Atmospheric profiling using the lidar technique

Tässä väitöskirjassa tutkittiin ilmakehän pienhiukkasten ominaisuuksia ja niiden vaikutusta pilviin hyödyntämällä useiden kaukokartoitusmenetelmien synergiaa. Tutkimuksessa käytettiin pääasiassa PollyXT–lidar-mittalaitetta. Tutkimus jakautui kolmeen kokonaisuuteen: 1) Arvioitiin eri kalibrointimenetelmien aiheuttamaa epävarmuutta lidar-mittauksiin pohjautuvissa vesihöyryprofiileissa. 2) Määritettiin Suomessa ja Yhdistyneissä arabiemiirikunnissa tehtyjen mittausten avulla siitepölyn ja aavikkopölyn optiset ominaisuudet. 3) Selvitettiin miten erilaiset pienhiukkastyypit vaikuttavat erityyppisten pilvien muodostumiseen Arktisella alueella hyödyntämällä satelliittipohjaisia lidar- (CALIOP) ja tutkahavaintoja (CloudSat). Vesihöyrytutkimus osoitti, että tarkat lidar-havainnot vesihöyrystä vaativat tarkan kalibroinnin muiden mittausten avulla. Parhaaseen tulokseen päästään käyttämällä radioluotauksia samalta asemalta mutta niiden puuttuessa voidaan käyttää myös radioluotauksia lähiseudulta tai mallinnettuja vesihyöryprofiileja. Heikoin tulos saatiin satelliittihavaintoja käyttämällä, mutta niistäkin on apua parempien tietolähteiden puuttuessa. Siitepölymittaukset osoittavat, että siitepölytyyppien tunnistaminen lidar-mittausten avulla saatavien optisten ominaisuuksien perusteella on mahdollista, vaikkakin haastavaa. Tyyppien tunnistamiseksi mittauksista täytyy saada tietoa hiukkasten muodosta, koosta sekä kyvystä absorboida valoa. Lisäksi pitää varmistaa, että havaintoja eivät ole häirinneet muut ei-pallomaiset hiukkaset, kuten aavikkopöly, käyttämällä tietoa ilmamassojen kulkureiteistä. Mittaukset Arabian niemimaan aavikkopölystä paljastivat, että sen optiset ominaisuudet poikkeavat Saharan pölystä, etenkin lidarsuhteen osalta. Täten lidar-mittausten analyyseissa usein käytetty lidarsuhde aavikkopölylle ei vastaa Arabian niemimaan aavikkopölyä. Tutkimus pienhiukkasten ja pilvien vuorovaikutuksesta Arktisella alueella osoitti, että pienhiukkasten määrän kasvaessa pilvet, jotka sisältävät sekä vettä että jäätä, lisääntyvät. Pienhiukkastyypin vaikutus pilviin oli huomattavasti pienempi. Sen sijaan ilmakehän ollessa epävakaa, esimerkiksi avomeren päällä, pilvien ominaisuudet riippuivat enemmän ilmakehän virtauksista kuin pienhiukkasten pitoisuudesta tai tyypistä.Atmospheric aerosol particles absorb and scatter solar radiation, altering directly the radiation balance. Indirectly, these particles have a complex interplay in cloud formation, affecting cloud reflectivity and cloud lifetime. Apart from the climatic effects, atmospheric particles pose negative health effects and they reduce visibility with adverse effects in road traffic and aviation safety. To improve the understanding of the aerosol effect on climate four different studies have been conducted. The main instrument utilized to retrieve vertical profiles of the aerosols was a multi-wavelength PollyXT lidar. The hygroscopic effect of the aerosol particles in the retrieved optical properties which is relevant to cloud studies can be assessed using the water vapor capabilities of the lidar. Lidar water vapor retrieval requires initial calibration. An evaluation of the different lidar water vapor signal calibration techniques was performed to quantify the uncertainty in the retrieved water vapor profiles. Moreover, two measurement campaigns were held in Finland and the United Arab Emirates in order to characterize the properties of understudied aerosol types (pollen and Arabian dust). Lastly, the effectiveness of the different aerosol types to the formation of ice, water, or mixedphase clouds in the Arctic was determined using a synergy of a spaceborne lidar (CALIOP) and a cloud radar (CloudSat). The study on water vapor showed that accurate water vapor retrievals are subject to the calibration factor. Operational on-site radiosondes are the best option, but robust retrievals are possible using data from the nearest radiosonde site or modelled data. Satellite-derived water vapor profiles performed the poorest, yet they could serve as an option in the absence of better information. The analysis of the pollen observations showed that the classification of various pollen types is possible, although challenging. Characterization requires shape information from at minimum two linear particle depolarization wavelengths, as well as external information such as airmass backward trajectories to ensure that other non-spherical aerosol particles such as dust are not present over the measurement site. Regarding the Arabian dust optical properties, it was found that this aerosol type exhibits different optical properties, specifically concerning the lidar ratios, than the dust originating from the Saharan region. Consequently, the universal lidar ratio of 55 sr currently used in lidar-based applications may lead to biases for dust originating from the Arabian Peninsula. The Arctic study on aerosol-cloud interactions showed that higher aerosol load was associated with higher occurrence of mixed-phase clouds. On the contrary, moderate association was found with varying the aerosol type. Nevertheless, meteorology outweighed the aerosol load importance over less stable atmospheric conditions, for example, over open ocean

