Layers of aerosol at heights between 2 and 11 km were observed with Raman
lidars in the UK between 23 and 31 May 2016. A network of these lidars,
supported by ceilometer observations, is used to map the extent of the
aerosol and its optical properties. Space-borne lidar profiles show that the
aerosol originated from forest fires over western Canada around 17 May, and
indeed the aerosol properties – weak volume depolarisation ( < 5 %) and a
lidar ratio at 355 nm in the range 35–65 sr – were consistent with
long-range transport of forest fire smoke. The event was unusual in its
persistence – the smoke plume was drawn into an atmospheric block that kept
it above north-western Europe for 9 days. Lidar observations show how the
smoke layers became optically thinner during this period, but the lidar ratio
and aerosol depolarisation showed little change. The results demonstrate the
value of a dense network of observations for tracking forest fire smoke, and
show how the dispersion of smoke in the free troposphere leads to the
emergence of discrete thin layers in the far field. They also show how
atmospheric blocking can keep a smoke plume in the same geographic area for
over a week.</p
