Spatial-temporal Distribution Of Indoor And Outdoor PM_2.5 Oxidative Potential And Key Constituents

Oxidative potential（OP）of PM_2.5 is an emerging health indicator representing its ability to induce oxidative stress and then cause adverse health effects,such as respiratory and cardiovascular diseases.Epidemiological studies have revealed that PM_2.5 OP is more strongly correlated with adverse health effects than PM_2.5 mass concentration.The mass-normalized OP representing the intrinsic property of PM_2.5.And the volume-normalized OP characterizes the overall PM_2.5 oxidative exposure burden,which is affected by both PM_2.5 mass concentration and OP_M(OP_V=C_PM2.5·OP_M).As we know,people spend more than 85%of their lifetime indoors.Indoor PM_2.5 is mainly from outdoor environments.It is essential to study indoor PM_2.5OP of outdoor origin to develop mitigation strategies and health estimation of PM_2.5.We examined pollution levels,temporal variations,and key constituents of PM_2.5 OP in both indoor and outdoor environments in Nanjing through DTT assay.The main contents are as follows:From December 2018 to October 2020,average outdoor PM_2.5 OP_V and OP_M in Nanjing were 1.9±1.1 nmol/min/m³ and 52±26 pmol/min/μg,respectively.Outdoor OP_V and OP_M of Nanjing were at a medium level to results in the literature for twenty-nine cities over the world.PM_2.5 mass concentration and OP_V consistently decreased during outdoor-to-indoor transport,outdoor and indoor difference percentages of OP_V and PM_2.5mass concentration are different,42%of the data showed that difference between these two percentages is more than 20%.OP_Mwas found to vary by a factor of up to 2 after PM_2.5 entering the room,which would be increase or decrease.And one-third of all OP_M has a difference greater than 50%between indoors and outdoors.This finding indicates that outdoor OP does not represent that of indoors.Only decrease PM_2.5 mass concentration cannot reduce OP_V in an equal proportion.Neglecting the existence of OP_M will make the health effect less than expected.Then,we measured the seasonal and day-night variation of PM_2.5 OP.Outdoor and indoor OPV has no significant seasonal variation.Both outdoor and indoor OP_M exhibited a significant seasonality as summer>autumn>spring>winter.Daytime and nighttime variation of indoor and outdoor PM_2.5 OP_V and OP_M fluctuated within a two-fold range.Seasonal and daytime-nighttime changes indicated temporal heterogeneity of PM_2.5 OP.Furthermore,seasonal rank of outdoor PM_2.5 mass concentration was winter（70.6±30.4μg/m³）>spring（54.3±25.7μg/m³）>autumn（36.7±18.1μg/m³）>summer（30.8±10.7μg/m³）,seasonal rank of outdoor OP_Vwas not consistent with that of OP_Vspring（2.5±1.6 nmol/min/m³）>winter（2.3±0.7nmol/min/m³）>summer（1.8±0.8nmol/min/m³）>autumn（1.7±1.0nmol/min/m³）.PM_2.5 and OP_V seasonal ranks were different,indicating that emission source of PM_2.5 and OP_V may be different in different seasons.Finally,Pearson correlation between the change of PM_2.5 compositions concentration and proportion and the change of OP_V and OP_M was analyzed during outdoor-to-indoor transport to provide useful information on building control of oxidative potential.The change in water-soluble Fe（ΔFe-WS）had a significant correlation withΔOP_M andΔOP_V（P<0.05）indicating that water-soluble Fe contributes to OP_M and OP_V.While the change in acid-soluble Fe had no significant correlation withΔOP_M andΔOP_V.This may be because the water-soluble iron is more easily absorbed by human body and participates in catalytic Fenton reaction in the body to produce oxidative species.Besides,water-soluble ions,such asΔNO₃^-,ΔSO₄^2-,ΔNH₄⁺,ΔCl^-exhibited a significant correlation withΔOP_V（P<0.01）,while onlyΔSO₄^2-shows a significant correlation withΔOP_M（r=0.60,P<0.01）,andΔSO₄^2-exhibited a positive significant correlation withΔFe-WS.This may be related to the water solubility of Fe is affected by the sulfate content.The above results indicated that the significance of controlling the main sources of Fe,such as dust and traffic pollution.