Forest fires occur every summer over the boreal forest of the Northern Hemisphere (; ). The smoke from these fires can be lifted to great heights by deep convection – indeed, the fires can amplify the storms leading to pyroconvection . Once deposited in the free troposphere or stratosphere, forest fire smoke can travel great distances, e.g. from North America to Europe , from Siberia to Europe , from Siberia to Japan or even around the globe . Long-range smoke transport has also been observed at lower latitudes, from Africa to South America . In this paper we discuss a transport event that occurred in May 2016, when smoke from intense fires in western Canada reached Europe and was observed by the UK lidar network. The fires in this case caused headlines around the globe due to the destruction of the Canadian town Fort McMurray (56.72∘ N, 111.38∘ W) in north-east Alberta (https://en.wikipedia.org/wiki/2016_Fort_McMurray_Wildfire, last access: 31 July 2018).

Raman lidars have been used extensively to study long-range smoke transport, measuring optical and microphysical properties and calculating the age and origin of the smoke e.g.. These studies have found smoke particles to have effective radii of less than 1 µm. A summary of 10 years of lidar measurements at Leipzig was provided by and , which included a number of long-range smoke events. Smoke was found to be twice as likely in summer as in winter and originated both from North America and Siberia. The extinction to backscatter ratio (lidar ratio, LR) of aged smoke was found to be lower at 355 nm (46±13 sr) than at 532 m (53±11 sr), in contrast to fresh smoke, for which values for both wavelengths are around 60 . Particle depolarisation values at 532 nm are generally <5 % , but there is evidence that for aged smoke this quantity can be substantially larger at 355 nm .

Here we show how a dense network of lidars and ceilometers tracked the evolution of a smoke episode from 23 to 31 May 2016 as an atmospheric block trapped the air over western Europe. Space-borne lidar data from the CALIOP and CATS instruments, supported by SEVERI images from Meteosat-10, enables the smoke to be tracked back unambiguously to the fires over Alberta on 17 May. We also examine the lidar and particle depolarisation ratios of the smoke for comparison with previous work.

The aerosol cloud was measured by a number of lidars around the UK (Fig. ):

the Raman lidar at the Natural Environment Research Council (NERC) Mesosphere–Stratosphere–Troposphere (MST) Radar facility at Capel Dewi (52∘25′ N, 4∘0′ W), Wales;

The basic principles of the retrieval method are shown in the Supplement. Retrieval of the aerosol optical depth uses the N2 Raman signals and a nearby radiosonde profile. From the latter, a synthetic molecular-only scattering profile may be constructed and fitted to the measured profile in a region of the atmosphere free from aerosol, here taken to be 13–16 km. The ratio Ram(z) of the Raman to the molecular profiles then leads to curves such as that shown in blue in Fig. . In principle, this ratio can be inverted to give a profile of aerosol extinction coefficient, but this approach tends to lead to large random errors. Here we take advantage of the fact that for the episode under discussion the aerosol was distributed in very distinct layers, so a method was devised to calculate only layer-average or layer-total quantities. Figure  also shows R(z), the ratio of the elastic channel to the Raman channel, normalised to 1 between 13 and 16 km. Aerosols show up in this curve as departures from 1 (the molecular background), clearly showing the layered structure. (A small correction has been applied to R(z) to account for the difference between the Raman and Rayleigh scattering cross sections, using the radiosonde profile.) Such a structure was observed at all sites during the course of this event. We therefore calculate the integrated aerosol optical depth (AOD) across each layer: AOD=-12ln⁡Ram(above)‾/Ram(below)‾, where the overbars indicate that Ram(z) was averaged in the aerosol-free regions above and below each layer. Each lidar profile used here was examined separately to determine the layer altitudes and the width of the aerosol-free regions, which were chosen as far as possible to be at least 1 km deep.

As the photon-counting signals can be assumed to follow Poisson statistics, the precision error in AOD is readily calculated from the number of photon counts in the regions above and below each layer. A further source of error comes from the choice of radiosonde profile used to normalise PR(z). For stations like Camborne and Watnall, where co-located radiosondes were released, this error is small, but for the other stations it is not negligible. As an example, Fig.  shows the same lidar data as Fig. , but using a radiosonde from Camborne rather than Castor Bay (Fig. ). The difference in AOD for the five layers is greater than the statistical uncertainty, showing that this source of error is important for free tropospheric aerosol measurements by Raman lidar.

