Unraveling Pathways of Guaiacol Nitration in Atmospheric Waters

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Environmental Science & Technology

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Unraveling pathways of guaiacol nitration in

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atmospheric waters: nitrite – a source of

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reactive nitronium ion in the atmosphere

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Ana Kroflič,a,* Miha Grilc,b and Irena Grgića

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a

Analytical Chemistry Laboratory, National Institute of Chemistry, Hajdrihova 19,

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SI-1001 Ljubljana, Slovenia b

Laboratory of Catalysis and Chemical Reaction Engineering, National Institute of Chemistry, Hajdrihova 19, SI-1001 Ljubljana, Slovenia

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Corresponding Author

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E-mail: [email protected] (A. Kroflič)

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Phone: +386 (1) 4760 361

Fax: +386 (1) 4760 300

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1 ACS Paragon Plus Environment


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Abstract

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The tropospheric aqueous-phase aging of guaiacol (2-methoxyphenol, GUA), a

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lignocellulosic biomass burning pollutant, is addressed in this work. Pathways of GUA

18

nitration in aqueous solution under atmospherically relevant conditions are proposed and

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critically discussed. The influence of NaNO2 and H2O2, hydroxyl radical scavenger, and

20

sunlight was assessed by an experimental-modeling approach. In the presence of the

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urban pollutant, nitrite, GUA is preferentially nitrated to yield 4- and 6-nitroguaiacol.

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After a short lag-time, 4,6-dinitroguaiacol is also formed. Its production accelerates after

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guaiacol is completely consumed, which is nicely described by the model function

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accounting for NO2• and NO2+ as nitrating agents. Although the estimated second-order

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kinetic rate constants of methoxyphenol nitration with NO2• are substantially higher than

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the corresponding rate constants of nitration with NO2+, nitration rates are competitive

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under nighttime and liquid atmospheric aerosol-like conditions. In contrast to

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concentrations of radicals, which are governed by the interplay between diffusion-

29

controlled reactions and are therefore mostly constant, concentrations of electrophiles are

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very much dependent on the ratio of NO2− to activated aromatics in solution. These

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results contribute substantially to the understanding of methoxyphenol aging in the

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atmospheric waters and underscore the importance of including electrophilic aromatic

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substitution reactions in atmospheric models.

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Introduction

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Aromatic compounds are abundant in atmospheric aerosols.1 They compose 20–50%

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of the non-methane hydrocarbon mass in urban air and are regarded as one of the main

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precursors to secondary organic aerosols (SOA), which constitute up to 80% of the

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total organic aerosol in the atmosphere.2 After being emitted into the troposphere,

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semi-volatile aromatics partition between gaseous and aqueous phases; different

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environmental conditions then determine their aging processes.3 Numerous studies

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have demonstrated the importance of aqueous-phase transformations for aging of semi-

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volatile organic compounds (SVOC) in conditions of cloud droplets, fog, and moist

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aerosol particles.4-7 During atmospheric processing, either gas or condensed phase, the

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oxidation state of the primary emitted aromatic pollutant usually increases, which often

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decreases its volatility and concurrently increases its water solubility.6,8 Therefore,

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within the pollutant’s lifetime in the troposphere, aqueous-phase reactions become

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more and more prominent for its aging. The fraction of organic carbon in the

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atmospheric waters was found to contain up to ~46% of total organic carbon in the

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atmosphere, whereas higher aqueous-phase organic carbon contents were associated

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with aged air masses, with the increased effective Henry’s law constant, in particular in

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remote locations.6

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Oxidized aromatics, either biogenic or anthropogenic in their origin, are promising

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candidates for forming SOA and, because they are mostly more hygroscopic than their

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precursor compounds, atmospheric transformations often improve the ability of

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airborne particles to act as cloud condensation nuclei.2,7,8 Some of the aged aromatic

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compounds are also colored and considered constituents of brown carbon, so they

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affect the radiative balance of the atmosphere.2,9-11 Besides altering the Earth’s climate,



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many of these products, especially nitrated aromatic compounds, are hazardous for

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human health and other living organisms.12,13

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Nitrated aromatics are directly emitted into the troposphere during combustion and

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fertilizing processes. In the atmosphere, they can also be secondarily formed via

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reactions with atmospheric nitrogen-containing reactive species (NRS).14-16 The

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tropospheric reactive nitrogen and its impact on the local and global climate are widely

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investigated recently, but studies are mostly focused on its gas-phase constituents.15,17-

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19

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aromatic compounds, which is not yet well understood. In the 1990s, being recognized

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as sources of nitronium ion (NO2+), N2O5 and ClNO2 were identified as potent nitrating

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agents of aromatic compounds in tropospheric waters.20 Very recently, N2O5 was found

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to ionize in the liquid layer on the surface of atmospheric particles, which was shown

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to substantially influence the distribution of nitration products of atmospheric

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polycyclic aromatic hydrocarbons.21 Reactive uptake of N2O5 into deliquescent

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aerosols was also parameterized22 and, very recently, the role of marine boundary layer

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in its chemical processing was assessed.23 An attempt has been also made to

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quantitatively resolve the kinetics of electrophilic aromatic substitution (SEAr) reaction

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of phenol with NO2+ produced from N2O5, unfortunately without a full success.24

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Although N2O5 is widely recognized as a primary nocturnal reservoir of the

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tropospheric reactive nitrogen,23,25 it is not the only known precursor of nitronium ion

