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

Abstract

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.

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“…According to previous studies (Borge et al, 2007;Guo et al, 2017;Osborne et al, 2019;Zheng et al, 2015), backward trajectories for identified particulate matter pollution episodes were computed using Numerical Atmospheric-dispersion Modelling Environment to identify the impacts of transboundary air pollution and local PM pollution in Hong Kong (Jones et al, 2007). Briefly, the Numerical Atmospheric-dispersion Modelling Environment model is a Lagrangian transport model that is driven by model forecast winds or reanalysis data with variable spatial resolutions (Walters et al, 2017).…”

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Multi-Sensor Observation of a Saharan Dust Outbreak over Transylvania, Romania in April 2019

“…Studies such as [14] show that transport over larger ranges are expected during April through June, while vertical transport is highest from April through September. Under certain meteorological conditions, significant amounts of Saharan dust are transported north, over the Mediterranean basin [15][16][17] and in severe cases, even over Western, Central, and Eastern Europe [18][19][20][21]. Dust outbreaks have been observed as far as northern Europe and the Baltic Sea [10,22].…”

A complex aerosol transport event over Europe during the 2017 Storm Ophelia in CAMS forecast systems: analysis and evaluation

“…On the morning of October 16 th 2017 a red sky phenomenon was reported on several sites of the United Kingdom (UK) (Telegraph, 2017), as a result of the high aerosol loadings in the overlying atmosphere (Harrison et al, 2018;Osborne et al, 2019), while similar reports on the morning of October 17 th were given for the Netherlands as well (NLTIMES, 2017). These high aerosol loadings were the result of transport of desert dust from Northern Africa and smoke from Iberian wildfires.…”