemical air pollutants in Houston. The performance of three versions of the SAPRC condensed chemical mechanism family, for predicting ozone and radical formation, was compared. Simulations were compared to environmental chamber data and ambient data. The analyses showed that separately modeling individual alkenes reactions (especially propene for southeast Texas) has the potential to lead to more accurate simulations of alkene chemistry. Caution must be exercised in un-lumping, however. Testing with different formulations of the 1-butene + O₃ reaction demonstrated the complexity and interconnectedness in choices of stoichiometric parameters for un-lumped species and the extent to which lumped mechanisms are un-lumped.     

Factors influencing the ground level distribution of ozone in Europe

Ozone is a widely distributed pollutant in the atmospheric boundary layer over north west Europe. Three main sources have been identified: the stratosphere, the free troposphere and boundary layer photochemical production. The pattern of ground level ozone concentrations resulting from these three sources cannot be accurately specified. Ozone shows significant variations in space and time but because of the high cost of continuous monitoring equipment, spatial variations on a national and international basis have not been studied in detail. Variations in ozone concentrations at individual monitoring sites have been given a great deal of attention and experience gained from United Kingdom monitoring sites is described in some detail. The averaging time statistical model of Larsen is employed to relate the exposure levels measured over different averaging periods. Diurnal variations have a major influence on exposure levels at sites nominally exposed to the same regional ozone distribution. The physical and chemical mechanisms which give rise to diurnal variations are detailed so that sites can be screened for different diurnal behaviour characteristics.     

A comparison of measured and simulated ozone concentrations in the rural areas of the eastern United States during summer 1995

The recent regulatory actions toward a longer-term (i.e., 8-hr) average ozone standard have brought forth the potential for many rural areas in the eastern United States to be in noncompliance. However, since a majority of these rural areas have generally few sources of anthropogenic emissions, the measured ozone levels primarily reflect the effects of the transport of ozone and its precursor pollutants and natural emissions. While photochemical grid models have been applied to urban areas to develop ozone mitigation measures, these efforts have been limited to high ozone episode events only and do not adequately cover rural regions. In this study, we applied a photochemical modeling system, RAMS/UAM-V, to the eastern United States from June 1-August 31, 1995. The purpose of the study is to examine the predictive ability of the modeling system at rural monitoring stations that are part of the Clean Air Status Trends Network (CASTNet) and the Gaseous Pollutant Monitoring Program (GPMP). The results show that the measured daily 1-hr ozone maxima and the seasonal average of the daily 1-hr ozone maxima are in better agreement with the predictions of the modeling system than those for the daily 8-hr ozone maxima. Also, the response of the modeling system in reproducing the measured range of ozone levels over the diurnal cycle is poor, suggesting the need for improvement in the treatment of the physical and chemical processes of the modeling system during the nighttime and morning hours if it is to be used to address the 8-hr ozone standard.     

Photochemical ozone creation potentials (POCPs) for different emission sources of organic compounds under European conditions estimated with a Master Chemical Mechanism

Through the combined application of a speciated VOC emission inventory and an explicit chemical mechanism, a picture has been put together of the different contributions to photochemical ozone formation from 248 VOC emission source categories. The study has shown that the different VOC emission source categories show vastly different propensities for forming photochemical ozone as indexed by their photochemical ozone creation potentials (POCPs). POCPs range from close to zero for numerous processes, including halocarbon solvent usage, through to over 70 for diesel combustion and some reactive solvent and other product usage applications. The consequences of the large range in POCPs are highlighted for cost-effective VOC emission control strategies across north west Europe.     

A Comparison of Measured and Simulated Ozone Concentrations in the Rural Areas of the Eastern United States during Summer 1995

ABSTRACT

The recent regulatory actions toward a longer-term (i.e., 8-hr) average ozone standard have brought forth the potential for many rural areas in the eastern United States to be in noncompliance. However, since a majority of these rural areas have generally few sources of anthropogenic emissions, the measured ozone levels primarily reflect the effects of the transport of ozone and its precursor pollutants and natural emissions. While photochemical grid models have been applied to urban areas to develop ozone mitigation measures, these efforts have been limited to high ozone episode events only and do not adequately cover rural regions. In this study, we applied a photochemical modeling system, RAMS/UAM-V, to the eastern United States from June 1-August 31, 1995. The purpose of the study is to examine the predictive ability of the modeling system at rural monitoring stations that are part of the Clean Air Status Trends Network (CASTNet) and the Gaseous Pollutant Monitoring Program (GPMP).

The results show that the measured daily 1-hr ozone maxima and the seasonal average of the daily 1-hr ozone maxima are in better agreement with the predictions of the modeling system than those for the daily 8-hr ozone maxima. Also, the response of the modeling system in reproducing the measured range of ozone levels over the diurnal cycle is poor, suggesting the need for improvement in the treatment of the physical and chemical processes of the modeling system during the nighttime and morning hours if it is to be used to address the 8-hr ozone standard.     

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.     