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in natural environments. In the presence of peroxynitrous acid (HOONO), substituted

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phenols are, among other possible pathways, often nitrated with NO2+, while in

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solutions of nitrous or nitric acid alone, SEAr reactions have been only proposed under

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rather extreme conditions.14,26,27 Instead, neutral reactive species such as nitrous acid

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(HNO2), N2O3, N2O4, NO2•, and NO3• have been mostly considered under conditions of

In this work, special attention is paid to the aqueous-phase nitration of atmospheric


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higher environmental relevance.14,27 Last but not least, nitrous acid ionization

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equilibrium was found to play an important role in nitrocatechol formation via Michael

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addition under mild acidic conditions.28 However, nitration mechanism of aromatic

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compounds in the atmospheric aqueous phase is still under debate.

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Among the sources of semi-volatile aromatic compounds in the atmosphere,

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emission rates of total methoxyphenols are reported in the range of 900−4200 mg/kg of

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wood;29 and wood smoke particulate matter was found to consist of up to 40% phenol

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derivatives.30 Concurrently, the role of nitrous acid in the atmospheric waters is often

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overlooked, although it is a common oxidation state of nitrogen in biological systems;

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presumably because of its lower abundance and stability in the environment.31

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However, nitrite (NO2−) in soil has been lately recognized as an important source of

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reactive species in the atmospheric gaseous phase32 and also a precursor of reactive

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nitronium ion in the atmospheric waters.33 Therefore, the tropospheric aqueous-phase

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aging of guaiacol (2-methoxyphenol, GUA), a lignocellulosic biomass burning

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pollutant, is addressed in this work. The investigation is based on the long-term kinetic

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study of GUA nitration in acidic H2SO4 solution, typical for the atmospheric waters

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(pH 4.5), which has been partially presented very recently.33 In that work, the impact of

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GUA aging on the ecosystem and climate has been discussed for the first time, whereas we

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now focus on different pathways of nitration of methoxyphenols in the atmospheric aqueous

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phase. This work thus contributes essentially to the understanding of tropospheric aqueous

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phase chemistry and underscores the importance of including electrophilic aromatic

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substitution reactions in atmospheric models. Experiments were performed in the dark and

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under simulated sunlight conditions upon addition of the mostly urban pollutant,

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sodium nitrite (NaNO2), and hydrogen peroxide (H2O2). Based on the proposed

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complex scheme of aromatic nitration in aqueous solution under atmospherically

109

relevant conditions, a set of differential equations was derived and kinetic rate 5 ACS Paragon Plus Environment


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constants included in a model function were fitted simultaneously to all experimental

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data obtained in 1 mM NaNO2 to reach the global minimum.

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Experimental

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Materials.

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tetrahydrofuran (Chromasolv Plus, for HPLC, ≥99.9%, inhibitor-free), ammonium

115

formate (Puriss p.a., eluent additive for LC/MS), formic acid (Puriss p.a., eluent

116

additive for LC/MS), and high purity water (18.2 MΩ cm), supplied by a Milli-Q water

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purification system, were used for mobile phase preparation. Sulfuric acid 98%

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(analysis grade), sodium nitrite (ACS reagent, ≥97.0%), hydrogen peroxide 30%

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(Perhydrol, for analysis), 2-propanol (gradient grade for LC, ≥99.9%), and vitamin C

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(ascorbic acid, puriss p.a., ≥99.0%) were used for reaction mixture preparation and

121

quenching. The following standard substances were used also as reactants: guaiacol

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(GUA), 4-nitroguaiacol (4NG), 2-methoxy-6-nitrophenol (6-nitroguaiacol, 6NG), and

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4,6-dinitroguaiacol (DNG, produced by Kitanovski et al.34). Purity of all standards was

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≥97% and they were used without further purification. Griess reagent (modified) was

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used for spectrophotometric determination of nitrite.

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Experimental methods. Nitration of GUA and its primary reaction products (4NG and

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6NG) in acidic H2SO4 solution (pH 4.5) was investigated in the dark and under

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simulated sunlight conditions. For performing the experiments under illumination a

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solar simulator LOT-QuantumDesign Europe equipped with an ozone free xenon short

130

arc lamp (300 W) was used. Initial concentrations of reactants in the reaction mixture

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were 0.1 mM GUA, 0.02 mM 4NG, or 0.023 mM 6NG; 0.5 mM, 1 mM, 2 mM, or 4

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mM NaNO2; and 0.5 mM, 1 mM, or 2 mM H2O2 (when added). In some experiments,

133

2-propanol (IPA) was also added into the reaction mixture in excess (20 mM) to

134

scavenge produced hydroxyl radicals (OH•). During the experiments concentrations of

Acetonitrile

(Chromasolv

gradient

grade,

for

HPLC,

≥99.9%),


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GUA, 6NG, 4NG, and DNG were determined by an Agilent 1100 Series HPLC system

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equipped with a UV/Vis diode-array detector (DAD). For nitrite mass balance control,

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concentration of nitrite was also measured spectrophotometrically. The experimental

138

procedure has been already explained in detail elsewhere.33

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Modeling. Influence of the tested reaction parameters (dark or illumination, absence or

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concentration dependence of NaNO2 and H2O2, and GUA, 6NG, 4NG, and/or IPA

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addition into the reaction mixture) on the course and kinetics of NRS formation and