Retrieval of the integrated aerosol backscatter (IAB) and hence the mean lidar ratio LR (LR = AOD/IAB) for each layer requires measurement of both polarisation components, and this was only possible for the Met Office Raymetrics lidars. Signals from the two elastic channels were added and used to generate an R(z) profile as before. The integral of B[R(z)-1]n(z) across each layer then gave IAB where n(z) is the number density of air at height z and B=3.31×10-31 m2 sr-1 is the molecular differential backscatter cross section, after. Errors in IAB come from the Poisson statistics in both the signals in the layer and the background noise subtracted from the measured lidar signals, which are treated differently under the integral (variances being added for the signals and standard deviations for the background). Finally, the two errors are added in quadrature to give the error in IAB and LR calculated for each layer in the usual way.

To gain an appreciation of the extent and persistence of the aerosol, we first examine the ceilometer measurements since continuous coverage was available from all of them throughout the period, with aerosol measurements limited only by the presence of cloud.

Thin layers of free-tropospheric aerosol began to appear over the UK during 22 May 2016 and persisted intermittently until the end of the month. As an example, Fig.  shows the variation of backscatter signal with height and time of the Lufft ceilometers at Lerwick (60∘8′ N, 1∘11′ W), Dishforth (54∘7′ N, 1∘24′ W) and Camborne (50∘12′ N, 5∘17′ W) on 23 May. Thin layers of enhanced backscatter due to aerosols are shown throughout the day at Camborne and from 10:00 UTC onwards at the other two stations. Similar patterns are seen in the layers at all three stations, suggesting that the aerosol layer was widespread over the UK during the second half of 23 May. To demonstrate this further, Fig.  summarises the ceilometer observations during 23 and 24 May. Four categories are shown: those with aerosol between 4 and 8 km (blue), those with aerosol between 2 and 4 km but no higher (green), those with no or only a trace of aerosol (red) and those where cloud cover precluded observations (yellow). High-altitude aerosol (>4 km) was indeed widespread across the UK on both days, with the suggestion of a clearance in the far north on 24 May.

To examine the duration of the event, the total number of ceilometers which observed definite aerosol layers, trace amounts or no aerosol, or were restricted by cloud cover, was plotted for each day from 22 to 27 May (Fig. ). This plot shows that the event seems to have peaked in terms of coverage on 24 May, although the increasing cloud cover thereafter means that some aerosol is likely to have been missed. None of the ceilometers detected aerosol after 27 May, although continuing low cloud cover restricted observations to around half of the stations until clear skies returned on 5 June.

The ceilometers only provide consistent measurements up to 8 km, and their infrared wavelength means they can only give qualitative information on the presence or not of aerosols. To extend the measurements to the tropopause and obtain quantitative information about the aerosol, we now turn to the Raman lidars. A qualitative inspection of the Met Office elastic channels (parallel and perpendicular polarisation) showed that there was extensive aerosol between 8 km and the tropopause, which was not captured by the ceilometers. This was observed at Camborne during 24–28 May, East Malling from 25 to 27 May, Exeter from 25 to 31 May and Watnall from 26 to 31 May. The Capel Dewi lidar also observed aerosol between 8 and 12 km up to the end of May. This shows that the event persisted from 22 to 31 May with aerosol layers found at all altitudes in the troposphere. We now concentrate on the night-time measurements from the Raman lidars to obtain quantitative information on these layers.

A similar analysis to that described in Sect.  was conducted for all the continuous night-time data collected by the Raman lidars between 23 and 31 May, after eliminating periods affected by cloud. Figure  shows the total aerosol optical depth above 2 km measured by the Raman lidars during this period, aggregated into whole-night averages. The greater sensitivity of these lidars means that aerosol was measured up to the night of 30–31 May at Capel Dewi with traces evident up to the same time at some of the other stations. However, the coverage is patchy due to the combination of intermittent sampling and low cloud cover.

The highest AOD measured was that at Capel Dewi on the night of 23–24 May, from the profile shown in Fig. . Although the value is sensitive to the choice of radiosonde profile, the total AOD of ∼0.13 above 2 km clearly results from aerosol observed throughout the free troposphere. All other AOD measurements at all the stations were below 0.1, with the values decreasing with time to below 0.05 after 28.

The evolution of lidar ratio as a function of optical depth is shown for the Met Office lidars in Figs.  and , for aerosol layers above and below 7 km respectively. Here, hourly average data are presented, as there could on occasion be considerable variability in AOD during a night; an example is 27–28 May at Camborne, where the total AOD varied between 0.02 and 0.09 over the 5 h measurement period. With the exception of a few outliers, the lidar ratios generally fall between 35 and 65 sr, consistent with the lidar ratios of 21–67 sr at 355 nm for Canadian and Siberian smoke reported by and with the values of 46±13 sr found in the 10-year study of forest fire smoke by using EARLINET data. There seems to be a greater spread of lidar ratios in the upper-tropospheric layers than in the mid-troposphere, where the spread is more like 40–60 sr. Note that the abscissa scales in Figs.  and  are different: most of the aerosol measured in this data set was found in the upper troposphere, where AOD values generally extended to 0.07, compared to 0.035 in the mid-troposphere.

