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Quantum chemical calculations resolved identification of methylnitrocatechols in atmospheric aerosols Sanja Frka, Martin Šala, Ana Krofli#, Matej Hus, Alen Cusak, and Irena Grgi# Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b00823 • Publication Date (Web): 02 May 2016 Downloaded from http://pubs.acs.org on May 4, 2016

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Environmental Science & Technology

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Quantum chemical calculations resolved identification of methylnitrocatechols in

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atmospheric aerosols

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Sanja Frka*†┴, Martin Šala†, Ana Kroflič†, Matej Huš‡, Alen Čusak§║ and Irena Grgić†

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†

6

┴

Analytical Chemistry Laboratory, National Institute of Chemistry, 1000 Ljubljana, Slovenia Division for Marine and Environmental Research, Ruđer Bošković Institute, 10000 Zagreb, Croatia

7 8

‡

Laboratory of Catalysis and Chemical Reaction Engineering, National Institute of Chemistry, 1000 Ljubljana, Slovenia

9 §

10 ║

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Alkemika Ltd., 3000 Celje, Slovenia

Acies Bio Ltd., 1000 Ljubljana, Slovenia

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Abstract

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Methylnitrocatechols (MNCs) are secondary organic aerosol (SOA) tracers and major

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contributors to atmospheric brown carbon; however, their formation and ageing processes in

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atmospheric waters are unknown. To investigate the importance of aqueous-phase

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electrophilic substitution of 3-methylcatechol with nitronium ion (NO2+), we performed

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quantum calculations of their favorable pathways. The calculations predicted the formation of

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3-methyl-5-nitrocatechol (3M5NC), 3-methyl-4-nitrocatechol (3M4NC), and a negligible

20

amount of 3-methyl-6-nitrocatechol (3M6NC). MNCs in atmospheric PM2 samples were

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further inspected by LC/(-)ESI-MS/MS using commercial as well as de novo synthesized

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authentic standards. We detected 3M5NC and, for the first time, 3M4NC. In contrast to

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previous reports, 3M6NC was not observed. Agreement between calculated and observed

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3M5NC/3M4NC ratios cannot unambiguously confirm the electrophilic mechanism as the

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exclusive formation pathway of MNCs in aerosol water. However, the examined nitration by 1 ACS Paragon Plus Environment


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NO2+ is supported by (1) the absence of 3M6NC in the ambient aerosols analysed and (2) the

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constant 3M5NC/3M4NC ratio in field aerosol samples, which indicates their common

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formation pathway. The magnitude of error one could make by incorrectly identifying

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3M4NC as 3M6NC in ambient aerosols was also assessed, suggesting the importance of

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evaluating the literature regarding MNCs with special care.

31 32

Introduction

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Organic aerosols (OA) are essential to many areas of science ranging from Earth’s climate to

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human health. Despite fundamental advances in understanding how OA form and evolve

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during atmospheric ageing, the specific ways in which they impact the atmosphere and

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climate are only beginning to be understood. A significant and highly variable fraction of OA

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named brown carbon (BrC), a light-absorbing form of particulate matter (PM), has recently

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attracted much interest because of its potentially large effect on Earth's climate.1–4 Yet, in

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many global models, organic particulates are still considered to be only light scattering.5 By

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absorbing solar radiation in (sub)visible wavelengths, BrC heats the atmosphere, significantly

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affecting the overall atmospheric energy distribution. The literature often refers to BrC as a

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primary organic aerosol (POA) because it is consistently observed in areas influenced by

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combustion of both biomass and fossil fuels.6–8 However, there is growing evidence that BrC

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also results from secondary aging processes. For example, volatile organic compounds

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(VOCs) in biomass-burning smoke have been oxidized to secondary organic aerosols (SOA)

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including brown carbon SOA.9–12 Such secondary aging processes often involve nitrogen, and

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brown SOA have been detected during photooxidation of volatile aromatic precursors (e.g.,

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toluene, benzene, or xylene) in the presence of NOx.13–16 The resulting product distribution

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strongly depends on the structure of the aromatic precursor, which substantially affects its

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reactivity and absorption of light in the near UV and visible regions of the spectrum.

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Nitroaromatic compounds (NACs) are among the largest and most important groups of

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industrial chemicals in use today. Especially catechol derivatives form one of the most

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ubiquitous families of natural antioxidants present in a variety of natural products, flavors,

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and pharmaceuticals. However, reactions involving NACs in the atmosphere are a matter of

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concern, as their interactions with DNA and the resulting mutagenicity render them hazardous

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to human health and wildlife.17 The mutagenicity and carcinogenicity of NACs have been

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specifically addressed, as influenced by the position of the nitro group on the aromatic ring,

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considering also presence of other functional groups.17 It has been suggested that NACs and,

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in particular, abundant methylnitrocatechols (MNCs), which are formed through atmospheric

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oxidation of biomass-burning plumes, are the major contributors to atmospheric BrC.18–20

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Recently, yellow-colored MNCs have drawn much attention due to their high concentrations

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in ambient aerosols,20,21 contributing (with other less abundant NACs) ~1% to the total

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organic aerosol mass.22,23 Moreover, MNCs rank among the most abundant chromophoric,

