Formation and ageing of nitroaromatic compounds as important constituents of atmospheric brown carbon

Vidović, Kristijan (author), Grgić, Irena (mentor)

The understanding of atmospheric brown carbon (BrC) formation is of crucial importance for the accurate assessment of its influence on the radiative forcing. In the present thesis, different formation and degradation pathways of methyl-nitrocatechols (MNC), as important constituents of BrC, under atmospherically relevant aqueous-phase conditions were investigated. The intriguing role of nitrous acid (HONO) in the dark aqueous-phase transformation of catechols was highlighted. HONO was found to have a dual role in the nitration of substituted aromatics; it acts as a catalyst and as an oxidant. In the dominant pathway, HONO is directly involved in the nitration of 3-methylcatechol (3MC) via consecutive oxidation and conjugated addition reactions (non-radical reaction pathway), forming two isomeric products, i.e. 3-methyl-5-nitrocatechol (3M5NC) as the main product and 3-methyl-4-nitrocatechol (3M4NC) as the minor one. The dominant pathway is expected to prevail at a pH typical for atmospheric aerosol (pH around the pKa of HONO). Under very acidic conditions, two other nitration pathways, oxidative aromatic substitution (electrophilic) and radical recombination, gain in importance. The subsequent electrochemical generation of 3-methil-o-quinone (3MoQ) at the electrode surface and its reaction with NO2− in the electrochemical cell allowed us to confirm unambiguously the proposed dominant reaction mechanism. In addition, a new pathway of nitrocatechol hydroxylation by oxidation and the addition of water was discovered. Because of the prolonged light absorption far in the visible light range, the new products (3M5NC-OH and ring cleavage products of 3M4NC) could significantly contribute to the atmospheric light absorption by BrC influencing the radiative balance of the Earth. Furthermore, the aqueous-phase formation of BrC from 3MC in the presence of HONO/NO2- under simulated sunlight conditions was investigated. Under illumination, the degradation of 3MC is faster in comparison to its degradation in the dark under the same solution conditions. On the other hand, the yield of the main two products of the dark reaction (3M5NC and 3M4NC) is low, suggesting different degradation pathways of 3MC. Besides the known primary reaction products with distinct absorption at 350 nm, second-generation products responsible for the absorption above 400 nm (e.g., 3M5NC-OH, and the oxidative cleavage products of 3M4NC) were also confirmed in the reaction mixture. The characteristic mass absorption coefficient (MAC) values were found to increase with the increase of NO2⁻/3MC concentration ratio and decrease with the increasing wavelength. Our findings reveal that the aqueous-phase processing of 3MC in the presence of HNO2/NO2-, both under the sunlight and in the dark, may significantly contribute to secondary organic aerosol (SOA) light absorption.

• Language: English
• Keywords: brown carbon chromophores, biomass burning, photooxidation, nitration, hydroxylation, nitrophenols, methylnitrocatechols, mass absorption coefficient
• Work type: Doctoral dissertation
• Typology: 2.08 - Doctoral Dissertation
• Organization: FKKT - Faculty of Chemistry and Chemical Technology
• Year: 2020
• PID: 20.500.12556/RUL-114778
• COBISS.SI-ID: 1538552515
• Publication date in RUL: 09.03.2020

Secondary language

• Language: Slovenian
• Title: Reakcije nastajanja in staranja nitroaromatskih spojin, ki predstavljajo pomemben delež rjavega ogljika v ozračju

Abstract

Razumevanje nastajanja rjavega ogljika (BrC) je ključnega pomena za natančno oceno njegovega vpliva na spremembe sevalnega ravnotežja. V doktorskem delu smo raziskali različne poti nastajanja in razgradnje metil-nitrokateholov (MNC), ki predstavljajo pomemben delež BrC, pri pogojih značilnih za atmosfersko vodno fazo. Izpostavili smo izjemno vlogo dušikove (III) kisline (HONO) pri transformaciji kateholov v temi v vodni fazi. Pokazali smo, da ima HONO pri nitraciji substituiranih aromatskih spojin dvojno vlogo; deluje kot katalizator in oksidant. V glavnem reakcijskem mehanizmu je HONO neposredno vključena v nitracijo 3-metilkatehola (3MC) preko zaporedne oksidacije in konjugirane adicije (neradikalska reakcijska pot), pri čemer nastaneta dva izomerna produkta, kjer je 3-metil-5-nitrokatehol (3M5NC) glavni produkt, 3-metil-4-nitrokatehol (3M4NC) pa predstavlja manjši delež. Glavni reakcijski mehanizem naj bi prevladoval pri pH vrednostih značilnih za aerosolske delce (pH okrog pKa HONO). Pri zelo kislih pogojih pa pridobita na veljavi dve drugi nitracijski poti in sicer oksidativna aromatska substitucija (elektrofilna) in radikalska rekombinacija. Nadaljnja elektrokemijska tvorba 3-metil-o-kinona (3MoQ) na elektrodni površini in njegova reakcija z NO2− v elektrokemijski celici nam je omogočila nedvoumno potrditev predlaganega glavnega reakcijskega mehanizma. Poleg tega smo odkrili novo pot hidroksilacije nitrokateholov, ki poteče z oksidacijo, ki ji sledi adicija vode. Novo nastali produkti (3M5NC-OH in produkti oksidativne cepitve 3M4NC), ki absorbirajo pri daljših valovnih dolžinah v vidnem območju, lahko bistveno prispevajo k atmosferskemu BrC in s tem k absorpciji svetlobe ter tako vplivajo na spremembe sevalnega ravnotežja na Zemlji. V nadaljevanju smo preiskovali kemijske procese nastajanja komponent BrC iz 3MC v prisotnosti HONO/NO2- v vodni fazi pri simuliranih pogojih sončne svetlobe. Razpad 3MC je pod vplivom sončne svetlobe hitrejši kot v temi pri enakih pogojih. Po drugi strani pa je prispevek dveh glavnih produktov temne reakcije (3M5NC in 3M4NC) nizek, kar kaže na drugačne poti razgradnje 3MC. Poleg primarnih reakcijskih produktov z izrazito absorpcijo pri 350 nm, smo v reakcijski mešanici potrdili tudi produkte druge generacije (3M5NC-OH in produkte oksidativne cepitve 3M4NC), ki so bili odgovorni za absorpcijo nad 400 nm. Ugotovili smo, da vrednosti karakterističnega masnega absorpcijskega koeficienta (MAC) naraščajo s povečanjem koncentracijskega razmerja NO2-/3MC in se znižujejo z naraščajočo valovno dolžino. Naše ugotovitve potrjujejo, da procesi 3MC v prisotnosti HNO2/NO2- v vodni fazi, tako v pogojih teme kakor tudi sončne svetlobe, lahko bistveno prispevajo k absorpciji sekundarnih organskih aerosolov (SOA).

• Keywords: kromofori rjavega ogljika, izgorevanje biomase, fotooksidacija, nitracija, hidroksilacija, nitrofenoli, metilnitrokateholi, masni absorpcijski koeficient