Source Apportionment and Influencing Factor Analysis of Residential Indoor PM2.5 in Beijing

1. Introduction

The majority of adults spend more than 80% of the day in a variety of indoor environments, mainly in their houses [1,2,3]. The contribution of indoor air is very important for accurate estimation of individual air pollution exposure. A vast number of epidemiological studies have shown that both short and long-term exposure to fine particulate matter (PM_2.5) are harmful to the human respiratory and cardiovascular systems [4,5,6,7,8,9,10]. However, the PM_2.5 exposure assessment in most of these studies is usually based on ambient PM_2.5 data, regardless of the difference between indoor and outdoor PM_2.5 concentration. And failure to account for the contribution of indoor PM_2.5 concentration may lead to exposure misclassifications, which have become source of measurement bias in the PM_2.5 epidemiologic studies linking health effects to PM_2.5 exposure [11,12,13,14].

Indoor PM_2.5 comes from both outdoor and indoor sources. Outdoor PM_2.5 typically comes from sources such as fossil fuels combustion, motor vehicle exhaust, industrial emissions, soil and dust, and particles from these sources can penetrate into indoor through building gap, door, window and mechanical ventilation [12,15,16,17,18]. A survey conducted in Beijing showed that 54-63% of indoor PM_2.5 originated from the outdoors with all windows closed [15]. Indoor sources such as smoking, cooking and indoor activities, also can generate particles to increase indoor PM_2.5 concentration, those sources have been recognized as another important contributor of indoor PM_2.5 [19,20,21,22,23].

The source apportionment of outdoor PM_2.5 have been explored in many studies across the world [16,17,18,24,25,26,27,28], but information about source apportionment of indoor PM_2.5, especially for residential houses, is relatively scarce. Chemical elements, including metal and metalloid elements, are common constituents of PM_2.5 in urban cities, and the use of elements as tracers to identify PM_2.5 from different sources has been applied in many studies [16,25,29,30,31,32,33,34,35,36]. Chemical elements in outdoor PM_2.5 can be divided into several tracer groups for different sources, such as, Al, Mg and Ca which are usually used as trace elements for soil dust, V and Se are usually used as trace elements for fossil fuel combustion; Pb and Cu are usually used as trace elements of motor vehicle exhaust [16,29,30,31,32,33,34,35,36]. Indoor sources can...