Nitrogen oxides, sulfur dioxide, ozone, light scattering and relative humidity were measured in the sphere of influence of the Radford Army Ammunition Plant using an airborne platform. Data were obtained on twelve days between May and July 1980 and were used to map the upwind-downwind pollutant concentrations around the arsenal. Results of this investigation indicate that an airborne platform can be used successfully in characterizing pollutant parameters in the sphere of influence of an area source. Results also indicate the complexity of NO-NO₂-O₃ cycle when hydrocarbons and sulfur dioxide are present. Benefits of this type of study may prove useful for the technical and administrative decision making processes of regulatory agencies and munitions manufacturing personnel. / M. S.

[pt] Neste trabalho foi feita a quantificação e a caracterização química de material particulado (PM, do inglês particulate matter), com foco no material particulado grosso (PM10) coletado em quatro diferentes locais da região metropolitana do Rio de Janeiro (Gericinó, Zona Oeste; Duque de Caxias, Baixada Fluminense; Bonsucesso, Zona Norte e Centro- região central da cidade), utilizando amostradores de grande volume e filtros de fibra de vidro fornecidos pelo Instituto Estadual do Ambiente (INEA). Utilizando filtros ao longo do ano de 2019, foram feitas análises em Cromatógrafo de íons (CI) e espectrometria de massas com plasma indutivamente acoplado (ICP-MS) para a caracterização das amostras e quantificação de íons e metais. Também foi feita avaliação de concentração de PM10 sob influência da precipitação ao longo de 10 anos de estudo (2010 a 2019) e os níveis comparados com os valores recomendados pelo CONAMA. Para complementar o estudo foi realizado um levantamento bibliográfico sobre poluição veicular relacionada ao PM10 no Brasil. Os resultados apontam para um aumento nos níveis de PM10 ao longo da década avaliada, principalmente na região de Bonsucesso, com concentrações médias de 62 (micro)g m-3 , em alguns casos ultrapassando os limites do CONAMA. Com relação á precipitação, houve diferença significativa estatisticamente em dias sob ocorrência de chuva e dias sob ausência de chuva. Os íons que apresentaram as maiores concentrações foram NO3 - , SO4 2- e Na+ com concentrações médias variando de 2,6-3,4; 3,0- 3,3; 2,4- 3,0 (micro)g m-3, respectivamente, em todas as estações do estudo. A análise dos metais definiu Fe e Cu como os metais em maior concentração nas amostras, com concentrações médias variando de 0,54 a 0,73 e de 0,023 a 0,047 (micro)g m-3 , respectivamente. Tais resultados são reforçados pela análise de Black carbon através de absorção ótica, que reportou uma concentração média variando de 3,6 a 6,0 (micro)g m-3 . A região de Bonsucesso revelou ser a que apresentou as maiores concentrações para os parâmetros supracitados, confirmando a influência do maior tráfego veicular reportado para a região. / [en] In this work, the quantification and chemical characterization of particulate matter (PM) was carried out, focusing on coarse particulate matter (PM10) collected in four different locations in the metropolitan region of Rio de Janeiro (Gericinó, West Zone; Duque de Caxias, Baixada Fluminense; Bonsucesso, North Zone and Center-central region of the city), employing large volume samplers and fiberglass filters, supplied by the State Institute of the Environment (INEA). Using filters throughout 2019, analyzes were carried out in Ion Chromatograph (IC) and inductively coupled plasma mass spectrometry (ICP-MS) for the characterization of the samples and quantification of ions and metals. An evaluation of PM10 concentration under the precipitation influence was also carried out over 10 years of study (2010 to 2019) and the levels compared with the values recommended by CONAMA. To complete the study, a bibliographic study on vehicular pollution related to PM10 in Brazil was implemented. The results point to an increase in PM10 levels over the evaluated decade, mainly in the Bonsucesso region, with average concentrations of 62 (micro)g m-3 , in some cases overreaching the limits of CONAMA. Concerning the precipitation, there was a statistically significant discrepancy in days with rain and days without rain. The ions that showed the highest concentrations were NO3 - , SO4 2- and Na+ with mean concentrations ranging from 2.6-3.4; 3.0-3.3; 2.4-3.0 (micro)g m-3 , respectively, in all groups of the study. The metal analysis defined Fe and Cu as the most represented metals in the samples, with mean concentrations ranging from 0.54 to 0.73 and from 0.023 to 0.047 (micro)g m3 , respectively. Such results are strengthened by the analysis of Black carbon through optical absorption, which reported an average concentration ranging from 3.6 to 6.0 (micro)g m-3 . The Bonsucesso region showed to be the one with the highest concentrations for the aforementioned parameters, confirming the influence of the greater vehicular traffic reported for the region.

