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.     

Development of a condensed SAPRC-07 chemical mechanism

The development of a condensed version of the SAPRC-07 mechanism, designated CS07A, is described. It is comparable in size to CB05 and was derived directly from detailed SAPRC-07, which serves as the basis for its predictive capability and evaluation against chamber data. It incorporates the more condensed and approximate peroxy radical lumped operator method employed in SAPRC-99, and condensations involving removing or lumping less reactive compounds, lumping some product species in isoprene or aromatic mechanisms with other species with similar mechanisms using reactivity weighting, removing some compounds and reactions that are rapidly reversed, and using fewer model species to represent emitted alkanes and similar species. It gives predictions of O₃, total PANs and OH radicals that are very close to the standard SAPRC-07 mechanism for airshed models used as the starting point, but predicts about 15% more H₂O₂. Use of CS07A is suitable for models where the priority is O₃ formation, while the less condensed version should be used if more accurate hydroperoxide predictions are a priority.     

Modelling the photooxidation of ULP,E5 and E10 in the CSIRO smog chamber

The photooxidation of fuel vapour was investigated in a smog chamber and simulated using three chemical mechanisms, the Master Chemical Mechanism (MCMv3.1), SAPRC-99 and the Carbon Bond chemical mechanism (CB05). Three varieties of fuel were used, unleaded petrol (ULP) and two ULP-ethanol blends which contained 5% and 10% ethanol (E5, E10). The fuel vapours were introduced into the chamber using two methods, by injecting the vapours from wholly evaporated fuel directly, and by injecting the headspace vapour from fuel equilibrated at 38 °C. The chamber experiments were simulated using the selected mechanisms and comparisons made with collected experimental data.The SAPRC-99 mechanism reproduced Δ(O₃–NO) more accurately for almost all fuel types and injection modes, with negligible model error for both injection modes. The average model error for MCM simulations was ?16% and for CB05 the average model error was ?34%. The predictions for the CB05 mechanism varied depending on injection mode, the Δ(O₃–NO) model error for wholly evaporated experiments was ?44%, compared to ?24% for headspace vapour experiments. The difference in aromatic content between experiments of different injection modes was likely to be the cause of the difference in model error for CB05. The model error for all headspace experiments was dependent upon the initial carbon monoxide concentrations.The results for Δ(O₃–NO) were matched by the prediction of other key products, with formaldehyde predicted to within 20% by both SAPRC and the MCM. The addition of ethanol to the base SAPRC mechanism altered the predictions of Δ(O₃–NO) by less than 2%. Changes observed in the concentrations of formaldehyde and acetaldehyde were consistent with the expected yields from ethanol oxidation.     

The impact of day-old dilute smog on fresh smog systems: An outdoor chamber study

The effect of dilute day-old smog on fresh smog systems was studied in the UNC dual outdoor chamber. Smog systems which contained fresh smog precursors plus dilute residuals from the day before (0.04 ppm O₃, 0.1–0.3 ppm NMHC and 0.1 ppmC aldehydes) produced almost twice the ozone as fresh systems which had the same initial NMHC and NO_x injections. It is hypothesized that as much as 15–25% of this O₃ increase is due to residual first day aldehydes and that the remaining increase results from an increased morning percentage of NO₂ and the extra residual paraffinic hydrocarbons. The combined interactive effect of changes in initial aldehydes to NO₂ and aged paraffins appears to be greater than if each of these are considered separately.     

Modeling smog chamber measurements of incremental reactivities of volatile organic compounds

