Air quality on UK diesel and hybrid trains

Concentrations of particulate matter (PM₁₀, PM_2.5), ultrafine (UFP), particle number (PNC), black carbon (BC), nitrogen dioxide (NO₂) and nitrogen oxides (NO_X) were measured in train carriages on diesel and bi-mode trains on inter-city and long-distance journeys in the United Kingdom (UK) using a high-quality mobile measurement system. Air quality on 15 different routes was measured using highly-time resolved data on a total of 119 journeys during three campaigns in winter 2020 and summer 2021; this included 13 different train classes. Each journey was sampled 4-10 times with approximatively 11,000 min of in-train concentrations in total. Mean-journey concentrations were 7.552 µg m^-3 (PM₁₀); 3.936 µg m^-3 (PM_2.5); 333-11,300 # cm^-3 (PNC); 225-9,131 # cm^-3 (UFP); 0.6-11 µg m^-3 (BC); 28-201 µg m^-3 (NO₂); and 130-3,456 µg m^-3 (NO_X). The impact of different factors on in-train concentrations was evaluated. The presence of tunnels was the factor with the largest impact on the in-train particle concentrations with enhancements by a factor of 40 greater than baseline for BC, and a factor 6 to 7 for PM and PNC. The engine fuel mode was the factor with the largest impact on NO₂ with enhancements of up to 14-times larger when the train run on diesel compared to the times running on electric on hybrid trains. Train classes with an age < 10 years observed the lowest in-train PM, BC and NO_X concentrations reflecting improvements in aspects of rail technology in recent years. Air quality on UK diesel trains is higher than ambient concentrations but has lower PM_2.5 and PNC than most other transport modes, including subway systems, diesel and petrol cars. This paper adds significantly to the evidence on exposure to poor air quality in transport micro-environments and provides the industry and regulatory bodies with reference-grade measurements on which to establish in-train air quality guidelines.