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© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 22 Aug 2018

Research article | 22 Aug 2018

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This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Saharan dust and biomass burning aerosols during ex-hurricane Ophelia: validation of the new UK lidar and sun-photometer network

Martin Osborne1,2, Mariana Adam3, Joelle Buxmann1, Jaqueline Sugier1, Franco Marenco1, and Jim Haywood1,2 Martin Osborne et al.
  • 1Met Office, FitzRoy Road,Exeter, Devon, EX1 3PB, UK
  • 2University of Exeter, Laver Building, North Park Road, Exeter, Devon, EX4 4QE, UK
  • 3National Institute for R&D in Optoelectronics INOE2000, Str. Atomistilor Nr. 409, Magurele, Ilfov, 077125, Romania

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 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. The observations, taken continuously over a period of 30 hours, show a complex picture, dominated by several 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. The event commenced with a hallow dust layer at 1–2km in altitude, and culminated in a deep and complex structure that lasted 12 hours at each site, correlated with the storm’s warm sector. For most of the time, the aerosol detected as mineral dust, as highlighted by depolarisation measurements, but an intense smoke layer was observed towards the end of the event, lasting around 3 hours at each site. The aerosol optical depth AOD) during the whole event ranged from 0.2 to 2.9, with the larger AOD correlated to the intense smoke plume. Such a large AOD is unprecedented in the United Kingdom according to AERONET records or the last 20 years. The Raman lidars permitted the measurement of the aerosol extinction coefficient at 355nm, the particle depolarisation ratio (PDR) and the lidar ratio (LR), and made possible the separation of the dust (depolarising) aerosol from other aerosol types. A specific extinction has also been computed to provide an estimate of the atmospheric concentration of both aerosols separately, which peaked at 500±100μgm−3 for the dust and 600±100μgm−3 for the smoke. 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.

Martin Osborne et al.
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Martin Osborne et al.
Martin Osborne et al.
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Publications Copernicus
Short summary
In this paper we present an analysis of the unusual "Red sky" event that occurred over the United Kingdom on the 15th and 16th of October 2017. We use measurements from the Met Office operational lidar / sun-photometer network, as well as other data and model output, to show that the event was caused by the passage of Ex-Hurricane Ophelia which transported unusual amounts of dust from the Sahara desert to the UK, as well as smoke from forest fires in Portugal.
In this paper we present an analysis of the unusual "Red sky" event that occurred over the...