Urban Bus Air Quality: A technical Guide based on Barcelona BUSAIR data

Urban public buses worldwide carry hundreds of millions of passengers, all of whom inhale atmospheric pollutants in the form of particles and gases during what is commonly a daily commute. These urban journeys can provide a disproportionate percentage of the individual’s average daily exposure to inhalable contaminants, and recognition of this fact is reflected in a large body of scientific literature on air pollution and city transport that goes back over 50 years (e.g. Haagen-Smit, 1966; Fernández-Iriarte et al., 2020). The number of such publications has increased rapidly in recent years, accompanying the recognition of the damage to human health being inflicted by inhaling polluted urban air whilst travelling by road (e.g. Alameddine et al., 2016; Cepeda et al., 2017; Fruin et al., 2011; Hudda et al., 2011, 2012; Hudda and Fruin 2018; Jo and Yu, 2001; Leavey et al., 2017; Lee et al., 2015; Madl et al., 2015; Moreno et al., 2015, 2020; Tartakovsky et al., 2013; Xing et al., 2018; Yang et al., 2015; Zhu et al., 2007). A subset of publications dealing with air quality associated with transport microenvironments has included information specifically on buses and bus stops (e.g. Adams et al 2001; Asmi et al., 2009; Bel and Holst 2018; Chernyshev et al., 2018; Choi et al., 2018; Dales et al., 2007; Dons et a., 2012; Fernández- Iriarte 2020, 2021; Gajewski 2013; Hess et al., 2010; Li et al., 2009; Lim et al., 2015; Merritt et al., 2019; Moore et al., 2012; Moreno et al., 2015, 2020, 2021; Nogueira et al., 2019; Rivas et al., 2017; Schimek et al., 2001; Van Ryswyk et al., 2020; Velasco and Tan 2016; Wang et al., 2011). Most of these studies, whilst all providing valuable data, are based on relatively short sampling campaigns or models and commonly focused on a limited number of specific contaminants such as PM10 or PM2.5 mass, number concentrations of ultrafine particles (UFP), and/or levels of gaseous pollutants. The ambition of the Barcelona BUSAIR project was to provide a more integrated study by utilising a broad spectrum of monitoring instruments measuring particulate and gaseous pollutants simultaneously inside vehicles under normal weekday operating conditions across four seasons of the year. The sampling phase of the project took place from May 2017 to April 2018, backed up by detailed data on background city air conditions, and produced the largest freely available database on urban bus air quality to date. This Technical Guide overviews these data, identifies key influences on bus air quality, and draws conclusions and recommendations aimed to stimulate future reductions in public transport passenger exposure to urban pollutants.This work is supported by the Spanish Ministry of Economy, Industry and Competitiveness with FEDER funds (BUSAIR CGL2016-79132-R) and by CSIC with the “Proyecto Intramural: Exposición a compuestos organicos volatiles en el interior de autobuses de transporte publico urbano” (201730I003). Funding from the Generalitat de Catalunya (AGAUR 2017 SGR41) is also acknowledged. We thank J. Grimalt and E. Marco (IDAEA), and R. Pintò and A. Bosch (University of Barcelona) for their research collaboration in the publications on VOCs and SARS-CoV-2 respectively. We are grateful to Transports Metropolitans de Barcelona (TMB) for their co-operation throughout the whole project. The editor wishes to thank Wes Gibbons for his constant support and positive ideas. IDAEA-CSIC is a Centre of Excellence Severo Ochoa (Spanish Ministry of Science and Innovation, Project CEX2018-000794-S).Peer reviewe