[pt] O intenso crescimento dos centros urbanos provocou aumento considerável da emissão de poluentes atmosféricos, conferindo destaque à poluição atmosférica. A qualidade do ar de uma determinada região pode ser afetada por fatores como tipos e quantidades de fontes de emissão, meteorologia e topografia, o que confere complexidade para essa área de estudo. Os principais poluentes legislados no Brasil são SO2, NO2, CO, O3 e o material particulado (MP). Dentre esses, o MP pode ser considerado como uma das mais importantes classes de poluentes devido sua composição heterogênea, que pode causar efeitos adversos à saúde e ao meio ambiente. O monitoramento dos poluentes atmosféricos para fins de fiscalização pode ser realizado utilizando métodos padronizados que incluem equipamentos capazes de determinar a concentração dos poluentes em tempo real. No Rio de Janeiro, as principais redes de monitoramento da qualidade do ar pertencem ao Instituto Estadual do Meio Ambiente (INEA) e à prefeitura do Rio de Janeiro, que dispõem de estações automáticas em diversas regiões da cidade e do estado. Entretanto, o monitoramento tradicional da poluição atmosférica demanda altos investimentos para compra e manutenção desses equipamentos, diminuindo as áreas passíveis de monitoramento. O biomonitoramento, por sua vez, que utiliza organismos vivos para avaliar mudanças ambientais, é considerado uma abordagem adequada para ampliar as áreas monitoradas e avaliar os impactos causados pela poluição. Assim, o objetivo deste estudo foi avaliar a qualidade do ar no Rio de Janeiro através do biomonitoramento ativo utilizando Tillandsia usneoides e Tillandsia stricta como biomonitores, investigando o estresse oxidativo causado pela exposição aos metais. Considerando o efeito das fontes de emissão na qualidade do ar, buscou-se também avaliar a qualidade do ar no Rio de Janeiro durante os primeiros meses da pandemia de COVID-19, quando a mobilidade urbana foi alterada consideravelmente. Nessa avaliação foi considerada ainda a influência das condições meteorológicas por meio da comparação com dados obtidos para os mesmos períodos do ano anterior. O biomonitoramento ativo foi realizado entre julho de 2019 e novembro de 2020 em cinco locais do Rio de Janeiro com diferentes características quanto a emissão de poluentes: Ramos, Urca, Niterói, Duque de Caxias e Santa Cruz. As plantas foram removidas de um local rural e instaladas nos locais de monitoramento em julho de 2019 e coletadas em outubro de 2019, janeiro e novembro de 2020. A avaliação do estresse oxidativo foi realizada através da quantificação dos biomarcadores Metalotioneína (MT), Glutationa Reduzida (GSH), H2O2 e Peroxidação Lipídica, medida pela concentração de malondialdeído (MDA). Para avaliar a fração biodisponível dos metais presente no tecido das plantas foi realizada distribuição subcelular através do procedimento de purificação da metalotioneína, que fornece três frações nas quais os elementos foram quantificados por espectrometria de massas com plasma indutivamente acoplado (ICP-MS), bem como a quantificação dos metais totais realizada em extratos ácidos dos tecidos das plantas. Na avaliação da qualidade do ar durante a pandemia, os dados de concentração de NO2, SO2, CO e O3 e variáveis meteorológicas foram fornecidos pelo INEA para março e abril de 2020 e usados para avaliar mudanças na qualidade do ar em comparação com o ano anterior e a contribuição das condições meteorológicas. A prefeitura do Rio de Janeiro forneceu dados de CO, O3 e PM10 e variáveis meteorológicas para o período entre março e setembro que foram usados para uma avaliação que considerou o impacto das medidas de distanciamento social na qualidade do ar através do acompanhamento desde o primeiro decreto de restrição até a abertura econômica total. Nos dois casos, os dados foram submetidos a destes estatísticos e análises temporais. O biomonitoramento da qualidade do ar indicou que os principais elementos encontrados nos biomonitores foram Fe, Na, K, Ca, Mg and Al. Elementos relacionados ao tráfego veicular, como Pb, Cr, Cu and V também foram encontrados em concentrações expressivas. A T. usneoides apresentou maiores concentrações em comparação com a T. stricta, entretanto, o enriquecimento das duas espécies em relação à amostra de referência foi similar, indicando que ambas são adequadas para o biomonitoramento. Duque de Caxias e Santa Cruz apresentaram os maiores enriquecimentos, o que pode estar relacionado com a contribuição das emissões industriais. A avaliação da distribuição subcelular dos metais demonstrou que Cr, Co, Cu, Cd, Mn, Ni, Se and Zn foram encontrados na fração