Natural Aerosols, Gaseous Precursors and Their Impacts in Greece: A Review from the Remote Sensing Perspective
Abstract
The Mediterranean, and particularly its Eastern basin, is a crossroad of air masses advected from Europe, Asia and Africa. Anthropogenic emissions from its megacities meet over the Eastern Mediterranean, with natural emissions from the Saharan and Middle East deserts, smoke from frequent forest fires, background marine and pollen particles emitted from ocean and vegetation, respectively. This mixture of natural aerosols and gaseous precursors (Short-Lived Climate Forcers—SLCFs in IPCC has short atmospheric residence times but strongly affects radiation and cloud formation, contributing the largest uncertainty to estimates and interpretations of the changing cloud and precipitation patterns across the basin. The SLCFs’ global forcing is comparable in magnitude to that of the long-lived greenhouse gases; however, the local forcing by SLCFs can far exceed those of the long-lived gases, according to the Intergovernmental Panel on Climate Change (IPCC). Monitoring the spatiotemporal distribution of SLCFs using remote sensing techniques is important for understanding their properties along with aging processes and impacts on radiation, clouds, weather and climate. This article reviews the current state of scientific know-how on the properties and trends of SLCFs in the Eastern Mediterranean along with their regional interactions and impacts, depicted by ground- and space-based remote sensing techniques.
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Citation: Amiridis, V.; Kazadzis, S.; Gkikas, A.; Voudouri, K.A.; Kouklaki, D.; Koukouli, M.-E.; Garane, K.; Georgoulias, A.K.; Solomos, S.; Varlas, G.; et al. Natural Aerosols, Gaseous Precursors and Their Impacts in Greece: A Review from the Remote Sensing Perspective. Atmosphere 2024, 15, 753. https://doi.org/10.3390/ atmos15070753 Academic Editor: Davide Zanchettin Received: 22 May 2024 Revised: 18 June 2024 Accepted: 19 June 2024 Published: 24 June 2024 Copyright: © 2024 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). atmosphere Review Natural Aerosols, Gaseous Precursors and Their Impacts in Greece: A Review from the Remote Sensing Perspective Vassilis Amiridis 1,* , Stelios Kazadzis 2,3 , Antonis Gkikas 4, Kalliopi Artemis Voudouri 1,5 , Dimitra Kouklaki 1,6, Maria-Elissavet Koukouli 5, Katerina Garane 5, Aristeidis K. Georgoulias 7, Stavros Solomos 4, George Varlas 8, Anna Kampouri 1, Dimitra Founda 3, Basil E. Psiloglou 3, Petros Katsafados 9, Kyriakoula Papachristopoulou 1, Ilias Fountoulakis 1,4 , Panagiotis-Ioannis Raptis 3, Thanasis Georgiou 1,10 , Anna Gialitaki 1,11 , Emmanouil Proestakis 1, Alexandra Tsekeri 1, Eleni Drakaki 1, Eleni Marinou 1, Elina Giannakaki 12 , Stergios Misios 4, John Kapsomenakis 4, Kostas Eleftheratos 6,13 , Nikos Hatzianastassiou 14, Pavlos Kalabokas 4, Prodromos Zanis 7, Mihalis Vrekoussis 15,16,17 , Alexandros Papayannis 18,19 , Andreas Kazantzidis 20 , Konstantinos Kourtidis 21 , Dimitris Balis 5, Alkiviadis F. Bais 5and Christos Zerefos 4,6,22,23 1Institute for Astronomy, Astrophysics, Space Applications and Remote Sensing, National Observatory of Athens, 152 36 Athens, Greece; [email protected] (K.A.V.); [email protected] (D.K.); [email protected] (A.K.); [email protected] (K.P.); [email protected] (I.F.); [email protected] (T.G.); [email protected] (A.G.); [email protected] (E.P.); [email protected] (A.T.); [email protected] (E.D.); [email protected] (E.M.) 2Physikalisch-Meteorologisches Observatorium Davos, World Radiation Center, Dorfstrasse 33, Davos, 7260 Dorf, Switzerland; [email protected] 3Institute for Environmental Research and Sustainable Development, National Observatory of Athens, Palaia Penteli, 152 36 Athens, Greece; [email protected] (D.F.); [email protected] (B.E.P.); [email protected] (P.-I.R.) 4Research Centre for Atmospheric Physics and Climatology, Academy of Athens, 106 79 Athens, Greece; [email protected] (A.G.); [email protected] (S.S.); [email protected] (S.M.); [email protected] (J.K.); [email protected] (P.K.); [email protected] (C.Z.) 5Laboratory of Atmospheric Physics, School of Physics, Aristotle University of Thessaloniki, 541 24 Thessaloniki, Greece; [email protected] (M.-E.K.); [email protected] (K.G.); [email protected] (D.B.); [email protected] (A.F.B.) 6Department of Geology and Geoenvironment, National and Kapodistrian University of Athens, 115 27 Athens, Greece; [email protected] 7Department of Meteorology and Climatology, School of Geology, Aristotle University of Thessaloniki, 541 24 Thessaloniki, Greece; [email protected] (A.K.G.); [email protected] (P.Z.) 8Hellenic Centre for Marine Research, Institute of Marine Biological Resources and Inland Waters, 46.7 km of Athens-Sounio Ave., 190 13 Anavissos, Greece; [email protected] 9Department of Geography, Harokopio University of Athens, 176 76 Athens, Greece; [email protected] 10 School of Physics, Aristotle University of Thessaloniki, 541 24 Thessaloniki, Greece 11 Department of Physics and Astronomy, Earth Observation Science Group, University of Leicester, Leicester LE1 7RH, UK 12 Department of Environmental Physics and Meteorology, Faculty of Physics, National and Kapodistrian University of Athens, 115 27 Athens, Greece; [email protected] 13 Biomedical Research Foundation, Academy of Athens, 115 27 Athens, Greece 14 Laboratory of Meteorology, Department of Physics, University of Ioannina, 451 10 Ioannina, Greece; [email protected] 15 Institute of Environmental Physics (IUP), University of Bremen, 28359 Bremen, Germany; [email protected] 16 Center of Marine Environmental Sciences (MARUM), University of Bremen, 28359 Bremen, Germany 17 Climate and Atmosphere Research Center (CARE-C), The Cyprus Institute, Nicosia 2121, Cyprus 18 Laser Remote Sensing Unit (LRSU), Physics Department, National Technical University of Athens, 157 80 Zografou, Greece; [email protected] 19 Laboratory of Atmospheric Processes and Their Impacts, School of Architecture, Civil and Environmental Engineering, École Polytechnique Fédérale de Lausanne, 1003 Lausanne, Switzerland 20 Laboratory of Atmospheric Physics, Department of Physics, University of Patras, 265 00 Patras, Greece; [email protected] 21 Department of Environmental Engineering, Democritus University of Thrace, 671 00 Xanthi, Greece; [email protected] 22 Navarino Environmental Observatory (N.E.O.), 240 01 Messinia, Greece Atmosphere 2024,15, 753. https://doi.org/10.3390/atmos15070753 https://www.mdpi.com/journal/atmosphere