The volume depolarisation ratio for the lidar profiles containing aerosol layers was a few percent at 355 nm, consistent with optically thin layers embedded within strong molecular backscatter. However, when converted to aerosol depolarisation ratios, δa, a remarkably consistent picture emerged across the different stations. Below 7 km, the value of δa was in the range 0.04–0.06, whereas above 7 km it was generally close to 0.20: for all four of the nights at Camborne and two of those at Watnall, δa lay in the range 0.18–0.21. Lower values were measured above 7 km at Exeter and East Malling on 25–26 May (0.15) and a high value at Watnall on 26–27 May (0.31) but the clear altitude difference remains. Values around 0.2 are consistent with those found at 355 nm by in aged smoke plumes, but the values around 0.05 are similar to the much lower δa values reported at 532 nm and references therein. It is clear that δa at 355 nm for smoke particles can vary considerably.

A higher value of δa means the particles are more depolarising, which suggests more irregular solid shapes. As all the measurements here were made using the same laser wavelength, we cannot infer anything about particle size from the data, but the greater δa at higher altitudes is consistent with some of the smoke particles having acquired an ice coating at the colder temperatures near the tropopause – consistent with the ice-nucleating potential of smoke particles identified by .

Having observed the presence of aerosol layers over the UK, three questions need to be answered. Where did they come from? How old are they? At what height(s) was the aerosol injected into the atmosphere?

Copernicus Atmospheric Monitoring Service (CAMS) daily fire products (available from http://macc.copernicus-atmosphere.eu/d/services/gac/nrt/fire_radiative_power, last access: 3 September 2016) showed extensive forest fires in Canada during May 2016, especially in Saskatchewan and Alberta (e.g. Fig. ). Given the prevailing westerly winds at midlatitudes and the presence of deep convection over Canada capable of lifting the smoke, these provide the most likely source for the aerosol found over the UK. This hypothesis will now be examined using meteorological charts, trajectory calculations and satellite observations.

The air flow in the free troposphere impinging the UK on 20 May (Fig. a) was zonal, with rapid flow across the Atlantic around a trough at 54∘ N, 32∘ W. This provided a route to bring smoke aerosol across from Canada. After this, the pattern became more complicated. The trough moved steadily eastward and deepened, with its axis along the Irish Sea by 12:00 UTC on 22 May (Fig. b). At the same time, a deepening depression east of Newfoundland resulted in a second trough near 45∘ N, 40∘ W. These two troughs and the ridge in between set up an omega block on 23 May which resulted in a split jet stream, with one branch heading north over Iceland towards the Norwegian Sea and the other heading south across the Mediterranean (Fig. c). (An omega block is characterised by an upper–level ridge or anticyclone flanked by two cut-off lows or troughs, to the south-west and south-east.) As the block moved and distorted, the UK lay first under the eastern trough (06:00 UTC on 22 May to 12:00 UTC on 23 May), then the anticyclone (18:00 UTC on 23 May to 12:00 UTC on 24 May, Fig. c), the western cut-off low (18:00 UTC on 24 May to 00:00 UTC on 29 May, Fig. d) and finally a broad area of almost no flow which persisted until a second, weaker omega block was established on 30 May as another depression developed in the western Atlantic and moved eastwards (not shown). From 23 to 31 May therefore the flow over the UK was slack and variable, which meant that smoke transported in the zonal jet up to 22 May was able to remain in the vicinity of the UK.

This study has presented observations of free-tropospheric aerosols by ceilometers and Raman lidars over the UK from 23 to 31 May 2016 and examined the origin of the aerosol. The principal conclusions are as follows.

Ceilometer measurements showed that much of the United Kingdom was covered by free-tropospheric smoke layers on 23 and 24 May.

The study shows the value of combining different kinds of lidars in following the evolution of long-range smoke transport events, since the 24 h capability of ceilometers allows maximum use to be made of breaks in cloud cover, while the quantitative information available from Raman lidars yields information on particle properties. The study shows how a smoke plume from Canadian wildfires was drawn into a pattern of thin layers far downstream of its source, due to stirring of the atmosphere by flow around a cyclone in the mid-Atlantic, and how these layers could be exploited in the lidar data analysis to produce robust estimates of particle properties. The study also shows how atmospheric blocking resulted in a smoke event remaining over the UK for over a week, during which time the layers became optically thinner but no change was observed in particle properties.