termoestável da purificação da metalotioneína, o que indica que as plantas estão em processo de destoxificação desses elementos. A T. stricta apresentou maiores percentuais da fração não biodisponível, o que indica que essa espécie é mais resistente aos efeitos tóxicos causados pelos metais. Foi observado ainda que em períodos secos a absorção das plantas é menor, devido ao metabolismo ácido das crassuláceas (CAM), que favorece o controle hídrico em condições de baixa umidade mantendo as organelas responsáveis pelas trocas gasosas fechadas durante o dia, período no qual se registram maiores concentrações de poluentes. Os biomarcadores de estresse oxidativo indicaram que as duas espécies estão em condições semelhantes de estresse oxidativo, pois suas concentrações não apresentaram diferença significativa entre as espécies na maioria dos casos. As correlações entre os biomarcadores indicaram que a principal função da GSH é no sistema antioxidante, embora em alguns caso ela tenha atuado auxiliando a MT na destoxificação de metais. As correlações de H2O2 com MT e GSH indicam que a exposição de metais estimula e produção de espécies reativas de oxigênio (ROS), como H2O2, cuja concentração pode ser regulada pela ação da GSH, entretanto, a correlação com MDA indica que o sistema antioxidante não está sendo eficiente na prevenção do estresse oxidativo. A avaliação da qualidade do ar durante a pandemia revelou que a redução de até 85 por cento na mobilidade urbana foi a principal responsável pela melhora da qualidade do ar, principalmente nos primeiros meses de isolamento social. Foi registrada diminuição na concentração dos poluentes primários e aumento da concentração de O3, um poluente secundário. Esse mesmo comportamento também foi registrado em outras cidades ao redor do mundo e é atribuído aos complexos processos de formação de ozônio, que dependem da concentração de alguns poluentes primários. Embora as concentrações de ozônio tenham aumentando, o índice de qualidade do ar durante a pandemia foi melhor do que em períodos com a rotina normal das cidades. A contribuição das condições meteorológicas não apresentou muita influência, uma vez que se mostrou semelhante ao mesmo período do ano anterior. A avaliação da qualidade do ar no Rio de Janeiro revelou que as emissões veiculares são as principais fontes de poluentes atmosféricos, e que a mudança no fluxo de veículos pode melhorar a qualidade do ar, embora empreendimentos industriais também apresentem expressiva contribuição. A poluição urbana pode causar desequilíbrio ambiental, uma vez que a vegetação está em constante exposição. Podese destacar ainda que o estado do Rio de Janeiro abriga importantes remanescentes de mata atlântica, o que demanda constante fiscalização visando a garantia da preservação ambiental. / [en] The intense growth of urban centers has caused a considerable increase in atmospheric pollutant emission, which can lead to human health and ecosystem risks. The air quality monitoring network in Brazil is limited, covering not even half of the states. Biomonitoring employing plants is, thus, an alternative to increase the monitored areas and still allow the ecotoxicological evaluations concerning air pollution exposure. In this sense, this study sought to evaluate the air quality of the metropolitan region of Rio de Janeiro using official atmospheric pollutant concentration data and biomonitoring efforts aiming to investigate oxidative stress in the bromeliad species used as biomonitors. During the social isolation period established as a contingency measure against the spread of the coronavirus, air quality was improved overall, although ozone concentrations increased relative to the previous year and the period preceding the lockdown. This change was attributed to an urban mobility reduction of up to 85 percent. The biomonitoring assessments indicate that the main elements taken up by the plants were those related to vehicular traffic. The correlation of these elements with oxidative stress biomarkers indicate that air pollution exposure represents a risk to local ecosystems. Tillandsia stricta and Tillandsia usneoides displayed similar behavior regarding metal accumulation, albeit presenting different detoxification processes. Although Rio de Janeiro has one of the most widespread air quality monitoring networks, studies aimed to assess the environmental air pollution impacts are required, considering that the state is home to important and endangered Atlantic Forest remnants. In additions, vehicular emission control can contribute to air pollution reduction, considering that this is one of the main pollutant sources.

