Environmental Simulation Chambers: Application to Atmospheric Chemical Processes

Photoreactors have been used for quite some time by scientists to study organic photochemistry (Bayes, Blacet, Calvert, Gunning, Hammond, Heicklen, Leighton, Okabe, Noyes, Steacy, and others mentioned in the text book by Calvert and Pitts, 1966). In industry, the photolytic initiation of product synthesis has been known since the beginning of the last century.

The UCR EPA chamber is a new large indoor environmental chamber constructed at the University of California at Riverside (UCR) under United States EPA funding for the purpose of evaluating gas-phase and secondary aerosol mechanisms for ground-level air pollution. The major characteristics of this chamber, the results of its initial characterization for gas-phase mechanism evaluation, and examples of initial gas-phase mechanism evaluation experiments, are described. It is concluded that the chamber has lower or at most comparable background effects than other chambers previously used for mechanism evaluation, and can provide useful mechanism evaluation data at NO_x levels as low as 2 ppb. Future research directions to utilize the capabilities of this chamber are discussed.

A large indoor environmental reactor was completed in 2002 at UC Riverside to investigate photochemical processes leading to ozone and secondary organic aerosol (SOA) formation. The chamber is briefly described by Carter (2004) in a separate contribution to this workshop and in a more detailed manuscript submitted to Atmospheric Environment (Carter et al., 2005). The contribution to the workshop covered some details of the UCR chamber wall characterization (Carter, 2004), selected approaches to studies of secondary organic aerosol formation (Cocker et al., 2001 and Song et al., 2005), the NO_x dependence of m-xylene SOA formation (Song et al, 2005), some acid catalyzed reaction work from Caltech (Gao et al., 2004), and select details of the Caltech environmental chamber (Cocker et al., 2001). This contribution will briefly describe the SOA components of the chamber facility and advantages of the new reactor with respect to SOA investigations, and will summarize the findings on the importance of hydrocarbon:NO_x ratio on SOA formation in the m-xylene photooxidation system

This paper describes the development of a chamber facility designed to investigate a range of atmospheric aerosol processes driven by their potential to affect radiative forcing. Development of a suite of models investigating coupled chemical and microphysical processes has informed the definition of a preliminary experimental programme. The chamber construction is underway and the experiments are phased to follow the chamber development and completion.

Since its initial operation in 1997, the AIDA aerosol and cloud chamber of Forschungszentrum Karlsruhe (Aerosol Interactions and Dynamics in the Atmosphere) has been established as a unique experimental facility to study multi-phase processes over a wide range of atmospheric conditions. Research activities include heterogeneous chemistry on aerosols, hygroscopic and optical properties of complex aerosol particles, homogeneous freezing of supercooled solution droplets, heterogeneous freezing and cirrus cloud formation, as well as formation and characterisation of polar stratospheric cloud constituents.

The atmosphere is a complex medium where chemicals are released, dispersed by physical processes and oxidized by photochemical reactions initiated by solar radiation. One of the most efficient oxidants is the OH radical produced by complex photochemical processes (Atkinson, 2000). In the past, the studies of Volatile Organic Compounds (VOCs) were focused on the gas phase reactivity. In the 1980s, inter-relations between gas and aqueous phases in the troposphere started to be considered (Graedel and Weschler, 1981). It was recognized that the aqueous phase photochemistry of Water Soluble Organic Compounds (WSOCs) has an impact on the gas phase concentrations of key species such as OH, HO₂ and O₃ (Lelieveld and Crutzen, 1990; Monod and Carlier, 1999; Herrmann, 2003). More recently, the contribution of WSOC to aerosol hygroscopicity and their ability to act as cloud condensation nuclei was found (Gelencser et al., 2003; Claeys et al., 2004; Ervens et al., 2004).