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organic, water-soluble constituents of PM from biomass-burning emissions.19 Based on

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laboratory and aerosol chamber experiments, MNCs are considered SOA contributors

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originating from NOx photooxidation of m-cresol, which is released during biomass burning

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as a thermal degradation product of lignin.20 MNCs have also been suggested as tracer

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compounds of anthropogenic SOA because the photooxidation of VOCs, such as toluene in

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the presence of NOx, can lead to the formation of MNCs as multigenerational SOA

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products.21, 24, 25

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Cloud droplets, rain, fogs, and water films associated with aerosol particles (wet aerosols)

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represent different forms of atmospheric liquid water. Concentrated mixtures of salts and

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organics, and low (acidic) pH are typical of wet aerosols and fogs, in general leading to a

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favorable chemical environment for the aqueous processing. Aqueous-phase reactions have

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been the subject of increased interest in the scientific community as they represent potential



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pathways contributing to the formation and processing of SOA, yet their magnitude in

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tropospheric models is often underestimated.26 Although aromatic nitration ranks among the

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most widely studied organic reactions with frequent occurrences in the biosphere, the exact

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mechanisms remain poorly understood. In aqueous solution, aromatic nitration is proposed to

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occur via a mechanistically well-defined electrophilic substitution reaction. It has been

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generally accepted that, under acidic conditions, the reaction involves the nitronium ion,

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NO2+, as a common nitrating agent, and the formation of Wheland intermediate (σ-

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complex).27 Alternatively, a free radical mechanism is often proposed to explain the

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mechanism of aromatic nitration by nitrous acid (HNO2) in the environment with aromatic

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pollutants, such as substituted phenols, forming the corresponding nitrophenols.28–31

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However, aqueous-phase nitration of aromatic molecules with nitrite/nitrous acid (NO2-

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/HNO2) and hydrogen peroxide (H2O2) has also been shown to lead to the formation of

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NACs,29,30 suggesting that nitrating electrophiles are possible reactive species in the acidic

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atmospheric waters. Electrophilic substitution has also been used to explain nitration of the

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wood-burning pollutant guaiacol, under atmospherically relevant conditions (pH = 4.5),

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providing additional evidence that electrophiles are important reactive species in acidic

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atmospheric aqueous environments and may contribute substantially to the transformation of

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aromatic compounds especially during the night time.32 Recent chamber observation33

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indicates that 4-methyl-5-nitrocatechol (4M5NC) is only formed from 4-methylcatechol after

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irradiation has stopped, thus, further highlighting that night-time chemistry may be important

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for this group of atmospheric pollutants.

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Processing of aromatic compounds is known to form SOA, but subsequent oxidation products

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are largely unknown or only tentatively identified. To our knowledge, there are no studies on

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the (photo)nitration of methylcatechols in atmospheric waters nor have their possible nitration

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products been unambiguously identified. Over the past decades, with increasing computing 4 ACS Paragon Plus Environment

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power and a better understanding of the dynamic processes that govern chemical reactions,

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computational quantum chemistry has become a reliable tool to support experimental

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atmospheric chemistry, particularly with regard to reaction mechanisms, conformational

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energies, thermodynamic properties, and transition states.34–36 Assuming that electrophilic

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nitration is also relevant for atmospheric methylcatechols (structural isomers of guaiacols), we

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performed quantum chemical calculations of energetically favorable pathways of electrophilic

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substitution of 3-methylcatechol (3MC) with NO2+ in aqueous medium and assessed the most

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favorable reaction products. Due to interest in NACs as important yellow-colored and

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potentially toxic SOA constituents and to better elucidate their night-time chemistry in the

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troposphere, we supplemented our investigation with a detailed chemical analysis of MNC

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compounds in ambient aerosols.

112 113

EXPERIMENTAL SECTION

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Computational method

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Ab initio quantum chemical calculations were performed with Gaussian 09 program37 at the

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MP2/6-31++g(d,p).38–42 Preliminary calculations were also performed with B3LYP density

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functional and yielded comparable energies and geometries. However, B3LYP is notorious

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for not identifying all energy minima in organic reactions. For aromatic nitration, B3LYP

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fails to locate certain energy minima, as previously also noted by Esteves et al.43 Taking this

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into account, MP2 was ultimately chosen as the best compromise between accuracy and

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computational cost. Aqueous environment was modeled with polarizable continuum model

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with effective dielectric constant 78.5. Thermodynamic properties were obtained from

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frequency calculations. Energies and thermodynamic quantities are reported relative to the

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infinitely separated nitronium ion and catechol molecule at 1 bar and 298 K, correcting for



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zero-point energy. This is a reasonable assumption because the reaction energies are large

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enough for small temperature variations not to influence results noticeably. Charges were

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calculated with the natural bond order analysis (NBO).

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Geometries of intermediates and transition states were fully optimized without constraints.

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The nature of each species was determined with vibrational analysis on the optimized

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geometry. Intermediates were characterized by not having any imaginary frequencies as

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minima of the potential energy surface (PES), whereas transition states (TS) had exactly one

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imaginary frequency, corresponding to their saddle position on PES. TS were located with

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synchronous transit-guided quasi-Newton method (STQN).