A series of experiments performed at the GM chamber facility provided useful data for the evaluation of two current chemical mechanisms used in airshed models (SAPRC97 and SAPRC93 mechanisms) and a test of their predictions of maximum incremental reactivities which describe the change in ozone caused by adding a small amount of a compound to a polluted urban mixture under high-NO_x conditions. In general, the SAPRC97 detailed mechanism performed well in simulating the volatile organic compound (VOC) reactivity experiments for most test species; however, it had a tendency to underpredict incremental reactivities. For base-case runs containing a nine-component urban-surrogate mixture under high-NO_x conditions, where maximum concentrations of either O₃ or the smog produced (SP=the initial NO oxidized plus the ozone produced) were not attained during a 12-h irradiation, the SAPRC97 performed well while the SAPRC93 underestimated SP or O₃ significantly. Under low-NO_x conditions where SP or O₃ maximums were attained, the SAPRC97 as well as the SAPRC93 underpredicted SP or O₃ for runs containing the urban-surrogate mixture. Simulations of incremental reactivity experiments and special chamber runs showed that the SAPRC97 mechanism performed poorly for n-octane and some aromatic isomers such as ethylbenzene and p-xylene, while it performed well for other aromatic isomers such as toluene, m-xylene and 1,3,5-trimethylbenzene. Although, additional chamber data for aromatic isomers is needed to further clarify the parameterized chemical mechanisms for aromatic isomers, the newer SAPRC97 mechanism appears to be much improved over the older SAPRC93 mechanism for simulating aromatic chemistry.     

A mechanism describing the photochemical oxidation of toluene in smog

The photo-oxidation of toluene/NO_x exhibits several features that distinguish it from olefin and paraffin smog systems: highly photolytic products, a relatively low production rate of peroxy radicals, and strong sinks for NO_x. The underlying chemical behavior of the toluene smog system is discussed and a kinetic simulation mechanism is presented. The mechanism simulates toluene smog chamber experiments conducted at two facilities: the University of California at Riverside evacuable chamber, and the outdoor smog chamber at the University of North Carolina.     

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.     

曝气沉砂池除砂机理分析

针对曝气沉砂池小试装置模型,通过配砂实验对沉砂池除砂率进行统计分析,研究曝气强度与HRT对沉砂池除砂率的影响。实验结果表明,不同曝气强度下,随着HRT变化,除砂率逐渐上升,并最终趋于平衡状态;不同HRT下,曝气强度与除砂率近似呈现一定的线性关系,且随着曝气强度的增大,除砂率降低;在HRT小于1min时,砂粒的运行路径和沉降时间缩短,不利于砂粒的去除;在HRT大于1min时,其中1、3和4min,随着气水比的增加除砂率降低幅度相对较小,曝气所引起的横向环流在一定程度上有利于砂粒的沉降,而HRT为2min时,除砂率降幅却很大,这与砂粒在曝气沉砂池池体断面分布有关,即断面处旋转流速和水平流速的大小变化将影响砂粒的运动;相比曝气强度,HRT对除砂率的影响较大。     

Evaluation of trajectories calculated from ecmwf data against constant volume balloon flights during etex

This paper validates trajectories calculated from ECMWF analyses against the tracks of constant volume balloons (CVBs) released during the European tracer experiment (ETEX). The altitudes of the calculated trajectories were adjusted to the altitudes of the respective balloons in short intervals to allow direct comparisons. The agreement between the calculated trajectories and the balloon tracks was very good for the first experiment (individual errors from 1 to 26%, average 15%), and excellent (errors from 2 to 11%, average 6%) for the second one. The agreement for the second experiment was probably partly better because the CVBs travelled above the planetary boundary layer, but the small errors also indicate that the ECMWF fields of the horizontal wind were of exceptionally good quality in the second experiment. This is in sharp contrast to the results of the dispersion models which all failed in the prediction of the perfluorocarbon tracer dispersion for the second experiment. A likely explanation for this is that vertical motions, possibly on small scales, were not correctly captured by the ECMWF analyses, but it is not possible to clarify this with the CVB data.     

凝并过程对亚微米灰霾颗粒中痕量重金属分布的影响

灰霾颗粒中痕量重金属物质对人体健康的危害,不仅仅与其重金属总含量有关,在很大程度上,与痕量重金属的尺度及其分布有直接的关联。为了研究凝并（coagulation）过程对不同初始条件的灰霾颗粒中重金属物质分布的影响,采用多重蒙特卡罗（multi-Monte Carlo,MMC）方法,模拟研究了亚微米灰霾颗粒中痕量重金属元素的分布规律。模拟发现,基于连续区布朗凝并机理下,在经过一段时间演化后,低初始数浓度的灰霾颗粒类中重金属物质质量百分比趋于均一,这表明凝并过程去除了初始重金属物质在灰霾颗粒中的富集效果;而在高初始数浓度灰霾颗粒类中重金属物质分布满足幂律关系。而且,进一步分析发现,在低初始数浓度灰霾颗粒类中重金属物质尺度分布趋于正态分布。     

