Modelling ultrafine particle concentrations near highways

Abstract

Air quality plays a large role in human health and lifespan. Millions of people die every year from diseases caused by air pollution, such as heart and lung diseases, and cancer. One of the components of air pollution which has come into the focus of attention recently, is ultrafine particles (UFPs). These are particles smaller than 100 nanometers. These are believed to be especially dangerous, because they may be able to enter our bodies directly, by passing through the lungs into the bloodstream, and cause serious health problems. One aspect of UFPs that is not yet well understood is how they move and behave. Therefore, the main topic of this thesis is understanding how these UFPs behave near highways, because highway traffic is a significant source of UFPs. These particles are then transported away by the wind. Our aim was to see how far these particles travel, and how their concentrations change over relatively short distances and times. We used a combination of measurements and a computer model called HAM/SALSA2.0 to study this. This model can simulate how particles behave in the air. We modified the model to include a constant source of particles, and to spread the particles through the system. By doing this, we hoped to simulate the observations made in the measurement data. It was found that the concentration of the smallest UFPs decreases quickly as you get farther away from the highway. It takes only a few hundred meters for their concentrations to drop to background levels. We were able to reproduce these findings with the model, where a stable background also quickly appeared. We also studied how weather conditions such as temperature and humidity affect the concentrations of these particles. We found that the patterns seen in the model matched those seen in measurements. However, this model also has some limitations. It is difficult to directly relate the model’s results to real-world conditions such as wind speed, or at what distance a stable background of particles appears. The model also assumes that the UFPs consist of six types of substances, in certain proportions. This consistency may vary depending on the source of the UFPs. Furthermore, the model cannot simulate the production of new particles from gaseous vehicle emissions, which is a large portion of air pollution. To improve the model, future research could adjust how particles move in and out of the modelled system. Finding a way to relate the model’s results to real-world conditions is important for understanding the model’s applicability. Besides this, more measurements at varying distances from highways are needed, to see if the patterns seen in this study occurs in more environments.