Mobile Sensor Networks for Air Quality Monitoring in Urban Settings

With urban populations rapidly increasing and millions of deaths being attributed annually to air pollution, there is a critical need for a deeper understanding of urban air quality. The locality of urban emissions sources, and the specific topography of cities lead to a very heterogeneous pollutant concentration landscape, the details of which cannot be captured by traditional monitoring stations. Although highly accurate, these systems are large, heavy and very expensive, which leads to a very sparse distribution of measurements. Mobile sensor networks hold the potential to allow a paradigm shift in our understanding of urban air pollution, through an augmentation of the spatial resolution of measurements. The road to achieving reliable high quality information from these type of systems is, however, full of challenges. These start from the system design, as the task of developing robust mobile sensing networks for continuous urban monitoring is arduous in itself. The limitations of existing sensor technology is another important source of hard problems. Chemical sensors suffer from many issues that make their use in a mobile scenario non-trivial. These include: instability, cross-sensitivity, low signal-to-noise ratios, and slow dynamic response. The latter problem, in particular, is a tough challenge when considering a mobile scenario, as it leads to significant measurement distortion. The question of maintaining the calibration of chemical sensors is another essential issue that derives from their instability. Finally, the development of appropriate modeling techniques that would enable us to generate high-resolution pollution maps based on mobile sensor network data is a highly difficult problem due to the inherently dynamic and partial coverage of such systems. The aim of this thesis is to show the feasibility of mobile sensor networks for monitoring air quality and their ability to achieve the goal of pushing our understanding of urban air pollution. We have taken a holistic approach, by studying the end-to-end system, and addressing each of the aforementioned challenges. Using public transportation vehicles for mobility, we have developed and deployed a full-scale mobile sensor network for monitoring the air quality in the city of Lausanne, Switzerland. We have carefully considered all steps of the system design process, starting from the choice of targeted pollutants, sensor selection, node design, server architecture, and system operation. For addressing the problem of mobility-caused distortion, we created a rigorous wind tunnel experimental set-up to study this effect and the techniques for mitigating it. We propose using deconvolution for recovering the underlying pollutant signal. Since the performance of this approach is limited by the signal-to-noise ratio of the measurements, we propose using an active sniffer to enhance the quality of the raw signal. On the topic of sensor calibration, we propose two improvements to online rendezvous calibration methodology. The first one is a model-based approach, which considers the use of more sophisticated sensor models, which are more faithful to the complex behavior of chemical sensors. The second one proposes the use of a pre-processing step, in which the mobile data is deconvolved. Finally, we study the problem of generating high-resolution maps based on mobile data. We propose five statistical modeling methods that use a heterogeneous list of explanatory variables.