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radical and electrophilic nitration and nitrosation of GUA and its primary reaction

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products was further quantitatively explored by the development of a novel kinetic

144

model. Concentrations of all compounds, aromatic and NRS, are considered equally

145

important for determining the rate of nitration and nitrosation according to the

146

proposed model, while the concept of lumping was only used to evaluate the global

147

reaction rate constants of formation and termination of each NRS. The set of

148

differential molar balances, derived according to the proposed reaction scheme, was

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numerically solved in Matlab 7.12.0 (MathWorks, Natick, MA, USA). Simultaneous

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numerical solving of differential molar balances of NRS and GUA and its derivatives

151

resulted

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formation/termination kinetic rates, which may differ by several orders of magnitude,

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in the same set of differential equations. Rosenbrock algorithm (orders 2 and 3) with

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adaptive step size control (step as low as 10−7 h was initially required) had to be used

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to solve this rather stiff system of differential equations in reasonable time, because

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particularly fast reactions caused serious problems in numerical stability of the system.

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During the modeling, experimentally determined initial concentrations of GUA, 6NG,

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or 4NG, and calculated concentrations of weighted NaNO2 and H2O2 were taken as

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initial values, while initial concentrations of NRS were all set to zero. Optimization

in

a

coexistence

of

aromatic

nitration/nitrosation

and

NRS



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procedure was performed in consecutive steps, starting with the optimization of rate

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constants of reactions with 4NG and 6NG in the dark and absence of H2O2. Reactions

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of GUA nitration and nitrosation were then added and finally the influence of H2O2

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and illumination was also considered. During the optimization of a new group of

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reaction rate constants, current step was repeated several times by using different

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combination of initial estimates that were systematically generated according to Box-

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Behnken experimental design, while optimized constants from the previous step were

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kept constant.

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Optimized kinetic rate constants correspond to the minimum of the objective

169

function, i.e. the sum of squares of the difference between the experimental and the

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calculated value for each followed component and for all investigated experiments.

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The Nelder-Mead method was initially applied for the approximate optimization of

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kinetic rate constants, followed by the Levenberg-Marquardt optimization method for

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the final parameters’ optimization and Jacobian matrix computation, required for the

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subsequent determination of confidence intervals.

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Reaction Model

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Reaction scheme. Proposed reaction pathways of GUA nitration in acidic aqueous

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solution of nitrite are represented in Fig. 1. Primary (4NG and 6NG) and mainly

178

secondary (DNG) nitration products of GUA were quantified throughout the

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experiments. On the left, proposed nitrosation side reactions, evolved from the

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experimental-modeling results under different conditions, are also shown, although

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nitrosated products were not monitored during the experiments. The model does not

182

distinguish between 4- and 6-nitrosoguaiacol (inseparably named nitrosoguaiacol

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(NOG)), which can be further nitrated to yield the same reaction products as

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nitrosation of 4NG and 6NG. Besides, oxidation of NOG into the corresponding 8 ACS Paragon Plus Environment

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nitrated derivatives is also possible, but is neglected in the current model. Following

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this short introduction, modeled reactions regarding formation and termination kinetics

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of NRS are discussed stepwise below.

188 189

Figure 1. Proposed reaction scheme of guaiacol transformations in acidic solution of

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sodium nitrite: 1 guaiacol (GUA), 2 4-nitroguaiacol (4NG), 3 6-nitroguaiacol (6NG),

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and 4 4,6-dinitroguaiacol (DNG). The model considers 4- and 6-nitrosoguaiacol

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(colored blue) indistinguishable (NOG). (Data used from Ref.33)

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Dark. It is well known that nitrous acid thermally decomposes into NO• and NO2•

194

radicals in acidic aqueous solution.35

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2 HNO NO• NO• H O

(1)

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Therefore, both HNO2/NO2− and NO2• are potential nitrating agents of aromatic

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compounds in the reaction mixture. In contrast to phenol nitration with nitrous

198

acid/nitrite in the dark, where HNO2/NO2− is the proposed reactive species,36,37 our

199

modeling study ruled out a HNO2-driven mechanism in GUA nitration. If HNO2 were

200

the reactive agent, significant nitration of 6NG as shown in Figs. S2b and S2e would

201

result in a similar DNG formation rate in the reactions shown in Figs. S2a and S2d

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after 12 h, because of nearly the same conditions applied (initial concentrations of 6NG

203

and HNO2). This was not the case.



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Concentration decays of NO2− and GUA in Figs. S1a and S1d reveal that the rate of

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nitrite consumption equals the rate of GUA conversion. Therefore, in the proposed

206

reaction model (Fig. 1), nitrosation is considered the only side-reaction pathway in the

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dark. Besides NO•, NO+ can also form in acidic solution, but is capable of attacking

208

only strongly activated aromatic rings.38,39

209

HNO H NO H O

210

Another nitrating agent is needed even in the dark, because: (i) according to the

211

literature, selectivity of reactions between NO2• and 4NG or 6NG is not likely to be

212

much different40 and (ii) light exerts distinct effects on the conversion of 4NG and

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6NG. A comparison of Figs. S1b and S1c and Figs. S1e and S1f shows that 6NG is

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rapidly converted into DNG, whereas 4NG is only slowly nitrated in the dark. Besides,

215

illumination substantially accelerates nitration of 4NG in acidic aqueous solution and

216

only a limited influence can be observed in the case of 6NG (compare Figs. S1 and

217

S2). Although NO2+ formation in the absence of H2O2 in the dark cannot be

218

unambiguously clarified, the following reaction is included into the reaction model,

219

which successfully accounts for the observed differences in the reaction rates.