The dynamic flow-through chamber system has been developed in response to a need to measure emissions of nitrogen, sulphur, and carbon compounds for a variety of field applications. The cylindrical chamber system is constructed of chemically inert materials and internally lined with 5mil thick transparent fluorinated ethylene polypropylene (FEP) Teflon to reduce chemical reactions and build up of temperature inside the chamber. The chamber (diameter = 27cm, height = 42 cm, volume = 24.05 L) is designed with an open-ended bottom that can penetrate either soil or liquid to a depth of ~6-8 cm, thus creating a completely enclosed system. Carrier gas (e.g. compressed zero-grade air) is pumped at a constant flow rate (~2 to ~5 lpm), depending on the season. The air inside the chamber is well mixed by a variable-speed, motor-driven Teflon impeller (~40 to ~100 rpm). Many different laboratory and field experiments have been conducted using this dynamic chamber system. Oxides of nitrogen (NO, NO₂, NO_y) emissions have been measured from agricultural soils where nitrogen-rich fertilizers have been applied. Ammonia-nitrogen (NH₃-N) and reduced organic sulphur compounds emissions have been measured using this same technique across a gas-liquid interface at swine waste treatment anaerobic storage lagoons, and agricultural fields. Similar chamber systems have also been deployed to measure uptake of nitrogen, sulphur, ozone, and hydrogen peroxide gases by crops and vegetation to examine atmospheric-biospheric interactions. Emissions measurements have been validated by a coupled gas-liquid transfer with chemical reaction model as well as a U.S. Environmental Protection Agency (EPA) WATER 9 model.

Aldehydes are emitted directly into the atmosphere from a variety of natural and anthropogenic sources and are also formed in situ from the atmospheric degradation of volatile organic compounds (VOCs). The atmospheric fate of aldehydes is controlled by photolysis and reaction with hydroxyl (OH) or nitrate (NO₃) radicals and, in the case of unsaturated compounds, reaction with ozone (Atkinson, 1994). The photolysis of aldehydes is of particular importance because it is a source of free radicals in the troposphere, and thus may significantly influence the oxidizing capacity of the lower atmosphere (Finlayson-Pitts and Pitts, 1986).

Small carbonyl compounds are formed during the photochemical oxidation of many volatile organic compounds (VOC’s), in urban as well as in rural areas. Photolysis and reaction with the OH radical are the most important initiation reactions for the atmospheric degradation of these compounds, and lead to the formation of peroxy radicals in the former case and either stable molecules and/or free radicals in the latter case (Finlayson-Pitts and Pitts, 1999).

Air pollution in many large cities is linked with the photochemical transformation of primary pollutants like VOCs (volatile organic compounds) and NO_x, which in the presence of sunlight foster the formation of secondary pollutants including ozone (O₃) and secondary organic aerosol (SOA) (Finlayson-Pitts and Pitts 2000; Molina and Molina 2002). ‘Photochemical smog’ has adverse effects on human health (Kunzli et al. 2000; Evans et al. 2002), the ecosystem (Middleton et al. 1950; Gregg et al. 2003) and regional climate (Lelieveld et al. 2001; Ramanathan and Crutzen 2003).

The Master Chemical Mechanism (http://mcm.leeds.ac.uk/MCM) has been updated to version MCMv3.1 in order to incorporate recent kinetic and mechanistic improvements in the understanding of aromatic photo-oxidation. Carefully designed experiments have been carried out in the European Photoreactor (EUPHORE) to investigate key subsets of the toluene system. These results have been used to evaluate the toluene mechanism in MCMv3.1 (and by extension the other aromatic mechanisms in MCMv3.1) and where appropriate to refine the degradation schemes. MCMv3 and MCMv3,1 have also been evaluated using a high quality EUPHORE dataset on the photo-oxidation of benzene, toluene, p-xylene and 1,3,5-trimenthylbenzene. Significant deficiencies have been identified in the mechanisms, in particular: (1) an over-estimation of the ozone concentration, (2) an under estimation of the NO oxidation rate and (3) an under-estimation of OH. The use of MCMv3.1 improves the model-measurement agreement in some areas but significant discrepancies remain. Ideas for additional modifications to the mechanisms and for future experiments to further our knowledge of the details of aromatic photo-oxidation are discussed.

