Aqueous-Phase Brown Carbon Formation from Aromatic Precursors under Sunlight Conditions

Abstract

At present, there are still numerous unresolved questions concerning the mechanisms of light-absorbing organic aerosol (brown carbon, BrC) formation in the atmosphere. Moreover, there is growing evidence that chemical processes in the atmospheric aqueous phase can be important. In this work, we investigate the aqueous-phase formation of BrC from 3-methylcatechol (3MC) under simulated sunlight conditions. The influence of different HNO2/NO2− concentrations on the kinetics of 3MC degradation and BrC formation was investigated. Under illumination, the degradation of 3MC is faster (k2nd(global) = 0.075 M−1·s−1) in comparison to its degradation in the dark under the same solution conditions (k2nd = 0.032 M−1·s−1). On the other hand, the yield of the main two products of the dark reaction (3-methyl-5-nitrocatechol, 3M5NC, and 3-methyl-4-nitrocatechol, 3M4NC) is low, suggesting different degradation pathways of 3MC in the sunlight. Besides the known primary reaction products with distinct absorption at 350 nm, second-generation products responsible for the absorption above 400 nm (e.g., hydroxy-3-methyl-5-nitrocatechol, 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 (at the concentration ratio of 50, MAC is greater than 4 m2·g−1 at 350 nm) and decrease with the increasing wavelength, which is characteristic for BrC. Yet, in the dark, roughly 50% more BrC is produced at comparable solution conditions (in terms of MAC values). 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.

Aging of Atmospheric Brown Carbon Aerosol

“…Aqueous photonitration of, for example, phenol, guaiacol, methylcatechol and vanillin, can occur via exposure to UV light in the presence of dissolved nitrite to form nitrophenol compounds. [153][154][155][156] These nitrated products absorb light more strongly in the visible region of the spectrum than the precursor compounds. 153 Nitration also occurs without the presence of light, for example, by reaction of aromatic compounds with nitrous acid/nitrite.…”

Ambient nitro-aromatic compounds – biomass burning versus secondary formation in rural China

“…NACs may also form in aqueous phases, particularly when the atmosphere has a high liquid water content (Harrison et al, 2005b;Vidović et al, 2018Vidović et al, , 2020. The contribution of aqueous-phase oxidation to NAC formation was found to be limited based on the negative relationship (r = −0.1 to −0.6) between the relative humidity (RH) and the mixing ratios of NACs in the gas and particle phases.…”

Secondary Formation of Aromatic Nitroderivatives of Environmental Concern: Photonitration Processes Triggered by the Photolysis of Nitrate and Nitrite Ions in Aqueous Solution

“…In particular, there has been interest towards photonitration in three successive periods-(i) from the early 1980s to the mid-1990s, when the main goal was to account for the formation of phytotoxic nitrophenols that, together with acid rains, play a role in 2 of 16 forest decline [6,7]; (ii) the 2000s, when the process attracted attention to account for the occurrence (and secondary formation) of toxic, mutagenic and potentially carcinogenic nitroaromatic compounds in atmospheric aerosols [8,9]; and (iii) the present time, when most interest is accounted for by the fact that the -NO 2 group of nitroderivatives enables them to absorb sunlight (visible radiation in addition to the UV, because most nitroderivatives are yellow both as solids and in aqueous solution). As part of the so-called "brown carbon", nitroaromatic compounds play a role in the absorption of sunlight by airborne aerosols and, by so doing, they affect to a potentially significant extent the still-debated and insufficiently defined climate feedback of the aerosols [10][11][12][13][14][15][16]. Another reason for the recent interest towards photonitration is the acknowledgement of the important role it may play in photochemical processes for water treatment, which are gaining increasing attention and where the occurrence of nitrate and nitrite under technical UV irradiation might favour the formation of harmful compounds [17][18][19][20][21].…”