Optimal Methodology to Generate Road Traffic Emissions for Air Quality Modeling

Quoc Bang Ho

Growing population, consuming a large amount of energy such as combustion of fossil fuel, increasing pollutant emissions in the atmosphere are the threats to the sustainable development of our planet in general, and of the air quality in particular. According to the World Health Organization, air pollution causes the death of more than 2 million people per year in developing countries, and millions of people also suffer from various respiratory illnesses. Road traffic is the main source of air pollution in cities and there are large uncertainties associated to this source of pollution. Different models are available to quantify the amount of pollutants released by road traffic. However, these models always require a large amount of information and work, and thus their use results expensive. These limitations are difficult for the study and the management of air quality in cities from the developing world. Thus, it is extremely difficult to design efficient abatement strategies to reduce air pollution. For this reason, it is necessary to develop a new methodology for generating road traffic emissions to contribute to a better management of the urban air quality. The first aim of this PhD thesis is to develop and to validate a new model for generating road traffic emissions in several steps with different levels of complexity. The developed model is called EMISENS. Its main specifications are: (i) EMISENS is able to compute a total amount of emissions and to distribute it in time and in space using a methodology which combines the top-down and the bottom-up approaches; (ii) the model is able to compute the emissions and the uncertainties within a reasonable computing time and (iii) the model formulation is based on well referenced methodology (COPERT IV). The validation of EMISENS model was carried out by its application over Strasbourg, France. The results of EMISENS have been compared with the results of the more complete and complex model Circul'air which is currently used to manage the Strasbourg air quality. After comparing the results of the two models, it appeared that they are very close. This example of application illustrates the capacity of EMISENS to calculate road traffic emission inventory (EI) for cities in developed countries as well as in developing countries. Further on, a complete EI is carried out over Ho Chi Minh City (HCMC) by applying an innovative methodology based on the application of the EMISENS model. HCMC is the largest city in Vietnam, it had more than 6 million inhabitants in 2006. It has more than three million vehicles and 28,500 factories in the city. High levels of air pollution are thus very often detected. The purpose of this part of the research consists in applying the EMISENS model to generate road traffic emissions. For the other emission sources, the top-down approach is used for generating the EI. The results show that the road traffic is the main emission source in the city. The motorcycles are responsible of the traffic emissions (94 % of CO, 68% of Non-Methane Volatile Organic Compound (NMVOC), 61 % of SO2 and 99 % of CH4). Two scenarios for reducing traffic emissions are evaluated to reduce the HCMC emissions for the year 2015 and 2020. In addition, two other scenarios are the Business as Usual scenarios for the year 2015 and 2020 are also studied to evaluate the traffic emissions in HCMC. The third part of this work consists in applying air quality models to the region of HCMC, the aim here is to study abatement strategies for emission reduction in the city. The results of the simulation show that the plume of O3 is developed in the north-western part of the city. These results are in good agreement with the measurements. Among the four previous emission scenarios, we chose only two reduction emission scenarios to study the effective abatement strategies for the year 2015 and 2020 in using air quality model. These two affective abatement strategies are adopted to help the local government to take decision for managing air quality in HCMC. The 100 Monte Carlo (MC) simulations are run for estimating the uncertainty in the results of air quality simulations. The results of these two abatement strategies showed that if the local government follows the emission control plan: For 2015, the O3 concentration in 2015 will be similar to the present O3 concentration. For 2020, the O3 concentration in 2020 will decrease of around 10% to 30% of O3 in comparison to the actual level. However, the O3 concentration in 2020 is still higher than standard limit. The developed methodology for generating road traffic emissions offers several advantages. It is able to calculate a road traffic emission for both developing and developed countries. The calculation is divided in several steps with different level of complexity. Therefore, this methodology provides the new approach to manage air quality in cities.

EPFL2010

Estimation of the total VOC reactivity with OH by pump-and-probe OH lidar to determine a new indicator for ozone production

Many of our most severe problems of environmental pollution involve the Earth's atmosphere. The adverse effects of atmospheric pollution are particularly evident in the low troposphere, especially in urban/suburban areas where many sources contribute to the atmospheric burden. Ozone (O3) is one of the major constituents of the air pollution over cities commonly called photochemical smog. It is a secondary pollutant, produced by the reaction of primary pollutants, nitrogen oxides (NOx) and hydrocarbons (VOC), in the presence of sunlight. Although hydrocarbons and NOx have long been identified as the two key precursors to photochemical O3, the development of an effective strategy for reducing ozone production in photochemical smog by controlling the anthropogenic emissions of these precursors has proved to be problematic. The objective of the present study is to present and demonstrate the robustness of a new observable "indicator" for determining the sensitivity of ozone formation to VOC and NOx (VOC- and NOx-limited regime). This indicator is based on the reactivity of VOC and NOx with respect to OH. Simulation tests of the ability of this new indicator Θ = τOHVOC/τOHNOx to distinguish between a NOx and a VOC-limited regime were performed with a box model and a 3D model applied to a specific episode of photochemical smog (Athens) and of a less polluted area (the Swiss plateau). Whereas the NOx reactivity 1/τOHNOx can easily be measured by standard methods, the total VOC reactivity 1/τOHVOC will be estimated by measuring the inverse lifetime of OH with a new Pump-and-Probe Lidar instrument. The method consists of producing high OH concentrations by the flash photolysis of ozone and the subsequent O(1D) reaction with H2O and by following the OH relaxation by laser-induced fluorescence (LIF). Numerical simulation of the experiment will show that the total VOC reactivity with OH can be obtained by measuring the inverse lifetime of the OH and its absolute initial concentration with the Pump-and-Probe experiment as well as the NOx, CO and O3 concentrations with standard trace-gas detectors. This total in-situ VOC reactivity will be used to determine the new indicator Θ but will also help to test the accuracy of the VOC-lumped chemical mechanism in the numerical atmospheric simulation. In order to demonstrate the feasibility of the Pump-and-Probe experiment, laboratory investigations have been performed in controlled atmosphere: OH absorption and fluorescence emission spectra allowed the OH transition bands and the non-resonant fluorescence wavelength to be identified. High-resolution OH absorption spectra allowed the OH effective absorption cross section to be determined for assessing the experimental sensitivity and retrieving the OH concentration. The experimental retrieval of the rate constant for the OH+CH4 and OH+NO2 reactions reveals the ability of the pump-and-probe experiment to monitor the chemical kinetics of OH at atmospheric pressure and room temperature. OH relaxation measured in different VOC/NOx/O3 gas mixtures confirm the relevance of the model predictions of the OH relaxation approximated by a first-order chemical decay and the validity of the total VOC reactivity with respect to OH estimated by such an approximation. A first in-situ OH relaxation measurement realized directly in the air of the laboratory showed that the pump-and-probe method could be performed under typical PBL conditions. Preliminary range-resolved measurements yielding vertical profiles of the OH concentrations produced by flash photolysis of ambient ozone suggest a promising future for the Lidar pump-and-probe technique. The perspectives for the Pump-and-Probe Lidar experiment are expressed in the concluding section.