Large quantities of harmful VOCs are emitted into the troposphere from anthropogenic sources (Calvert et al., 2002). Aromatic hydrocarbons are an important class of VOCs present in the atmosphere, which contribute significantly to the chemistry of urban air (Atkinson, 2000; Atkinson and Arey, 2003). Estimations of the global emissions of aromatic hydrocarbon suggest that they comprise between 17-25% of the total anthropogenic NMVOC emissions (Calvert et al., 2002). The degradation of aromatic hydrocarbons is mainly initiated during the day by reaction with the hydroxyl radical (OH). Based on the presently accepted but inadequate mechanism for the atmospheric degradation of aromatic hydrocarbons, it has been calculated that this class of VOC could account for up to 30% of the photooxidant formation in urban areas (Derwent et al., 1996, 1998). Besides the photooxidant formation, this class of hydrocarbon is also assumed to make a significant contribution to secondary organic aerosol (SOA) formation in urban areas (Odum et al., 1996; Forstner et al., 1997; Hurley et al., 2001). This ranks aromatic hydrocarbon as one of the most important classes of hydrocarbons emitted into the urban atmosphere. Formulation of a realistic photo-oxidation mechanism for aromatic hydrocarbons represents one of the most important and challenging problems remaining to be solved in chemical models of tropospheric photooxidant formation.

Oxygenated volatile organic compounds form a major component of the trace gases found in the troposphere (Singh et al., 2001). They are emitted directly into the atmosphere from biogenic sources, solvent and fuel additives use and are also formed in the tropospheric oxidation of all hydrocarbons. A number of oxygenated compounds are used as alternatives to aromatic and halocarbon solvents both in terms of toxicity problems and in the case of aromatic compounds as a means of reducing the levels of oxidant formation in the troposphere. It is apparent that oxygenated compounds will thus play an increasing role in determining the oxidising capacity of the troposphere both on a regional and global scale.

Chlorinated solvents are widely used in metal degreasing, dry cleaning, paint stripping, extraction of pharmaceuticals and foodstuffs, and electronic circuit board production (Sidebottom and Franklin, 1996). Emission data for trichloroethene (CHCl=CCl₂) and tetrachloroethene (CCl₂=CCl₂) show that emissions of these compounds are declining steadily, largely as a result of constant improvements in the efficiency with which they are being used and recycled. While 1,1,1-trichloroethane (CH₃CCl₃) was formerly widely used as a solvent, it is now strictly regulated as an ozone-depleting compound under the Montreal Agreement. Despite the decline in emissions of these chlorinated solvents their atmospheric fate and impact is still of concern.

The branching ratio for the reaction of OH radicals with CH₃COOD was determined at 298 2±K and at atmospheric pressure using an indoor smog chamber (300 L) coupled to two different detection systems: (i) Gas Chromatograph (GC) with FTIR detection for determination of [CH₃COOD], (ii) Cavity Ring Down Spectroscopy (CRDS) using a telecommunication diode laser as a source for the determination of [HDO] and [H₂O]. The reaction CH₃COOD + OH occurs via D-atom abstraction with an efficiency of 36±20 %.

Alkyl halides (RX: X = Cl, I) are an important source of halogens in the atmosphere. The major tropospheric sinks of these compounds are photolysis (RBr, RI) and reaction with OH radicals. In the case of alkyl iodides (RI) relative kinetic studies of their OH reactions in photoreactors are complicated by fast reactions with the O(3P) atoms generated by the photochemical OH radical sources. Figure 1 below shows a ln-ln plot of the kinetic data from an experiment performed in a large photoreactor to determine the OH rate coefficient for the reaction OH + CH₃CH₂CH₂I relative to OH + ethene using the photolysis of methyl nitrite (CH₃ONO) as the OH radical source. A recent example of the implementation of the relative kinetic technique for the determination of OH radical rate coefficients in a photoreactor can be found in Olariu et al. (2000).

The reaction between CH₃OH and CF₂O has been studied in the temperature range between 20 and 122°C. A considerable decrease in the reaction rate with increase in temperature was noted, indicating a contribution of heterogeneous character. Different types of surfaces and coatings have been used. The possible involvement of aerosols for the reaction to occur in the atmosphere is discussed.

CF₃O_x radicals are present in the atmosphere as oxidized species of a series of CFC's, HFC's and HCFC's. They react with NO_x species. In particular, the reaction between CF₃O₂ and NO which shows two reaction channels

Dimethyl sulphide (CH₃SCH₃, DMS) is the dominant natural sulphur compound emitted from the world’s oceans (Berresheim et al., 1995, Urbanski and Wine, 1999), accounting for about one quarter of global sulphur gas emissions. Oceanic DMS, through its oxidation products, is proposed to play a key role in climate regulation, especially in the remote marine atmosphere (Charlson et al., 1987).

