Development of Broadband Aerosol Optical Depth Models
Date
2016-05-05
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Abstract
Päike on väga stabiilne valgusallikas. Suvalisel kuupäeval Maa atmosfääri ülapiirile langeva päiksekiirguse kiiritustihedust võib arvutada suure täpsusega. Samasuguse täpsusega võib mõõta ka atmosfääri läbinud ja aluspinnani jõudnud päiksekiirgust. Suurust, mis iseloomustab atmosfääri rolli päiksekiirguse nõrgendamises, nimetatakse atmosfääri optiliseks paksuseks. Otstarbekas on jagada atmosfääri optilise paksuse põhjused kolmeks komponendiks:
a) atmosfääri moodustavad püsigaasid (lämmastik, hapnik, argoon) ehk kuiv ja puhas atmosfäär ehk ideaalne atmosfäär,
b) atmosfääris leiduv veeaur,
c) atmosfääris hõljuvad aerosooliosakesed (suitsud, tolmud, udud).
Suhteliselt kergesti on leitavad esimese kahe kihi nõrgestavad panused. Keerukam, aga informatiivsem, on aerosooli optiline paksus, millest on olemas omakorda kaks varianti. Lihtsam hõlmab summaarselt kõiki päiksekiirguse lainepikkusi ning selle arvutamise võimekus oli Eestis juba 1930tel. Spektraalse optilise paksuse mõõtmiseks on aga vaja keerukamaid instrumente, milliseid Eestis kuni aastani 2002 ei olnud. Tänu USA NASA projektile AERONET töötab Tõraveres alates 2002. aastast spektraalne päikesefotomeeter, mille analüüsile ongi pühendatud see väitekiri.
Selgus, et nii laiaribaline kui spektraalne aerosooli optiline paksus on omavahel korreleeruvad, mis võimaldab viimase ligikaudset arvutamist esimese kaudu. Antud töös on arvutatava spektraalse kiirguse lainepikkuseks 500 nm ning saadud suurust tähistatakse AOD500. Selline ligikaudne arvutamine võimaldab anda hinnanguid, mis on abiks näiteks klimatologilistes uuringutes, aga ka spektraalmõõtmiste kontrolliks.
Mudelarvutusi üleminekuks aerosooli laiaribaliselt optiliselt paksuselt spektraalsele on tehtud mitmetes teadusrühmades. Töös on neid võrreldud ulatuslikus testis, mis põhineb 10 aasta mõõtmistele. Võrdlustesti tulemusena selgus, et ka teiste uurijate poolt koostatud AOD500 arvutusskeemid annavad häid tulemusi, kuid väitekirjas pakutav mudel oli siiski pisut parem.
In meteorological practice the Sun is considered as a stable source of light. The amount of broadband solar energy encompassing all wavelengths, incident on a unit area at the top of the atmosphere (TOA) may be calculated with a high precision. The ratio between the solar intensity at the Earth’s surface after attenuation in the air, and this at the TOA, easily gives atmospheric optical depth. It is convenient to divide atmosphere into three virtual layers based on attenuation reasons: a) atmospheric base-gases (mainly nitrogen, oxygen and argon), also known as a clean and dry atmosphere (CDA), b) water vapor, c) aerosol particles (smokes, dusts, fogs). In terms of atmospheric optics, each layer has its own broadband optical depth. The most interesting is the layer of aerosol particles. Its characterization with broadband aerosol optical depth (BAOD) was in Estonia feasible already in the 1930s. Measurements of BAOD’s spectral counterpart, AOD, became available only since June 2002, after enlargement of NASA AERONET project to Tõravere. Compared to BAOD, spectral approach allows considerably more detailed investigation of aerosols. Starting writing the thesis, it was logical to look for a correlation between BAOD and AOD500. This correlation was found and it allowed transition from BAOD to AOD500. Considerable part of the thesis is dedicated to test runs of created computational scheme and to comparisons with other broadband models elaborated in USA and Russia. Although all models gave reasonable results for estimating AOD500, this work's model performed slightly better. Concerning environmental research, a practical benefits from use of created model for estimation of AOD500 would be: a) retrospective retrieval of AOD500 for periods in the past when spectral measurements were not available, b) a quick AOD500 estimation for correction of satellite remotely sensed data, c) quality assurance of recorded spectral data.
In meteorological practice the Sun is considered as a stable source of light. The amount of broadband solar energy encompassing all wavelengths, incident on a unit area at the top of the atmosphere (TOA) may be calculated with a high precision. The ratio between the solar intensity at the Earth’s surface after attenuation in the air, and this at the TOA, easily gives atmospheric optical depth. It is convenient to divide atmosphere into three virtual layers based on attenuation reasons: a) atmospheric base-gases (mainly nitrogen, oxygen and argon), also known as a clean and dry atmosphere (CDA), b) water vapor, c) aerosol particles (smokes, dusts, fogs). In terms of atmospheric optics, each layer has its own broadband optical depth. The most interesting is the layer of aerosol particles. Its characterization with broadband aerosol optical depth (BAOD) was in Estonia feasible already in the 1930s. Measurements of BAOD’s spectral counterpart, AOD, became available only since June 2002, after enlargement of NASA AERONET project to Tõravere. Compared to BAOD, spectral approach allows considerably more detailed investigation of aerosols. Starting writing the thesis, it was logical to look for a correlation between BAOD and AOD500. This correlation was found and it allowed transition from BAOD to AOD500. Considerable part of the thesis is dedicated to test runs of created computational scheme and to comparisons with other broadband models elaborated in USA and Russia. Although all models gave reasonable results for estimating AOD500, this work's model performed slightly better. Concerning environmental research, a practical benefits from use of created model for estimation of AOD500 would be: a) retrospective retrieval of AOD500 for periods in the past when spectral measurements were not available, b) a quick AOD500 estimation for correction of satellite remotely sensed data, c) quality assurance of recorded spectral data.
Description
Väitekirja elektrooniline versioon ei sisalda publikatsioone.
Keywords
atmosfäärioptika, aktinomeetria, aerosoolid, modelleerimine (teadus), atmospheric optics, actinometry, aerosol pollutants, modelling (science)