Profiling water vapor mixing ratios in Finland by means of a Raman lidar, a satellite and a model

We present tropospheric water vapor profiles measured with a Raman lidar during three field campaigns held in Finland. Co-located radio soundings are available throughout the period for the calibration of the lidar signals. We investigate the possibility of calibrating the lidar water vapor profiles in the absence of co-existing on-site soundings using water vapor profiles from the combined Advanced InfraRed Sounder (AIRS) and the Advanced Microwave Sounding Unit (AMSU) satellite product; the Aire Limitée Adaptation dynamique Développement INternational and High Resolution Limited Area Model (ALADIN/HIRLAM) numerical weather prediction (NWP) system, and the nearest radio sounding station located 100 km away from the lidar site (only for the permanent location of the lidar). The uncertainties of the calibration factor derived from the soundings, the satellite and the model data are < 2.8, 7.4 and 3.9 %, respectively. We also include water vapor mixing ratio intercomparisons between the radio soundings and the various instruments/model for the period of the campaigns. A good agreement is observed for all comparisons with relative errors that do not exceed 50 % up to 8 km altitude in most cases. A 4-year seasonal analysis of vertical water vapor is also presented for the Kuopio site in Finland. During winter months, the air in Kuopio is dry (1.15±0.40 †kg-1); during summer it is wet (5.54±1.02 †kg-1); and at other times, the air is in an intermediate state. These are averaged values over the lowest 2 km in the atmosphere. Above that height a quick decrease in water vapor mixing ratios is observed, except during summer months where favorable atmospheric conditions enable higher mixing ratio values at higher altitudes. Lastly, the seasonal change in disagreement between the lidar and the model has been studied. The analysis showed that, on average, the model underestimates water vapor mixing ratios at high altitudes during spring and summer

Evaluating modelled winds over an urban area using ground-based Doppler lidar observations

Wind information in urban areas is essential for many applications related to air pollution, urban climate and planning, safety of drone-related operations, and assessment of urban wind energy potential. These applications require accurate wind forecasts, and obtaining this information in an urban environment is challenging as the morphology of a city varies from street to street, altering the wind flow. Remote sensing techniques such as Doppler lidars (light detection and ranging) provide a unique opportunity for wind forecast verification as they can provide both the vertical profile of the horizontal wind and the spatial variation in the horizontal domain at high resolution. In this study, the performance of numerical weather prediction (NWP) models, analysis systems, and large-eddy simulation (LES) models have been analysed by comparing the modelled winds against Doppler lidar observations under various atmospheric conditions and from season to season, in the coastal environment of Helsinki, Finland. The long-term mean vertical profile of the modelled horizontal wind shows good agreement with observations; the NWP model and the analysis systems selected here exhibit different strengths and weaknesses depending on the atmospheric conditions but no significant diurnal variation in performance. However, both the model and analysis systems show differences in their spatially-averaged bias when investigating different wind directions. LES verification shows that these models can potentially provide winds down to street level, given pre-computed scenarios of atmospheric conditions. For Helsinki, the observed winds are stronger during winter than summer, and, on average, higher wind speeds were observed at the urban site than the sub-urban site.Peer reviewe

Optical and geometrical aerosol particle properties over the United Arab Emirates

One year of ground-based night-time Raman lidar observations has been analysed under the Optimization of Aerosol Seeding In rain enhancement Strategies (OASIS) project, in order to characterize the aerosol particle properties over a rural site in the United Arab Emirates. In total, 1130 aerosol particle layers were detected during the 1-year measurement campaign which took place between March 2018 and February 2019. Several subsequent aerosol layers could be observed simultaneously in the atmosphere up to 11 km. The observations indicate that the measurement site is a receptor of frequent dust events, but predominantly the dust is mixed with aerosols of anthropogenic and/or marine origin. The mean aerosol optical depth over the measurement site ranged at 0.37±0.12 and 0.21±0.11 for 355 and 532 nm, respectively. Moreover, mean lidar ratios of 43±11 sr at a wavelength of 355 nm and 39±10 sr at 532 nm were found. The average linear particle depolarization ratio measured over the course of the campaign was 15±6% and 19±7% at the 355 and 532 nm wavelengths, respectively. Since the region is both a source and a receptor of mineral dust, we have also explored the properties of Arabian mineral dust of the greater area of the United Arab of Emirates and the Arabian Peninsula. The observed Arabian dust particle properties were 45±5 (42±5) sr at 355 (532) nm for the lidar ratio, 25±2% (31±2 %) for the linear particle depolarization ratio at 355 (532) nm, and 0.3±0.2 (0.2±0.2) for the extinction-related Angstrom exponent (backscatterrelated Angstrom exponent) between 355 and 532 nm. This study is the first to report comprehensive optical properties of the Arabian dust particles based on 1-year long observations, using to their fullest the capabilities of a multi-wavelength Raman lidar instrument. The results suggest that the mineral dust properties over the Middle East and western Asia, including the observation site, are comparable to those of African mineral dust with regard to the particle depolarization ratios, but not for lidar ratios. The smaller lidar ratio values in this study compared to the reference studies are attributed to the difference in the geochemical characteristics of the soil originating in the study region compared to northern Africa. © 2020 Royal Society of Chemistry. All rights reserved

