title: 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.
authors.affiliation: Analytical Chemistry Laboratory, National Institute of Chemistry , 1000 Ljubljana, Slovenia.|Division for Marine and Environmental Research, Ruđer Bošković Institute , 10000 Zagreb, Croatia.
authors.forename: Sanja
authors.initials: S
authors.lastname: Frka
authors.affiliation: Analytical Chemistry Laboratory, National Institute of Chemistry , 1000 Ljubljana, Slovenia.
authors.forename: Martin
authors.initials: M
authors.lastname: Šala
authors.affiliation: Analytical Chemistry Laboratory, National Institute of Chemistry , 1000 Ljubljana, Slovenia.
authors.forename: Ana
authors.initials: A
authors.lastname: Kroflič
authors.affiliation: Laboratory of Catalysis and Chemical Reaction Engineering, National Institute of Chemistry , 1000 Ljubljana, Slovenia.
authors.forename: Matej
authors.initials: M
authors.lastname: Huš
authors.affiliation: Alkemika, Ltd. , 3000 Celje, Slovenia.|Acies Bio, Ltd. , 1000 Ljubljana, Slovenia.
authors.forename: Alen
authors.initials: A
authors.lastname: Čusak
authors.affiliation: Analytical Chemistry Laboratory, National Institute of Chemistry , 1000 Ljubljana, Slovenia.
authors.forename: Irena
authors.initials: I
authors.lastname: Grgić
mesh_terms: D000336:Aerosols; D002853:Chromatography, Liquid; D009930:Organic Chemicals; D053719:Tandem Mass Spectrometry; D014867:Water
chemical_list: D000336:Aerosols; D009930:Organic Chemicals; D014867:Water