135 136

Field samples

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Size-segregated aerosol samples were collected for 48–72 h on prebaked Al-foils at an urban

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background site of Ljubljana, Slovenia (approx. 270,000 inhabitants, 298 m a.s.l.) with a

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Berner cascade impactor (HAUKE, LPI 25/0,015/2; nominal flow rate of 25.8 L min-1)

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between December 3, 2014 and March 2, 2015. Although the impactor has 10 collection

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stages of the nominal size ranges of aerodynamic diameter (Dae) from 0.038 to 15.6 µm, for

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the purpose of this study only stages with PM of Dae up to 2.1 µm were further processed and

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represented as PM2 values obtained by summing up the corresponding deposits of stages from

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1 to 6. To ensure a sufficient amount of material for chemical analysis, corresponding stages

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of 2–4 samples were extracted jointly with high purity Milli-Q water (Millipore, Bedford,

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MA, USA) and further diluted in the injection solvent containing 50 µg L-1 picric acid

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(internal standard, IS; Sigma-Aldrich), 7.5 mM ammonium formate buffer pH 3 (ammonium

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formate: puriss p.a., eluent additive for LC-MS, Fluka; and formic acid: eluent additive for

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LC-MS, Fluka), and 200 µM ethylenediaminetetraacetic acid (EDTA; 99.995%; Sigma-


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Aldrich) for the analysis of MNCs by LC-MS/MS. For details regarding the sample collection

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and pretreatment please refer to SI material.

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LC-MS/MS analysis

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The water extracts of aerosol samples (as well as commercially available standards and the de

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novo synthesized standard isolated with the semi-preparative HPLC, see below) were

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analyzed by UltiMate™ 3000 UHPLC system (Thermo Scientific, USA) coupled with a triple

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quadrupole/linear ion trap mass spectrometer (4000 QTRAP LC-MS/MS System; Applied

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Biosystems/MDS Sciex, Ontario, Canada). An analytical HPLC column Atlantis T3 column

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(3.0 × 150 mm, 3 µm particle size; Waters) with Atlantis T3 guard column (3.0 × 10 mm, 3

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µm particle size; Waters) was used with the flow rate of 0.3 ml min-1. A mobile phase

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consisted of methanol (MeOH, Chromasolv® LC-MS grade, ≥ 99.9%, Fluka)/tetrahydrofuran

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(THF, Chromasolv® Plus, for HPLC, ≥ 99.9%, Fluka)/water (30/15/55, V/V/V) with 7.5 mM

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ammonium formate buffer, pH 3. The elution program was adopted from Kitanovski et al.44

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and is given in detail in Table S1. The injection volume and the column temperature were 10

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µL and 30 °C, respectively.

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After the separation, MNCs were detected by using negative polarity electrospray ionization

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((-)ESI) in the selected reaction monitoring (SRM) mode. The SRM transition m/z 168 → m/z

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138, characteristic of MNCs, was monitored as well as the MS/MS product ion spectra. The

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operating conditions of mass spectrometer were as follows: -45.0 V for declustering potential

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(DP), -22.4 V for collision energy (CE), -9.6 V for collision cell exit potential (CXP). Data

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were acquired and processed using Analyst 1.5 Software (Applied Biosystems/MDS

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Analytical Technologies Instruments).

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The quantification of MNCs present in field samples was performed by running a series of

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commercially available authentic standards: 4-methyl-5-nitrocatechol (4M5NC, 0.5–50 µg 7 ACS Paragon Plus Environment


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L-1, 6 calibration points, linear fitting, R2 > 0.999; Santa Cruz Biotechnology, INC., CA,

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USA), 3-methyl-5-nitrocatechol (3M5NC, 0.5–50 µg L-1, 6 calibration points, linear fitting,

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R2 > 0.998; Atomax Chemicals, Shenzhen, China), and 3-methyl-6-nitrocatechol (3M6NC,

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50–1500 µg L-1, 6 calibration points, linear fitting, R2 > 0.989; Atomax Chemicals, Shenzhen,

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China) as well as de novo synthesized 3-methyl-4-nitrocatechol (3M4NC, 0.5–50 µg L-1, 6

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calibration points, linear fitting, R2 > 0.998; synthesized, purified, and identified within this

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study) containing fixed concentration of 50 µg L-1 of picric acid (aqueous solution 1.0%;

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Sigma-Aldrich) used as an internal standard. Method limits of detection (LODs) were

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determined as analyte concentrations that correspond to a signal-to-noise ratio (S/N) of 3. The

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recovery efficiencies of MNCs water extraction from the foils was also taken into account and

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varied between 95.3 and 99.6%.