No / In this paper we propose a unified approach for integrating implicit and explicit knowledge in neurosymbolic systems as a combination of neural and neuro-fuzzy modules. In the developed hybrid system, training data set is used for building neuro-fuzzy modules, and represents implicit domain knowledge. The explicit domain knowledge on the other hand is represented by fuzzy rules, which are directly mapped into equivalent neural structures. The aim of this approach is to improve the abilities of modular neural structures, which are based on incomplete learning data sets, since the knowledge acquired from human experts is taken into account for adapting the general neural architecture. Three methods to combine the explicit and implicit knowledge modules are proposed. The techniques used to extract fuzzy rules from neural implicit knowledge modules are described. These techniques improve the structure and the behavior of the entire system. The proposed methodology has been applied in the field of air quality prediction with very encouraging results. These experiments show that the method is worth further investigation.

This case study documents the entire indoor air quality (IAQ) problem experienced by the students, faculty and staff of Cowgill Hall on the campus of Virginia Polytechnic Institute and State University from August 1987 to August 1988, recommends a general IAQ solution process and makes several specific suggestions to prevent the reoccurrence of the problem in Cowgill Hall. Background information on Cowgill Hall and the indoor air quality issue are also provided. This document is addressed to students of architecture, engineering and related disciplines. It emphasizes the growing importance and possible repercussions of their design decisions on the total environment; both in and out of doors. / Master of Science

Management of the air quality surrounding Petroleum Refineries deserves attention because the industry contributes almost five percent of the total emissions from all anthropogenic sources. A document containing a complete set of guidelines for use in the refining industry which satisfies the current and anticipated air quality legislations and regulations in the U.S. is necessary. In the past, several documents have been prepared, but have not included a complete coverage of the air quality management as currently needed. Furthermore, due to the continuing revisions of the Clean Air Act, a document with current, updated regulations and air quality management principles is necessary. This study dealt with a broad range of topics including characteristics of emissions, control technology applied, regulations and legislative issues, monitoring and modeling practices, and issues of the 1980s together with future projections and implications. Air quality regulations and standards are periodically revised and are becoming more stringent with time. Issues like acid rain may lead to even more stringent emission standards if investigations carried out currently reveal that the refineries are significant contributors. Great measures are taken to control emissions from the refineries either by using good control equipment or using other alternative control strategies. Small operating refineries are closing down due to changing conditions. An agglomeration and or expansion of the existing refining capacity is occuring. The air quality trends associated with this transition in the industry are discussed. / Master of Science

According to the United Nations, 55% of the world's population currently lives in urban areas and which is projected to increase to 67% by 2050. Thus, it is imperative that effective strategies are developed to mitigate urban pollution. Complementing field experiments, computational fluid dynamics (CFD) analyses are becoming an effective strategy for identifying critical factors that influence urban pollution and its mitigation. This thesis focuses on two scales of the urban micro-climate environment: (i) evaluation of LES simulations with a simplified grid for modeling pollution dispersion in a street canyon and (ii) investigation of the effects of leaf surface micro-characteristics, wind speed, and particle sizes on the dry deposition of fine particulate matter (PM2.5). The first of these studies focuses on reproducing the pollution dispersion in a street canyon measured in a wind tunnel at Karlsruhe Institute of Technology (KIT), Germany. A simplified grid with the Large Eddy Simulations (LES) approach for canyon ratio W/H = 1 is proposed with the goal to reduce the computational cost by eliminating the need to model the entire canyon while striving to preserve the mixing induced by individual jets used to model vehicle emission in the experiment. LES is also capable of providing transient flow field and pollution concentration data not available with widely-used steady approaches such as RANS. The time-dependent information is crucial for pollution mitigation since pedestrians are usually exposed to pollution on a short-time basis. The predictions are in satisfactory agreement with the experiment for W/H = 1, yielding the