Atmosphere 2024,15, 753 2 of 28 23 Mariolopoulos-Kanaginis Foundation for the Environmental Sciences, 106 75 Athens, Greece *Correspondence: [email protected] Abstract: The Mediterranean, and particularly its Eastern basin, is a crossroad of air masses advected from Europe, Asia and Africa. Anthropogenic emissions from its megacities meet over the Eastern Mediterranean, with natural emissions from the Saharan and Middle East deserts, smoke from frequent forest fires, background marine and pollen particles emitted from ocean and vegetation, respectively. This mixture of natural aerosols and gaseous precursors (Short-Lived Climate Forcers—SLCFs in IPCC has short atmospheric residence times but strongly affects radiation and cloud formation, contributing the largest uncertainty to estimates and interpretations of the changing cloud and precipitation patterns across the basin. The SLCFs’ global forcing is comparable in magnitude to that of the long-lived greenhouse gases; however, the local forcing by SLCFs can far exceed those of the long-lived gases, according to the Intergovernmental Panel on Climate Change (IPCC). Monitoring the spatiotemporal distribution of SLCFs using remote sensing techniques is important for understanding their properties along with aging processes and impacts on radiation, clouds, weather and climate. This article reviews the current state of scientific know-how on the properties and trends of SLCFs in the Eastern Mediterranean along with their regional interactions and impacts, depicted by groundand space-based remote sensing techniques. Keywords: short-lived climate forcers; Mediterranean 1. Introduction Observing SLFCs [ 1 ] over the Eastern Mediterranean is of importance because their concentrations are typically 2to 10-times higher than in the hemispheric background troposphere [ 2 ]. There has been a significant number of studies on SLCF abundance, properties, and spatiotemporal distributions and trends over Greece and the extended Eastern Mediterranean in the last 20 years, utilizing groundand satellite-based remote sensors. Satellite remote sensing has been utilized to study the SLCF spatiotemporal distributions over the greater Eastern Mediterranean region, based on data from Meteosat [ 3 ], SeaWIFS (e.g., [ 4 ]), TOMS (e.g., [ 5 – 9 ]), MODIS Terra and Aqua (e.g., [ 6 – 8 , 10 – 26 ]), OMI/AURA (e.g., [ 8 , 9 , 27 ]), CALIOP/CALIPSO (e.g., [ 28 – 30 ]), MISR/Terra (e.g., [ 31 ]), as well as NOAA/ AVHRR, MERIS/ENVISAT, AATSR/ENVISAT, PARASOL/POLDER, MSG/SEVIRI and Landsat satellite observations (e.g., [ 32 , 33 ]). Ground-based remote sensors have also been used to study the aerosol and trace gas abundance and characterization over Greece, utilizing sensors, such as lidars (e.g., [ 34 – 56 ]), MAX-DOAS ([ 57 – 62 ]), sun photometers (e.g., [ 48 , 63 – 69 ]), Brewer spectrophotometers (e.g., [ 6 , 65 , 70 – 75 ]), Multi-Filter Radiometers (e.g., [ 76 – 78 ]) and ceilometers (e.g., [ 79 , 80 ]). In addition to the systematic ground-based observations [ 81 ], a number of experimental campaigns have been conducted in the last few decades in Greece, supported by the PANhellenic infrastructure for Atmospheric Composition and climatE chAnge (PANACEA) National Research Infrastructure (the Greek component of the Aerosol, Clouds and Trace Gases Research Infrastructure—ACTRIS European Research Infrastructure), to address specific science objectives of interest. A list of the recent experiments includes the Pre-TECT ([ 82 ]) experiment in Finokalia focusing on desert dust characterization and the CALISHTO experiment in Helmos for aerosol–cloud interactions ([83]). This article is structured as follows: the current state of scientific know-how on the properties and trends of the natural aerosols and the gaseous precursors over the Mediterranean is described in Section 1. Section 2describes their regional interactions and impacts as, depicted by groundand space-based remote sensing techniques. Finally, Section 3contains the summary and main recommendations for future research.
Atmosphere 2024,15, 753 3 of 28 1.1. Natural Aerosols 1.1.1. Aerosol Types and Sources The diversity of aerosol species that we see in Greece (Figure 1) results from the convergence of air masses carrying aerosols from natural sources situated in the surrounding regions (e.g., [ 84 , 85 ]). Residing in the proximity of the Sahara Desert, Greece receives massive amounts of mineral dust particles, mainly in spring and summer [ 8 , 30 , 86 ]. Smoke from local forest fires also plays a critical role in the accumulation of light-absorbing soot particles in summer [ 87 – 89 ]. Transboundary smoke advection over Greece from sources in Ukraine, Portugal, the Balkans and Canada has also been reported [ 22 , 44 , 51 , 54 , 69 , 90 – 94 ]. Volcanic particles (composed of ash and sulphates) constitute another natural aerosol component affecting Greece after Etna or Icelandic volcanic eruptions [ 42 , 47 , 95 – 98 ]. Among the recorded aerosol species, a significant contribution results from marine particles produced by bursting bubbles during whitecap formation, attributed to wind–wave interactions (e.g., [ 99 ]). Biogenic particles consisting mainly of airborne fungi and pollen grains also contribute to the natural aerosol burden in the Greek area [100–104]. Atmosphere2024,15,xFORPEERREVIEW3of30 1.1.NaturalAerosols 1.1.1.AerosolTypesandSources ThediversityofaerosolspeciesthatweseeinGreece(Figure1)resultsfromtheconvergenceofairmassescarryingaerosolsfromnaturalsourcessituatedinthesurrounding regions(e.g.,[84,85]).ResidingintheproximityoftheSaharaDesert,Greecereceivesmassiveamountsofmineraldustparticles,mainlyinspringandsummer[8,30,86].Smoke fromlocalforestfiresalsoplaysacriticalroleintheaccumulationoflight-absorbingsoot particlesinsummer[87–89].TransboundarysmokeadvectionoverGreecefromsources inUkraine,Portugal,theBalkansandCanadahasalsobeenreported[22,44,51,54,69,90– 94].Volcanicparticles(composedofashandsulphates)constituteanothernaturalaerosol componentaffectingGreeceafterEtnaorIcelandicvolcaniceruptions[42,47,95–98]. Amongtherecordedaerosolspecies,asignificantcontributionresultsfrommarineparticlesproducedbyburstingbubblesduringwhitecapformation,attributedtowind–wave interactions(e.g.,[99]).Biogenicparticlesconsistingmainlyofairbornefungiandpollen grainsalsocontributetothenaturalaerosolburdenintheGreekarea[100–104]. Figure1.ColumnarclimatologicalestimationoftheaerosoltypesobservedinThessaloniki,Athens andCrete,asretrievedfromtheAErosolROboticNETwork(AERONET)data.BasedonOPAC ([105,106],source:[107]. 