Chemical Characteristics And Relationship Of Indoor-Outdoor PM_2.5 During Haze-Fog Episodes

Haze-fog (HF) episodes have been frequent occurrences in China in recent years, atmospheric fine particulate matter (PM2.5) pollution is serious, but approximately 85%-90% of our daily life is spent indoors. The study about the chemical composition and relationship of indoor and outdoor PM2.5 during HF episodes is thus crucial for a proper evaluation of individuals’actual exposure to PM2.5 and the concomitant health risk. Research on chemical characteristics of indoor PM2.5 during HF episodes is still insufficient, in order to understand the characteristics and relationship of indoor and outdoor PM2.5 during HF events, PM2.5 samples inside and outside typical lab were collected simultaneously from 12th January to 4th February 2013, subsequently analyzing 10 inorganic water-soluble ions (WSIIs) including F-, Cl-, NO2-, NO3-, SO4-, Na+, NH4+, K+, Mg2+ and Ca2+, organic and element carbon(OC and EC) and trace elements including Al, Na, Cl, Mg, Si, K, Ca, Mn, Fe, Cu, Zn, Se, Br and Pb. Meteorological parameters including temperature, relative humidity and wind speed was also measured. The main conclusions are displayed as follows:The average mass concentration of indoor and outdoor PM2.5 in non-haze-fog (NHF) episodes were 62 and 87 μg·m-3, respectively, and increased to 236 and 409 μg·m-3, respectively, in HF events, which significantly exceeded the outdoor PM2.5 daily average limit of 75 μg·m-3 recommended by the Chinese Ministry of Environmental Protection in 2012 by a factor of 3.14 and 5.45, respectively, indicating that indoor PM2.5 pollution is serious when outdoor PM2.5 level is high. So people should stay in rooms in HF events to reduce the exposure to PM2.5.The predominant component is (NH4)2SO4, which accounted for 55.07% and 44.43% of the total measured species of indoor and outdoor PM2.5, respectively, in the HF episodes, and 40.31% and 32.93%, respectively, in the HF episodes. The indoor NH4NO3 level is significantly lower than the outdoor level due to volatility. The OC and EC levels obviously increased and the ratio of the secondary organic carbon (SOC) to OC also augmented in HF events, which suggests HF events could promote the accumulation of carbon compositions and the formation of SOC. Cl, K, Fe and Si are the most abundant among all measured elements in PM2.5, the concentrations of elements are higher in HF episodes than those in NHF episodes. The elements derived from coal and motor vehicles enrich apparently.The indoor and outdoor concentration ratios (I/O) of PM2.5, NO3-, SO42-, NH4+, OC and EC are lower than one during HF and NHF episodes. The infiltration rate (FINF) of PM2.5 is 0.53, that is to say that approximately half of the outdoor PM2.5 enter the lab during the HF period, the indoor-generated PM2.5 accounts for 8% of the total indoor PM2.5, which maybe come from human activities and re-suspended particles. The I/Os of Cl under different weather conditions are lower than those of other elements, the I/Os of Cu during HF and NHF events are the highest among elements. The FINFs of Si, Ca, Al and Mg are similar, but I/Os of them are obviously different, which result from indoor-generated sources of Ca and Mg. the FINFs of Pb and Zn are closed to 0.62 and higher than the FINFS of Si, Ca, Al and Mg.The FINFs of three main WSIIs are in the order of FINF (SO42-)> FINF (NH+> FINF (NO3-), NH4+ and NO3- are simi-volatile constituents, and transform from the particle phase to gas phase from outdoor to indoor environment. EC and SO42- have similar FINF values due to chemical stability and their FINFs higher than the other species. The indoor sources of NO3- and SO42- could be neglected. However, NH4+ has significant indoor-generated sources such as building material and toilet emission. The use of muffle furnace and organic chemistry could explain indoor-generated OC.