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Synthesis and identification of the 3-methylcatechol nitration product

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Nitration of 3MC in aqueous solution using a NaNO2/H2SO4 system led to the formation of a

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MNC reaction mixture.20,44 An Agilent 1100 HPLC system with a quaternary pump and a

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UV–vis diode array detector (DAD) coupled to an analytical-scale fraction collector was used

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for semi-preparative isolation of the minor reaction product extract. For details regarding the

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synthesis and product isolation please refer to SI material. The structure of the purified

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reaction product was elucidated by combining high-resolution mass spectrometric (HRMS)

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data and NMR spectra. HRMS measurements were performed with a hybrid quadrupole

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orthogonal acceleration time-of-flight mass spectrometer (QTOF Premier, Waters, Milford,

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MA, USA) and the compound was analyzed in negative mode. The capillary voltage was set

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to 3.0 kV, whereas the sampling cone voltage was 40 V. 1H-NMR,

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gHMBC were recorded on a Varian Unity Inova 300 MHz NMR spectrometer in the

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deuterated methanol (CD3OD) at ambient temperature. Chemical shifts (δ) are given in ppm 8 ACS Paragon Plus Environment

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C-NMR, gHSQC, and

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and calibrated using the signal of undeuterated solvent or internal reference (CD3OD/0.03%

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Me4Si). Chemical shifts of residual undeuterated solvent are δH = 3.31 ppm and δC = 49.15

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ppm. 13C-NMR signals with complete proton decoupling are described with a chemical shift

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in ppm.

204 205

RESULTS AND DISCUSSION

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Theoretical predictions of 3-methylcatechol nitration products in aqueous phase

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As we were less interested in the formation of reactive species (NO2+) per se, and more in

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different affinity of all available sites on the aromatic ring for the electrophilic attack of a

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nitrating agent, we limited theoretical calculations to the interplay of 3MC and NO2+ which

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has been long known as the key electrophile in the process.27 In addition to the original

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Ingold-Hughes mechanism, a single-electron transfer process45,46 and several other

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mechanisms have been proposed (e.g., the Olah mechanism47) which all feature the arenium

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ion as the key intermediate for reaction rate and selectivity. Different pathways that lead to 4-,

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5-, and 6-nitrated products were studied. We identified six different stable intermediates

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(Figure 1) and five transition states (Figure 2), thermodynamic and geometric properties of

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which are presented in Table S2.

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219 220 221

Figure 1. Geometries of intermediates for NO2+/3-methylcatechol (3MC) complexes, obtained with

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energy minimization and verified with vibrational analysis. All distances are in Å.

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224 225 226

Figure 2. Geometries of transition states for NO2+/3-methylcatechol (3MC) complexes, obtained with

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STQN and verified with vibrational analysis. All distances are in Å

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The geometries of the reactants (and products) were optimized, finding the most stable

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structures for the different species. Thus, isolated nitronium cation is perfectly linear with the

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calculated equilibrium O–N distance 1.15 Å. The positive charge is mostly retained on the

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nitrogen atom (+1.06) with oxygen atoms being only slightly negative (-0.03). Isolated

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catechol adopts the expected structure with the aromatic ring, hydroxyl groups, and methyl

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carbon lying in the same plane. Structure 1 is a shallow energetic minimum, corresponding to

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the perpendicular approach of NO2+ to the delocalized electronic system of catechol. This

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early intermediate represents a weakly bounded π-complex of NO2+ and 3MC, and is common

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to all studied pathways. Nitronium ion is situated 2.52 Å above the center of the aromatic

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ring, oriented perpendicularly to it. Its geometry remains unperturbed when compared to that

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of the isolated NO2+, as well as atomic charges. Formation of this complex is energetically

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and kinetically favorable. As it has already been shown for general aromatic nitration,48 there

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is no activation barrier for this first step. Instead, the relative energy of the complex

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monotonically decreases to -28.1 kJ mol-1 as the nitronium species approaches the 3MC

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molecule.

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After this step, the reaction mechanism bifurcates into two pathways. Intermediate structure 1

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can convert into two different δ-reactant complexes, shown as structures 4 and 5 where the

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nitronium is located approximately parallel to the aromatic ring plane. In structures 4 and 5,

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the nitrogen atom resides 2.38 Å and 2.43 Å above the ring, halfway between C1–C2 or C4–C5

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atoms, respectively. The O–N–O angle is reduced to 147°, while the O–N bond is slightly

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elongated to 1.18 Å. In these δ-reactant complexes, the interaction between the nitronium

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group and the aromatic ring is stronger, as evidenced by the aromatic ring compensation for

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the electron deficiency at the nitronium. The nitrogen atom has a charge +0.75 and the oxygen

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atoms -0.26 each. Thermodynamically, the formation of structure 4 is more favorable than the

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formation of structure 5 by 13.3 kJ mol-1. Kinetically, however, the reverse is true. The



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structure 2, corresponding to the transition state between 1 and 4, has a higher energy than the

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structure 3, corresponding to the transition state between 1 and 5, by 1–2 kJ mol-1.