220

HNO H NO H O

(2)

(3)

221

Electrophilic nitration in the dark is also supported by the experiments performed upon

222

addition of a hydroxyl radical scavenger, which is discussed below. A possible

223

explanation for Reaction 3 is existence of a redox system in the presence of dissolved

224

oxygen from air. Preliminary results of the experiment performed under N2 atmosphere

225

without any added oxidant show good matching with the simulated nitration of GUA,

226

if Reactions 2 and 3 are not accounted for in the model. Oxidation of HNO2 and

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formation of NO2+ from HNO3 in the presence of H2SO4 as a catalyst usually proceeds

228

under extremely low pH and is therefore unlikely.38,39 Nevertheless, analogous to the 10 ACS Paragon Plus Environment

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industrial H2O2 production, where anthraquinone acts as a reaction carrier,41 in the

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reaction mixture GUA or its impurities can be efficiently oxidized with dissolved O2 to

231

quinone(s), giving hydrogen peroxide, which can yield NO2+ in nitrite aqueous solution

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as shown below.

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Reverse reactions, converting NRS into either HNO2 or HNO3, controlled by the

234

apparent rate constants (k23–k26) that implicitly account for the net contributions of the

235

reactions reported by Vione et al.31 are included into the reaction model to control low

236

bulk concentrations of NRS during experiments.

237

NO• NO

238

NO• NO

239

NO NO

240

NO NO

(4)

(5)

(6)

(7)

241

Hydrogen peroxide. Nitration of GUA, 4NG, and 6NG is faster in the presence of

242

NaNO2 and H2O2 than in the presence of nitrite alone (compare Figs. S1a–c and S1d–

243

f). This is attributable to the formation of peroxynitrous acid in acidic aqueous

244

solution.

245

HNO H O → HOONO H O

(8)

246

HOONO isomerizes quickly into nitric acid, which is its main transformation product,

247

but can also yield OH• and NO2• radicals and other nitrating agents such as NO+ and

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NO2+.42 To overcome the problem of the interrelated kinetic constants of HOONO

249

formation and decomposition, apparent direct conversions of nitrous acid into NRS are

250

modeled in the presence of H2O2 in this study.

251

HNO H O NO•

252

HNO H O NO•

(9)

(10)



253

HNO H O NO

254

HNO H O NO

255

HNO H O HNO H O

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(11)

(12)

(13)

256

Mass balance of hydroxyl radicals is kept out on purpose, because it would demand

257

additional fitting parameters, while OH• are not of the utmost importance for the

258

matching of the proposed theoretical model with experimental data. On the contrary,

259

mass balance of H2O2 is considered in the model but does not affect the results much.

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Sunlight. Upon illumination, nitrite and in particular nitrous acid decompose into OH•

261

and NO• and also yield NO2• in the reaction with OH•.42

262

NO H ℎ → NO• OH •

(14a)

263

NO OH • → NO• OH

(14b)

264

HNO ℎ → NO• OH •

(14c)

265

HNO OH• → NO• H O

(14d)

266

In order to avoid the unnecessary fitting parameters accounting for each elementary

267

step, the global reaction rate is only modeled in this study.

268

2 HNO ℎ NO• NO• H O

(14)

269

Experimental data also reveal that oxidation of HNO2 with H2O2 into HNO3 is

270

enhanced upon illumination (compare concentration decays of NO2− in Figs. S1d and

271

S2d), which is additionally addressed in the reaction model.

272

HNO H O ℎ HNO H O

(15)

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Photolysis of nitrate/nitric acid (NO3−/HNO3), which yields NO2• + OH• and NO2− +

274

O(3P), is not considered in the model.27 Namely, NO3− concentration in solution is

275

obtained by subtraction and not measured, which could result in inaccuracies in the

276

optimized objective function and determined kinetic parameters. Formation of


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radiation-excited nitrite (NO2−*) cannot be excluded under illuminated conditions as

278

well, although, in order to avoid the excess fitting parameters, it is not accounted for in

279

the reaction model. Consequences of neglecting these NRS in the proposed reaction

280

model are discussed in the Experimental-modeling study section of Results and

281

Discussion.

282

Results and Discussion

283

Dark. In the dark, 4NG is barely nitrated in the absence of H2O2 (Fig. S1c), whereas

284

6NG converts rapidly into DNG under the same conditions (Fig. S1b). Addition of 1

285

mM H2O2 into the reaction mixture accelerates all of the examined transformations

286

(compare Figs. S1a–c and S1d–f; predicted GUA lifetime is decreased from 22.4

287

(Fig.S1a) to 13.3 h (Fig.S1d)), but still a large difference between conversion of 4NG

288

and 6NG is retained. The lifetime is defined as a time in which concentration of the

289

compound decreases for a factor of 1/e and is used for easier comparison of kinetic

290

data at different conditions. No lag-time in DNG formation is detected in either the

291

4NG or 6NG nitration experiments (Figs. S1b, S1c, S1e, and S1f). In contrast, a

292

substantial lag-time is observed in formation of DNG in GUA nitration experiments

293

represented in Figs. S1a and S1d (refer also to inset Fig. 3) without and with added

294

H2O2, respectively. Therefore, DNG is mainly considered a secondary reaction product

295

of GUA. Furthermore, concentration decay of total solution nitrite in the dark

296

corresponds well to the consumption of nitrogen by the nitrated and nitrosated reaction

297

products

298

concentration decays of NO2− and GUA in Figs. S1a and S1d).