PollyNET - an emerging network of automated raman-polarizarion lidars for continuous aerosolprofiling

PollyNET is a network of portable, automated, and continuously measuring Ramanpolarization lidars of type Polly operated by several institutes worldwide. The data from permanent and temporary measurements sites are automatically processed in terms of optical aerosol profiles and displayed in near-real time at polly.tropos.de. According to current schedules, the network will grow by 3-4 systems during the upcoming 2-3 years and will then comprise 11 permanent stations and 2 mobile platforms

A methodology for investigating dust model performance using synergistic EARLINET/AERONET dust concentration retrievals

Systematic measurements of dust concentration profiles at a continental scale were recently made possible by the development of synergistic retrieval algorithms using combined lidar and sun photometer data and the establishment of robust remote-sensing networks in the framework of Aerosols, Clouds, and Trace gases Research Infra-Structure Network (ACTRIS)/European Aerosol Research Lidar Network (EARLINET). We present a methodology for using these capabilities as a tool for examining the performance of dust transport models. The methodology includes considerations for the selection of a suitable data set and appropriate metrics for the exploration of the results. The approach is demonstrated for four regional dust transport models (BSC-DREAM8b v2, NMMB/BSC-DUST, DREAM-ABOL, DREAM8-NMME-MACC) using dust observations performed at 10 ACTRIS/EARLINET stations. The observations, which include coincident multi-wavelength lidar and sun photometer measurements, were processed with the Lidar-Radiometer Inversion Code (LIRIC) to retrieve aerosol concentration profiles. The methodology proposed here shows advantages when compared to traditional evaluation techniques that utilize separately the available measurements such as separating the contribution of dust from other aerosol types on the lidar profiles and avoiding model assumptions related to the conversion of concentration fields to aerosol extinction values. When compared to LIRIC retrievals, the simulated dust vertical structures were found to be in good agreement for all models with correlation values between 0.5 and 0.7 in the 1-6 km range, where most dust is typically observed. The absolute dust concentration was typically underestimated with mean bias values of -40 to -20 mu g m(-3) at 2 km, the altitude of maximum mean concentration. The reported differences among the models found in this comparison indicate the benefit of the systematic use of the proposed approach in future dust model evaluation studies

Airborne Pollen Observed by PollyXT Raman Lidar at Finokalia, Crete

In order to document and study airborne pollen in the Mediterranean region, a pollen measurement campaign was performed during February-May 2018, at the Finokalia station in Crete. A ground-based multi-wavelength Raman polarization lidar PollyXT performed continuous measurements, together with a Hirst-type Burkard pollen sampler. The optical properties of pollen layers with presence of airborne pollen are retrieved and presented. Dust-free condition is applied for pollen study, using the dust models

CONSISTENCY OF THE SINGLE CALCULUS CHAIN FOR CLIMATOLOGICAL STUDIES USING LONG-TERM MEASUREMENTS FROM THESSALONIKI LIDAR STATION

The long term analysis of 15 years of lidar data derived from a Raman lidar at Thessaloniki is presented here. All measurements have been processed with the latest version 4 of the EARLINET Single Calculus Chain algorithm and are compared with the results from the current operational retrieval algorithm. In this paper we investigate the consistency between the EARLINET database and SCC for the case of Thessaloniki and we identify the issues to be considered when switching from current operations to SCC

Consistency of the single calculus chain for climatological studies using long-term measurements from thessaloniki lidar station

The long term analysis of 15 years of lidar data derived from a Raman lidar at Thessaloniki is presented here. All measurements have been processed with the latest version 4 of the EARLINET Single Calculus Chain algorithm and are compared with the results from the current operational retrieval algorithm. In this paper we investigate the consistency between the EARLINET database and SCC for the case of Thessaloniki and we identify the issues to be considered when switching from current operations to SCC

Airborne Pollen Observed by Polly

In order to document and study airborne pollen in the Mediterranean region, a pollen measurement campaign was performed during February-May 2018, at the Finokalia station in Crete. A ground-based multi-wavelength Raman polarization lidar PollyXT performed continuous measurements, together with a Hirst-type Burkard pollen sampler. The optical properties of pollen layers with presence of airborne pollen are retrieved and presented. Dust-free condition is applied for pollen study, using the dust models