| 1. |
- Achtert, Peggy, 1982-, et al.
(författare)
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Assessing lidar-based classification schemes for polar stratospheric clouds based on 16 years of measurements at Esrange, Sweden
- 2014
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Ingår i: Journal of Geophysical Research: Atmospheres. - 2169-8996. ; 119:3, s. 1386-1405
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Tidskriftsartikel (refereegranskat)abstract
- Lidar measurements of polar stratospheric clouds (PSCs) are commonly analyzed in classification schemes that apply the backscatter ratio and the particle depolarization ratio. This similarity of input data suggests comparable results of different classification schemes—despite measurements being performed with a variety of mostly custom-made instruments. Based on a time series of 16 years of lidar measurements at Esrange (68°N, 21°E), Sweden, we show that PSC classification differs substantially depending on the applied scheme. The discrepancies result from varying threshold values of lidar-derived parameters used to define certain PSC types. The resulting inconsistencies could impact the understanding of long-term PSC observations documented in the literature. We identify two out of seven considered classification schemes that are most likely to give reliable results and should be used in future lidar-based studies. Using polarized backscatter ratios gives the advantage of increased contrast for observations of weakly backscattering and weakly depolarizing particles. Improved confidence in PSC classification can be achieved by a more comprehensive consideration of the effect of measurement uncertainties. The particle depolarization ratio is the key to a reliable identification of different PSC types. Hence, detailed information on the calibration of the polarization-sensitive measurement channels should be provided to assess the findings of a study. Presently, most PSC measurements with lidar are performed at 532 nm only. The information from additional polarization-sensitive measurements in the near infrared could lead to an improved PSC classification. Coincident lidar-based temperature measurements at PSC level might provide useful information for an assessment of PSC classification.
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| 2. |
- Achtert, Peggy, 1982-, et al.
(författare)
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Hygroscopic growth of tropospheric particle number size distributions over the North China Plain
- 2009
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 114, s. D00G07-
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Tidskriftsartikel (refereegranskat)abstract
- The hygroscopic growth of atmospheric submicrometer particle size distributions (diameter D-p ranging from 22 to 900 nm) was studied at a rural/suburban site in the North China Plain within the framework of the international Campaigns of Air Quality Research in Beijing and Surrounding Region 2006 (CAREBeijing-2006) research project. The goal was to characterize the regional aerosol in the polluted northeastern plain in China. Size descriptive hygroscopic growth factors (DHGFs) were determined as a function of relative humidity (RH) by relating the particle number size distribution at a dry condition ( 100 nm), the DHGF are substantially higher than in the Aitken particle mode (D-p < 100 nm) as a result of different chemical composition. The size-dependent behavior of the DHGF highlights the relevance of particulate sulfate production over the North China Plain, accomplished by secondary formation from the gas phase and, potentially, liquid phase processes in convective clouds. Furthermore, all results concerning the DHGF show a significant dependency on meteorological air masses. The hygroscopic growth of accumulation mode particles correlates significantly with the PM1-mass fraction of sulfate ions determined by chemical analysis. Finally, this investigation provides a parameterization of the hygroscopic growth of 250-nm particles, which might be useful when predicting visibility and radiative forcing and performing atmospheric aerosol model validations.
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| 3. |
- Achtert, Peggy, 1982-, et al.
(författare)
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Investigation of polar stratospheric clouds in January 2008 by means of ground-based and spaceborne lidar measurements and microphysical box model simulations
- 2011
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 116, s. D07201-
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Tidskriftsartikel (refereegranskat)abstract
- Polar stratospheric clouds (PSCs) play a key role in heterogeneous chemistry and ozone depletion in the lower stratosphere. The type of PSC as well as their temporal and spatial extent are important for the occurrence of heterogeneous reactions and, thus, ozone depletion. In this study a combination of ground-based and spaceborne lidar measurements were used together with microphysical box model simulations along back trajectories to investigate the formation and alteration of Arctic PSCs. The measurements were made by the Rayleigh/Mie/Raman lidar system at Esrange and by the Cloud-Aerosol Lidar with Orthogonal Polarization aboard the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite. Between 20 and 23 January 2008 PSCs composed of liquid particles were observed by CALIPSO between Greenland and the western side of the Scandinavian Mountains. Between 21 and 23 January 2008 the Esrange lidar observed a PSC composed of distinct layers of liquid and solid particles on the eastern side of the mountain range. Microphysical box model simulations along air parcel back trajectories indicate that liquid particles had formed at least 40 h before the observation at Esrange. Furthermore, the model indicates a high HNO(3) uptake into the liquid layer between 10 and 20 h before the observation. The PSC was formed when the air mass was over Greenland. On two occasions during these 20 h, CALIPSO observed PSCs when its measurement tracks crossed the air parcel back trajectory ending at the location of the Esrange lidar. Backscatter ratios calculated from the output of the box model simulation indicate good agreement with the values observed with the Esrange lidar and by CALIPSO. The box model simulations along the back trajectories from Esrange to the CALIPSO ground track and beyond provide us with the unique opportunity to relate ground-based and spaceborne lidar measurements that were not performed at the same spatial location and time. Furthermore, possible differences in the observations from ground and space can be traced to temporal and/or geographically induced changes in particle microphysics within the measured PSCs.
