Outdoor–Indoor VOC Dynamics: Insights from multi-site measurements and impacts of long-range transport on indoor air quality

Indoor concentrations are consistently higher than outdoors for most VOCs due to indoor sources and limited ventilation. In Finland, outdoor VOC concentrations are low even in urban areas. However, sometimes outdoor air can be impacted by long-range transported (LRT) pollution and concentration levels of VOCs increase. Some of these VOCs also impact the indoor air. In this study we compared indoor-outdoor ratios of VOCs in different kind of environments in Finland (Vantaa and Tampere) Czech Republic (Prague) and Germany (Düsseldorf).Carbopack B tubes were collected at five different locations, both indoors and outdoors; two daycare centers in Tampere, office in Vantaa, office in Düsseldorf and a high school in Prague. In Vantaa, an online thermal desorption–gas chromatography–mass spectrometry (TD-GC-MS) system was additionally employed to measure indoor and outdoor air with a time resolution of 2 hours over the two-week period.During the Vantaa campaign with more intensive online VOC measurements a LRT event occurred providing an opportunity to investigate its impact on indoor air quality. For certain compounds, such as benzene, which is not effectively removed by building ventilation, a clear correlation was observed: as outdoor concentrations increased, indoor concentrations rose accordingly, closely matching the outdoor levels. Up to four times higher concentrations were measured both indoors and outdoors during the event. Benzene is a known carcinogenic and regulated by the EU. Similar levels in indoor and outdoor air were also observed in passive samples in all locations for benzene and tetrachloromethane indicating that the main source is in outdoor air. For most other compounds higher indoor air levels were detected. Especially for monoterpenes (α-pinene, 3Δ-carene and limonene) and octane clearly higher concentrations were measured in indoor air indicating strong indoor sources.