Indoor and Outdoor Aerosol Particle Size Characterization in Helsinki

This thesis presented analysis and results from long-term and short-term aerosol particle measurements of indoor-outdoor total particle number and size distributions in Helsinki. This thesis focused on the temporal–spatial variations of aerosol particle number size distributions, the factors (including local wind, ambient temperature, and traffic density) influencing the particle number size distributions, the modal structure of aerosol particles, the indoor-to-outdoor relationship of aerosol particles, and the emission rates and fate of aerosol particles in the indoor air. Two mathematical models were developed: a multi log-normal distribution model and an indoor aerosol particle transport model. The highest levels of ultrafine particle number concentrations were observed nearby urban centers or close to a major road. The total particle number concentrations showed a slightly decreasing trend that was related to the improved engine technology used in new cars. On average, the UFP number concentrations contributed 70–95% in the urban and suburban atmosphere of Helsinki. In general, the particle number size distributions in the urban and suburban atmosphere of Helsinki were characterized by two, three, or more than three log-normal modes depending on different ambient conditions, traffic influence, and mixing between background and local emission of aerosol particles. The modal structure of the particle number size distribution that is directly influenced by traffic emissions is characterized by small geometric mean diameters. In general, indoor aerosol particles originate from outdoors and their number concentrations were found to follow similar temporal variations as those encountered outdoors. However, the number concentrations of indoor aerosol particles can not be solely estimated from the outdoor aerosol particle number concentrations during intensive indoor activities. In contrary to natural ventilation, mechanical ventilation systems provide well-controlled relationship between indoor and outdoor particle number concentrations. The variation of the quartile values of the indoor-to-outdoor particle number concentration ratio (I/O) can be used as a measure of the stability of the relationship between indoor and outdoor particle number concentrations. The time-lag between the temporal variations of indoor and outdoor particle number concentrations can be neglected in the I/O analysis when the ventilation rate is relatively high (> 2 h). Based on long-term data analysis, the I/O values vary from season to another. In general, the modal structure of indoor particle number size distributions in the indoor air had larger geometric mean diameters than those outdoors, mainly because of the filtration and penetration processes that reduce the number concentrations of ultrafine particles. Indoor aerosol models are capable of reproducing the measured indoor particle number concentrations with a good accuracy, and they are useful to predict the best-fit values of the parameters (penetration factor, air exchange rate, and deposition rate) that control the relationship between indoor and outdoor aerosol particle number concentrations. The emission rate of aerosol particles can be as high as 200 particle/cms during grilling in a fireplace and sauna heating. Indoor activities take place in another room may significantly increase the aerosol particle number concentrations in other rooms of the same building. The results obtained in this thesis are important to the exposure assessments to harmful atmospheric aerosol particles indoors and outdoors. The long-term aerosol data sets and the analysis of the modal structure of the ambient particle number size distributions are also useful in the development of urban aerosol models. Indoor aerosol models and indoor-outdoor aerosol data sets presented in this thesis provided more understanding to the physical characterization of particle number size distributions.