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| 4. |
- Achtert, Peggy, 1982-
(författare)
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Lidar Measurements of Polar Stratospheric Clouds in the Arctic
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Polar Stratospheric Clouds (PSCs) play a key role for ozone depletion in the polar stratosphere. Its magnitude depends on the type of PSC and its lifetime and extent. This thesis presents PSC observations conducted with the Esrange lidar and the space-borne CALIPSO lidar.PSCs are separated into three types according to their optical properties. The occurrence rate of the different types which are often observed simultaneously as well as their interaction and connection is not well understood. To better understand the processes that govern PSC formation, observations need to be combined with a detailed view of the atmospheric background in which PSCs develop, exist, and are transformed from one type to another.This thesis introduces a new channel of the Esrange lidar for temperature profiling at heights below 35 km. The design of this channel and first temperature measurements within PSCs and cirrus clouds are presented. This is an important step since the majority of PSC-related literature extracts temperatures within PSCs from reanalysis data.In contrast to ground–based measurements space–borne lidar does not rely on cloud–free conditions. Hence, it provides an unprecedented opportunity for studying the connection between PSCs and the underlying synoptic–scale conditions which manifest as tropospheric clouds. This thesis shows that most of the PSCs observed in the Arctic during winter 2007/08 occurred in connection with tropospheric clouds.A combined analysis of ground-based and space-borne lidar observation of PSCs in combination with microphysical modeling can improve our understanding of PSC formation. A first case study of this approach shows how a PSC that was formed by synoptic-scale processes is transformed into another type while passing the Scandinavian mountains.Today a variety of classification schemes provides inconsistent information on PSC properties and types. This thesis suggests a unified classification scheme for lidar measurements of PSCs.
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| 5. |
- Achtert, Peggy, 1982-, et al.
(författare)
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On the linkage between tropospheric and Polar Stratospheric clouds in the Arctic as observed by space-borne lidar
- 2012
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Ingår i: Atmospheric Chemistry And Physics. - : Copernicus GmbH. - 1680-7316 .- 1680-7324. ; 12:8, s. 3791-3798
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Tidskriftsartikel (refereegranskat)abstract
- The type of Polar stratospheric clouds (PSCs) as well as their temporal and spatial extent are important for the occurrence of heterogeneous reactions in the polar stratosphere. The formation of PSCs depends strongly on temperature. However, the mechanisms of the formation of solid PSCs are still poorly understood. Recent satellite studies of Antarctic PSCs have shown that their formation can be associated with deep-tropospheric clouds which have the ability to cool the lower stratosphere radiatively and/or adiabatically. In the present study, lidar measurements aboard the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite were used to investigate whether the formation of Arctic PSCs can be associated with deep-tropospheric clouds as well. Deep-tropospheric cloud systems have a vertical extent of more than 6.5 km with a cloud top height above 7 km altitude. PSCs observed by CALIPSO during the Arctic winter 2007/2008 were classified according to their type (STS, NAT, or ice) and to the kind of underlying tropospheric clouds. Our analysis reveals that 172 out of 211 observed PSCs occurred in connection with tropospheric clouds. 72% of these 172 observed PSCs occurred above deep-tropospheric clouds. We also find that the type of PSC seems to be connected to the characteristics of the underlying tropospheric cloud system. During the Arctic winter 2007/2008 PSCs consisting of ice were mainly observed in connection with deep-tropospheric cloud systems while no ice PSC was detected above cirrus. Furthermore, we find no correlation between the occurrence of PSCs and the top temperature of tropospheric clouds. Thus, our findings suggest that Arctic PSC formation is connected to adiabatice cooling, i.e. dynamic effects rather than radiative cooling.
