Characterization of aerosol hygroscopicity using Raman lidar measurements at the EARLINET station of Payerne

This study focuses on the analysis of aerosol hygroscopicity using remote sensing techniques. Continuous observations of aerosol backscatter coefficient (ßaer), temperature (T) and water vapor mixing ratio (r) have been performed by means of a Raman lidar system at the aerological station of MeteoSwiss at Payerne (Switzerland) since 2008. These measurements allow us to monitor in a continuous way any change in aerosol properties as a function of the relative humidity (RH). These changes can be observed either in time at a constant altitude or in altitude at a constant time. The accuracy and precision of RH measurements from the lidar have been evaluated using the radiosonde (RS) technique as a reference. A total of 172 RS profiles were used in this intercomparison, which revealed a bias smaller than 4¿%¿RH and a standard deviation smaller than 10¿%¿RH between both techniques in the whole (in lower) troposphere at nighttime (at daytime), indicating the good performance of the lidar for characterizing RH. A methodology to identify situations favorable to studying aerosol hygroscopicity has been established, and the aerosol hygroscopicity has been characterized by means of the backscatter enhancement factor (fß). Two case studies, corresponding to different types of aerosol, are used to illustrate the potential of this methodology. The first case corresponds to a mixture of rural aerosol and smoke particles (smoke mixture), which showed a higher hygroscopicity (f355ß=2.8 and f1064ß=1.8 in the RH range 73¿%–97¿%) than the second case, in which mineral dust was present (f355ß=1.2 and f1064ß=1.1in the RH range 68¿%–84¿%). The higher sensitivity of the shortest wavelength to hygroscopic growth was qualitatively reproduced using Mie simulations. In addition, a good agreement was found between the hygroscopic analysis done in the vertical and in time for Case I, where the latter also allowed us to observe the hydration and dehydration of the smoke mixture. Finally, the impact of aerosol hygroscopicity on the Earth's radiative balance has been evaluated using the GAME (Global Atmospheric Model) radiative transfer model. The model showed an impact with an increase in absolute value of 2.4¿W¿m-2 at the surface with respect to the dry conditions for the hygroscopic layer of Case I (smoke mixture).Peer ReviewedPostprint (published version

Tropospheric water vapour and relative humidity profiles from lidar and microwave radiometry

In this paper, we outline an iterative method to calibrate the water vapour mixing ratio profiles retrieved from Raman lidar measurements. Simultaneous and co-located radiosonde data are used for this purpose and the calibration results obtained during a radiosonde campaign in summer and autumn 2011 are presented. The water vapour profiles measured during night-time by the Raman lidar and radiosondes are compared and the differences between the methodologies are discussed. Then, a new approach to obtain relative humidity profiles by combination of simultaneous profiles of temperature (retrieved from a microwave radiometer) and water vapour mixing ratio (from a Raman lidar) is addressed. In the last part of this work, a statistical analysis of water vapour mixing ratio and relative humidity profiles obtained during 1 year of simultaneous measurements is presented.This work was supported by the Andalusian Regional Government through projects P12-RNM-2409 and P10-RNM-6299, by the Spanish Ministry of Science and Technology through projects CGL2010-18782, CSD2007-00067, CGL2011-13580-E/CLI and CGL2011-16124-E; and by the EU through the ACTRIS project (EU INFRA-2010-1.1.16-262254)

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

The vertical aerosol type distribution above Israel – 2 years of lidar observations at the coastal city of Haifa

For the first time, vertically resolved long-term lidar measurements of the aerosol distribution were conducted in Haifa, Israel. The measurements were performed by a PollyXT multi-wavelength Raman and polarization lidar. The lidar was measuring continuously over a 2-year period from March 2017 to May 2019. The resulting data set is a series of manually evaluated lidar optical property profiles. To identify the aerosol types in the observed layers, a novel aerosol typing method that was developed at TROPOS is used. This method applies optimal estimation to a combination of lidar-derived intensive aerosol properties to determine the statistically most-likely contribution per aerosol component in terms of relative volume. A case study that shows several elevated aerosol layers illustrates this method and shows, for example, that coarse dust particles are observed up to 5ĝ€¯km height over Israel. From the whole data set, the seasonal distribution of the observed aerosol components over Israel is derived. Throughout all seasons, coarse spherical particles like sea salt and hygroscopically grown continental aerosol were observed. These particles originate from continental Europe and were transported over the Mediterranean Sea. Sea-salt particles were observed frequently due to the coastal site of Haifa. The highest contributions of coarse spherical particles are present in summer, autumn, and winter. During spring, mostly coarse non-spherical particles that are attributed to desert dust were observed. This is consistent with the distinct dust season in spring in Israel. An automated time-height-resolved air mass source attribution method identifies the origin of the dust in the Sahara and the Arabian deserts. Fine-mode spherical particles contribute significantly to the observed aerosol mixture during all seasons. These particles originate mainly from the industrial region at the bay of Haifa

