Abstract. On 15–16 October 2017, ex-hurricane Ophelia passed to the
west of the British Isles, bringing dust from the Sahara and smoke from
Portuguese forest fires that was observable to the naked eye and reported in
the UK's national press. We report here detailed observations of this event
using the UK operational lidar and sun-photometer network, established for the
early detection of aviation hazards, including volcanic ash. We also use
ECMWF ERA5 wind field data and MODIS imagery to examine the aerosol
transport. The observations, taken continuously over a period of 30 h,
show a complex picture, dominated by several different aerosol layers at
different times and clearly correlated with the passage of different
air masses associated with the intense cyclonic system. A similar evolution
was observed at several sites, with a time delay between them explained by
their different location with respect to the storm and associated
meteorological features. The event commenced with a shallow dust layer at
1–2 km in altitude and culminated in a deep and complex structure that lasted
∼12 h at each site over the UK, correlated with the storm's warm sector.
For most of the time, the aerosol detected was dominated by mineral dust
mixtures, as highlighted by depolarisation measurements, but an intense
biomass burning aerosol (BBA) layer was observed towards the end of the
event, lasting around 3 h at each site. The aerosol optical depth at
355 nm (AOD355) during the whole event ranged from 0.2 to 2.9, with the
larger AOD correlated to the intense BBA layer. Such a large AOD is
unprecedented in the UK according to AERONET records for the last
20 years. The Raman lidars permitted the measurement of the aerosol
extinction coefficient at 355 nm, the particle linear depolarisation ratio
(PLDR), and the lidar ratio (LR) and made the separation of the dust
(depolarising) aerosol from other aerosol types possible. A specific extinction has
also been computed to provide an estimate of the atmospheric concentration of
both aerosol types separately, which peaked at 420±200 µg m−3 for
the dust and 558±232 µg m−3 for the biomass burning aerosols.
Back trajectories computed using the Numerical Atmospheric-dispersion
Modelling Environment (NAME) were used to identify the sources and strengthen
the conclusions drawn from the observations. The UK network represents a
significant expansion of the observing capability in northern Europe, with
instruments evenly distributed across Great Britain, from Camborne in
Cornwall to Lerwick in the Shetland Islands, and this study represents the
first attempt to demonstrate its capability and validate the methods in use.
Its ultimate purpose will be the detection and quantification of volcanic
plumes, but the present study clearly demonstrates the advanced capabilities
of the network.
A long Saharan dust event over the western Mediterranean: Lidar, Sun photometer observations, and regional dust modeling
