Investigation of PM10, PM2.5, PM1 in an unoccupied airflow-controlled room: How reliable to neglect resuspension and assume unreactive particles?

Abstract

It is now well recognized that Particulate Matter (PM) is one of the main air pollutants affecting both ambient and indoor air quality. While ambient PM mass concentration measurements are often performed by air quality monitoring networks, current regulations do not address their indoor concentrations. The latter can be estimated nonetheless from a mass balance analysis accounting for (i) the transfer of particles from outdoor and (ii) their typical indoor sources (emissions, resuspension) and sinks (deposition, removal by air exchange). Inherently, the mass balance analysis is valid with inert atmospheric species, i.e. mass is conserved and described by sources and sinks, thus assuming no physicochemical transformations. To check that the relative imbalance (RI) is not significant over different size fractions, a series of careful measurements were conducted in an unoccupied room within a building incorporating an energy-efficient design (minimal heat losses, negligible air leaks, etc.). We show that by carefully characterizing the room properties, i.e. the air exchange rate, penetration factors and deposition rates for sized-resolved particles, the PM10 and PM2.5 fractions, which are typically regulated outdoors, as well as PM1, can be well estimated indoors (with RI between measured and expected values < 19%) under moderate ambient temperatures (<22 °C in this study). However, RI increases significantly, especially for submicron particles, at higher temperatures, indicating possible transformations in the particulate phase, which are not accounted for by the mass balance model. Therefore caution is recommended regarding the mass balance analysis to estimate PM fractions indoors, especially for PM1.