Research On The Spatial-temporal Characteristics Of Outdoor PM_2.5 Pollution And Indoor Control Methods In Typical Cities

According to a great number of existing studies on epidemiology and toxicology,a long-term exposure to PM_2.5 polluted environments may cause damage to the human respiratory system,cardiovascular system,nervous system,immune system,reproductive system,etc.Therefore,relevant research have gradually received considerable attentions.At present,in China,although it is difficult to adjust the outdoor PM_2.5 concentration to meet the world health organization?WHO?guidelines within a short period,the indoor PM_2.5 has the characteristics as easy control,long exposure time to occupants,significant health effects,etc.Carrying out the indoor PM_2.5 pollution prevention and control is an important way to reduce the risk of diseases induced by PM_2.5.Therefore,this research focuses on indoor particulate pollution and control methods based on the spatial-temporal distribution of outdoor PM_2.5pollution in typical cities in China.Firstly,the spatial-temporal distribution of outdoor PM_2.5 pollution in China is investigated by monitoring the big data.The results show that in 2019,the outdoor PM_2.5 pollution distribution has distinctive regional and seasonal characteristics.For cities at and above prefecture level,the outdoor PM_2.5 pollution distribution has a strong global spatial autocorrelation,while a strong local spatial autocorrelation was associated with most cities in northern China,southwestern China and western Xinjiang.Among them,a high pollution level was observed in the places like Beijing,Tianjin,Hebei and their surrounding cities,middle-lower Yangtze Plain,Fenwei Plain,Hotan and Kashi prefecture in Xinjiang Province,etc.,to which the specific attentions should be paid with prompt measures.Moreover,the trend of outdoor PM_2.5 concentration in typical cities and its correlation with meteorological factors are analyzed.Both the variation of average concentration of outdoor PM_2.5 in the past three years and Spearman's rank correlation coefficient have demonstrated that the average annual concentration of outdoor PM_2.5 in typical cities showed downward trends.Relative humidity,wind speed can pose great impacts to the PM_2.5 pollution.Moreover,outdoor relative humidity,wind speed and PM_2.5 mass concentration have the largest Pearson correlation coefficient in winter among the four seasons.Secondly,the indoor and outdoor particle size distribution,transport process,as well as the characteristics of indoor and outdoor pollutant sources have been studied.The comparative analysis of the sampling results indicated that the indoor and outdoor particle size distribution present obvious seasonal characteristics.In the transition season and non-epidemic heating season,the peak value of outdoor particle average concentration is between 2.5?5.0?m,and the peak value of outdoor practical average concentration in epidemic heating season lies between 0.5?1.0?m.Besides,personnel activities such as cleaning have a significant impact on the distribution of indoor large-size particles.Based on the fractal theory,the characteristics of indoor and outdoor particle size distribution have been extensively studied.Results show that the fractal dimension of indoor and outdoor particles in the heating season is larger than that in transition season,and the outdoor small size particle level in heating season is higher than that in transition season.In addition,the indoor PM_2.5 source emission intensity and PM_2.5 outdoor source contribution rate in a typical house were quantitatively analyzed through the method of PM_2.5 source intensity and source contribution rate estimation.In the past 6 years,the rebounded values were observed in some of the 74 cities in terms of the outdoor PM_2.5 average annual concentration,the PM_2.5daily average concentration without guarantee for 5 days,and the 95th percentile of PM_2.5 daily average concentration.Taking into account the influential factors(e.g.outdoor PM_2.5calculation concentration value,the actual unguaranteed time in the next year,the number of cities that exceeds the design unguaranteed time)and combining with the design specifications of heating ventilation and air conditioning?HVAC?,the average statistical method of the average daily mass concentration of PM_2.5 that does not guarantee 5 days in the past 3 years was proposed for the calculation of PM_2.5 outdoor parameters.This thesis further calculates the outdoor PM_2.5 parameters of the 74 cities by using this statistical method.Outdoor PM_2.5 mass concentration obviously possesses the characteristics of randomness,time variability and regionality.Through statistical methods,it is found out that there is an"inflection point"in the average daily outdoor PM_2.5 mass concentration of different cities in 2014-2019.There are differences in the distributions of the"inflection point"of the daily average mass concentration of outdoor PM_2.5 among different cities.The average"inflection point"of the 35 cities is between18 and 20 days from 2014 to 2019.This thesis also calculates the outdoor PM_2.5parameters of the 74 cities by using this statistical method of"average daily mass concentration of PM_2.5 that does not guarantee 18 days in the past 4 years".Finally,combined with PM_2.5 outdoor and indoor calculation parameters,penetration coefficient,air change rates and other key design parameters,the design methods of indoor PM_2.5 pollution control are developed for centralized,semi-centralized and decentralized systems respectively.Meanwhile,according to the characteristics of PM_2.5 pollution in different regions,the configuration and selection methods of air filters for fresh air units and air purifiers for residential buildings are recommended respectively.Combined with the estimated intensity of PM_2.5 indoor source,the air purifier selection schemes for different gears are proposed.In this thesis,different research methods have been adopted to find out the spatial-temporal distribution of outdoor PM_2.5 pollution in typical cities.The research results provided a scientific foundation for exposure research,as well as joint prevention and control of outdoor PM_2.5 pollution.Meanwhile,the size distribution and source characteristics of indoor and outdoor particles,indoor PM_2.5 pollution control design methods and equipment selection are systematically studied,which provides strategies and technical support for indoor PM_2.5 pollution control.The results and conclusions in the thesis have a good engineering application prospect.