Selected results of environmental chamber experiments carried out in the new large indoor environmental chamber at the University of California at Riverside (UCR) that are relevant to quantifying ozone impacts of volatile organic compounds (VOCs) are described. Issues and data needs for quantification of VOC reactivities towards ground-level ozone are described. The ability of the current SAPRC-99 chemical mechanism to simulate recent data from this chamber concerning the ozone formation from irradiations of an ambient reactive organic gas (ROG) - NO_x mixture and several new aromatics experiments are discussed. It was found that the mechanism consistently under predicts ozone formation in ambient surrogate - NO_x experiments at low ROG/NO_x ratios, and also under predicts the effects of adding CO to aromatic - NO_x irradiations. The two problems may be related and suggest problems with the formulation of current mechanisms for atmospheric reactions of aromatics.

The chemistry of ground level atmospheric ozone formation is highly complex and nonlinear and has many uncertainties. As a result no chemical model can be relied upon to give accurate predictions unless it has been evaluated by comparison with experimental data. There are essentially two ways in which a photochemical oxidant model can be evaluated. One compares the predictions of the complete model against field data taken during a historic ozone pollution episode. Another approach relies on the evaluation of individual pollutant compounds, separately. In the case of gas-phase chemical mechanisms, this means evaluating the predictions of the mechanism against results of environmental chamber experiments (Carter et al., 1995a).

Atmospheric chemistry has not yet been fully understood. One of the main reasons is the complex interaction between components of the gas phase and of the condensed phases. In this contribution we would like to consider, by example of the reaction between aqueous SO₂ and molecular oxygen, occurring in the presence of nitrate, some aspects of such aninteraction which may be of interest both to the researchers and modellers of cloud chemistry.

Isoprene is a conjugated diene (2-methyl-buta-1,3-diene), volatile and hardly soluble in water; under normal pressure it boils at 34 °C (Merck, 1999) and dissolves up to 1.47x10-2 M at 21.5 °C, with a Henry’s constant of 0.027 mole kg-1 atm-1 at 25 °C (NIST, 2001). Isoprene is a metabolite in plants, microbes, animals and humans, and a major biogenic trace compound emitted to the atmosphere. It is very reactive towards atmospheric gas-phase oxidants such as hydroxyl and nitrate radicals or ozone. At higher concentrations, 220 – 7000 ppm, it is carcinogenic to rodents and possibly carcinogenic to humans (Melnick and Sills, 2001).

Once released into the atmosphere, primary diesel emissions (or any other direct emissions) are subject to dispersion and transport and, at the same time, to various physical and chemical processes, which determine their ultimate environmental fate. To elucidate potential health effects of diesel exhaust, it is insufficient to characterize only the primary pollutants that are directly emitted from diesel vehicles. Secondary compounds, formed during the transport of emissions through the atmosphere, may also affect human health.

Emissions from the combustion of fossil fuels, especially road traffic emissions, are major contributors to air pollution. Though exhaust-cleaning techniques for vehicles have been significantly improved during the last years and the averaged fuel consumption per single vehicle has significantly decreased, the contribution of road traffic emissions to tropospheric photosmog formation is still high, due to the increasing total number of vehicles.

The ability of Europe’s research teams to remain at the forefront of all fields of science and technology depends on their being supported by state-of-the-art infrastructures. The term “research infrastructures” refers to facilities and resources that provide essential services to the research community in both academic and/or industrial domains.

Historically, some of the new EU Member States and the Candidate Countries have experienced high levels of pollution in the past. Enhanced management measures were and are needed to improve their air quality. A survey was recently conducted (Batchvarova et al., 2005) to list the current research activities on atmospheric chemistry in these counties, as well as the groups and institutions involved in it. Air chemistry is an essential element of air quality, climate change modelling, industrial energy planning, and health risk assessments. Atmospheric chemistry research is also related to air quality monitoring. Therefore, the survey has also retrieved information on air quality monitoring networks and their management of those countries involved. Some information on air pollution modelling research is also discussed.