Uncertainties in predicted ozone concentrations due to input uncertainties for the UAM-V photochemical grid model applied to the July 1995 OTAG domain

The photochemical grid model, UAM-V, has been used by regulatory agencies to make decisions concerning emissions controls, based on studies of the July 1995 ozone episode in the eastern US. The current research concerns the effect of the uncertainties in UAM-V input variables (emissions, initial and boundary conditions, meteorological variables, and chemical reactions) on the uncertainties in UAM-V ozone predictions. Uncertainties of 128 input variables have been estimated and most range from about 20% to a factor of two. 100 Monte Carlo runs, each with new resampled values of each of the 128 input variables, have been made for given sets of median emissions assumptions. Emphasis is on the maximum hourly-averaged ozone concentration during the 12–14 July 1995 period. The distribution function of the 100 Monte Carlo predicted domain-wide maximum ozone concentrations is consistently close to log-normal with a 95% uncertainty range extending over plus and minus a factor of about 1.6 from the median. Uncertainties in ozone predictions are found to be most strongly correlated with uncertainties in the NO₂ photolysis rate. Also important are wind speed and direction, relative humidity, cloud cover, and biogenic VOC emissions. Differences in median predicted maximum ozone concentrations for three alternate emissions control assumptions were investigated, with the result that (1) the suggested year-2007 emissions changes would likely be effective in reducing concentrations from those for the year-1995 actual emissions, that (2) an additional 50% NO_x emissions reductions would likely be effective in further reducing concentrations, and that (3) an additional 50% VOC emission reductions may not be effective in further reducing concentrations.     

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.     

Sensitivity of ozone to precursor emissions in urban Beijing with a Monte Carlo scheme

In order to understand the formation mechanisms of high surface ozone and identify the main contributor sources in Beijing, this study investigates the sensitivity of surface ozone to NO, NO₂ and nine types of NMVOC emissions during a photochemical smog episode. Monte Carlo sensitivity analysis scheme with fifty simulations is established based on the Nested Air Quality Prediction Model System (NAQPMS). At every simulation, each of the eleven precursor emissions is perturbed with a distinct set of perturbations. The sensitivities of ozone to emissions are identified by multiple linear regressions. The stability of sensitivity results is validated by two experiments with standard deviations of log-normal perturbations set as 30% and 50% respectively. The sensitivity results suggest that the current high surface ozone is strongly stimulated by NMVOC emissions. Among NMVOC emissions, formaldehyde, ethylene and olefins emissions present the greatest impacts on ozone. On the other hand, NOx emissions have a strong inhibitory effect on ozone formation, even after 50% NOx emission reduction. This indicates that the current ozone formation in Beijing is under NOx-saturated conditions. A transition of ozone formation is observed from NOx-saturated to NOx-limited sensitivity behavior with a 75% reduction of NOx emissions. This study gives the implication that abatement of the four NMVOC types mentioned above could be efficient on reducing the high levels of surface ozone in central urban Beijing, while inadequate abatement in NOx emissions probably induces reverse effects.     

Photochemical modelling of summer ozone episodes in the Greater Madrid area

The mesoscale meteorological model TVM, coupled to a photochemical/transport module in which different chemical mechanisms (RACM, EMEP) are implemented, has been evaluated. Field measurements and numerical results are used to determine the impact of the mesoscale flows on the photochemical smog episodes observed in the Greater Madrid Area for two selected days, characterised by the presence of a thermal low-pressure system over the Iberian Peninsula. During the 14 July 1992, the synoptic flow from the southeast favoured the transport of the precursors to the Guadarrama mountain range, where high concentrations of ozone were registered, exceeding the population information threshold. On the 15 July 1995, the synoptic wind from the northwest interacted with the local thermally driven flows, pushing the pollutants far away from the metropolitan area with the result that high ozone concentrations were measured to the east-southeast of the city.     

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.     

The photochemical pollution episode of 5–16 July 1983 in North-West England

Ground level ozone, NO_x and specific C₂-C₆ hydrocarbon measurements from a rural site in N-W England during a photochemical pollution episode are presented. Maximum hourly ozone concentrations exceeded 80 ppb for ten consecutive days with a maximum of 156 ppb. Mid-morning ozone concentrations were found to be indicative of the amount of ozone from continental sources. The air mass trajectories, total NMHC and alkane : alkene ratios all indicate that in the early to middle stages of the episode the air had been exposed to recent precursor emissions relative to more aged air before and after this period. The measurements are compared with the predictions of recent theoretical models of ozone formation over England.     

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.     

Analysis of photochemical pollution in summer and winter using a photochemical box model in the center of Tokyo, Japan.

In order to give an effective and rapid analysis of the photochemical pollution and information for emission control strategies, a photochemical box model (PBM) was applied to one moderate summer episode, 11 July 1996, and one typical winter episode, 3 December 1996, in the center of Tokyo, Japan. The box model gave a good prediction of the photochemical pollution with minimal investment. As expected, the peak ozone in summer is higher than in winter. The NOx concentrations in winter are higher than those in summer. In summer, NO and NO2 have one peak in the morning. In winter, NO and NO2 show two peaks during the day. Three model runs including no reactions, a zero ozone boundary condition and dark reactions were conducted to understand the photochemical processes. The effects of emission reduction on the formation of the photochemical pollution in the center of Tokyo have been studied. The results show that the reduction of NMHC emission can decrease the ozone, however, the reduction of NOx emission can increase the ozone. It can be concluded that if the NOx emission are reduced, the reduction of NMHC should be more emphasized in order to decrease the ozone concentration in the center of Tokyo, Japan, especially the reduction of the NMHC from stationary source emission.