Evaluation of Daytime Measurements of Aerosols and Water Vapor made by an Operational Raman Lidar over the Southern Great Plains

Raman lidar water vapor and aerosol extinction profiles acquired during the daytime over the Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) site in northern Oklahoma (36.606 N, 97.50 W, 315 m) are evaluated using profiles measured by in situ and remote sensing instruments deployed during the May 2003 Aerosol Intensive Operations Period (IOP). The automated algorithms used to derive these profiles from the Raman lidar data were first modified to reduce the adverse effects associated with a general loss of sensitivity of the Raman lidar since early 2002. The Raman lidar water vapor measurements, which are calibrated to match precipitable water vapor (PWV) derived from coincident microwave radiometer (MWR) measurements were, on average, 5-10% (0.3-0.6 g/m(exp 3) higher than the other measurements. Some of this difference is due to out-of-date line parameters that were subsequently updated in the MWR PWV retrievals. The Raman lidar aerosol extinction measurements were, on average, about 0.03 km(exp -1) higher than aerosol measurements derived from airborne Sun photometer measurements of aerosol optical thickness and in situ measurements of aerosol scattering and absorption. This bias, which was about 50% of the mean aerosol extinction measured during this IOP, decreased to about 10% when aerosol extinction comparisons were restricted to aerosol extinction values larger than 0.15 km(exp -1). The lidar measurements of the aerosol extinction/backscatter ratio and airborne Sun photometer measurements of the aerosol optical thickness were used along with in situ measurements of the aerosol size distribution to retrieve estimates of the aerosol single scattering albedo (omega(sub o)) and the effective complex refractive index. Retrieved values of omega(sub o) ranged from (0.91-0.98) and were in generally good agreement with omega(sub o) derived from airborne in situ measurements of scattering and absorption. Elevated aerosol layers located between about 2.6 and 3.6 km were observed by the Raman lidar on May 25 and May 27. The airborne measurements and lidar retrievals indicated that these layers, which were likely smoke produced by Siberian forest fires, were primarily composed of relatively large particles (r(sub eff) approximately 0.23 micrometers), and that the layers were relatively nonabsorbing (omega(sub o) approximately 0.96-0.98). Preliminary results show that major modifications that were made to the Raman lidar system during 2004 have dramatically improved the sensitivity in the aerosol and water vapor channels and reduced random errors in the aerosol scattering ratio and water vapor retrievals by an order of magnitude

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

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 Limitee Adaptation dynamique Developpement 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 g kg(-1)); during summer it is wet (5.54 +/- 1.02 g 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.Peer reviewe

Origin, Transport, and Vertical Distribution of Atmospheric Polluntants over the Northern Sourth China Sea During the 7-SEAS-Dongsha Experiment

During the spring of 2010, comprehensive in situ measurements were made for the first time on a small atoll (Dongsha Island) in the northern South China Sea (SCS), a key region of the 7-SEAS (the Seven South East Asian Studies) program. This paper focuses on characterizing the source origins, transport processes, and vertical distributions of the Asian continental outflows over the region, using measurements including mass concentration, optical properties, hygroscopicity, and vertical distribution of the aerosol particles, as well as the trace gas composition. Cluster analysis of backward trajectories classified 52% of the air masses arriving at ground level of Dongsha Island as having a continental origin, mainly from northern China to the northern SCS, passing the coastal area and being confined in the marine boundary layer (0-0.5 km). Compared to aerosols of oceanic origin, the fine mode continental aerosols have a higher concentration, extinction coefficient, and single-scattering albedo at 550 nm (i.e., 19 vs. 14 microg per cubic meter in PM(sub 2.5); 77 vs. 59 M per meter in beta(sub e); and 0.94 vs. 0.90 in omega, respectively). These aerosols have a higher hygroscopicity (f at 85% RH = 2.1) than those in the upwind inland regions, suggesting that the aerosols transported to the northern SCS were modified by the marine environment. In addition to the near-surface aerosol transport, a significant upper-layer (3-4 km) transport of biomass-burning aerosols was observed. Our results suggest that emissions from both China and Southeast Asia could have a significant impact on the aerosol loading and other aerosol properties over the SCS. Furthermore, the complex vertical distribution of aerosols-coinciding-with-clouds has implications for remote-sensing observations and aerosol-cloud-radiation interactions