Quantum Chemical Calculations Resolved Identification of Methylnitrocatechols in Atmospheric Aerosols

Abstract

Methylnitrocatechols (MNCs) are secondary organic aerosol (SOA) tracers and major contributors to atmospheric brown carbon; however, their formation and aging processes in atmospheric waters are unknown. To investigate the importance of aqueous-phase electrophilic substitution of 3-methylcatechol with nitronium ion (NO2(+)), we performed quantum calculations of their favorable pathways. The calculations predicted the formation of 3-methyl-5-nitrocatechol (3M5NC), 3-methyl-4-nitrocatechol (3M4NC), and a negligible amount of 3-methyl-6-nitrocatechol (3M6NC). MNCs in atmospheric PM2 samples were further inspected by LC/(-)ESI-MS/MS using commercial as well as de novo synthesized authentic standards. We detected 3M5NC and, for the first time, 3M4NC. In contrast to previous reports, 3M6NC was not observed. Agreement between calculated and observed 3M5NC/3M4NC ratios cannot unambiguously confirm the electrophilic mechanism as the exclusive formation pathway of MNCs in aerosol water. However, the examined nitration by NO2(+) is supported by (1) the absence of 3M6NC in the ambient aerosols analyzed and (2) the constant 3M5NC/3M4NC ratio in field aerosol samples, which indicates their common formation pathway. The magnitude of error one could make by incorrectly identifying 3M4NC as 3M6NC in ambient aerosols was also assessed, suggesting the importance of evaluating the literature regarding MNCs with special care.

Evolution of the chemical fingerprint of biomass burning organic aerosol during aging

“…Nitro-aromatic compounds have been observed at sites impacted by biomass burning before (Iinuma et al, 2010;Kitanovski et al, 2012Kitanovski et al, , 2014Mohr et al, 2013;Kahnt et al, 2013;Frka et al, 2016;Gaston et al, 2016) at concentration levels typically two orders of magnitude lower than that of levoglucosan (4-NC / LG ≈ 0.01-0.1, MNC / LG ≈ 0.01-0.08). Here, the 4-NC / LG and MNC / LG ratios vary from 0.25 to 0.5 and 0.06 to 0.15, respectively.…”

Electrochemistry as a Tool for Studies of Complex Reaction Mechanisms: The Case of the Atmospheric Aqueous-Phase Aging of Catechols

“…In contrast to the commonly applied LC-MS, which is known to often fail in the quantification of a group of compounds by use of a surrogate standard, the electrochemical signal can be fairly well quantified by the standard addition method with the use of a single surrogate molecule. 54 The represented analytical technique thus promises a better quantitative method for the determination of total nitroaromatics in real atmospheric samples (aerosol particles, fog and cloud water, or precipitation). The anticipated limit of detection is in the nanomolar range, which is sufficient and comparable to the LC-MS.…”

Composition and mass size distribution of nitrated and oxygenated aromatic compounds in ambient particulate matter from southern and central Europe – implications for the origin

“…4-NC, MNCs, nitroguaiacols (NGs) and nitrosalicylic acids (NSAs) are predominantly formed by secondary oxidation of lignin thermal decomposition products (e.g. m-cresol, phenols, methoxyphenols, catechols, salicylic acid) in the gas and aqueous phases (Iinuma et al, 2010;Kelly et al, 2010;Kroflič et al, 2015;Frka et al, 2016;Teich et al, 2017;Finewax et al, 2018;Xie et al, 2017;Wang et al, 2019). Therefore, the yellow-coloured watersoluble 4-NC and MNCs have been proposed as suitable tracers for highly oxidized secondary BB aerosols (Iinuma et al, 2010;Kitanovski et al, 2012b;Kahnt et al, 2013;Caumo et al, 2016;Chow et al, 2016).…”