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
(<3 %), whereas the dry irregularly shaped soot particles in the
stratosphere lead to high depolarization ratios of 22 % at 355 nm and
18 % at 532 nm and a comparably low value of 4 % at 1064 nm. The lidar
ratios were 40–45 sr (355 nm), 65–80 sr (532 nm), and 80–95 sr
(1064 nm) in both the tropospheric and stratospheric smoke layers
indicating similar scattering and absorption properties. The strong
wavelength dependence of the stratospheric depolarization ratio was probably
caused by the absence of a particle coarse mode (particle mode consisting of
particles with radius >500 nm). The stratospheric smoke particles
formed a pronounced accumulation mode (in terms of particle volume or mass)
centered at a particle radius of 350–400 nm. The effective particle radius
was 0.32 µm. The tropospheric smoke particles were much smaller
(effective radius of 0.17 µm). Mass concentrations were of the
order of 5.5 µg m−3 (tropospheric layer) and
40 µg m−3 (stratospheric layer) in the night of
22 August 2017. The single scattering albedo of the stratospheric particles
was estimated to be 0.74, 0.8, and 0.83 at 355, 532, and 1064 nm,
respectively.
Triple-wavelength lidar observations of the depolarization ratio and the backscatter coefficient of marine aerosol as a function of relative humidity (RH) are presented with a 5 min time resolution. ...The measurements were performed at Barbados (13° N, 59° W) during the Saharan Aerosol Long-range Transport and Aerosol-Cloud interaction Experiment (SALTRACE) winter campaign in February 2014. The phase transition from spherical sea salt particles to cubic-like sea salt crystals was observed with a polarization lidar. The radiosonde and water-vapor Raman lidar observations show a drop in RH below 50 % in the marine aerosol layer simultaneously with a strong increase in particle linear depolarization ratio, which reaches values up to 0.12 ± 0.08 (at 355 nm), 0.15 ± 0.03 (at 532 nm), and 0.10 ± 0.01 (at 1064 nm). The lidar ratio (extinction-to-backscatter ratio) increased from 19 and 23 sr for spherical sea salt particles to 27 and 25 sr (at 355 and 532 nm, respectively) for cubic-like particle ensembles. Furthermore the scattering enhancement due to hygroscopic growth of the marine aerosol particles under atmospheric conditions was measured. Extinction enhancement factors from 40 to 80 % RH of 1.94 ± 0.94 at 355 nm, 3.70 ± 1.14 at 532 nm, and 5.37 ± 1.66 at 1064 nm were found. The enhanced depolarization ratios and lidar ratios were compared to modeling studies of cubic sea salt particles.
For the first time, a dense data set of particle extinction-to-backscatter ratios (lidar ratios), linear depolarization ratios, and backscatter- and extinction-related Ångström exponents for a ...Central Asian site are presented. The observations were performed with a continuously running multiwavelength polarization Raman lidar at Dushanbe, Tajikistan, during an 18-month campaign (March 2015 to August 2016). The presented seasonally resolved observations fill an important gap in the database of aerosol optical properties used in aerosol typing efforts with spaceborne lidars and ground-based lidar networks. Lidar ratios and depolarization ratios are also basic input parameters in spaceborne lidar data analyses and in efforts to harmonize long-term observations with different space lidar systems operated at either 355 or 532 nm. As a general result, the found optical properties reflect the large range of occurring aerosol mixtures consisting of long-range-transported dust (from the Middle East and the Sahara), regional desert, soil, and salt dust, and anthropogenic pollution. The full range from highly polluted to pure dust situations could be observed. Typical dust depolarization ratios of 0.23–0.29 (355 nm) and 0.30–0.35 (532 nm) were observed. In contrast, comparably low lidar ratios were found. Dust lidar ratios at 532 nm accumulated around 35–40 sr and were even lower for regional background dust conditions (20–30 sr).
Detailed correlation studies (e.g., lidar ratio vs. depolarization ratios, Ångström exponent vs. lidar ratio and vs. depolarization ratio) are presented to illuminate the complex relationships between the observed optical properties and to identify the contributions of anthropogenic haze, dust, and background aerosol to the overall aerosol mixtures found within the 18-month campaign.
The observation of 532 nm lidar ratios (<25 sr) and depolarization ratios (around 15 %–20 %) in layers with very low particle extinction coefficient (<30 sr) suggests that direct emission and emission of resuspended salt dust (initially originated from numerous desiccating lakes and the Aralkum desert) have a sensitive impact on the aerosol background optical properties over Dushanbe.