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Nevertheless, the activation energy in both cases is small (less than 10 kJ mol-1) and is not the

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rate-determining step. It should be noted that it is also possible for the nitronium ion to attack

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the 3MC molecule from a different orientation, effectively skipping 1 and equilibrating

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directly at 4 or 5. In all cases, the reaction rate was found to be determined by the subsequent

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step, where an arenium ion is formed. This step is also known to influence the

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regioselectivity.43 A δ-reactant complex is converted into the corresponding δ-intermediate

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complex, with nitro group bound to the carbon atom, yielding structures 9, 10, and 11, which

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are attained through the transition states 6, 7, and 8, respectively. The hydrogen atom remains

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bound to the carbon and is only cleaved afterwards in a fast, non-rate-limiting step, leading to

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the formation of respective 4-nitro (3M4NC), 5-nitro (3M5NC), and 6-nitro (3M6NC)

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substituted 3-methylcatechols. Thermodynamically, the formation of 9 and 10 is favored, as

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both have similar lower energies (-86.7 and -91.4 kJ mol-1, respectively) in comparison to the

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nitration on site 6 yielding the structure 11 of higher energy -70.3 kJ mol-1. Kinetic

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considerations show the same trends, with transition states 6 and 7 having considerably lower

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energies (-41 and -43 kJ mol-1, respectively) in comparison to 8 (-22 kJ mol-1).

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The potential energy surface change during nitration of 3-methylcatechol (3MC) is shown in

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Figure 3 while discussed reaction pathways are schematically presented in Figure 4. Due to

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lower activation barriers for nitration of 3MC on sites 4 and 5 (+19 and +16 kJ mol-1,

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respectively) than for nitration on site 6 (+51 kJ mol-1), only negligible amounts of 3M6NC

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are expected to be formed through this reaction mechanism. Conversely, 3M4NC and 3M5NC

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should both be produced in larger quantities.

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Figure 3. The potential energy surface change during nitration of 3-methylcatechol (3MC)

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with NO2+. Three different pathways corresponding to the formation of 3-methyl-4-

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nitrocatechol (3M4NC, 9), 3-methyl-5-nitrocatechol (3M5NC, 10), and 3-methyl-6-

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nitrocatechol (3M6NC, 11) are shown.

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Figure 4. Schematic presentation of proposed electrophilic nitration pathways of 3-

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methylcatechol in aqueous solution.

289 290

To our knowledge, neither the energetics of the considered reaction nor the reaction products

291

have been reported previously. According to theoretical predictions and presuming that

292

electrophilic nitration is relevant for atmospheric methylcatechols, as it was previously shown

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for guaiacols,49 we should expect a dominant presence of 3M5NC and 3M4NC isomers in

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ambient aerosol samples. Namely, ignoring the effects of fast hydrogen abstraction, a rough

295

estimate for the relative proportion of the final products can be made from Gibbs free energies

296

of the transition states in the rate-determining step.

297

Using Eyring equation ,

298

(1)

299

kB, h, and R representing Boltzman, Planck and gas constants, T is the reference temperature

300

298.15 K, and G# Gibbs free energy (kJ mol-1) taken from Table S2, we obtain the following

301

reaction

rate

constants:

,

,

(corresponding to the relative reaction rates

302 303

: 1). One should note that the reported values agree with a

304

microscopic description of the studied reaction pathways according to the following kinetic

305

expression

306

aware that these kinetic rate constants cannot be used when considering the same reactions

307

macroscopically because macroscopic first order reaction rate constants are usually reported

308

for equilibrium concentrations of measurable reactant species, resulting in the kinetic equation

309

(subscript M refers to macroscopic). Still, due to low temperatures and

310

concentrations in the atmosphere, the actual ratio of products is expected to be in agreement

311

with the presented kinetic analysis (3M4NC:3M5NC=1:2.4; the amount of 3M6NC formed is

(subscript m refers to microscopic). One should be also


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negligible). Due to necessary approximations and inherent limitations of the method, the ratio

313

should be considered accurate within an order of magnitude.

314

There is a limited number of papers referring to the presence of MNCs in field samples,19, 20,

315

22, 23

316

al.22 and Iinuma et al.20 Previous studies report on the overall domination of 4M5NC,

317

3M5NC, as well as 3M6NC in nitroaromatic fraction of atmospheric aerosols, together with

318

4-nitrocatechol (4NC). However, to date there has been no evidence of 3M4NC in field

319

aerosols; only a chamber study identified the 3M4NC isomer as a third generation SOA

320

product of toluene photooxidation, based on the knowledge of gas-phase mechanisms and the

321

structural elucidation of the mass spectral fragmentation patterns.33 These facts motivated our

322

further evaluation of theoretical predictions through a detailed chemical analysis of MNC

323

compounds in ambient PM.

whereas an effort to quantify specific MNC isomers has been only made by Kitanovski et

324 325

Identification of methylnitrocatechols in atmospheric particulate matter

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Identification of MNC isomers present in water extracts of aerosol samples from Ljubljana,

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Slovenia was performed by LC-(-)ESI-MS/MS. The typical SRM chromatogram of MNCs

328

(m/z 168 → m/z 138) in the field sample is given in Figure 5a.

329



330 331 332

Figure 5. SRM chromatograms of (a) ambient PM sample, Ljubljana, Slovenia (collected 14–

333

25/1/2015), (b) 30 µg L-1 reference mixture of 4-methyl-5-nitrocatechol (4M5NC), 3-methyl-

334

6-nitrocatechol (3M6NC) and 3-methyl-5-nitrocatechol (3M5NC), and (c) reaction mixture

335

after 3-methylcatechol (3MC) nitration as well as MS/MS data of m/z 168 compounds in (d)

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ambient PM sample peak eluted at 20.1 min, (e) 30 µg L-1 3M6NC reference compound

337

eluted at 20.6 min and (f) minor peak in nitration reaction mixture eluted at 20.1 min.

