"There is an obvious risk associated with exhausting diesel emissions into a semi-enclosed environment, yet it has not received much study," Adam Boies of the University of Cambridge, UK, told environmentalresearchweb. "These results show that regulators should consider setting standards for train station air quality in the same way we regulate outdoor air quality. It is too soon to state what the health impacts for workers or patrons of the station are, but further study could provide those answers."

Paddington Station is a terminus, with trains only entering from one side and the rest of the building largely enclosed. The station delivers 38 million passengers to their destinations each year, acting as a hub for train services to the west and southwest. Around 70% of the trains at Paddington are powered by diesel engines; trains manufactured before 2006 are not subject to European regulations on their emissions.

"If the same standards that apply for outdoor air applied to indoor air, we found that pollution levels within Paddington Station exceeded such levels multiple times throughout the week we analysed," said Boies, "and it is likely that if a longer study was permitted by station operators, significantly more exceedances would have been recorded."

Boies and colleagues from the University of Cambridge, Minnesota State University Mankato and University of Minnesota installed kit at five locations inside the station to measure the mass of particulate matter (PM2.5) – on two platforms, near food outlets that cook with gas, on the ramp of the main station exit and on the roadside outside. They also monitored oxides of nitrogen (NOx), sulphur dioxide, and particulate matter size and number at two locations. The measurement campaign ran from 17 to 21 September 2012, with measurements beginning at around 4 am and lasting until the kit’s batteries ran out roughly 8 hours later.

Paddington Station’s hourly mean PM2.5 mass concentrations averaged 16 µg/cubic metre, whilst the hourly mean NO2 concentrations averaged 73 ppb and SO2 concentrations averaged 25 ppb. There were five instances when the hourly mean NO2 concentrations exceeded the 106 ppb hourly mean limit set by the EU for outdoor air quality. These standards allow 18 such hourly exceedances per year. At times the hourly PM2.5 averages exceeded the annual average EU standard for ambient air quality of 25 µg/cubic metre.

The team compared the station results with those from Marylebone Road, a busy street about 1.5 km away. PM2.5 from at least one measurement site within the station was higher than at Marylebone Road on three out of four days, whilst NO2 was higher within Paddington on four out of five days and SO2 was higher within the station on all three days it was measured.

"While the long-term solution for reduced pollution levels is likely electrification, we need not wait until electrification comes to take action," said Boies; there is currently a £6 billion programme of electrification on the Great Western Main Line that terminates at Paddington, with completion due in 2018. "The majority of diesel trains operating in Paddington Station were built prior to modern regulations and have no exhaust after-treatment whatsoever. We would never allow diesel vehicles into London that had such poor emissions control."

Adding diesel particulate filters with catalytic regeneration to trains could cut their particulate matter emissions by more than 90%, the team believes. "Another of our recent studies on London buses has shown that exhaust after-treatment devices are extremely effective at reducing pollution from diesel engines, while cost effective," said Boies. "These technologies can be retrofitted onto trains, which is something that should be strongly considered."

Around half the trainlines in Europe remain non-electrified, with 97% of Irish train lines, 71% of Danish, 47% of French, 41% of German and 29% of Italian train lines running by other means. Around 6.5% of all European travel is by train.

Boies and colleagues ran this study as part of the Energy Efficient Cities Initiative (EECi), which aims to determine the impacts on UK cities of energy use for transportation and in buildings. "We began to examine the Paddington area and realised that the unique function of train stations that house diesel locomotives may serve to trap emissions that would otherwise be dispersed by diffusion or wind," said Boies. "We found that very little was known about the air quality of stations housing diesel trains, thus we set out to conduct the first known study to determine whether air quality concerns existed with such stations."

Boies is now part of a UK working group on this topic. "There are many solutions to air quality problems, so long as we understand the root cause," he said. "I am hopeful that with further work, air quality in one of the UK’s most important transportation modes can be drastically improved."

The findings are published in Environmental Research Letters (ERL).

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