EPFL2000

Alternative techniques to assess road traffic emissions

Luis Carlos Belalcazar Ceron

Today, one of the most important environmental problems in urban areas is air pollution. According to the World Health Organization, air pollution represents a serious risk to human health in many cities of the world. Road traffic is one of the main sources of pollution in cities. Besides, vehicular emissions are released in close proximity to population, increasing the adverse health effects. Road traffic emissions are highly uncertain in developing countries. Existing techniques to assess this source of pollution are expensive and not always accurate. It results difficult for a city from the developing world to afford these techniques and thus, adequate abatement strategies can not be adopted. It is essential to develop methodologies to reduce these uncertainties to manage air quality more effectively. This PhD thesis aims to develop and to implement alternative techniques to assess road traffic emissions. This work focuses on the study of Volatile Organic Compounds (VOCs), this family of pollutants have not been sufficiently studied in cities. The developed techniques were tested during an intensive measuring campaign conducted in Ho Chi Minh City (HCMC), Vietnam. The motorcycles are the main mode of transport in HCMC, this means of transport has contributed to the progress of this rapidly developing city. The high levels of air pollution, the elevated number of motorcycles and especially the limited funding available, made HCMC an interesting place to test the developed methodologies. In a first stage the results of the measuring campaign were used to assess the roadside levels of pollution in HCMC. During the HCMC campaign, 19 C2-C6 VOCs were monitored on-line at roadside level together with other important pollutants. Results show that there is a severe air pollution problem in HCMC. The pollutants that are produced by road traffic are identified by means of a Principal Components Analysis (PCA) receptor Model. The PCA shows that all the VOCs monitored (except isoprene) are produced by motorcycles. It is necessary to develop and to encourage the use of alternative modes of transport in HCMC. The use of the on-line roadside pollutants monitoring together with the PCA showed to be a good first approach to assess the road traffic emissions. This is an affordable technique and it is strongly recommended to use it in cities with limited financial means to study the air pollution problem. This work also presents a new method to estimate road traffic emission factors (EFs). This method is based on a long term tracer experiment conducted during the HCMC campaign. The results of the HCMC tracer experiment were used together with traffic counts and pollutant measurements to calculate the dispersion factors and afterwards the EFs. Estimated EFs for HCMC are within the range of EFs estimated in other studies. Additionally, a Computational Fluid Dynamics Model (CFD) is used to critically evaluate the proposed methodology. The evaluations show that it is possible to accurately estimate the EFs from tracer studies. The methodology proposed here results in an interesting and promising alternative to estimate the EFs. All the techniques presented in this work offer several advantages. Data obtained serve for different purposes at the same time and their use can provide valuable information for urban air quality assessment. For example, concentrations of pollutants are determined at roadside level, as well as their evolution in time, this information is useful for exposure studies. Roadside monitoring can be used to identify the pollutants that are directly emitted by road traffic. Results from the tracer study can be used to estimate the EFs under real urban conditions and to validate dispersion models which in turn can be used in the future to evaluate abatement strategies for such streets.

EPFL2009