1.1.2.CurrentKnowledgePerAerosolType DesertDust:DustoutbreaksfrequentlyaffectthebroaderGreekregion,asrevealed bypassiveandactiveremotesensingtechniques.Onaseasonalbasis,dustopticaldepths (DODs)aremaximizedinspring,whencyclonicpressuresystems,inducingsouth–southwesterlywinds,favortheadvectionofSaharanmineralparticlestowardsGreece[9,21,77]. Similarairmasspathwaysarerecordedinwinter[108],whereasduringsummermonths, dust-ladenairmasses,originatinginthenorth-westernpartsoftheSahara,carrymineral particlesoverGreeceaftercrossingtheItalianpeninsula[30,37,39–41,43].Underfavorable conditions(i.e.,dustsourcesactivation,strongwinds,weakremovalmechanisms),DODs canyieldextremevalues(>2)[8,109,110],withthecoreportionofthedustburdenresiding upto6km,whilemineralparticlesatlowconcentrationsaredetectedupto10km [30,111,112].Suchextremeeventstakeplaceabout9days/year(mainlyinspring)inthe southernpartsofGreece(Crete),whiletheirfrequencydropsdownto1–2days/yearin thenorth[8,111].Dusthasbeenshowntoaccountforaboutone-thirdofthetotalaerosol opticaldepth(AOD)overGreeceusingsatelliteobservationsfromtheModerate-ResolutionImagingSpectroradiometer(MODIS)andreanalysis/modeldata[23,26].Basedonthe MIDASdustdataset(2003–2017,[15]),themostnegativetrendsovertheextendedGreek areaarecomputedinsummerandspring(downto—0.010/year)([24];Figure2).Thezonal distributionofthedustextinctionverticaldistributionfromCALIPSO-LIVASdustproduct[113]fortheregion20°–30°Eforthelatitudinalregionfrom10°–60°Nisdepictedin Figure2(seasonaldistribution[30]). Figure 1. Columnar climatological estimation of the aerosol types observed in Thessaloniki, Athens and Crete, as retrieved from the AErosol RObotic NETwork (AERONET) data. Based on OPAC ([105,106], source: [107]). 1.1.2. Current Knowledge Per Aerosol Type Desert Dust: Dust outbreaks frequently affect the broader Greek region, as revealed by passive and active remote sensing techniques. On a seasonal basis, dust optical depths (DODs) are maximized in spring, when cyclonic pressure systems, inducing south–southwesterly winds, favor the advection of Saharan mineral particles towards Greece [ 9 , 21 , 77 ]. Similar airmass pathways are recorded in winter [ 108 ], whereas during summer months, dust-laden air masses, originating in the north-western parts of the Sahara, carry mineral particles over Greece after crossing the Italian peninsula [ 30 , 37 , 39 – 41 , 43 ]. Under favorable conditions (i.e., dust sources activation, strong winds, weak removal mechanisms), DODs can yield extreme values (>2) [ 8 , 109 , 110 ], with the core portion of the dust burden residing up to 6 km, while mineral particles at low concentrations are detected up to 10 km [ 30 , 111 , 112 ]. Such extreme events take place about 9 days/year (mainly in spring) in the southern parts of Greece (Crete), while their frequency drops down to 1–2 days/year in the north [ 8 , 111 ]. Dust has been shown to account for about one-third of the total aerosol optical depth (AOD) over Greece using satellite observations from the Moderate-Resolution Imaging Spectroradiometer (MODIS) and reanalysis/model data [ 23 , 26 ]. Based on the MIDAS dust dataset (2003–2017, [ 15 ]), the most negative trends over the extended Greek area are computed in summer and spring (down to—0.010/year) ([24]; Figure 2). The zonal distribution of the dust extinction vertical distribution from CALIPSO-LIVAS dust product [ 113 ] for the region 20 ◦ –30 ◦ E for the latitudinal region from 10 ◦ –60 ◦ N is depicted in Figure 2(seasonal distribution [30]).
Atmosphere 2024,15, 753 4 of 28 Atmosphere2024,15,xFORPEERREVIEW4of30 Figure2.(Upperleft):Annualmeandustopticaldepth(DOD)at550nmfromMIDAS(2003–2017); (upperright):trends(statisticallysignificantat95%confidencelevel)ofdeseasonalizedDODfrom MIDAS(2003–2017);(lower)zonalseasonaldistributionoftheconditionaldustextinctionvertical distributionfromCALIPSO–LIVASdustproduct. Amorecomprehensivedescriptionofdustopticalandmicrophysicalpropertiesis depictedbyground-basedlidarsandsunphotometersoperatinginGreece,providingverticallyresolvedandcolumnarinformation,respectively(e.g.,[5,35,39,41– 43,45,49,63,68,69,79,114–120]).Amongotherconclusions,theaforementionedstudiesrevealthefollowing:dustoutbreakscauseexceptionalexceedancesofAODlevels(>0.3)and reductionsintheÅngströmexponent(<0.5);dustlayersaremainlyconfinedbetween1.5 and6.5kma.s.l.overGreece;theirregularshapeandthelargedustparticlesizeledto significantlyhighdepolarizationvalues(~30%)andmediumlidarratiovalues(~45sr). FireSmoke:SmokefromlocalforestfiresandacrossEasternEuropedevelopedunder dryconditions,andstrongwindsplayacriticalroleintheaccumulationoflight-absorbing sootparticlesinsummer[75,87–89].Althoughoriginatingfrommedium-strengthfires, theseintensesmokeplumesusuallypenetratethePlanetaryBoundaryLayer(PBL)to reachthefreetroposphere(e.g.,[90]).ThisispossibleduetothecomplexGreektopographyandtheinterchangebetweenlandandmarineboundarylayersandalsoduetoconvectiveprocessesandPyro-Cbformation[121,122].ThedispersionofsmokefromwildfiresoverGreeceisdetectedandforecastedbytheFirehub-smokeplatform([123,124]; Figure3). Figure 2. (Upper left): Annual mean dust optical depth (DOD) at 550 nm from MIDAS (2003–2017); (upper right): trends (statistically significant at 95% confidence level) of deseasonalized DOD from MIDAS (2003–2017); (lower) zonal seasonal distribution of the conditional dust extinction vertical distribution from CALIPSO–LIVAS