Non-methane volatile organic compounds (NMVOC) were measured at various sites representing different areas and different emission sources in the city of Wuppertal, Germany. The measurements covered volatile hydrocarbons in the range of C₂-C₁₀ and oxygenated hydrocarbons such as alcohols, ketones and esters. Samples were collected using Carbotrap and Carbosieve SIII solid adsorption tubes and analysed off-line by thermal desorption and GC-FID analysis. Measurement results were used to create the input data for the source apportionment analysis with the Chemical Mass Balance Modelling technique. Emission profiles for traffic and solvent use were calculated.

The ozone problems (stratospheric, tropospheric and surface O₃) are an important part of atmospheric air pollution problems. One of the goals, outlined in the Recommendations of the meetings of the Ozone Research Managers (under World Meteorological Organization – WMO) was: the conducting of systematic measurements, which provide the basis for understanding the ozone regime and its trends.

The “heavy metal in the environment” collocation refers to any metallic chemical element and some metalloids (e.g. arsenic) that are toxic or poisonous for living organisms even at low concentration, e.g. Pb, Cd, Hg, As, Tl, Cr. They originate in the Earth’s crust as well as in the majority of wastes resulting from anthropogenic activities. Toxic effects of other heavy metals (Cr, Mo, Ni, As, Se etc.) have to be considered separately from the effects of biologic doses in which they exert their vital role.

The degradation of environmental quality in many regions of the world has accelerated during the past decades especially due to the industrial development, which has led to important changes in different compartments of the environment. Important atmospheric species are considered to be responsible for wide spread environmental effects including, changes in pH deposition, corrosion of buildings material etc. Deposition of air pollutants is an important loss process for most of the species present in the atmosphere that can cause severe damage to ecosystems. Air pollutants are deposited to the earth’s surface especially through wet and dry processes. Deposition rates are determined in order to estimate the impact of these pollutants on ecological systems. Deposition of pollutants by wet processes is relatively easy to determine through analysis of precipitation samples. However, it is well recognised that less is known about dry deposition, which is much more difficult to measure (estimated using measured air concentration and the deposition velocity concept) and which appears to predominate near strong emission sources with wet deposition predominating further downwind (Whelpdale et al., 1997). Direct measurement of pollutants deposition by dry processes is more difficult and requires extensive instrumentation and technical resources.

During last years the negative effects of anthropogenic pollution on the environment has intensified. The effects are observed on the regional and local levels in the change of the environment quality, as well as on the global level as in the change of the planet climate. Urban agglomerations play an important role in these processes.

Since many years there has been increased interest in the investigation of chemical processes taking place in the polluted atmosphere. Definite relationships exist between chemical reactions which occur in the atmosphere in the gas, liquid and solid phases under stationary conditions.

At present, in the mountain regions of Tien-Shan and Pamir reduction and vanishing of glaciers is occurring. According to glaciological data, 1081 glaciers disappeared at Pamir and Alai between 1957 and 1990, and 71 glaciers have completely thawed at Zailijsky Ala Tau. The glaciers recede with an average velocity of 8 meters per year. Not only is the linear size of glaciers reduced, but also their volume. This influences significantly river waters, vegetation and Central Asian climate.

Investigations were conducted at urban and rural sites of the Dniepropetrovsk Province (Ukraine) during the last decade, and were aimed at an assessment of the air pollution and the buffer capacity of local ecosystems. The following themes were considered in the study: the regional distribution of industrial emissions, the structure of the ecosystems, an estimation of the buffer capacity of the soil and vegetation cover.

The measurement of air pollution parameters is a costly process. Due to several reasons, the devices may not take measurements for certain days. In such cases robust estimation methods are quite necessary in order to fill the gaps in the time series. Artificial neural networks have been employed successfully for this purpose for hydrometeorological time series, as reported in literature. In this study, modelling of the time series of air pollution parameters was investigated using two ANN methods; a radial basis function algorithm (RBF) and feed forward back propagation method (FFBP). The ANN methods were employed to estimate the PM10 values using the NO and CO values. The data were from a measurement station in Istanbul, Turkey. The results of an initial statistical analysis were considered in the determination of the input layer node number. In the estimation study, values corresponding to other air pollution parameters were included in the input layer. The results were compared to those obtained with a conventional multi-linear regression (MLR) method.