For the first time, continuous, vertically resolved long-term aerosol measurements were conducted with a state-of-the-art multiwavelength lidar over a Central Asian site. Such observations are ...urgently required in efforts to predict future climate and environmental conditions and to support spaceborne remote sensing (ground truth activities). The lidar observations were performed in the framework of the Central Asian Dust Experiment (CADEX) at Dushanbe, Tajikistan, from March 2015 to August 2016. An AERONET (AErosol RObotic NETwork) sun photometer was operated at the lidar field site. During the 18-month campaign, mixtures of continental aerosol pollution and mineral dust were frequently detected from ground to cirrus height level. Regional sources of dust and pollution as well as long-range transport of mineral dust mainly from Middle Eastern and the Saharan deserts determine the aerosol conditions over Tajikistan. In this study, we summarize our findings and present seasonally resolved statistics regarding aerosol layering (main aerosol layer depth, lofted layer occurrence); optical properties (aerosol and dust optical thicknesses at 500–532 nm, vertically resolved light-extinction coefficient at 532 nm); profiles of dust and non-dust mass concentrations and dust fraction; and profiles of particle parameters relevant for liquid water, mixed-phase cloud, and cirrus formation such as cloud condensation nuclei (CCN) and ice-nucleating particle (INP) concentrations. The main aerosol layer over Dushanbe typically reaches 4–5 km height in spring to autumn. Frequently lofted dust-containing aerosol layers were observed at heights from 5 to 10 km, indicating a sensitive potential of dust to influence cloud ice formation. Typical dust mass fractions were of the order of 60 %–80 %. A considerable fraction is thus anthropogenic pollution and biomass burning smoke. The highest aerosol pollution levels (in the relatively shallow winter boundary layer) occur during the winter months. The seasonal mean 500 nm AOT (aerosol optical thickness) ranges from 0.15 in winter to 0.36 in summer during the CADEX period (March 2015 to August 2016); DOTs (dust optical thicknesses) were usually below 0.2; seasonally mean particle extinction coefficients were of the order of 100–500 Mm−1 in the main aerosol layer during the summer half year and about 100–150 Mm−1 in winter but were mainly caused by anthropogenic haze. Accordingly, the highest dust mass concentrations occurred in the summer season (200–600 µg m−3) and the lowest during the winter months (20–50 µg m−3) in the main aerosol layer. In winter, the aerosol pollution mass concentrations were 20–50 µg m−3, while during the summer half year (spring to autumn), the mass concentration caused by urban haze and biomass burning smoke decreases to 10–20 µg m−3 in the lower troposphere. The CCN concentration levels are always controlled by aerosol pollution. The INP concentrations were found to be high enough in the middle and upper troposphere to significantly influence ice formation in mixed-phase and ice clouds during spring and summer seasons.
Triple-wavelength polarization lidar measurements in Saharan dust layers were performed at Barbados (13.1° N, 59.6° W), 5000–8000 km west of the Saharan dust sources, in the framework of the Saharan ...Aerosol Long-range Transport and Aerosol-Cloud-Interaction Experiment (SALTRACE-1, June–July 2013, SALTRACE-3, June–July 2014). Three case studies are discussed. High quality was achieved by comparing the dust linear depolarization ratio profiles measured at 355, 532, and 1064 nm with respective dual-wavelength (355, 532 nm) depolarization ratio profiles measured with a reference lidar. A unique case of long-range transported dust over more than 12 000 km is presented. Saharan dust plumes crossing Barbados were measured with an airborne triple-wavelength polarization lidar over Missouri in the midwestern United States 7 days later. Similar dust optical properties and depolarization features were observed over both sites indicating almost unchanged dust properties within this 1 week of travel from the Caribbean to the United States. The main results of the triple-wavelength polarization lidar observations in the Caribbean in the summer seasons of 2013 and 2014 are summarized. On average, the particle linear depolarization ratios for aged Saharan dust were found to be 0.252 ± 0.030 at 355 nm, 0.280 ± 0.020 at 532 nm, and 0.225 ± 0.022 at 1064 nm after approximately 1 week of transport over the tropical Atlantic. Based on published simulation studies we present an attempt to explain the spectral features of the depolarization ratio of irregularly shaped mineral dust particles, and conclude that most of the irregularly shaped coarse-mode dust particles (particles with diameters > 1 µm) have sizes around 1.5–2 µm. The SALTRACE results are also set into the context of the SAMUM-1 (Morocco, 2006) and SAMUM-2 (Cabo Verde, 2008) depolarization ratio studies. Again, only minor changes in the dust depolarization characteristics were observed on the way from the Saharan dust sources towards the Caribbean.
Quasi‐simultaneous vertically resolved multiwavelength aerosol Raman lidar observations were conducted in the near field (Praia, Cape Verde, 15°N, 23.5°W) and in the far field (Manaus, Amazon basin, ...Brazil, 2.5°S, 60°W) of the long‐range transport regime between West Africa and South America. Based on a unique data set (case study) of spectrally resolved backscatter and extinction coefficients, and of the depolarization ratio a detailed characterization of aerosol properties, vertical stratification, mixing, and aging behavior during the long‐distance travel in February 2008 (dry season in western Africa, wet season in the Amazon basin) is presented. While highly stratified aerosol layers of dust and smoke up to 5.5 km height were found close to Africa, the aerosol over Manaus was almost well‐mixed, reached up to 3.5 km, and mainly consisted of aged biomass burning smoke.