338 339

The corresponding fragmentation patterns, obtained from aerosol MS/MS experiment, of the

340

minor (retention time, RT 20.1 min) and two major (RT 15.7 and 23.1 min) m/z 168 peaks are

341

presented in Figures 5d and S1a,b, respectively. In order to structurally distinguish between

342

the three distinct peaks attributed to positional isomers of MNCs in the field samples, their


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343

chromatographic behaviors and fragmentation patterns were first compared to commercially

344

available reference compounds 4M5NC, 3M6NC, and 3M5NC.

345

The SRM chromatogram of the reference compounds is presented in Figure 5b. Only the

346

corresponding fragmentation pattern of 3M6NC is shown in Figure 5e because fragmentations

347

of reference compounds 4M5NC and 3M5NC agree well with published data.20,44 The two

348

major peaks in the field samples were assigned to 4M5NC (RT 15.7 min, Figure S1a) and

349

3M5NC (RT 23.1 min, Figure S1b) isomers. As it has also been previously documented, their

350

MS/MS product ion spectra differ only in relative abundances of ions corresponding to the

351

neutral losses of NO (m/z 138) and HNO (m/z 137), the loss of NO2 (m/z 122) as well as the

352

combined loss of NO and CO (m/z 109).20,44 However, the retention time of 3M6NC reference

353

compound (RT 20.6 min) did not satisfactory match with the minor peak occurring in the field

354

samples at 20.1 min. Furthermore, the fragmentation patterns were distinctly different as well,

355

indicating the presence of another, closely eluting, isobaric MNC in the atmospheric aerosols.

356

3M6NC has a quite similar fragmentation pattern (Figure 5e) as the other two investigated

357

positional isomers, containing all of the fragments also found in the fragmentation patterns of

358

4M5NC and 3M5NC (vide supra). However, two additional peaks at m/z 151 and 150,

359

corresponding to neutral losses of •OH and H2O, respectively, can be found in the

360

fragmentation pattern of 3M6NC. All of the above agrees with the fact that the compounds

361

are closely related, whereas the latter difference can be attributed to the ortho effect, i.e. the

362

nitro group in 3M6NC is adjacent to one of the hydroxyl groups.50 However, MS/MS

363

spectrum of the compound eluting at 20.1 min in the field sample extracts (Figure 5d) shows

364

no peaks for neutral losses of •OH and H2O, only fragments with different relative

365

abundances, seen also in 4M5NC and 3M5NC fragmentation patterns (Figures S1a,b).

366

Therefore, it can be certainly concluded that there is no 3M6NC in the field samples, but

367

rather another possible MNC species. Noteworthy, in previous studies of ambient MNCs,



368

chromatographic peaks have been assigned to 4M5NC, 3M5NC, and 3M6NC. According to

369

Iinuma et al.,20 the presence of 3M6NC was postulated based on the comparison of field

370

sample LC-MS characteristics with those of synthesized reaction products isolated after the

371

nitration of 3MC. Although Kitanovski et al.44 unambiguously identified the major reaction

372

product of the same nitration reaction as 3M5NC, by combining NMR and LC-MS

373

techniques, the identification of the minor reaction product is still lacking detailed analysis.

374

Figure 5c illustrates SRM chromatogram of a reaction mixture after the similar nitration

375

procedure as mentioned above (see Experimental section). The synthesized compounds are

376

eluting at 20.1 and 23.1 min, agreeing with the field samples. The fragmentation of the major

377

peak (RT 23.1 min) is shown in Figure S1c and corresponds to the already identified 3M5NC

378

isomer,44 while the retention time and the fragmentation pattern of the peak corresponding to

379

the minor reaction product (Figure 5f) agree with those of the yet unidentified peak in the

380

ambient samples eluting at 20.1 min and not to the 3M6NC standard. Therefrom it follows

381

that the minor reaction product is not 3M6NC as it has previously been thought, but can be,

382

taking into account its synthesis from 3MC, only assigned to 3M4NC isomer.

383

To unambiguously identify the unknown MNC isomer of our concern, the minor product was

384

isolated from the reaction mixture and structurally characterized by HRMS (deprotonated

385

molecule: [C7H6NO4]- exact mass measured: 168.0300 Da; exact mass calculated: 168.0297

386

Da; ppm 1.8) and 1H-NMR, 13C-NMR, and 2D correlation (gHSQC and gHMBC) NMR. The

387

combination of 1D- and 2D-NMR experiments unequivocally confirmed that the exact

388

molecular structure of the isolated compound corresponds to 3M4NC (for details please refer

389

to SI material). All the resonances, multiplicities, and coupling constants in 1H-NMR (300

390

MHz) and

391

(Table S3). In addition, excellent agreement with experimental values was observed for

392

calculated 13C-NMR resonances. The structural assignment of 3M4NC was finally supported

13

C-NMR (75 MHz) spectra were in accordance with the structure of 3M4NC


Page 18 of 34

Page 19 of 34


393

by gHSQC and gHMBC 2D correlation spectroscopy (1H-13C), see Figures S4, S5 and S6.

