We present spectrally resolved optical and microphysical properties of
western Canadian wildfire smoke observed in a tropospheric layer from
5–6.5 km height and in a stratospheric layer from 15–16 km height during
a record-breaking smoke event on 22 August 2017. Three polarization/Raman
lidars were run at the European Aerosol Research Lidar Network (EARLINET)
station of Leipzig, Germany, after sunset on 22 August. For the first time,
the linear depolarization ratio and extinction-to-backscatter ratio (lidar
ratio) of aged smoke particles were measured at all three important lidar
wavelengths of 355, 532, and 1064 nm. Very different particle depolarization
ratios were found in the troposphere and in the stratosphere. The obviously
compact and spherical tropospheric smoke particles caused almost no
depolarization of backscattered laser radiation at all three wavelengths
(
A record-breaking Canadian wildfire smoke event was observed over central
European lidar stations of the European Aerosol Research Lidar Network
(EARLINET) on 21–22 August 2017
The study is an important contribution to lidar-based efforts of aerosol
classification
Fundamental input parameters in the CALIOP data analysis are the particle
extinction-to-backscatter ratio (lidar ratio) at 532 and 1064 nm
Particle lidar-ratio and linear depolarization-ratio data sets for 355, 532, and 1064 nm for all basic aerosol types are required in efforts to harmonize long-term time series of aerosol observations with CALIOP at 532 and 1064 nm, and later on with ATLID at 355 nm. Only with good knowledge of the spectral dependencies of particle backscatter, extinction, lidar ratio, and depolarization ratio, long-term data sets can be harmonized and normalized to permit aerosol trend analysis by combing the NASA and ESA lidar missions, spanning the time period from 2006 (start of the CALIPSO mission) to 2025 (probable end of the EarthCARE mission). Our triple-wavelength lidar observations, presented here, can be regarded as a contribution and first step towards an aerosol library containing the requested spectrally resolved aerosol optical parameters.
The paper is organized as follows: in the next section, we briefly describe
the three lidars involved in the smoke study, the basic data analysis
methods, and the retrieved products. In Sect. 3, the main results in terms of
optical and microphysical parameters of the tropospheric and stratospheric
smoke layers are presented and discussed. In Sect. 4, we make an attempt to
explain the unexpected spectral dependence of the smoke linear depolarization
ratio in the stratosphere. The measurement case from 22 August 2017 provides
the favorable opportunity to test and improve optical models and
particle-shape parameterizations. Modeling of the optical properties of
irregularly shaped dust and smoke particles is a big and unsolved problem.
Trustworthy parameterizations that allow us to simulate the optical
properties of irregularly shaped mineral dust and soot particles at
180
In the evening and night of 22 August 2017, three polarization/Raman lidars
were run at the EARLINET station at Leipzig (51.3
The second lidar was the dual receiver field-of-view (RFOV) multiwavelength
polarization/Raman lidar MARTHA (Multiwavelength Tropospheric Raman lidar for
Temperature, Humidity, and Aerosol profiling;
The third Leipzig lidar is the triple-wavelength polarization/Raman lidar
BERTHA (Backscatter Extinction lidar-Ratio Temperature Humidity profiling
Apparatus;
The laser beams of Polly and BERTHA were tilted to an off-zenith angle of
5
Details of the determination of the particle optical properties and the
uncertainties in the products can be found in the articles mentioned above.
An overview of the retrieval methods is given in
In Sect. 3.2, the lidar results for the time period
from 20:45 to 23:15 UTC on 22 August 2017 (shown in
Fig.
