From outdoor to indoor air pollution source apportionment: Answers to ten challenging questions

Indoor air pollution source apportionment (SA) is an evolving field and remains challenging to adjust the gained knowledge from atmospheric studies to the indoor environment. This paper identifies ten key-questions on indoor SA. Firstly, the scientific challenges of indoor SA are presented, including the differences between indoor and outdoor SA (Q1), the appropriate tracers (Q2), the challenges for indoor organic PM and VOCs apportionment (Q3) and the effect of oxidative reactions on indoor SA (Q4). Afterwards, the advances of indoor air monitoring/analysis are discussed towards an optimized indoor air SA (Q5-6). Q7 and Q8 review the type of receptor models best applying for indoor SA and discuss how the advances in online outdoor SA can benefit indoor SA. Q9 deals with the optimization of the estimation of the outdoor sources' contribution to indoor air. Finally, the challenge of using machine learning techniques for indoor SA is discussed in Q10.