394

Thus, precise structural identification of 3M4NC isomer, synthesized as a minor nitration

395

product of 3MC, together with its chromatographic and fragmentation properties described

396

above unambiguously confirm the presence of 3M4NC in the atmospheric aerosols, which has

397

never been done before.

398 399

Quantification of methylnitrocatechols in atmospheric particulate matter

400

Atmospheric concentrations of 4M5NC, 3M4NC, and 3M5NC in urban winter aerosols were

401

determined by LC-MS analysis of ambient PM2 samples using commercially available

402

authentic standards (4M5NC and 3M5NC) and de novo synthesized 3M4NC standard

403

compound by means of an internal calibration method and are shown in Table S4. The

404

concentrations of 3M5NC and 3M4NC ranged from 10.2 to 43.2 ng m-3 and from 2.0 to 8.2

405

ng m-3, respectively. It should be noted that 3M6NC was not detected in any of the PM2

406

samples although the ESI source was also exchanged by an atmospheric pressure chemical

407

ionization interface, which resulted in lower LOD of 3M6NC. In addition, we quantified

408

4M5NC isomer despite its formation pathway is different from those of 3M5NC, 3M4NC and

409

3M6NC. Namely, in contrast to the studied isomers which contain the methyl group attached

410

to the position 3-, 4M5NC compound has the methyl group attached to the position 4- of the

411

catechol ring. Thus, the formation mechanism of the 4M5NC should include analogue

412

nitration of 4-methylcatechol (4MC). However, beside 3M5NC and 3M6NC, 4M5NC

413

concentrations considerably contributed to the overall MNCs concentration in our samples.

414

Thus, during the sampling period, the average total concentration of MNCs in ambient PM2

415

was 43 ng m-3, contributing on average 0.15% (range: 0.11–0.21%) to the PM2 mass. For

416

comparison, Iinuma et al.20 determined on average 5.2 ng m-3 MNCs in wintertime PM10 from

417

a rural village Seiffen, Germany, which is for an order of magnitude lower than the values



418

obtained within this work. On the other hand, Kitanovski et al.22 measured a comparable

419

amount of MNCs in winter PM10 at the same sampling site of Ljubljana (69.2 ng m-3).

420

We next assessed the magnitude of error one could make by the incorrect assignment of

421

3M6NC instead of 3M4NC in an aerosol sample. If authentic 3M6NC standard compound

422

was used for quantification of 3M4NC peak by LC-MS, about two orders of magnitude higher

423

concentration would be determined, introducing also a significant error (of about the factor of

424

10) to the total ambient MNCs concentration. 3M6NC obviously ionizes poorly under ESI

425

conditions leading to the significantly lower response with respect to the other considered

426

isomers. The same can be deduced from the obtained LODs, being 0.15 mg L-1 for 4M5NC,

427

3M5NC, and 3M4NC and 40 mg L-1 for 3M6NC. As MNCs, along with 4NC, are the major

428

nitroaromatic compounds in ambient aerosols, contributing together more that 95% to the

429

total nitroaromatic mass,22 the substantial error due to the incorrect MNCs identification

430

would further lead to the overestimation of the overall nitroaromatic contribution to the

431

ambient PM and OC. Calculated for Ljubljana winter samples, contribution of total MNCs to

432

PM2 would increase to l.2% (from 0.15%). Assuming the 40% fraction of organic material in

433

total aerosol mass as relevant for this study,22,51 the actual 0.4% ratio of OC would be grossly

434

overestimated as >3%, if we used the value from the incorrect quantification of MNC. As

435

reported for several urban sites, the latter value is in the range of (di)carboxylic acids in

436

ambient PM,52,53 which are well known as major SOA components formed in both the gas and

437

aerosol phases.54-56

438 439

Environmental significance

440

We theoretically predicted the possible reaction products of the electrophilic nitration of 3MC

441

in aqueous solution and further assessed the relevance of this reaction pathway for the

442

formation of atmospheric MNCs by comparing the theoretical calculations with field


Page 20 of 34

Page 21 of 34


443

measurements. According to quantum chemical calculations, energetically favorable

444

pathways of electrophilic substitution of 3MC by NO2+ lead to the preferential formation of

445

3M5NC and 3M4NC isomers, whereas negligible amounts of 3M6NC are expected to form.

446

After a detailed reassessment of MNCs in ambient aerosols, both 3M5NC and 3M4NC were

447

determined in winter PM2 from Ljubljana, Slovenia; at the same time, no 3M6NC could be

448

detected in these samples. The presented theoretical calculations anticipate the ratio of

449

3M5NC to 3M4NC according to the electrophilic formation pathway in favor of 3M5NC (the

450

ratio 1:2.4 was obtained and should be considered accurate within one order of magnitude).

451

On average, concentrations 5.6 ± 0.8 times higher of 3M5NC to 3M4NC were observed in

452

ambient aerosols, which agrees well with the theoretical calculations. Although the predicted

453

kinetics of aqueous-phase formation of MNCs cannot point directly to the prevailing

454

electrophilic nitration of 3MC in the atmospheric wet aerosols, a very good correlation (R2 =

455

0.99) obtained between 3M5NC and 3M4NC concentrations (Figure 6) indicates their

456

common formation pathway(s) and supports this assumption.