Canadian wildfire smoke layers in the troposphere, mostly between
the boundary-layer top (at a height of 1.8 km) and 6.5 km, and in the
stratosphere (15–16 km) observed with lidar at Leipzig on
22–23 August 2017, 20:45–00:30 UTC. Shown is the range-corrected
cross-polarized 532 nm backscatter signal measured with temporal and
vertical resolution of 10 s and 7.5 m, respectively. The indicated
tropopause height
In the retrieval of the extinction coefficient, a least-squares linear
regression method was applied to the respective Raman signal profiles. The
regression window length was 750 m (532 nm) to 1200 m (355 nm) in the
troposphere and 1200 m for both wavelengths in the stratosphere. To obtain
the lidar ratios at 355 and 532 nm, the extinction profiles were combined
with the respective backscatter profiles. In this procedure, we applied the
optimum effective resolution concept
In the case of the 1064 nm extinction coefficient, only smoke-layer mean
extinction values could be derived. Profiles could not be obtained because
the 1058 nm Raman signals were too weak and noisy. The retrieval window
lengths are indicated by vertical bars in the figures in Sect. 3.2. Retrieval window lengths of 750–1500 m in the
troposphere and 2500 m in the stratosphere had to be applied to obtain the
1064 nm layer-mean extinction coefficient with an uncertainty of about
10 % (troposphere) and
Different expressions for the Ångström exponent, a well-established
parameter to characterize the spectral dependence of aerosol optical
properties, are shown in Sect. 3.2. The
Ångström exponent
The lidar inversion method of
The 2.5 h mean profiles (20:45–23:15 UTC, see
Fig.
Same as Fig.
The record-breaking Canadian wildfire smoke event over Leipzig on
22 August 2017 was discussed by
Optical properties of smoke aerosol in the tropospheric layer
(5–6.5 km) and stratospheric smoke layer (15–16 km). Layer-mean values of
the particle extinction coefficient
Figure
In Figs.
As can be seen in Figs.
In the case of the 1064 nm extinction coefficient, we only can show a few
values in Figs.
The key findings shown in Figs.
The most surprising finding is the strong difference between the
depolarization spectrum in the tropospheric and stratospheric smoke layers as
shown in Figs.
Table
As can be seen in Table
Figure
Lidar inversion products (assuming spherical particles) for the
tropospheric layer (5–6.5 km) and stratospheric smoke layer (15–16 km).
Layer-mean values (and retrieval uncertainties) of the particle volume
concentration
The AERONET observation describes the aerosol properties in the entire
vertical column from the surface to the top of the stratospheric layer. To
convert the AERONET column values to stratospheric volume and mass
concentrations so that we can compare sun-photometer-derived and
lidar-derived stratospheric volume and mass concentrations, we assumed that
(a) the stratospheric smoke contributed 60 % to the total AOT (as observed
with lidar) and also 60 % to the column volume concentration, and (b) that
these 60 % can be assigned to the 1 km thick stratospheric layer between
15 and 16 km. With this information, the AERONET column volume values for each
size bin were converted into volume and mass concentrations as shown in
Fig.
Particle mass size distribution derived from column
(tropospheric
The lidar-derived size distribution (from the nighttime measurements) fits
very well into the AERONET observations at Lindenberg, 180 km northeast of
Leipzig, in the early morning of 23 August 2017. The effective radius
The lidar-derived and AERONET-derived mass size distributions in
Fig.
The lidar inversion results in Table
Figure
Comparison of the spectral dependence of the tropospheric (5–6 km)
and stratospheric (15–16 km) particle lidar ratio
As already mentioned in Sect. 3.2, the particle depolarization ratio was low
at all three wavelengths in the tropospheric layer. These low depolarization
values are indicative for spherical particles dominating the measured optical
effects. The particles must have been compact in shape
The strong spectral slope of the depolarization ratio of the stratospheric
smoke particles was also measured with a triple-wavelength polarization lidar
at Lille, northern France, in Canadian wild fire smoke layers from 24–31 August 2017
The question arises: what is the reason for this unexpectedly strong spectral
dependence of the particle linear depolarization ratio in the stratosphere?
Usually, the observed wavelength dependence of the depolarization ratio of
irregularly shaped particles (such as mineral dust or volcanic ash) is weak.
The depolarization ratio for desert dust is lower at 355 nm (20 %–25 %)
than at 532 nm (30 %–35 %;
We hypothesize that the specific size distribution of the stratospheric smoke
particles shown in Fig.
Our hypothesis is corroborated by Fig.
Spectral dependence of particle linear depolarization ratio for fine-mode desert dust and stratospheric accumulation-mode smoke particles.
As can be seen, quite similar values of the soot and fine-mode dust depolarization ratios at 355 and 532 nm are found. The rather different composition of the particles (soot vs. dust particles) is not visible in the two depolarization spectra. Obviously particle shape and size widely control the strength of depolarization of backscattered laser radiation at the different wavelengths.