299

Simulated sunlight. In comparison to the reactions performed in the dark, the

300

predicted lifetime of GUA is substantially decreased under illumination (compare Figs.

301

S1a and S1d with Figs. S2a and S2d; predicted GUA lifetime is decreased from 22.4

with

one

or

two

nitrogen-containing

functional

groups

(compare



Page 14 of 33

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and 13.3 h to 4.3 and 4.0 h without and with added H2O2, respectively) and the lag-

303

time observed in the formation of DNG is subsequently shortened (inset Fig. 3). In

304

contrast to nitration in the dark, reactivity of 4NG and 6NG is similar under simulated

305

sunlight conditions (Figs. S2b and S2c and Figs. S2e and S2f without and with added

306

H2O2, respectively), which implies that there are at least two nitrating agents with

307

different influence of illumination on their concentration profiles. In the reaction

308

model, NO2• and NO2+ are proposed. At this point it can be assumed that NO2+ is much

309

more selective towards 6NG while 4NG is presumably mostly nitrated with NO2•.

310

After addition of 1 mM H2O2, reaction kinetics of aromatic compounds in the

311

illuminated reaction mixture is very slightly accelerated, but the consumption of

312

solution nitrite is substantially increased (compare concentration decays of NO2− in

313

Figs. S2a and S2d); presumably because of the enhanced oxidation of nitrite into

314

nitrate.

315

Hydrogen peroxide and sodium nitrite. Dependence of reaction profiles on the

316

concentration of added H2O2 and NaNO2 is represented in Figs. S3 and S4.

317

Experimental data show a remarkable effect of H2O2 addition on the nitration of GUA

318

in the dark (Figs. S3a–d), whereas reaction kinetics is not significantly affected by the

319

concentration of added peroxide under illumination (Figs. S4a–d). The predicted

320

lifetimes of GUA are 22.4, 16.6, 13.3, and 10.8 h in the dark and 4.3, 4.5, 4.0, and 3.9

321

h under simulated sunlight conditions for 0, 0.5, 1.0, and 2.0 mM H2O2, respectively.

322

In contrast, a pronounced effect of NaNO2 concentration on the nitration of GUA in the

323

dark and under illumination can be observed in Figs. S3e–g and S4e–g. The predicted

324

lifetimes of GUA are 30.2, 13.3, 6.9, and 3.3 h in the dark and 8.6, 4.0, 2.1, and 1.0 h

325

under simulated sunlight conditions for 0.5, 1.0, 2.0, and 4.0 mM NaNO2, respectively.


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Hydroxyl radical scavenger. IPA is a known scavenger of OH•.42 Its effect on the

327

studied reaction kinetics is represented in Fig. S5. Experimental data show that IPA

328

does not affect GUA nitration in the dark, even in the presence of H2O2 (small symbols

329

in Fig. S5b). On the other hand, OH• scavenger inhibits slightly the investigated

330

processes under illumination without and with added H2O2 (Figs. S5a and S5b),

331

although not to the expected extent, if radical mechanism through HNO2 photolysis

332

was predominant in the investigated systems. Noteworthy, twenty times higher

333

concentration of IPA in comparison to NaNO2 should substantially suppress the

334

reaction in the case of OH• mediated nitration of GUA with NO2•, because IPA

335

competes with NO2− and HNO2 for OH• in Reactions 14b and 14d. Furthermore,

336

radical mechanism through HNO3 photolysis, which is not accounted for in the

337

proposed reaction model and is also not expected to be inhibited by IPA (IPA can even

338

enhance radical nitration upon nitrate photolysis), should show stronger influence on

339

the experiments performed upon addition of H2O2, because much higher concentration

340

of NO3− (and lower concentration of NO2−) is estimated under such conditions. The

341

impact of H2O2 on GUA reactivity upon illumination is actually not seen on any of the

342

diagrams. Therefore, the investigated reaction is believed to be at least partially

343

accounted for by the non-radical nitration mechanism (electrophilic NRS and/or

344

NO2−*), which was also assumed by Vione et al.42,43 in the cases of phenol and

345

benzene. It should be mentioned, that nitration with NO2• under simulated sunlight

346

conditions is probably overestimated in the proposed reaction model, because its

347

inhibition does not result in the concentration profiles obtained in the dark (Fig. S5).

348

Experimental-modeling study. The model function proposed fits well to the

349

experimental data obtained in the dark (Figs. S1 and S3), whereas under simulated

350

sunlight conditions few deviations can be found which are discussed in details below.



Page 16 of 33

351

Nevertheless, the approach used allowed us to determine kinetic parameters of the

352

proposed reaction pathways with a fair amount of confidence (R2 = 0.9958).

