Kemijske pretvorbe aerosolov biogenega izvora

Pavlica, Damjan Jan (author), Pompe, Matevž (mentor), Podlipnik, Črtomir (co-mentor)

V doktorskem delu sem preučeval pomen singletnega kisika in hidrotrioksi radikala pri pretvorbah organskih aerosolov. Kemijske pretvorbe s singletnim kisikom sem preučeval eksperimentalno, pretvorbe s hidrotrioksi radikalom pa teoretično s semiempiričnimi kvantnomehanskimi metodami in metodami, ki so osnovane na teoriji gostotnega funkcionala. Kot modelno spojino sem izbral α-pinen, ki je količinsko najbolj pomemben monoterpen in eden glavnih prekurzorjev sekundarnih organskih aerosolov. Razvil sem enostaven postopek za oksidacijo terpenov v tekoči fazi s singletnim kisikom, ki sem ga sintetiziral fotokemično z žarnice z volframovo nitko in acetonitrilno raztopino metilensko modrega. Za oksidacijske produkte, štiri hidroperokside α-pinena, sem razvil ločbi s tekočinsko kromatografjo (LC) in plinsko kromatografijo (GC), ter za zaznavo uporabil ultravijolični (UV), plamensko ionizacijski (FID) in masnospektrometrični (MS) detektor. Za analizo nestabilnih sistemov je primerna tudi kvantitativna nuklearna magnetna resonanca (NMR), ki kot absolutna tehnika omogoča določitev absorpcijskih koeficientov, in sem jo v doktorskem delu uporabil za identifikacijo hidroperoksidnih izomerov α-pinena. Za njihovo kvantifikacijo sem razvil GC-FID metodo s predkolonskim sililiranjem in kvantifikacijo z odzivnimi faktorji na osnovi efektivnega števila ogljikovih atomov. Z razvitim analiznim pristopom sem zaobšel težave z reaktivnostjo analitov, njihovo temperaturno neobstojnost, neznane in nizke absorpcijske koeficiente in šibko MS ionizacijo. Linearnost metode sem preveril med 1 in 90 mg/L, kjer so bili korelacijski koeficienti nad 0,99. Metoda je ponovljiva z relativnimi standardnimi odmiki pod 5 %. Celoten postopek, od sinteze hidroperoksidov do njihove kvantifikacije, je prenosljiv tudi na druge terpenske hidroperokside. Metodo sem uporabil za spremljanje staranja terpentina, pri katerem je masni delež raziskovanih hidroperoksidov α-pinena narasel na 5 %. Pri radikalski oksidaciji in oksidaciji s singletnim kisikom sem opazil razliko v deležu posameznih hidroperoksidov in jo uspel mehanistično razložiti. Poleg tega mi je uspelo dokazati, da omenjeni hidroperoksidi nastajajo tudi pri fotokemičnih reakcijah s PM10 delci. V raztopini hidroperoksidov sem z LC-MS in GC-MS opazil dimerizacijo hidroperoksidnih derivatov α-pinena in jo uspel opisati s semiempirično kvantnomehansko metodo PM6. Do sedaj so bile določitve 1O2 v zraku zelo redke, v znanstvenem članku je bila objavljena le določitev z α-terpinenom. Visoke koncentracije so lahko posledica reakcij z drugimi oksidanti, ki reagirajo na podoben način kot 1O2, zato je za potrditev pravilnosti določenih koncentracij potreben razvoj novih metod. Pri razvoju nove metode sem preizkusil tri različne kemijske pasti: N,N–dimetil-4-nitrozoanilin, sečno kislino in furfurilni alkohol. Kot najprimernejši reagent se je izkazal furfurilni alkohol, s katerim sem določil koncentracije singletnega kisika v raztopini med 2x10-12 in 4,5x10-12 mol/L. Pri meritvah v zraku so potrebne še nadaljnje izboljšave metode. Pomembnost hidrotrioksi radikala (•OOOH) sem ovrednotil in silico. Dokazal sem, da •OOOH ne nastaja pri reakciji α-pinena z radikalsko obliko ozona. Na dvojni vezi poteka adicija •OOOH, saj je abstrakcija alilnega vodika termodinamsko in kinetično neugodna. Hidrotrioksi radikal je manj reaktiven od hidroksilnega in hidroperoksi radikala, najbolj kritična pa je njegova majhna stabilnost, zato verjetno ne igra pomembne vloge v atmosferski kemiji.