Pearson correlation coefficient R = 0.81, with better performance near the leeward wall. Due to the small span modeled, three-dimensional instabilities fail to develop which could probably explain the overprediction of pollution concentration near ground level. However, other LES investigations where the full canyon was modeled also observed over-predictions. The use of a discrete emission source was not observed to provide benefits. The current model could be further improved by using a larger spanwise domain with a continuous line source to allow large wavelength instabilities to develop and increase turbulent diffusion. The second part of this thesis investigates the impact of trichome morphology and wind speed on the deposition of 0.3 μm and 1.0 μm particles on leaves. Using the one-way coupling approach to predict the fluid-particle interactions with the assumption that all particles that impact the leaf or trichome surface deposit, trichomes of 5 μm and 20 μm in diameter are modeled as equally spaced and uniform cylinders on an infinitely large plane. The results show that trichome diameter, density, and wind speed have a favorable impact on deposition velocity. Comparing to the smooth leaf, the presence of the thicker 20 μm hairs increases the deposition velocity by 1.5 – 4 times, whereas, the presence of short 5um trichomes reduces the deposition by 15 - 45%. Increasing trichome height from H/D = 20 to 30 shows benefits for the thinner trichomes but lowers the deposition for the densely packed thicker trichomes. Less aerosol deposition is also observed when the particle diameter increases from 0.3 μm to 1.0 μm. Due to the non-uniform contributions of these various traits, a non-dimensional ratio Rhp is proposed to model the aerosol deposition on leaf surface at wind speed of 1 m/s which yields a satisfactory linear correlation coefficient of 0.89 for 0 < R_hp < 0.3. Comparing to other published field and wind tunnel experiments conducted on a much larger scale, the deposition velocities predicted are at the lower end (U_dep^* = 0.002 to 0.012 cm/s) because of the idealized conditions. Nonetheless, the results still offer valuable insight into the effects of trichome morphology on pollutant deposition in isolation from other macro-factors. / Master of Science / According to the United Nations, 55% of the world’s population currently lives in urban areas and which is projected to increase to 67% by 2050. Thus, it is imperative that effective strategies are developed to mitigate urban pollution. Complementing field experiments, computational fluid dynamics (CFD) analyses are becoming an effective strategy for identifying critical factors that influence urban pollution and its mitigation. This thesis focuses on two scales of the urban micro-climate environment: (i) evaluation of Large Eddy Simulation (LES) with a simplified method for modeling pollution dispersion in a street canyon and (ii) investigation of the effects of leaf surface micro-characteristics, wind speed, and particle sizes on the dry deposition of fine particulate matter (PM2.5). The first of these studies focuses on reproducing the pollution dispersion in a street canyon measured in a wind tunnel at Karlsruhe Institute of Technology (KIT), Germany. A simplified grid with the LES approach for canyon ratio W/H = 1 is proposed. The goal of this study is to reduce the computational cost by modelling the canyon with a very thin span instead of the entire canyon while providing time-dependent information which is crucial for pollution mitigation since pedestrians are usually exposed to pollution on a short-time basis. The predictions are in satisfactory agreement with the experiment for W/H = 1 with better performance near the leeward wall (i.e. the left wall) and overprediction of pollution concentration near ground level – as observed by other LES investigations. The current model could be further improved by using a larger spanwise domain with a continuous line source to allow instabilities to develop, thus improve prediction accuracy. The second part of this thesis investigates the impact of trichome (i.e. a hair or an outgrowth from leaf surface) morphology and wind speed on the deposition of 0.3 mm and 1.0 mm particles on leaves. The results show that trichome diameter, density, and wind speed have a favorable impact on deposition velocity. Less aerosol deposition is also observed when the particle diameter increases from 0.3 mm to 1.0 mm. No clear effects is observed by altering the trichome height. Due to the non-uniform contributions of these various traits, a non-dimensional ratio D∗ �D∗ �2 Rhp = hair hair is proposed to model the aerosol deposition on leaf surface at wind speed of D∗ H∗ S∗ p hair hair 1 m/s which yields a satisfactory linear correlation coefficient of 0.89 for 0 < Rhp < 0.3. This ratio includes trichome diameter (D∗ ), height (H∗ ), spacing (S∗ ) as well as the ratio of hair hair hair trichome diameter to particle diameter (D∗ /D∗ ). The results offer valuable insight into the hair p effects of trichome morphology on pollutant deposition in isolation from other macro-factors.