353

First of all, special attention should be drawn to the unpredictable DNG

354

concentration profiles obtained experimentally, which are satisfactorily described by

355

the model function derived (Figs. S2a and S2d). The most noticeable is drastic change

356

in DNG formation that matches perfectly with the total GUA conversion in the reaction

357

mixture; it seems like nitration of 4NG and 6NG is substantially accelerated when

358

GUA is completely consumed. However, the observed phenomenon cannot be

359

explained by the radical reaction mechanism, because: (i) more or less constant steady-

360

state concentration is usually expected for radical species in solution (as a consequence

361

of their fast reactions with present compounds and their near-diffusion-controlled

362

recombination reactions) and (ii) second order kinetic rate constants of reactions

363

between radicals and aromatic compounds in the atmospheric aqueous phase usually

364

fall in a narrow range of orders of amplitude (108–109 M−1 s−1).31,40 On the other hand,

365

susceptibility of aromatic compounds for electrophilic attack is closely related to the

366

substituents on the aromatic ring.44 This makes 4NG and 6NG, which contain the

367

deactivating electron-withdrawing nitro substituent, much less reactive towards

368

electrophilic reactive species than their precursor GUA with two electron-donating

369

functional groups attached to the benzene ring (hydroxyl and methoxy).39 Furthermore,

370

the affinity of aromatic compounds to undergo SEAr and reaction regioselectivity

371

depend also on the orientation of the (de)activating substituents on the aromatic ring,

372

which more or less affect the electron density distribution of the conjugated π electron

373

system and stabilize the intermediate σ complex.44

374

As a matter of fact, quantitative results gathered in Table 1 can be nicely correlated

375

with the schematically represented electrostatic surface potentials of the investigated


Page 17 of 33


376

molecules in Fig. S6. Electrophilic nitration of GUA on the ortho position in relation to

377

its hydroxyl group is slightly preferred (higher electron density), which corresponds to

378

the 1.6-times higher second order kinetic rate constant of 6NG formation through SEAr

379

mechanism (k4') in relation to the analogous kinetic rate constant of 4NG formation

380

(k3'). A 50-times higher second order kinetic rate constant is determined for the

381

reaction of NO2+ with 6NG (k4') in comparison to 4NG (k3') and the reactivity of much

382

more nucleophilic GUA (k1' and k2') towards NO2+ is found even approximately 500-

383

times higher than that of 6NG (Table 1). As it has been already mentioned, NO+ is a

384

weak electrophile and can only attack strongly activated aromatic ring;39 in fact, its

385

second order kinetic rate constant for the reaction with GUA (k10') is found similar to

386

the kinetic rate constant of 6NG reacting with NO2+ (k4'). Furthermore, less deactivated

387

nitrosoguaiacol is nitrated faster with NO2+ than its nitro derivatives (compare k13' with

388

k3' and k4'), which is still slow in comparison to electrophilic nitration of strongly

389

activated GUA (k1' and k2').

390 391

Table 1. Best-fit kinetic rate constants with 95% confidence valid at the experimental

392

conditions applied, i.e. 25 °C and pH 4.5. Reaction 1 1' 2 2' 3 3' 4 4' 5 6

ri k1·[GUA]·[NO2•] k1'·[GUA]·[NO2+] k2·[GUA]·[NO2•] k2'·[GUA]·[NO2+] k3·[4NG]·[NO2•] k3'·[4NG]·[NO2+] k4·[6NG]·[NO2•] k4'·[6NG]·[NO2+] k5·[GUA]·[NO2+]·[NO2−] k6·[DNG]

ki (4.01±0.04)·109 (2.52±0.02)·105 (5.74±0.04)·109 (4.07±0.03)·105 (7.04±0.08)·108 (1.42±0.01)·101 (1.190±0.009)·108 (7.01±0.04)·102 (3.03±0.03)·106 (7±1)·10−6

10 10' 11 12

k10·[GUA]·[NO•] k10'·[GUA]·[NO+] k11·[4NG]·[NO•] k12·[6NG]·[NO•]

(6.65±0.05)·109 (5.46±0.04)·102 (9.18±0.08)·108 (3.86±0.03)·109

* * * *

* * *

units Ref. L mol−1 s−1 33 L mol−1 s−1 33 L mol−1 s−1 33 L mol−1 s−1 33 L mol−1 s−1 33 L mol−1 s−1 33 L mol−1 s−1 33 L mol−1 s−1 33 L2 mol−2 s−1 33 33 s−1 L mol−1 s−1 L mol−1 s−1 L mol−1 s−1 L mol−1 s−1

33 33 33 33



13 13'

k13·[NOG]·[NO2•] k13'·[NOG]·[NO2+]

(1.095±0.007)·1010 *,† L mol−1 s−1 † (4.09±0.05)·104 L mol−1 s−1

20 21 22 23 24 25 26 27 28 29 30 31 32 33

k20·[NO2−] k21·[NO2−] k22·[NO2−] k23·[NO•] k24·[NO2•] k25·[NO+] k26·[NO2+] k27·[NO2−]·[H2O2] k28·[NO2−]·[H2O2] k29·[NO2−]·[H2O2] k30·[NO2−]·[H2O2] k31·[NO2−]·[H2O2] k32·[NO2−] k33·[NO2−]·[H2O2]

(5.1±0.3)·10−7 (1.6±0.1)·10−5 (1.7±0.2)·10−7 (7.37±0.05)·105 (2.40±0.02)·105 (2.50±0.02) (3.86±0.03)·10−3 (0±9)·10−5 (6±1)·10−4 (3.9±0.5)·10−4 (0±2)·10−3 (2±1)·10−3 (3.73±0.08)·10−6 (6±1)·10−3

Page 18 of 33

33 33

s−1 s−1 s−1 s−1 s−1 s−1 s−1 L mol−1 s−1 L mol−1 s−1 L mol−1 s−1 L mol−1 s−1 L mol−1 s−1 s−1 L mol−1 s−1

393

* Kinetic rate constants are correlated with k23 and k24. Still, their values are set reasonable for

394

radical reactions, the ratios between them are reliable, and this does not affect the conclusions stated in

395

the manuscript.