Dual‐wavelength Raman lidar observations were regularly carried out at Leipzig (51.3°N, 12.4°E) from May to August 2003. The measurements showed that particle backscatter and extinction coefficients ...in the free troposphere were higher compared to values in 2000–2002. Backward dispersion modeling indicates that intense forest fires that occurred in Siberia and Canada in spring/summer 2003 were the main cause of these free tropospheric haze layers. Measurements on 3 days were selected for an optical and microphysical particle characterization of these well‐aged particle plumes. Particle lidar ratios measured at 532 nm wavelength were higher than at 355 nm. This property seems to be a characteristic feature of aged biomass‐burning particles observed over central Germany. Mean particle Ångström exponents calculated for the wavelength range from 355 to 532 nm varied from 0 to 1.3. Particle effective radii varied between 0.24 and 0.41 μm. Pollution advected from North America on 25 August 2003, in contrast, was characterized by considerably smaller particles. Mean effective radii were ≤0.2 μm, and Ångström exponents were 1.8–2.1. Lidar ratios in that case were lower at 532 nm compared to those at 355 nm. Such signatures are characteristic for anthropogenic particles. At the moment, however, it cannot be completely ruled out that extremely hot forest fires in western areas of Canada generated comparably small particles. Except for this specific case the forest fire particles were considerably larger than what is usually reported from in situ observations of biomass‐burning smoke. Possible explanations for this difference could be the kind of burning process, which could generate much larger particles in the source region, condensation of organic vapors on existing particles, and coagulation processes during the long transport time of more than a week. Relative humidity measured in these layers was very low. Hygroscopic growth of the particles therefore seemed to have little influence on the size of the particles. The forest fire smoke consisted of moderately absorbing material. Real parts of the complex refractive index of the particles were mostly <1.5, and imaginary parts were <0.01i. Single‐scattering albedo in all cases varied between 0.9 and 0.98 at 532 nm.
Abstract
The microwave radiometers HATPRO (Humidity and Temperature Profiler) and MiRAC-P (Microwave Radiometer for Arctic Clouds - Passive) continuously measured radiation emitted from the ...atmosphere throughout the Multidisciplinary drifting Observatory for the Study of the Arctic Climate (MOSAiC) expedition on board the research vessel Polarstern. From the measured brightness temperatures, we have retrieved atmospheric variables using statistical methods in a temporal resolution of 1 s covering October 2019 to October 2020. The integrated water vapour (IWV) is derived individually from both radiometers. In addition, we present the liquid water path (LWP), temperature and absolute humidity profiles from HATPRO. To prove the quality and to estimate uncertainty, the data sets are compared to radiosonde measurements from Polarstern. The comparison shows an extremely good agreement for IWV, with standard deviations of 0.08–0.19 kg m
−2
(0.39–1.47 kg m
−2
) in dry (moist) situations. The derived profiles of temperature and humidity denote uncertainties of 0.7–1.8 K and 0.6–0.45 gm
−3
in 0–2 km altitude.
On June 29–30, 2019, the Barcelona Dust Forecast Center with Non‐hydrostatic Multiscale Model (NMMB/BSC‐Dust) and the Navy Aerosol Analysis and Prediction System forecasted huge amounts of mineral ...dust over Poland. The Hybrid Single Particle Lagrangian Integrated Trajectory model confirmed uniquely fast (120 hr) long‐range air‐mass transport form North Africa to Poland. This remarkable dust event was observed using lidar at the Aerosol, Clouds and Trace Gases Research InfraStructure site in Warsaw, Central Poland; the only site equipped with Raman‐Mie polarization water vapor lidar in East‐Central Europe. The excellent capabilities of PollyXT lidar allowed to obtain an impressive number of 31 full sets of aerosol optical properties profiles, which enabled study of dust properties evolution on a rare hourly scale. The analyses were completed with the separation of fine and coarse mode dust particles form non‐dust particles using the POlarization‐LIdar PHOtometer Networking algorithm. Huge amount of an exceptionally pure mineral dust from Sahara measured in the free troposphere was characterized by a gradually decreasing coarse dust fraction (76%–21%) with a peak of fine dust fraction (67%) and particle linear depolarization ratio (26%) in the middle of the event. Within the boundary layer, a local urban dust mixed with pollution was observed with fine mode dust particles dominating (44%) and lower particle linear depolarization ratio (7.4%). The influx of pure mineral dust has been unique to this geographical region and will therefore be a reference point for future research and comparative studies.
Key Points
Unusually fast, uniform, and quasi‐stable pure mineral dust inflow form Sahara Desert to East‐Central Europe
Advected Saharan dust composed of significant fractions of fine and coarse mode dust particles evolving in time
Local urban/agro dust with no evidence of coarse mode particles
We present a new method to determine the hygroscopic growth of atmospheric particles. This method combines lidar measurements with high temporal resolution of the particle backscatter coefficient and ...water vapor mixing ratio with temperature measurements from radiosondes and a microwave radiometer. The hygroscopic growth is described by an equation that represents the two observed branches of the growth curve with different dependencies on the relative humidity. An example is presented to illustrate a first result from a continental air mass case.
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