An automated tungstic acid technique (TAT) has been successfully used to measure gaseous HNO₃ in the presence of water vapor. The TAT is based on the diffusion of gaseous HNO₃ to the interior walls of a tube coated with tungsten VI oxide (WO₃), where it is selectively chemisorbed. The collected HNO₃ sample is thermally desorbed from the WO₃ surface, as NO, and measured by a chemiluminescent oxides of nitrogen analyzer. The integrated analyzer response is directly proportional to the nitric acid collected. Based on nitric acid hydration characteristics, a decrease in the diffusion coefficient and thus collection efficiency for denuder type measurement techniques may result with increased atmospheric water vapor (i.e., relative humidity). This study emphasizes the effect of water vapor (i.e., relative humidity) as a potential interferent for HNO₃ collection with the TAT system. The effect of water vapor (< 78% RH) on the collection efficiency for HNO₃ with the tungstic acid technique is negligible at 25°C, but is significant only at elevated sampling temperatures. This threshold effect is further substantiated and eliminated when a modified sampling collection system was designed with coolant capabilities. The new design has been tested to sub-part-per-billion (NO_x analyzer detection limit) levels with minimal loss of gaseous HNO₃ signal, thereby increasing sensitivity to atmospheric HNO₃ concentrations and maintaining the gas/aerosol sample integrity. / Ph. D.

This dissertation consists of three separate research projects. Each paper uses a different applied econometric technique to investigate problems related to air quality economics. The first chapter is a general introduction to all three studies. The second chapter explores adopting an environmentally-friendly public transportation system in Europe. The Bayesian econometric methods show that willingness to pay for a new public transportation system is primarily driven by improvements to public goods, such as air quality and greenhouse gas emission reduction. The third chapter uses the red tide-related stated experience and satellite imagery of chlorophyll-a concentration as well as field data of respiratory irritation. This chapter illustrates that ancillary scientific information can be efficiently combined with choice experimental data. The fourth chapter uses panel fixed-effect models to investigate the short-term effect of air pollution on students' cognitive performance in China. It is shown that PM2.5 has a significantly negative impact on students' exam performance. / Doctor of Philosophy / This dissertation consists of three separate research projects. The first chapter is a general introduction to all three chapters. The second chapter assesses residents' support for environmentally-friendly public transportation (EFPT) upgrades across Europe. We develop a novel Bayesian logit model to investigate residents' willingness to pay for local EFPT upgrades. We find evidence that WTP is primarily driven by expected improvements to public goods, such as air quality and greenhouse gas abatement, as opposed to private ridership benefits. WTP distributions are strongly positive in all nations suggesting implicit public support for EFPT in Europe. The third chapter presents a unique opportunity to validate stated experiences by Florida Gulf coast residents with red tide-related air toxins with satellite imagery of chlorophyll-a concentration, as well as field data on respiratory irritation at local beaches. We find that respondents are more likely to choose our proposed new harmful algal blooms forecast system when the chlorophyll-a concentration or respiratory irritation is higher at nearby coastal locations. Moreover, we illustrate that this ancillary scientific information can be efficiently combined with choice experimental data and consider this research a first step in a broader effort to directly link scientific data on environmental conditions with nonmarket economic outcomes. The fourth chapter investigates short-term exposure of air pollution on students' cognitive performance in a high-stakes exam: China's College English Test (CET). We use student fixed effects in the panel-data model to estimate the effect of air pollution on students' test scores. Our findings indicate a statistically significant negative effect of PM2.5 on exam performance and also show PM2.5 is equally harmful to listening and reading section, and maybe even more for writing section. We also find that short-term exposure causes negative cognitive effects, suggesting that temporary preventative measures could be effective in avoiding the negative effects of PM2.5.

Air Quality Models are defined for prediction and simulation of air pollutant concentrations over a certain period of time. The predictions can be used in setting limits for the emission levels of industrial facilities. The input data for the air quality models are very large and encompass various environmental conditions like wind speed, turbulence, temperature and cloud density. Most air quality models are based on advection-diffusion equations. These differential equations are moderately stiff and require appropriate techniques for fast integration over large intervals of time. Implicit time stepping techniques for solving differential equations being unconditionally stable are considered suitable for the solution. However, implicit time stepping techniques impose certain data dependencies that can cause the parallelization of air quality models to be inefficient. The current approach uses Runge Kutta Chebyshev explicit method for solution of advection diffusion equations. It is found that even if the explicit method used is computationally more expensive in the serial execution, it takes lesser execution time when parallelized because of less complicated data dependencies presented by the explicit time-stepping. The implicit time-stepping on the other hand cannot be parallelized efficiently because of the inherent complicated data dependencies. / Master of Science