396

†

Apparent kinetic rate constants for nitration of both NOGs (4- and 6-nitrosoguaiacol) are reported.

397 398

The observed sigmoidal concentration profile of DNG is actually attributable to the

399

substantial difference between kinetic rate constants of electrophilic nitration of GUA

400

(k1' and k2') and 4NG and 6NG (k3' and k4'); besides the relatively slow removal of

401

NO2+ through Reaction 7 (k26). In the beginning of the experiment, NO2+ reverse

402

reaction rate, = " #NO $, is much slower than electrophilic nitration of GUA and

403

its derivatives yet present in solution. Accounting for the initial experimental

404

GUA conditions in the equation for the rate of SEAr nitration of GUA (init ):

405

GUA init = %"&' "' "( NO )GUA#NO $ = "app #NO $

(16)

406

a limiting value of kapp = 66.2 s−1 can be determined, which is much higher than k26 =

407

3.86·10−3 s−1. Because of the significantly higher kinetic rate constants of formation of

408

mononitro derivatives (MNG) compared to kinetic rate constants of DNG formation

409

(k2'>k1'>>k4'>k3') and appreciably higher concentration of GUA in the reaction mixture 18 ACS Paragon Plus Environment

Page 19 of 33


410

MNG in comparison to 4NG and 6NG, SEAr formation rate of mononitro derivatives (init )

411

DNG is also much faster than SEAr formation rate of DNG (init ).

412

MNG DNG init = %"&' "' )GUA#NO = %"' 4NG "*' 6NG)#NO $ >> init $

(17)

413

The major portion of NO2+ (more than 99%) is therefore scavenged by GUA in the

414

MNG formation pathways, which temporally regulate #NO $ in solution, whereas

415

SEAr formation of DNG is only minor consumer of nitronium ion in the beginning of

416

the experiment. On the other hand, when GUA is completely consumed, only 4NG,

417

6NG, NOG, and other undetectable and less activated reaction products compete for

418

NO2+ and regulate its bulk concentration in the reaction mixture; consequently higher

419

steady-state concentration of nitronium ion substantially increases the formation rate of

420

DNG. For modeled NRS concentration profiles during the experiment refer to Fig. 2.

421 422

Figure 2. Experimental data (symbols) upon addition of H2O2 under simulated sunlight

423

conditions and modeled concentration profiles according to the proposed reaction



Page 20 of 33

424

scheme (lines) together with differential quotients of DNG experimental points

425

model (+, ⁄+- DNG ) and modeled DNG production rates (DNG ).

exp

426 427

Figure 3. DNG experimental data (symbols) and modeled concentration profiles

428

according to the proposed reaction scheme (lines) at different experimental conditions:

429

in the absence or presence of H2O2 (colors) in the dark (full symbols and dashed lines)

430

and under simulated sunlight conditions (open symbols and solid lines).

431

Close inspection of DNG formation profiles in Fig. 3 shows their unexpected

432

logarithmic shape at the beginning of the experiment that could be only obtained if

433

DNG were a primary reaction product of GUA. Contradictory, the lag-time detected in

434

DNG production is typical for secondary reaction product formation, which is also

435

supported by the nitration reactions of 4NG and 6NG, which show that DNG is

436

definitely formed from MNG. Therefore, direct conversion of GUA into DNG is also

437

considered in the reaction model; although it cannot fully account for the pronounced

438

logarithmic shape of the data obtained experimentally. This is also supported by the

439

modeled DNG production rates and differential quotients of DNG experimental data

440

over time shown next to the concentration profiles of GUA and its nitrated derivatives

441

in Fig. 2. After GUA is completely consumed, slopes of experimental DNG

442

differentials and its modeled formation rates show similar trends and are also pretty


Page 21 of 33


443

much alike the slopes of the MNG concentration profiles; this indicates that, in the

444

absence of GUA, DNG is formed from 4NG and 6NG. On the other hand, in the

445

beginning of the experiment, slopes of experimental DNG differentials and its modeled

446

formation rates differ substantially, which arises from the mismatching of the modeled

447

and experimental data shown in Fig. 2. Besides, both of them also disagree with either

448

of the GUA/MNG concentration profiles and point out the complex mechanism of

449

DNG formation in the presence of GUA. Surprisingly, experimental DNG differentials

450

approach zero after 5 h, which indicates that formation of DNG temporally almost

451

stops. Unfortunately, at this point no reasonable explanation can be given for the

452

observed dinitrated product formation profile in the beginning of the experiments.