Evaluation of total volatile organic compound emissions from adhesives based on chamber tests

In 1997, Homeswest in western Australia and Murdoch University developed a project to construct low-allergen houses (LAHs) in a newly developed suburb. Before the construction of LAHs, all potential volatile organic compound (VOC) emission materials used in LAHs are required to be measured to ensure that they are low total VOC (TVOC) emission materials. This program was developed based on this purpose. In recent times, the number of complaints about indoor air pollution caused by VOCs has increased. A number of surveys of indoor VOCs have indicated that many indoor materials contribute to indoor air pollution. Although some studies have been conducted on the characteristics of VOC emissions from adhesives, most of them were focused on VOC emissions from floor adhesives. Few measurements of VOC emissions from adhesives used for wood, fabrics, and leather are available. Furthermore, most research on VOC emissions from adhesives has been done in countries with cool climates, where ventilation rates in the indoor environment are lower than those in Mediterranean climates, due to energy conservation. VOCs emitted from adhesives have not been sufficiently researched to prepare an emission inventory to predict indoor air quality and to determine both exposure levels for the Australian population and the most appropriate strategies to reduce exposure. An environmental test chamber with controlled temperature, relative humidity, and airflow rate was used to evaluate emissions of TVOCs from three adhesives used frequently in Australia. The quantity of TVOC emissions was measured by a gas chromatography/flame ionization detector. The primary VOCs emitted from each adhesive were detected by gas chromatography/mass spectrometry. The temporal change of TVOC concentrations emitted from each adhesive was tested. A double-exponential equation was then developed to evaluate the characteristics of TVOC emissions from these three adhesives. With this double-exponential model, the physical processes of TVOC emissions can be explained, and a variety of emission parameters can be calculated. These emission parameters could be used to estimate real indoor TVOC concentrations in Mediterranean climates.     

Evaluation of the CAR-FMI model against measurements near a major road

A field measurement campaign was conducted near a major road in southern Finland from September 15 to October 30, 1995. The concentrations of NO, NO₂ and O₃ were measured simultaneously at three locations, at three heights (3.5, 6 and 10 m) on both sides of the road. Traffic densities and relevant meteorological parameters were also measured on-site. We have compared measured concentration data with the predictions of the road network dispersion model CAR-FMI, used in combination with a meteorological pre-processing model MPP-FMI. In comparison with corresponding results presented previously in the literature, the agreement of measured and predicted datasets was good, as measured using various statistical parameters. For all data (N=587), the index of agreement (IA) was 0.83, 0.82 and 0.89 for the measurements of NO_x, NO₂ and O₃, respectively. The IA is a statistical measure of the correlation of the predicted and measured time series of concentrations. However, the modelling system overpredicts NO_x concentrations with a fractional bias FB=+13%, and O₃ concentrations with FB=+8%, while for NO₂ concentrations FB=−2%. We also analyzed the difference between model predictions and measured data in terms of meteorological parameters. Model performance clearly deteriorated as the wind direction approached a direction parallel to the road, and for the lowest wind speeds. The range of variability concerning atmospheric stability, ambient temperature and the amount of solar radiation was modest during the measurement campaign. As expected, no clear dependencies of model performance were therefore detected in terms of these parameters. The experimental dataset is available for the evaluation of other roadside dispersion models.     

Evaluation of a Gaussian and a Lagrangian model against a roadside data set,with emphasis on low wind speed conditions