• Language: Slovenian
• Keywords: sekundarni organski aerosoli, singletni kisik, organski peroksidi, masna spektrometrija
• Work type: Doctoral dissertation
• Typology: 2.08 - Doctoral Dissertation
• Organization: FKKT - Faculty of Chemistry and Chemical Technology
• Year: 2021
• PID: 20.500.12556/RUL-133610
• COBISS.SI-ID: 90414339
• Publication date in RUL: 03.12.2021

Secondary language

• Language: English
• Title: Chemical transformation of aerosols from biogenic precursors

Abstract

In my PhD thesis, I investigated the importance of singlet oxygen and hydrotrioxy radicals in the chemical transformation of aerosols. Chemical transformations involving singlet oxygen were studied experimentally and transformations involving hydrotrioxy radicals were studied theoretically, using semiempirical quantum mechanical methods and methods based on density functional theory. Alfa-pinene was chosen as a model compound because it is very abundant in the atmosphere and important for the formation of secondary organic aerosols. I have developed a simple procedure for liquid phase oxidation with singlet oxygen using a light bulb with tungsten filament and an acetonitrile solution of methylene blue. For the oxidation products, four α-pinene hydroperoxides, I have developed a liquid chromatography and a gas chromatography separation with an ultraviolet (UV), flame-ionization (FID), and mass spectrometry (MS) detection. The quantitative NMR technique, as an absolute technique, allows the determination of absorption coefficients, and was used to identify the hydroperoxide isomers of α-pinene. The analytes had low absorption coefficients, weak MS ionization, and low thermal stability. To overcome these limitations, I proposed a GC-FID approach involving pre-column silylation and quantification via the effective carbon number approach. The linearity of the method was confirmed between 1 and 90 mg/L, with correlation coefficients above 0.99. The method is reproducible with relative standard deviations below 5%. The entire process, from the synthesis of the hydroperoxides to their quantification, is transferable to other terpene hydroperoxides. The validated method was applied to study the aging of turpentine in which the mass fraction of α-pinene hydroperoxides increased to 5%. It was found that the differences in abundance of each hydroperoxide were due to radical oxidation or a singlet oxygen oxidation mechanism. Furthermore, I was able to show that hydroperoxides are also formed by photochemical reactions with PM10 particles. In solution, the dimerization of α-pinene hydroperoxides was observed by LC-MS and GC-MS and the mechanism was explained by the PM6 semiempirical quantum mechanical method. So far, determinations of 1O2 in the air have been very rare. Only one determination using the chemical trap α-terpinene was published in a scientific article. Since high concentrations may result from reactions with oxidants that react in a similar manner to 1O2, the development of new methods is needed to confirm the accuracy of determinations. In developing the new method, I tested three 1O2 chemical traps: N, N-dimethyl-4-nitrosoaniline, uric acid, and furfuryl alcohol. Furfuryl alcohol proved to be the most suitable reagent and was used to determine concentrations of singlet oxygen in solution, ranging from 2x10-12 to 4.5x10-12 mol/L. Further improvements to the method are required for air measurements. The importance of the hydrotrioxy radical (•OOOH) was investigated in silico. It was shown that •OOOH is not formed by the reaction of α-pinene with the radical form of ozone. The addition of •OOOH occurs at the double bond because the allylic hydrogen abstraction is thermodynamically and kinetically unfavorable. The hydrotrioxy radical is less reactive than the hydroxyl and hydroperoxy radicals and has very low stability, so it is unlikely to play a significant role in atmospheric chemistry.

• Keywords: secondary organic aerosols, singlet oxygen, organic peroxides, mass spectrometry