Study On Outdoor-indoor Correlation And Tracking Characteristics Of PM_2.5-associated Iron

In recent years,many studies have shown that the iron element in PM_2.5 is closely related to human oxidative stress and endangers people’s health.Because the human body is exposed indoors for a long time,and outdoor PM_2.5 is the main source of indoors,it is particularly necessary to study the indoor and outdoor correlation and traceability of iron in PM_2.5.This article sampled PM_2.5 in indoor and outdoor environments from July 2018 to December 2019,and continuously measured the iron concentrations（total iron content and water-soluble content）in the equipment using ICP ion chromatograph and other equipment.This paper analyzed the indoor and outdoor correlation characteristics of total iron in PM_2.5 and its tracking effect on PM_2.5.In addition,a preliminary study on the proportion of water-soluble iron ions in total iron and influencing factors,the main contents are as follows:（1）The indoor and outdoor distribution characteristics of total iron concentration in PM_2.5 in Nanjing were obtained.The indoor and outdoor correlation characteristics of total iron elements were analyzed by I/O ratios.It is found that the indoor and outdoor I/O ratios of total iron elements in PM_2.5 ranged from 0.17 to 0.96,and the average I/O ratio is 0.51±0.22,with a median value of 0.47.The ranking of indoor and outdoor correlation intensity of total iron in different seasons is:spring>autumn>winter>summer.The ranking of total iron element I/O ratio in different indoor places is:dormitory place>office place>meeting place.The indoor and outdoor correlation intensity of total iron and PM_2.5 have the characteristics of spatiotemporal changes,but their variation laws are not the same.In addition,a survey was conducted on relevant global literature in the past decade,and the influence of different regions and indoor sampling places on the concentration of total iron in PM_2.5 was analyzed.It was found that the ranking of indoor total iron concentration in different regions was:North America<Europe<Asia<South America;the ranking in different indoor places was:Dormitory<Conference Room<Office<Residence<School<Laboratory.（2）The trace effect of total iron on outdoor PM_2.5 transmission from outdoor to indoor was analyzed.The standardized ratio is defined as the ratio between the I/O ratio of the total iron element concentration in PM_2.5and the I/O ratio of PM_2.5 concentration.The range of Nanjing standardized ratio is 0.22 to 1.90,with an average value of 0.92±0.42.The standardized ratio is close to 1,indicating that the traceability of iron in Nanjing is good.At the same time,through a survey of relevant literature in the world for the past 10 years,it is found that the average value of the standardized ratio of iron in other literatures is 0.93,which is very close to this article（0.92）.Therefore,we believe that the total iron element is good for the tracking of outdoor PM_2.5 from outdoor to indoor transmission.In addition,the temporal and spatial variation characteristics of the total iron tracer effect are also discussed in the article.It is found that the tracer effect in autumn and winter is significantly better than that in spring and summer.The ranking of tracking effect in different seasons is:autumn>winter>spring>summer.The ranking of the tracking effect in different regions is:Asia>South America>Europe>North America.The ranking of the tracking effect in different indoor fields is Office>Lab>Meeting Room>Hospital>Student Dormitory>School.The tracer effect of total iron has obvious space-time heterogeneity.（3）The proportion of water-soluble iron ions in total iron and its influencing factors were initially explored.First,the characteristics of the spatial-temporal concentration of water-soluble iron ions and insoluble metal iron were analyzed.We calculated a special value（ratio of indoor water-soluble iron ions to total iron minus ratio of outdoor water-soluble iron ions to total iron）.The ratios ranged from-0.64 to 0.54,with an average value of-0.03±0.27.Compared with temperature and relative humidity,the p H value may be an important factor that affects the proportion of water-soluble iron ions in total iron.That is,when the indoor p H is greater than the outdoor p H,the ratio of indoor water-soluble iron ions to total iron tends to be less than the ratio of outdoor water-soluble iron ions to total iron.