dust product. A more comprehensive description of dust optical and microphysical properties is depicted by ground-based lidars and sun photometers operating in Greece, providing vertically resolved and columnar information, respectively ( e.g., [5,35,39,41–43,45,49,63,68,69,79,114–120]) . Among other conclusions, the aforementioned studies reveal the following: dust outbreaks cause exceptional exceedances of AOD levels (>0.3) and reductions in the Ångström exponent (<0.5); dust layers are mainly confined between 1.5 and 6.5 km a.s.l. over Greece; the irregular shape and the large dust particle size led to significantly high depolarization values (~30%) and medium lidar ratio values (~45 sr). Fire Smoke: Smoke from local forest fires and across Eastern Europe developed under dry conditions, and strong winds play a critical role in the accumulation of light-absorbing soot particles in summer [ 75 , 87 – 89 ]. Although originating from medium-strength fires, these intense smoke plumes usually penetrate the Planetary Boundary Layer (PBL) to reach the free troposphere (e.g., [ 90 ]). This is possible due to the complex Greek topography and the interchange between land and marine boundary layers and also due to convective processes and Pyro-Cb formation [ 121 , 122 ]. The dispersion of smoke from wildfires over Greece is detected and forecasted by the Firehub-smoke platform ([123,124]; Figure 3). Smoke detections over Greece from distant sources include the plumes from biomass burning of agricultural waste in Eastern Europe and grassland/shrubland fires along the coasts of the Black Sea [ 44 , 50 , 92 , 94 , 125 – 129 ], Portugal wildfires [ 122 ], and Canadian wildfires [ 51 , 54 , 69 , 93 , 118 ]. The studies report that the smoke particles were found to be relatively small, with high Ångström exponents and high lidar ratios (of the order of 70 ± 20sr)duetotheirabsorption andsizedistribution[ 22 , 45 , 55 , 120 , 127 , 128 ]. Siomos et al. [48] report AOD values at 355 nm ranging from 0.18 to 0.24, with the exception of summer in the free troposphere, where the largest AOD value occurs (~0.4). The observed variability in smoke properties is attributed to the particle age, which affects the absorption efficiency and particle hygroscopic growth processes (e.g., [ 44 ]). For the same reasons, large variability in the depolarization ratio of smoke has been reported (e.g., [ 128 , 129 ]) due to particle age or the particle water uptake due to different humidity conditions. Gialitaki et al. [ 54 ] shows, however, that for the Canadian stratospheric smoke observed above Europe in August 2017,
Atmosphere 2024,15, 753 5 of 28 the observed depolarization and lidar ratio values (along with their spectral dependence) can be successfully reproduced with a proposed model of compact near-spherical particles. Atmosphere2024,15,xFORPEERREVIEW5of30 Figure3.(a)MODISimageon25August200720:00UTC;(b)columnconcentrationofsmoke (mg/m 2 )fromaconcurrentFLEXPART-FireHubsimulation;(c)MODISimageon26August2007 09:30UTC;(d)columnconcentrationofsmoke(mg/m 2 )fromaconcurrentFLEXPART-Fire. SmokedetectionsoverGreecefromdistantsourcesincludetheplumesfrombiomass burningofagriculturalwasteinEasternEuropeandgrassland/shrublandfiresalongthe coastsoftheBlackSea[44,50,92,94,125–129],Portugalwildfires[122],andCanadianwildfires[51,54,69,93,118].Thestudiesreportthatthesmokeparticleswerefoundtoberelativelysmall,withhighÅngströmexponentsandhighlidarratios(oftheorderof70±20 sr)duetotheirabsorptionandsizedistribution[22,45,55,120,127,128].Siomosetal.[48] reportAODvaluesat355nmrangingfrom0.18to0.24,withtheexceptionofsummerin thefreetroposphere,wherethelargestAODvalueoccurs(~0.4).Theobservedvariability insmokepropertiesisattributedtotheparticleage,whichaffectstheabsorptionefficiency andparticlehygroscopicgrowthprocesses(e.g.,[44]).Forthesamereasons,largevariabilityinthedepolarizationratioofsmokehasbeenreported(e.g.,[128,129])duetoparticle ageortheparticlewateruptakeduetodifferenthumidityconditions.Gialitakietal.[54] shows,however,thatfortheCanadianstratosphericsmokeobservedaboveEuropein August2017,theobserveddepolarizationandlidarratiovalues(alongwiththeirspectral dependence)canbesuccessfullyreproducedwithaproposedmodelofcompactnearsphericalparticles. Marine:Marineaerosolsareamajorcomponentofnaturalaerosols,especiallyin countriessurroundedbysea,suchasGreece(e.g.,[99]).Georgouliasetal.[22]reportsthat marineaerosolsaccountforaboutone-fourthofthetotalAODovertheGreekseas.The twomainprocessesresponsiblefortheformationofmarinesea-spraydropletsare:(1)the burstingofairbubblesinwhitecaps,whichreleasesfilmandjetsea-spraydroplets,and (2)thedirecttearingofspumesea-spraydropletsfromtheedgesofbreakingwaves[130]. Theejectedsea-spraydropletsevaporatewhentheyenteradrieratmosphericenvironment,releasingsea-saltparticlesthatcirculateintheatmosphereand,finally,dropback tothesurfacethroughdryandwetdeposition[131].Sea-saltparticlesfeatureapredominantcoarsemodeandaresphericalunderhumidconditions.Haarigetal.[132]reportthat themarineparticledepolarizationratioshowsastrongincrease(upto10%)duringthe Figure 3. (a) MODIS image on 25 August 2007 20:00 UTC; (b) column concentration of smoke (mg/m 2 ) from a concurrent FLEXPART-FireHub simulation; (c) MODIS image on 26 August 2007 09:30 UTC; (d) column concentration of smoke (mg/m2) from a concurrent FLEXPART-Fire. Marine: Marine aerosols are a major component of natural aerosols, especially in countries surrounded by sea, such as Greece (e.g., [ 99 ]). Georgoulias et al. [ 22 ] reports that marine aerosols account for about one-fourth of the total AOD over the Greek seas. The two main processes responsible for the formation of marine sea-spray droplets are: (1) the bursting of air bubbles in whitecaps, which releases film and jet sea-spray droplets, and (2) the direct tearing of spume sea-spray droplets from the edges of breaking waves [ 130 ]. The ejected sea-spray droplets evaporate when they enter a drier atmospheric environment, releasing sea-salt particles that circulate in the atmosphere and, finally, drop back to the surface through dry and wet deposition [ 131 ]. Sea-salt particles feature a predominant coarse