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| 6. |
- Achtert, Peggy, 1982-, et al.
(författare)
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Pure rotational-Raman channels of the Esrange lidar for temperature and particle extinction measurements in the troposphere and lower stratosphere
- 2013
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Ingår i: Atmospheric Measurement Techniques. - : Copernicus GmbH. - 1867-1381 .- 1867-8548. ; 6:1, s. 91-98
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Tidskriftsartikel (refereegranskat)abstract
- The Department of Meteorology at Stockholm University operates the Esrange Rayleigh/Raman lidar at Esrange(68° N, 21° E) near the Swedish city of Kiruna. This paper describes the design and first measurements of the newpure rotational-Raman channel of the Esrange lidar. The Esrange lidar uses a pulsed Nd:YAG solid-state laser operating at 532 nm as light source with a repetition rate of 20 Hz and a pulse energy of 350 mJ. The minimum vertical resolution is 150m and the integration time for one profile is 5000 shots. The newly implemented channel allows for measurements of atmospheric temperature at altitudes below 35 km and is currently optimized for temperature measurements between 180 and 200 K. This corresponds to conditions in the lower Arctic stratosphere during winter. In addition to the temperature measurements, the aerosol extinction coefficientand the aerosol backscatter coefficient at 532 nm can be measured in dependently. Our filter-based design minimizes the systematic error in the obtained temperature profile to less than 0.51 K. By combining rotational-Raman measurements (5–35 km height) and the integration technique (30–80 kmheight), the Esrange lidar is now capable of measuring atmospheric temperature profiles from the upper troposphere up to the mesosphere. With the improved setup, the system can be used to validate current lidar-based polar stratospheric cloud classification schemes. The new capability of the instrument measuring temperature and aerosol extinction furthermore enables studies of the thermal structure and variability of the upper troposphere/lower stratosphere. Although several lidars are operated at polar latitudes, there are few instruments that are capable of measuring temperature profiles in the troposphere, stratosphere, and mesosphere, as well as aerosols extinction in the troposphere and lower stratospherewith daylight capability.
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| 7. |
- Khosrawi, Farahnaz, et al.
(författare)
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Denitrification and polar stratospheric cloud formation during the Arctic winter 2009/2010
- 2011
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Ingår i: Atmospheric Chemistry And Physics. - : Copernicus GmbH. - 1680-7316 .- 1680-7324. ; 11:16, s. 8471-8487
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Tidskriftsartikel (refereegranskat)abstract
- The sedimentation of HNO3 containing PolarStratospheric Cloud (PSC) particles leads to a permanent re-moval of HNO3 and thus to a denitrification of the strato-sphere, an effect which plays an important role in strato-spheric ozone depletion. The polar vortex in the Arctic win-ter 2009/2010 was very cold and stable between end of De-cember and end of January. Strong denitrification between 475 to 525 K was observed in the Arctic in mid of Januaryby the Odin Sub Millimetre Radiometer (Odin/SMR). Thiswas the strongest denitrification that had been observed inthe entire Odin/SMR measuring period (2001–2010). Lidarmeasurements of PSCs were performed in the area of Kiruna,Northern Sweden with the IRF (Institutet för Rymdfysik) li-odar and with the Esrange lidar in January 2010. The measurements show that PSCs were present over the area of Kirunaduring the entire period of observations. The formation ofPSCs during the Arctic winter 2009/2010 is investigated using a microphysical box model. Box model simulationsare performed along air parcel trajectories calculated sixdays backward according to the PSC measurements with the ground-based lidar in the Kiruna area. From the temperaturehistory of the backward trajectories and the box model simulations we find two PSC regions, one over Kiruna accordingto the measurements made in Kiruna and one north of Scandinavia which is much colder, reaching also temperatures below Tice. Using the box model trajectories together with the observations of Odin/SMR,Aura/MLS (Microwave Limb Sounder), CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations)and the ground-based lidar we investigate how and by whichtype of PSC particles the denitrification that was observedduring the Arctic winter 2009/2010 was caused. From ouranalysis we find that due to an unusually strong synopticcooling event in mid January, ice particle formation on NATmay be a possible formation mechanism during that particu-lar winter that may have caused the denitrification observed in mid January. In contrast, the denitrification that was observed in the beginning of January could have been caused by the sedimentation of NAT particles that formed on moun-tain wave ice clouds.
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| 8. |
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