Mapping of traffic emissions from a busy road in a rural village using gaseous perfluorocarbon tracers

The exposure of populations to toxic traffic emissions is an important concern and significant research effort has been put into measuring and modelling exposure to traffic related pollutants within urban areas. It is also the case that busy trunk roads can pass through villages and towns subjecting those populations to both particulates and pollutant gases. Often, smaller villages in urban areas have complex topographies and provide a different environment to cities with regard to pollutant dynamics. Perfluorocarbon (PFC) trace gases are useful to measure the flow of gases within an area [1]. PFCs are inert, non-depositing and non-toxic and can be detected at low levels using sufficiently sensitive mass spectrometers and preconcentration devices [2]. To understand the passage of gaseous pollutants from a busy road passing through a rural village in Southern England, PFCs were released from a fixed and a moving source and sampled in several locations downwind.

Eight experiments occurred over three different measurement periods, three in June 2021, two in February 2022 and two in May 2022, covering different times of day and meteorological and road conditions. In each experiment, perfluoromethylcyclohexane was released from a fixed position approximately 400 m downwind of the road (July, Feb) or on the road west of the village (May) for 15 minutes, while 1,3-perfluorodimethylcyclohexane was released from the passenger side of a moving vehicle travelling with the flow of traffic through the village. 30-minute Tedlar bag samples were collected in up to 10 locations, some of these locations also measured PM₁₀, PM_2.5 and PM₁ using an Alphasense N3 optical particle sensor. Bag samples were stored separately from release equipment and transported to Bristol University School of Chemistry for analysis using the methodology described in [3].

Wind directions during the experiments were south westerly and westerly. The highest tracer concentrations from the moving source were often measured within a bus stop roadside in the centre of the village, whereas the static release was highest at a residential roadside sample on the western side of the village. Two samples were taken roadside in a church yard and at the top of the spire, the moving release was consistently higher at ground level, but from the stationary release the tracer was predominantly higher in concentration at roof height.  

[1] Shallcross, D. E. et al. 2009. Atmospheric Science Letters, 10(2), 59-65.

[2] Simmonds, P. G. et al .1995. Analytical Chemistry, 67(4), 717-723.

[3] Matthews, J. C. et al. 2020. Boundary-Layer Meteorology, 175(1), 113-134.