mode and are spherical under humid conditions. Haarig et al. [ 132 ] report that the marine particle depolarization ratio shows a strong increase (up to 10%) during the phase transition from spherical sea-salt particles to cubic-like sea-salt crystals under low relative humidities in the marine boundary layer (below 50%). However, the lidar ratio shows very small variability, within 20–25 sr for all states. During the last decade, the monitoring of sea-salt particles in Greece with groundbased remote sensing instruments that operated during the CHARADMExp and Pre-TECT campaigns in Finokalia and PANGEA stations provided critical datasets for constraining modeled emissions [ 117 , 133 , 134 ]. As presented by Varlas et al. [ 99 ], the consideration of wind–wave interactions in models drives towards a better agreement with observations, mainly due to the increase in emissions at low-to-moderate wind intensities. Varlas et al. [99] corroborated Regayre et al. [ 135 ], who suggested that the default sea-spray emissions in wind-only-dependent models need to be increased by a factor of approximately three when wind speeds are relatively low. However, they have to be reduced in winter (usually characterized by relatively higher winds). The convergence of the two
Atmosphere 2024,15, 753 6 of 28 studies strengthens the aspect that there is a necessity of revisiting sea-spray emissions by reconsidering the dependence of sea-salt aerosol emissions on the wind regime, while also designing more complete modeling methods, including not only wind speed but also physical processes such as waves and white capping. Volcanic aerosols: Volcanic eruptions can inject large amounts of volcanic ash and gases (e.g., sulfur dioxide, SO 2 ) into the atmosphere, which can influence the radiation budget and climate [ 136 ], ecosystems, agriculture and aviation but even air quality and health [137–142] . Greece is often affected by the long-range transport of volcanic aerosols, mainly due to continuous Etna volcanic activity. Mt. Etna is the largest point source of particulate matter in the atmosphere of the Mediterranean, affecting the atmospheric levels of airborne particles and their deposition rates at both local and regional scales [97,98,143,144] . The optical properties of volcanic ash aerosols are generally similar to those of desert dust, as shown by Wang et al. [ 145 ] and Ansmann et al. [ 146 ] for fresh ash, with particle linear depolarization ratios reaching 0.37 and lidar ratio at 532sr of 50–65sr. Aged volcanic particles transported over Greece from the Icelandic volcano eruption of 2010 indicate lower depolarization ratios of 0.1–0.25 and lidar ratios for 355nm within the range 55–70sr [ 42 , 147 ]. Fresh volcanic ash plumes injected from Etna are systematically recorded in the PANGEA observatory. The plumes usually travel at free tropospheric heights, while WRF-FLEXPART forecasts of the projected pathways, constrained by satellite and PANGEA observations, are provided by NOA (Figure 4; [ 97 ]). Occasional transport over Greece of volcanic aerosols at stratospheric heights can also occur (e.g., [53,84]). Atmosphere2024,15,xFORPEERREVIEW6of30 phasetransitionfromsphericalsea-saltparticlestocubic-likesea-saltcrystalsunderlow relativehumiditiesinthemarineboundarylayer(below50%).However,thelidarratio showsverysmallvariability,within20–25srforallstates. Duringthelastdecade,themonitoringofsea-saltparticlesinGreecewithgroundbasedremotesensinginstrumentsthatoperatedduringtheCHARADMExpandPreTECTcampaignsinFinokaliaandPANGEAstationsprovidedcriticaldatasetsforconstrainingmodeledemissions[117,133,134].AspresentedbyVarlasetal.[99],theconsiderationofwind–waveinteractionsinmodelsdrivestowardsabetteragreementwithobservations,mainlyduetotheincreaseinemissionsatlow-to-moderatewindintensities. Varlasetal.[99]corroboratedRegayreetal.[135],whosuggestedthatthedefaultseasprayemissionsinwind-only-dependentmodelsneedtobeincreasedbyafactorofapproximatelythreewhenwindspeedsarerelativelylow.However,theyhavetobereduced inwinter(usuallycharacterizedbyrelativelyhigherwinds).Theconvergenceofthetwo studiesstrengthenstheaspectthatthereisanecessityofrevisitingsea-sprayemissionsby reconsideringthedependenceofsea-saltaerosolemissionsonthewindregime,whilealso designingmorecompletemodelingmethods,includingnotonlywindspeedbutalso physicalprocessessuchaswavesandwhitecapping. Volcanicaerosols:Volcaniceruptionscaninjectlargeamountsofvolcanicashand gases(e.g.,sulfurdioxide,SO 2 )intotheatmosphere,whichcaninfluencetheradiation budgetandclimate[136],ecosystems,agricultureandaviationbutevenairqualityand health[137–142].Greeceisoftenaffectedbythelong-rangetransportofvolcanicaerosols, mainlyduetocontinuousEtnavolcanicactivity.Mt.Etnaisthelargestpointsourceof particulatematterintheatmosphereoftheMediterranean,affectingtheatmosphericlevelsofairborneparticlesandtheirdepositionratesatbothlocalandregionalscales [97,98,143,144].Theopticalpropertiesofvolcanicashaerosolsaregenerallysimilarto thoseofdesertdust,asshownbyWangetal.[145]andAnsmannetal.[146]forfreshash, withparticlelineardepolarizationratiosreaching0.37andlidarratioat532 srof50–65 sr. AgedvolcanicparticlestransportedoverGreecefromtheIcelandicvolcanoeruptionof 2010indicatelowerdepolarizationratiosof0.1–0.25andlidarratiosfor355 nmwithinthe range55–70 sr[42,147].FreshvolcanicashplumesinjectedfromEtnaaresystematically recordedinthePANGEAobservatory.Theplumesusuallytravelatfreetropospheric heights,whileWRF-FLEXPARTforecastsoftheprojectedpathways,constrainedbysatelliteandPANGEAobservations,areprovidedbyNOA(Figure4;[97]).Occasional transportoverGreeceofvolcanicaerosolsatstratosphericheightscanalsooccur(e.g., [53,84]). Figure4.FLEXPARTsimulationsofthevolcanicashcolumnarconcentrations(µg/m −2 )originating fromEtna,transportedoverGreece,usingmeteorologicalfieldswithAeoluswindassimilation(12 March2021,19:30UTC).