Indoor Air Quality Monitoring And Ventilation Control Strategies For Residential Buildings In Xi’an City

Affected by the superimposition of the Qinling Mountains and the Loess Plateau,the situation of outdoor air pollution in Xi’an is grim.Besides,with the improvement of the residents’economic level,large amount of decorative materials and composite wooden furniture are used for decoration,leading to serious over-standard emission of indoor formaldehyde and other volatile organic compounds.In addition,the building airtightness has increased because of the development of green building concepts and the tightening of corresponding national standards,residents are beginning to be threatened by over concentrated indoor CO₂.Such indoor and outdoor air pollution poses serious threat to the occupants is posed,and thus urgent study on the exposure levels of various indoor pollutants and their influencing factors is needed to propose reasonable and effective indoor ventilation control strategies to protect the health of residents in Xi’an.This paper used a combination of household measurement and online monitoring to study indoor pollutants,and carried out household measurement in 19 typical households in Xi’an in four typical seasons,spring,summer,autumn and winter,and 31households from January 2017 to 2019 Continuous online monitoring in June.The distribution rules of indoor formaldehyde,CO₂ and indoor and outdoor PM2.5concentrations were studied from the two time dimensions of full period and season,and the most unfavorable working conditions of each pollutant were determined.The role of CO₂ and PM2.5 elimination.The simulation research method was used to obtain the control strategies under various ventilation modes.The specific research results are as follows.The analysis of the whole monitoring period shows that the measured annual average concentration of formaldehyde in the household ranges from 40.6 to 62.8μg/m³,and the annual average concentration of online monitoring range is 30.1～42μg/m³;the annual average concentration of CO₂ concentration ranges from 563.3 to 705.9 ppm;PM2.5 indoor annual average concentration range is 59.6～71.4μg/m³,outdoor average annual concentration range is 66～545.7μg/m³,indoor and outdoor PM_2.5.5 concentrations show a sharp increase in autumn and winter,and a sharp drop in spring and summer distribution,showing a"wave-shaped"distribution trend.In the seasonal distribution law,the seasonal distribution of formaldehyde is the highest in summer and the lowest in winter;the distribution law of CO₂ concentration is negatively correlated with the change of temperature.The rate of exceeding the standard at night is about 3 times than that of daytime.Ventilated residents are better.The indoor and outdoor PM2.5concentration changes have a strong negative correlation with the outdoor temperature.In winter and spring,the outdoor PM2.5 concentration is greater than the indoor PM2.5concentration,and vice in summer and autumn.In view of the most unfavorable working conditions of various pollutants,the operation strategy and effect of the existing ventilation methods were studied.The results show that:1)For formaldehyde,the natural ventilation mode can reduce the indoor formaldehyde concentration within 1h after opening the window.The national standard value is 100μg/m³,and the decrease range is more than 50%;in the natural ventilation+air purifier ventilation mode,the usage of air purifier cannot effectively reduce the indoor formaldehyde concentration;in the mechanical ventilation mode,the formaldehyde concentration reaches lowest point 3h after the fresh air fan is turned on and the decline was about 19.1%.2)For CO₂,after opening the window for 30 minute in natural ventilation mode,the indoor CO₂ concentration can be below the national standard limit of 1000ppm.Therefore,the window can be opened regularly without affecting the comfort requirements of the indoor occupants.For natural ventilation plus air purifier ventilation mode,the best is still based on natural ventilation（opening and closing purifier）,supplemented by air purifier.In the mechanical ventilation mode,turning on the fresh air fan,especially at night,can control the indoor CO₂ concentration.3)For PM2.5,there is no obvious relationship between the natural ventilation behavior like switching windows and indoor PM_2.5.5 concentration changes,and the main factor is still the influence of outdoor PM_2.5.5 concentration.In the natural ventilation plus air purifier ventilation mode,it is better to use a window-opening purifier or a window-shutting purifier,but the best time to use the latter is when the outdoor pollution is not serious.In the mechanical ventilation mode,the effect of the fresh air fan on the indoor PM2.5 concentration is much greater than that of the switch window;the best way is to close the window and open the fan,which can make the indoor PM2.5concentration the lowest.Regarding the control strategy of the ventilation mode,the simulation results show that the two-resident with full-window mode（Scheme 1）cannot meet the indoor PM2.5concentration control;When the temperature is 16℃≦T≦28℃and the PM_2.5concentration is less than 75μg/m³,open the window（Scheme 2）.Compared with scheme 1,it can reduce the indoor PM2.5 concentration and the unsatisfactory rate,but the CO₂ unsatisfied rate is increasing,especially in the bedroom.Add an air purifier(PM_2.5.5 concentration<75μg/m³)and two specific time periods（7:00 to 8:00 in the morning and 22:00 to 23:00 at night）to pen the window,add a fan to the mechanical ventilation(PM_2.5.5 concentration<75μg/m³)（Scheme 3）.The results show that both can make the indoor PM_2.5.5 annual average concentration below 35μg/m³,and the indoor CO₂ annual unsatisfactory rate below 10%.