The evaluation of the high percentiles of concentration distributions is required by most national air quality guidelines, as well as the EU directives. However, it is problematic to compute such high percentiles in stable, low wind speed or calm conditions. This study utilizes the results of a previous measurement campaign near a major road at Elimäki in southern Finland in 1995, a campaign specifically designed for model evaluation purposes. In this study, numerical simulations were performed with a Gaussian finite line source dispersion model CAR-FMI and a Lagrangian dispersion model GRAL, and model predictions were compared with the field measurements. In comparison with corresponding results presented previously in the literature, the agreement of measured and predicted data sets was good for both models considered, as measured using various statistical parameters. For instance, considering all NO_x data (N=587), the so-called index of agreement values varied from 0.76 to 0.87 and from 0.81 to 1.00 for the CAR-FMI and GRAL models, respectively. The CAR-FMI model tends to slightly overestimate the NO_x concentrations (fractional bias FB=+14%), while the GRAL model has a tendency to underestimate NO_x concentrations (FB=−16%). The GRAL model provides special treatment to account for enhanced horizontal dispersion in low wind speed conditions; while such adjustments have not been included in the CAR-FMI model. This type of Lagrangian model therefore predicts lower concentrations, in conditions of low wind speeds and stable stratification, in comparison with a standard Lagrangian model. In low wind speed conditions the meandering of the flow can be quite significant, leading to enhanced horizontal dispersion. We also analyzed the difference between the model predictions and measured data in terms of the wind speed and direction. The performance of the CAR-FMI model deteriorated as the wind direction approached a direction parallel to the road, and for the lowest wind speeds. However, the performance of the GRAL model varied less with wind speed and direction; the model simulated better the cases of low wind speed and those with the wind nearly parallel to the road.     

Validation of the lagrangian particle dispersion model FLEXPART against large-scale tracer experiment data

A comprehensive validation of FLEXPART, a recently developed Lagrangian particle dispersion model based on meteorological data from the European Centre for Medium-Range Weather Forecasts, is described in this paper. Measurement data from three large-scale tracer experiments, the Cross-Appalachian Tracer Experiment (CAPTEX), the Across North America Tracer Experiment (ANATEX) and the European Tracer Experiment (ETEX) are used for this purpose. The evaluation is based entirely on comparisons of model results and measurements paired in space and time. It is found that some of the statistical parameters often used for model validation are extremely sensitive to small measurement errors and should not be used in future studies. 40 cases of tracer dispersion are studied, allowing a validation of the model performance under a variety of different meteorological conditions. The model usually performs very well under undisturbed meteorological conditions, but it is less skilful in the presence of fronts. The two ETEX cases reveal the full range of the model’s skill, with the first one being among the best cases studied, and the second one being, by far, the worst. The model performance in terms of the statistical parameters used stays rather constant with time over the periods (up to 117 h) studied here. It is shown that the method used to estimate the concentrations at the receptor locations has a significant effect on the evaluation results. The vertical wind component sometimes has a large influence on the model results, but on the average only a slight improvement over simulations which neglect the vertical wind can be demonstrated. Subgrid variability of mixing heights is important and must be accounted for.     

A numerical analysis of the combined open-top chamber data from the USA and Europe on ambient ozone and negative crop responses

Statistical analysis was performed using selected sets of combined data from the US National Crop Loss Assessment Network and the European Open-Top Chambers Programme to examine the relationships between the occurrences of hourly ambient ozone (O3) concentrations and adverse crop yield responses. The results suggest that the frequency of occurrences of relatively low hourly O3 concentrations ( approximately <35 ppb) are not as important as moderate to higher concentrations in eliciting negative crop biomass responses. They also suggest that daily peak (highest) hourly O3 values ( approximately >90 ppb) may not be as critical, most likely because they frequently do not occur during time periods when conditions that promote atmospheric conductivity (O3 deposition) and plant uptake (O3 absorption) are in coherence.     

Anatomy of a Los Angeles smog episode: Pollutant transport in the daytime sea breeze regime

The three-dimensional distribution of aerosols and trace gases in the Los Angeles air basin was mapped out during a smoggy day by instrumented aircraft. Strong vertical and horizontal gradients were observed in the concentrations of both primary and secondary pollutants.The day began with much of the basin occupied by polluted air carried over from the day before. New emissions accumulated in the stagnant air until late morning, when a well-organized sea breeze developed at the surface. Onshore flow during the afternoon carried heavily polluted air into inland receptor areas, ahead of a shallow, well-defined, advancing layer of cleaner marine air. The highest ozone concentrations of the day were observed just above the marine layer in stagnant air decoupled from the surface and just ahead of the marine front in photochemically aged air transported into low emission density areas.The data show that air pollution in the basin is a regional problem and that emissions in the western portion of the basin can result in high concentrations of secondary pollutants (e.g. > 0.25 ppm ozone) in areas over 50 km downwind. Layers of well aged pollutants are also shown to occur aloft. These layers can remain overnight and be re-entrained the next day by a deepening mixing layer, contributing to surface concentrations.