(Right:)AshmassconcentrationscalculatedbythePANGEAdepolarizationlidar(orange)andmodeledwithFLEXPART(green). Figure 4. FLEXPART simulations of the volcanic ash columnar concentrations ( µ g/m −2 ) originating from Etna, transported over Greece, using meteorological fields with Aeolus wind assimilation (12 March 2021, 19:30 UTC). (Right): Ash mass concentrations calculated by the PANGEA depolarization lidar (orange) and modeled with FLEXPART (green). Pollen: Pollen is an essential part of plant reproduction as it stores and transports the genetic information of the plant. Pollen grains transferred by winds have various effects on climate and human health (e.g., [ 148 , 149 ]). Pollen can be lifted to several kilometers by turbulent mixing within the boundary layer and above and, thus, transported by wind over thousands of kilometers [ 150 – 153 ]. Dispersed in the atmosphere, they can affect the climate by acting as ice nucleation and cloud condensation nuclei, promoting the formation of clouds [ 154 – 158 ]. In the Mediterranean region, olive (Olea europaea) pollen is considered as one of the most important causes of respiratory allergic disease, whereas cypress (cupressus) releases enormous amounts of anemophilous pollen and has been recognized to be responsible for a significant portion of the overall airborne pollen [ 159 ]. Recently, an increasing interest in pollen has arisen in the aerosol lidar community. Bohlmann et al. [ 160 ] showed that in the absence of other non-spherical particles, light detection and ranging (lidar) measurements and especially the particle
Atmosphere 2024,15, 753 7 of 28 depolarization ratio can be used to track pollen grains in the atmosphere. However, the atmospheric aerosol population is always a mixture of several particle types and, thus, identifying pure pollen optical properties is still difficult. Shang et al. [ 161 ] reported that the depolarization ratio at 532 nm for pure birch (Betula) was in the order of 25% and 35% for pine pollen. Airborne pollen measurements in the Mediterranean region were performed at Finokalia station in Crete to characterize the optical properties of pollen. The linear particle depolarization ratio of pollen was relatively small, with a maximum of 0.15, since the shape of the majority of pollen types in the region is quasi-spherical. 1.2. Gaseous Precursors Ground-based and satellite remote sensing has been used for the monitoring and evaluation of NO 2 , total and tropospheric ozone, methane and SO 2 levels in the Mediterranean, as discussed in the following. Total Ozone: The study of ozone variations in Greece started in the early 1980s, using ground-based measurements from Dobson [ 162 ] and Brewer spectrophotometers, along with various satellite missions (e.g., [ 163 – 170 ] etc.) and allowed for the detection of signs of the ozone layer depletion over Thessaloniki and Athens in the 1980s and the 1990s. The associated changes in solar UV-B radiation were also analyzed (e.g., [ 171 – 173 ]). More recent studies investigated longer-term measurements of total ozone in Thessaloniki, Greece [ 174 – 177 ] and detected a non-significant increase in total ozone between 1997 and mid-2010, likely associated with ozone recovery [ 178 ]. In Figure 5, the time series of the monthly mean departures (in percent) of total ozone from the climatological means are shown for Thessaloniki for the period 1982–2024. Total ozone measurements for this figure were performed by the first commercially available Brewer spectrophotometer, serial number #005, which is a single monochromator type MKII operating at the Laboratory of Atmospheric Physics [ 179 ], Thessaloniki, since 1982 and regularly calibrated via its systematic participation in international intercomparison campaigns. The analysis revealed a sharp decrease in stratospheric ozone in the early 1990s, which was attributed to the effect of the Mt. Pinatubo volcanic eruption. The 41-year record shows that total ozone has a statistically significant overall negative trend of − 0.6% per decade, driven by the strong negative change of − 3.8% per decade until 1996. Since then, the ozone layer record at Thessaloniki has been recovering at a (non-statistically significant) slower rate of − 0.2% per decade. Atmosphere2024,15,xFORPEERREVIEW8of30 ozoneretrievinginstrumentsthatprovidelargerspatialcoveragethantheground-based observations(e.g.,[181–183]). Figure5.Monthlymeanpercentagedeparturesfromtheclimatologicalvaluesoftotalozoneand therespectiveestimatedtrend,retrievedbytheBrewerspectrophotometer#005,operatinginThessalonikisince1982. TroposphericOzone:Theinter-connectivityoftroposphericO 3 andaerosoldirecteffectsismanifold;ononehand,theincreasedpresenceofaerosolsreducesradiationreachingtheground,andthesubsequentphotolysisreductioncandecreasetroposphericO 3 in pollutedareasduringsummer.Ontheotherhand,enhancedaerosolpresenceleadsto cooling,whichsuppressesatmosphericventilationandresultsinincreasedsurface-level O 3 inwintertime[184].TheEasternMediterraneanisamongtheregionswiththehighest levelsofbackgroundtroposphericozoneworldwide[185].Threeatmosphericprocesses controllingitsformationintheregion,namelythelong-rangetransportofpollutedair masses,thedynamicsubsidenceatmid-troposphericlevelsandthestratosphere-to-troposphereexchange,havebeenextensivelystudiedfortheregion(e.g.,[186–190]).Recent space-borneobservationshaveshownthatintheMediterraneantroposphere,betweenthe surfaceand2km,O 3 ismostlyformedfromanthropogenicemissions,whileabove4km, itismostlytransportedfromoutsidethedomainorfromstratosphericorigins[191]. InFigure6,theseasonalsummertimemeanobservationsofsurfaceO 3 byAIRS/Aura arepresented,forthedaytimeobservationsintheleftandthenighttimeobservationsin theright.ThemeandaytimesurfaceO 3 spansbetween50and66ppb,withameanof55 ±8ppb,whilethenighttimeexhibitslowerlevels,between44and56ppb,withameanof 53±6ppbforthewholeregionshowninthisfigure. (a)(b) Figure 5. Monthly mean percentage departures from the climatological values of total ozone and the respective estimated trend, retrieved by the Brewer spectrophotometer #005, operating in Thessaloniki since 1982. Eleftheratos et al. [ 180 ] analyzed the ground-based Brewer measurements of total ozone in the urban environment of Athens for the period July 2003–July 2019 and estimated a 16-year climatological mean of total ozone of about 322 Dobson Units (DU), with no significant change since 2003. An update of the data until July 2023 confirmed these findings.