453

It has been already mentioned that OH• (and NO2−*) are intentionally omitted from

454

the reaction model, because this is not fatal for the overall matching of the model

455

function with the experiments. We are aware that such approach neglects the very

456

important reactive species in the environment, but still we tried to keep the number of

457

fitting parameters minimal in order to ensure reliable results. Next mismatch between

458

the model function and the experimental data points can be in fact assigned to the

459

neglected OH•. The results for 4NG nitration with added H2O2 under simulated

460

sunlight conditions show that model function fits nicely to DNG experimental points,

461

while the modeled decay of 4NG is too slow (Fig. S2f). We believe that nitration of

462

4NG is appropriately considered, whereas 4NG should be additionally depleted

463

through another side reaction, which is ignored in the proposed reaction model. The

464

authors believe that direct photolysis does not contribute much to the conversion of

465

aromatic compounds in the reaction mixture; namely, GUA is stable at pH 4.5 in the

466

dark and its concentration does not change for more than 5% in 10 h under simulated

467

sunlight conditions. In contrast, concentration of GUA is significantly decreased upon



Page 22 of 33

468

addition of H2O2 into the illuminated acidic solution. It is well known that photolysis

469

of hydrogen peroxide yields OH• radicals, which could hydroxylate 4NG considerably,

470

because it is mainly susceptible for radical attack.

471

Another mismatching between the proposed model and the experimental data is seen

472

in Figs. S4f and S4g. Similar dependence of nitration kinetics on NaNO2 concentration

473

has been already reported by Vione et al.43 for benzene and was correlated to the

474

concentration dependent absorbance of radiation by nitrite. The first insight into our

475

initial formation rates seemed promising as well (Fig. S7), but the modeling revealed

476

that accounting for the absorption factor of NO2− (89:; = 1 − 10?∙A∙9: )45 in

477

Reactions 14 and 15 is not sufficient to overcome the drastic slowdown in GUA

478

conversion obtained experimentally (especially at 4 mM NaNO2). The only reasonable

479

explanation that can be given at this point is the missing reaction with NO2−*, which:

480

(i) might account for the reaction kinetics dependence on 89:; and (ii) is concurrently

481

not expected to be inhibited by the OH• scavenger.43 Nevertheless, we believe that,

482

similar to the case of OH•, this study would not benefit from the inclusion of new NRS

483

into the reaction model.

;

484

Simulated concentration profiles with k32 being set to zero are shown in Fig. S5 for

485

comparison with experimental data upon addition of OH• scavenger. According to our

486

expectations, simulation in the dark (dashed lines) corresponds well to the

487

experimental data, whereas the impact of the radical nitration mechanism under

488

simulated sunlight conditions (dash-dotted lines) is overestimated in the proposed

489

reaction scheme. As it was correctly assumed, radiation-excited nitrite or any other

490

NRS also form in aqueous solution under illumination, that are not accounted for in the

491

reaction model.


Page 23 of 33


492

Kinetics. Best-fit kinetic parameters according to the proposed reaction scheme

493

represented in Fig. 1 are gathered in Table 1. Noteworthy, formation of NO2• and NO+

494

through Reactions 9 and 12 is not crucial for matching of the model with the

495

experimental data, whereas Reactions 10 and 11 through which NO2+ and NO• are

496

formed have to be accounted for in the proposed reaction scheme. It can be therefore

497

concluded, that radical and electrophilic species form during the decomposition of

498

HOONO.

499

Environmental relevance. Environmental importance of the observed sigmoidal

500

profile of DNG formation was studied by simulating bulk concentration of NO2+ vs.

501

nitrite concentration at different nitrite-to-GUA ratios (Fig. 4). In a context of

502

atmospheric relevance, GUA is only a representative choice for a mixture of strongly

503

activated aromatic compounds in wet aerosols. Concentration ratios of nitrite to

504

activated aromatics (Re) from 1 to 1.000.000 are shown in Figs. 4 and S8, where the

505

red lines represent experimental Re of 10. The bulk concentration of NO2+ in Fig. 4

506

does not depend on the amount of its consumers at low nitrite concentration and

507

similarly at very high Re (above 20.000, depicted with the violet line). Nevertheless,

508

the concentration window of the highest environmental relevance of electrophilic

509

nitration (for details please refer to our previous paper33), where aromatic compounds

510

compete for NO2+ in tropospheric aqueous phase, is relatively broad. At higher, but

511

still environmentally relevant, nitrite concentration and lower Re, steady-state

512

concentration of NO2+ is approached in solution, which is strongly dependent on Re

513

and can result in drastic changes of the reaction rates of less activated aromatics in the

514

environment. Similar simulation is also shown for NO2• in Fig. S8. In contrast to

515

• #NO $, bulk concentration of NO2 is dependent on the concentration of its consumers

516

only at very high nitrite concentration and very low Re (below 50). Therefore, constant



Page 24 of 33

517

NO• is expected at distinct environmental conditions, independent on the presence of

518

organic compounds in the atmospheric waters. Note that production of NO2• and NO2+

519

from nitrite only is accounted for in these simulations. Besides, based on the

520

preliminary studies, the importance of SEAr nitration strengthens at lower pH, which is

521

even more relevant for polluted and usually more concentrated atmospheric aerosols.46

522 523

Figure 4. Bulk NO2+ concentration vs. nitrite concentration at different nitrite-to-GUA

524

ratios (Re = 1–1.000.000). Red line represents experimental Re of 10 and violet line

525

limiting Re of 20.000, indicating absence of its influence on the concentration of NO2+.

526

Acknowledgment

527

This work was supported by the Slovenian Research Agency (Contract Nos. P1-0034

528

and P2-0152), which is gratefully acknowledged.

529

Supporting Information Available

530

Additional figures are included in the Supporting Information. This information is

531

available free of charge via the Internet at http://pubs.acs.org.


Page 25 of 33

532


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