457

458 459

Figure 6. Correlation between 3M5NC and 3M4NC concentrations determined for PM2

460

samples (n=7) collected during winter 2014/2015 from Ljubljana, Slovenia.



461 462

Aqueous-phase nitration of 3MC by nitronium ion is corroborated as a formation pathway of

463

MNCs in the wet atmospheric aerosols as (1) no 3M6NC could be detected in any of the

464

ambient samples and (2) the fixed ratio of 3M5NC to 3M4NC agrees well with theoretical

465

predictions and indicates their common formation pathway. A potential discrepancy between

466

the theoretical predictions and the field measurements could be explained by distinct vapor

467

pressures and/or solubilities of the isomers under consideration, which would affect their

468

Henry's law constants and consequently their partitioning between the atmospheric aqueous

469

and gaseous phases. Unfortunately, none of these data (neither vapor pressures or solubilities

470

nor Henry's law constants) are available for the investigated compounds to support our

471

explanation. As estimation methods have already been recognized as one of the major

472

contributors to introducing errors into predictive atmospheric models, they will be avoided.24

473

Still, formation pathways and the multiphase partitioning are not the sole factor to be

474

discussed when considering the ratios between organics in the atmospheric samples; losses of

475

organic compounds through their ageing and degradation processes, influenced by their

476

individual chemical stabilities, are also governing the chemical equilibrium of the

477

atmosphere. However, this study cannot rule out any of other possible condensed- or gas-

478

phase MNC formation pathways, proceeding in the dark or being photochemically induced

479

and involving different reactive species (e.g., •NO3, •NO2, HOONO, and •OH; organic radicals

480

or excited triplet states; H2O2; and metal ions) that could result in different product

481

yields.20,28,57–59

482

A review paper by Noziere et al.4 comprehensively discussed and highlighted the importance

483

of maximum level identification of organic compounds in the atmosphere. Accurate

484

identification is needed to reveal correct sources and precursors, to properly represent

485

formation pathways of specific SOA tracers in atmospheric models, and to elucidate


Page 22 of 34

Page 23 of 34


486

corresponding climate and health effects.4 Moreover, the position of aromatic substitution was

487

found to be crucial for controlling not only the formation but also the losses of MNCs in the

488

aerosols.33 Such findings underscore the relevance of studying SOA formation and evolution

489

on a molecular level and the importance of identifying tracer organic compounds of interest.

490

Semi-volatile MNCs are contributing to the water-soluble organic fraction of atmospheric

491

PM; they are capable of forming SOA and regarded as tracers of processed biomass-burning

492

emissions as well as aged anthropogenic aerosols.20,33 They are prone to inappropriate

493

analysis by the most commonly used analytical techniques, which could introduce a

494

significant misconception about their sources, atmospheric chemistry leading to their

495

formation and aging, and/or their potential hazardous impact on the environment. We

496

evaluated the eventual errors in quantification of particular MNCs and consequently in

497

calculating NACs contributions to the OC and PM due to the incorrect identification of

498

3M4NC as 3M6NC. Moreover, MNCs rank among the most abundant chromophoric

499

substances in the atmospheric aerosols.19 As relative positions of –OH and –NO2 functional

500

groups on the aromatic ring in particular affect the absorbance of (methyl)nitrocatechols,60

501

incorrect MNCs quantification could also lead to incorrect estimations of their contribution to

502

the overall absorption of water soluble BrC. As already pointed out by Noziere et al.,4

503

speculative or careless analytics resulting in misidentification of chemical species in

504

atmospheric samples could lead to misunderstanding of basic environmental processes and

505

thus inappropriate evaluation of state-of-the-art predictive models, which could consequently

506

introduce important fundamental errors to beliefs of the scientific community. Bearing the

507

survey of published data on MNCs and discussion above in mind, we find it of utmost

508

importance to consider the up-to-date literature regarding MNCs notation with special care.

509 510

Corresponding Author



511

*Address: Analytical Chemistry Laboratory, National Institute of Chemistry, Hajdrihova 19,

512

SI-1001 Ljubljana, Slovenia

513

Division for Marine and Environmental Research, Ruđer Bošković Institute, Bijenička 54,

514

PO-10000 Zagreb, Croatia

515

Phone: +386 (1) 4760 361; Fax: +386 (1) 4760 300; E-mail: [email protected]

516 517

Acknowledgments

518

Financial support provided from the European Commission and the Croatian Ministry of

519

Science, Education and Sports through Marie Curie FP7-PEOPLE-2011-COFUND project

520

NEWFELPRO (author SF), and from the Slovenian Research Agency (Contract Nos. P1-0034

521

and P2-0152; authors SF, MŠ, AK, MH, IG), is gratefully acknowledged.

522 523

Supporting Information

524

Detailed procedures of the ambient aerosol collection and pretreatment; synthesis and

525

isolation of the 3MC nitration product; NMR characterization of the purified reaction product,

526

4 Tables and 6 Figures. This information is available free of charge via the Internet at

527

http://pubs.acs.org/.

528 529

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