Atmosphere 2024,15, 753 8 of 28 The high-quality ground-based measurements of total ozone that are performed on a daily basis in Athens and Thessaloniki are used to systematically validate satellite-based ozone retrieving instruments that provide larger spatial coverage than the ground-based observations (e.g., [181–183]). Tropospheric Ozone: The inter-connectivity of tropospheric O 3 and aerosol direct effects is manifold; on one hand, the increased presence of aerosols reduces radiation reaching the ground, and the subsequent photolysis reduction can decrease tropospheric O 3 in polluted areas during summer. On the other hand, enhanced aerosol presence leads to cooling, which suppresses atmospheric ventilation and results in increased surface-level O 3 in wintertime [ 184 ]. The Eastern Mediterranean is among the regions with the highest levels of background tropospheric ozone worldwide [ 185 ]. Three atmospheric processes controlling its formation in the region, namely the long-range transport of polluted air masses, the dynamic subsidence at mid-tropospheric levels and the stratosphere-to-troposphere exchange, have been extensively studied for the region (e.g., [ 186 – 190 ]). Recent space-borne observations have shown that in the Mediterranean troposphere, between the surface and 2 km, O 3 is mostly formed from anthropogenic emissions, while above 4 km, it is mostly transported from outside the domain or from stratospheric origins [191]. In Figure 6, the seasonal summertime mean observations of surface O 3 by AIRS/Aura are presented, for the daytime observations in the left and the nighttime observations in the right. The mean daytime surface O 3 spans between 50 and 66 ppb, with a mean of 55 ±8 ppb , while the nighttime exhibits lower levels, between 44 and 56 ppb, with a mean of 53 ±6 ppb for the whole region shown in this figure. Atmosphere2024,15,xFORPEERREVIEW8of30 ozoneretrievinginstrumentsthatprovidelargerspatialcoveragethantheground-based observations(e.g.,[181–183]). Figure5.Monthlymeanpercentagedeparturesfromtheclimatologicalvaluesoftotalozoneand therespectiveestimatedtrend,retrievedbytheBrewerspectrophotometer#005,operatinginThessalonikisince1982. TroposphericOzone:Theinter-connectivityoftroposphericO 3 andaerosoldirecteffectsismanifold;ononehand,theincreasedpresenceofaerosolsreducesradiationreachingtheground,andthesubsequentphotolysisreductioncandecreasetroposphericO 3 in pollutedareasduringsummer.Ontheotherhand,enhancedaerosolpresenceleadsto cooling,whichsuppressesatmosphericventilationandresultsinincreasedsurface-level O 3 inwintertime[184].TheEasternMediterraneanisamongtheregionswiththehighest levelsofbackgroundtroposphericozoneworldwide[185].Threeatmosphericprocesses controllingitsformationintheregion,namelythelong-rangetransportofpollutedair masses,thedynamicsubsidenceatmid-troposphericlevelsandthestratosphere-to-troposphereexchange,havebeenextensivelystudiedfortheregion(e.g.,[186–190]).Recent space-borneobservationshaveshownthatintheMediterraneantroposphere,betweenthe surfaceand2km,O 3 ismostlyformedfromanthropogenicemissions,whileabove4km, itismostlytransportedfromoutsidethedomainorfromstratosphericorigins[191]. InFigure6,theseasonalsummertimemeanobservationsofsurfaceO 3 byAIRS/Aura arepresented,forthedaytimeobservationsintheleftandthenighttimeobservationsin theright.ThemeandaytimesurfaceO 3 spansbetween50and66ppb,withameanof55 ±8ppb,whilethenighttimeexhibitslowerlevels,between44and56ppb,withameanof 53±6ppbforthewholeregionshowninthisfigure. (a)(b) Figure 6. AIRS/Aura seasonal summertime mean surface O 3 vmr levels [ppb] over the Eastern Mediterranean from 2002 to 2022. (a) Daytime and (b) nighttime observations. Data acquired from ([192]). Methane: The study of methane variations over the Eastern Mediterranean, including Greece, was initially based on 2003–2004 data from SCIAMACHY on ENVISAT [ 193 ]. A summer–autumn peak was observed for both 2003 and 2004, August being the month with the highest methane concentrations. Recently, Kourtidis et al. [ 194 ] used 2018–2022 data from the Tropospheric Monitoring Instrument (TROPOMI) on Sentinel 5P (S5P) to study methane concentrations over the greater Thessaloniki area. They found increased concentrations over the rice fields of Chalastra, biological waste treatment units and biogas plants, and garbage burial sites.
Atmosphere 2024,15, 753 9 of 28 Since early 2019, methane levels over Thessaloniki and the surrounding areas are monitored via the Bruker EM27/SUN ground-based low-resolution Fourier-Transform spectrometer operated according to the requirements of the Collaborative Carbon Column Observing Network (COCCON) in the Laboratory of Atmospheric Physics, Aristotle University of Thessaloniki [ 195 ]. In a recent study, based on four years of observations, the methane levels were found to have increased by approximately 4%, with the highest concentrations of ~1.92 ppm during early 2022 [ 196 ]. Excellent agreement was also reported against collocated TROPOMI/S5P observations, with a mean of − 0.01 ± 0.6%, underlying the significance of satellite measurements as a valuable supplement to ground-based data for the purpose of greenhouse gas monitoring. In Figure 7, the monthly mean methane levels over the Eastern Mediterranean based on the Atmospheric Infrared Sounder (AIRS) on board the Aura satellite are presented. A very pronounced positive trend of ~5 ppb per annum is found for the timeframe 2012 to 2023, alongside a stable seasonal pattern, with a peak-to-peak amplitude of ±15.5 ±2.5 ppb . According to the latest European State of the Climate report [ 197 ], the annual increase in atmospheric concentrations for methane has been about 9 ppb/year (0.5%/year) since 2010, with the growth rates being larger than 10 ppb/year in the last three years, with a record high growth rate of about 17 ppb/year in 2021. Atmosphere2024,15,xFORPEERREVIEW9of30 Figure6.AIRS/AuraseasonalsummertimemeansurfaceO 3 vmrlevels[ppb]overtheEasternMediterraneanfrom2002to2022.(a)Daytimeand(b)nighttimeobservations.Dataacquiredfrom ([192]). Methane:ThestudyofmethanevariationsovertheEasternMediterranean,including Greece,wasinitiallybasedon2003–2004datafromSCIAMACHYonENVISAT[193].A summer–autumnpeakwasobservedforboth2003and2004,Augustbeingthemonthwith thehighestmethaneconcentrations.Recently,Kourtidisetal.