However, to obtain clear answers concerning the role of particle size, shape,
and composition on light depolarization, we need extended simulation studies
with advanced optical particle models
To compare our lidar and depolarization ratio observations in
Sect. 3.2 with previous lidar observations of
wildfire smoke, we performed a literature review. Numerous articles on
tropospheric biomass-burning smoke are available. In
Table
Literature overview of multiwavelength lidar observations of smoke
lidar ratios and particle linear depolarization ratios of fresh and aged
biomass-burning smoke in the troposphere and
stratosphere. Our values (indicated by “this study”) are obtained from the BERTHA measurements.
For better comparison,
the tropospheric triple-wavelength depolarization ratio observation of
Only a few observations of the particle depolarization ratio in aged and
fresh tropospheric smoke layers are available as can be seen in
Table
A record-breaking stratospheric smoke event with aerosol layers from
tropospheric heights around 3 km to about 16–17 km allowed us to
characterize Canadian wildfire smoke (after long-range transport) in great
detail in terms of optical and microphysical properties. The case study
demonstrates the unique potential of advanced aerosol lidars to contribute to
atmospheric aerosol research. There is no alternative to lidar regarding a
continuous aerosol profiling over long time periods providing a clear
separation of tropospheric and stratospheric aerosol effects. Our advanced triple-wavelength
polarization/Raman lidar delivered height profiles of particle backscatter
and extinction coefficients, respective lidar ratios, and linear
depolarization ratios at all three lidar wavelengths, and, in addition,
microphysical, morphological, and composition-related information about the
smoke layers. Very different smoke properties were observed in the
tropospheric and stratospheric smoke layers. For the first time, measured
lidar ratios for smoke at 1064 nm are now available, and can be used in the
CALIOP data analysis of the spread of the smoke over the northern hemisphere
occurring during the second half of 2017
The spectrally resolved optical data sets for stratospheric smoke is an important new contribution to the aerosol classification library used in lidar remote sensing from space and by ground-based networks such as EARLINET. The smoke observations also provide a favorable opportunity to test and validate optical models regarding their potential to reproduce the observed data sets of smoke lidar and depolarization ratios. Improved modeling will in turn help to better interpret aerosol lidar observations and support the development of new lidar retrieval algorithms, and also to improve climate modeling by using improved optical aerosol models for non-spherical particles.
It should finally be mentioned that the lidar-derived optical properties for stratospheric smoke are rather different from the ones of non-absorbing and non-depolarizing spherical volcanic aerosol particles (liquid sulfuric-acid-containing droplets). A clear and unambiguous discrimination of biomass-burning smoke and volcanic aerosol is possible based on polarization lidar observations and thus allows for a clear and unambiguous identification of these major contributors to stratospheric aerosol perturbations and contamination.
As an outlook, we plan to use the triple-wavelength polarization/Raman lidar
to characterize other fundamental aerosol types such as desert dust and urban
haze in terms of lidar ratios and depolarization ratios at 355, 532, and
1064 nm to support the CALIOP observations and future spaceborne missions
and space lidar data harmonization efforts. First attempts in pure marine
environments and in lofted mineral dust layers have been published
The lidar data are available at TROPOS upon request
(info@tropos.de). AERONET sun-photometer data were downloaded from the AERONET web page
MH, HB, CJ, RE, and DA run the lidars and collected the observational data. MH, HB, and AA analyzed the lidar data and IV computed the lidar inversion products. MH and AA prepared the manuscript in close cooperation with HB.
The authors declare that they have no conflict of interest.
This article is part of the special issue “EARLINET aerosol profiling: contributions to atmospheric and climate research”. It is not associated with a conference.
We are grateful to AERONET for providing high-quality sun-photometer observations, calibrations, and products. Special thanks to the Lindenberg AERONET team to carefully run the station. This activity is supported by ACTRIS Research Infrastructure (EU H2020-R&I) under grant agreement no. 654109. The development of the lidar inversion algorithm was supported by the Russian Science Foundation (project 16-17-10241). Edited by: Eduardo Landulfo Reviewed by: two anonymous referees