[194]used2018–2022data fromtheTroposphericMonitoringInstrument(TROPOMI)onSentinel5P(S5P)tostudy methaneconcentrationsoverthegreaterThessalonikiarea.TheyfoundincreasedconcentrationsoverthericefieldsofChalastra,biologicalwastetreatmentunitsandbiogas plants,andgarbageburialsites. Sinceearly2019,methanelevelsoverThessalonikiandthesurroundingareasare monitoredviatheBrukerEM27/SUNground-basedlow-resolutionFourier-Transform spectrometeroperatedaccordingtotherequirementsoftheCollaborativeCarbonColumn ObservingNetwork(COCCON)intheLaboratoryofAtmosphericPhysics,AristotleUniversityofThessaloniki[195].Inarecentstudy,basedonfouryearsofobservations,the methanelevelswerefoundtohaveincreasedbyapproximately4%,withthehighestconcentrationsof~1.92ppmduringearly2022[196].Excellentagreementwasalsoreported againstcollocatedTROPOMI/S5Pobservations,withameanof−0.01±0.6%,underlying thesignificanceofsatellitemeasurementsasavaluablesupplementtoground-baseddata forthepurposeofgreenhousegasmonitoring. InFigure7,themonthlymeanmethanelevelsovertheEasternMediterraneanbased ontheAtmosphericInfraredSounder(AIRS)onboardtheAurasatellitearepresented.A verypronouncedpositivetrendof~5ppbperannumisfoundforthetimeframe2012to 2023,alongsideastableseasonalpattern,withapeak-to-peakamplitudeof±15.5±2.5 ppb.AccordingtothelatestEuropeanStateoftheClimatereport[197],theannualincrease inatmosphericconcentrationsformethanehasbeenabout9ppb/year(0.5%/year)since 2010,withthegrowthratesbeinglargerthan10ppb/yearinthelastthreeyears,witha recordhighgrowthrateofabout17ppb/yearin2021. Figure7.AIRS/AuramonthlymeanCH4vmrlevels[ppb]overtheEasternMediterraneanfrom 01.2012to12.2023.Dataacquiredfrom([198]). Figure 7. AIRS/Aura monthly mean CH4 vmr levels [ppb] over the Eastern Mediterranean from 01.2012 to 12.2023. Data acquired from ([198]). Nitrogen dioxide: Recent studies have unequivocally shown the ability of spaceborne air quality observations to monitor both long-term and short-term changes in NOx emissions over Greece while also identifying the specific emission sector [ 199 – 202 ]. These observations also revealed the adverse results of the 2008–2010 economic crisis on air quality over Greek urban sites [ 203 , 204 ], reflecting the reduction in traffic activity due to economic factors in Greek cities and the subsequent drop in oil consumption. For Athens, Georgoulias et al. [205] , using multi-satellite data, showed that tropospheric NO 2 stabilized after 2010 following a consistent decline in the preceding years (1996–2010), with a rate of −1.76% yr−1 . On the contrary, for Greece as a whole, the tropospheric NO 2 trend reversed from positive (0.52% yr −1 ) to strongly negative ( − 4.22% yr −1 ) after 2012, according to the same study. Recently, Alexandri et al. [ 206 ], using satellite NO2 observations from a single sensor (OMI/AURA), indicated strong decreasing trends over various locations in Greece over the fifteen-year period of 2005-2019 (e.g., − 3.95% yr −1 for Kozani, −3.57% yr−1 for Athens and − 2.89% yr −1 for Thessaloniki). The implementation of lockdown measures subsequent to the 2020 outbreak of COVID-19 resulted in abrupt alterations to nitrogen
Atmosphere 2024,15, 753 16 of 28 projects are imperative. In addition, a denser network of stations would be beneficial for improving the spatiotemporal representation of SLFCs and for enhancing satellite calibration and validation (Cal/Val) activities in the region. The challenges for reducing the aerosol uncertainties through the enhancement of our knowledge can be summarized as follows: 1. Enhancing and upgrading ground-based monitoring infrastructures through global and European network initiatives, driven by a significant national mandate; 2. Obtaining systematic aircraft and UAV in situ measurements of aerosol microphysical and chemical properties combined with meteorology for the major aerosol types and air masses, to facilitate closure studies that will enhance our knowledge on aerosol variability and concomitant effects; 3. Focusing on research for integrating satellite observations, suborbital measurements and modeling (including data assimilation). Major aspects to be addressed concern the coherent and continuous monitoring of SLCFs in Greece, aerosol typing and separation of anthropogenic and natural aerosols, aerosol load temporal variability, aerosol model downscaling aspects, aerosol–cloud– radiation interactions, solar energy high-spatiotemporal-resolution forecasting and satellite validation activities in the diverse aerosol-wise Greek environment. Author Contributions: Conceptualization, V.A. and S.K.; methodology, A.G. (Antonis Gkikas), M.-E.K., K.G. and A.K.G.; formal analysis, K.A.V., D.K., A.K. (Anna Kampouri) and K.P.; investigation, D.F., B.E.P., P.K. (Petros Katsafados), P.K. (Pavlos Kalabokas) and I.F.; data curation, P.-I.R., A.G. (Antonis Gkikas), S.S., E.P., G.V., A.T., E.D., E.M., E.G., S.M., J.K. and T.G.; writing—review and editing, K.E., V.A., S.K., A.G. (Anna Gialitaki), M.-E.K., N.H., A.G. (Antonis Gkikas), P.K. (Petros Katsafados), P.K. (Pavlos Kalabokas), P.Z., M.V., A.P., A.K. (Andreas Kazantzidis), K.K., D.B., A.F.B. and C.Z. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Hellenic Foundation for Research and Innovation (H.F.R.I.) under the “3rd Call for H.F.R.I. Research Projects to support Post-Doctoral Researchers” (Project Acronym: REVEAL, Project Number: 07222). V. A. acknowledges support by the projects: “Support for Enhancing the Operation of the National Network for Climate Change (CLIMPACT)”, National Development Program, General Secretariat of Research and Innovation, Greece (2023NA11900001—N. 5201588); the Harmonia Action (no. CA21119) supported by COST (European Cooperation in Science and Technology); the Horizon Europe programme under Grant Agreement No 101137680 via project CERTAINTY (Cloud-aERosol inTeractions & their impActs IN The earth sYstem); the AIRSENSE which is a part of Atmosphere Science Cluster of ESA’s EO Science for Society programme, and the Hellenic Foundation for Research and Innovation (Project Acronym: StratoFIRE, Project Number: 3995). E. P. acknowledges support by the AXA Research Fund for postdoctoral researchers under the project entitled “Earth Observation for Air-Quality—Dust Fine-Mode (EO4AQ-DustFM). KAV acknowledges support by the PANGEA4CalVal (grant agreement no. 101079201) funded by the European Union. A. Gkikas acknowledges support by the Hellenic Foundation for Research and Innovation (H.F.R.I.) under the “2nd Call for H.F.R.I. Research Projects to support Post-Doctoral Researchers” (Project Number: 544). Acknowledgments: This research was supported by data and services obtained from the ACTRIS Research Infrastructure and the PANhellenic Geophysical Observatory of Antikythera (PANGEA) of the National Observatory of Athens (NOA). Conflicts of Interest: The authors declare no conflicts of interest. References 1. Szopa, S.; Naik, V.; Adhikary, B.; Artaxo, P.; Berntsen, T.; Collins, W.D.; Fuzzi, S.; Gallardo, L.; Kiendler-Scharr, A.; Klimont, Z.; et al. Short-Lived Climate Forcers. In Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change; Masson-Delmotte, V., Zhai, P., Pirani, A., Connors, S.L., Péan, C., Berger, S., Caud, N., Chen, Y., Goldfarb, L., Gomis, M.I., et al., Eds.; Cambridge University Press: Cambridge, UK; New York, NY, USA, 2021; pp. 817–922. [CrossRef]
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