Small α-dicarbonyls represent the major
precursors of secondary organic aerosol (SOA) and brown carbon (BrC) in the
atmosphere, but the chemical mechanisms leading to their formation remain
unclear. Here we elucidate the fundamental kinetics and mechanisms for
aqueous-phase oligomerization of glyoxal (GL) using quantum chemical and
kinetic rate calculations. Our results identify several essential isomeric
processes for GL, including protonation to yield diol / tetrol and carbenium
ions, nucleophilic addition of carbenium ions to diol / tetrol as well as to
free methylamine / ammonia (MA / AM), and deprotonation to propagate oligomers
and N-heterocycles. Both protonation and nucleophilic addition occur without
activation barriers and are dominantly driven by electrostatic attraction.
Deprotonation proceeds readily via water molecules in the absence of MA / AM
but corresponds to the rate-limiting step for N-containing cationic
intermediates to yield N-heterocycles. On the other hand, the latter occurs
readily via a catalytic process by acidic anions (e.g., SO42-). A
carbenium ion-mediated reaction rate of GL is 4.62 × 10-3 s-1 under atmospheric conditions, in good agreement with the
experimental data. Our results provide essential mechanistic and kinetic
data for accurate assessment of the role of small α-dicarbonyls in
SOA and BrC formation.
Introduction
Volatile organic compounds (VOCs) from biogenic and anthropogenic sources
are of particular importance due to their chemical reactivity and high
abundance in the atmosphere (Piccot, 1992; Acosta Navarro et al., 2014).
Gas-phase reactions of VOCs associated with photochemical oxidant cycles
generally produce secondary organic aerosol (SOA) particle mass and increase
tropospheric ozone concentration (Lim et al., 2005; Ziemann and Atkinson,
2012; S. Wang et al., 2020; Y. Wang et al., 2020; Ge et al., 2021; Ma et al.,
2021). Hence, the emission of VOCs is a key process in controlling the
formation and growth of new particles over continental regions. Oxidation of
biogenic terpenes from terrestrial vegetation and aromatics from human
activities generates a large number of organic carbonyls, which are major
precursors of SOA (Altieri et al., 2006; Volkamer et al., 2001; Gomez
Alvarez et al., 2007; Sareen et al., 2016; Yang et al., 2020). These organic
carbonyls engage in a variety of reactions, including new particle
formation, condensation / equilibrium partitioning, particle- and
aqueous-phase reactions, leading to the formation of additional products in the
particle phase (Loeffler et al., 2006; Kroll et al., 2005; Nozière et
al., 2009; Marrero-Ortiz et al., 2019; Tuguldurova et al., 2019; Kua et al.,
2011; Li et al., 2020; Shi et al., 2019; Xia et al., 2021).
Glyoxal (GL), an important and simple carbonyl compound, originates from
the gas-phase oxidation of biogenic isoprene and anthropogenic aromatics (Volkamer et al., 2001; Gomez Alvarez et al., 2007; Lim et al., 2005).
The global source of GL is estimated to be 45–56 Tg yr-1 (Fu et al.,
2008; Myriokefalitakis et al., 2008), with a predicted contribution of 2.6 Tg C yr-1 to global SOA mass (Fu et al., 2008). GL and
methylglyoxal contribute to 16 % of SOA mass during a severe haze episode
in Hebei Province, China (J. Li et al., 2021). In Mexico City, GL
contributes at least 15 % of SOA mass (Volkamer et al.,
2007), and in the Pearl River Delta, China, 21 % of SOA formation
originates from the heterogeneous reactions of GL and methylglyoxal (Ling et al., 2020). Hence, GL is a primary contributor to the rapid
and efficient formation of SOA under urban environments. The aqueous-phase
reaction of GL starts with hydration, subsequently forming several
high-molecular-weight oligomers, such as dimers, trimers, tetramers, and
pentamers (Hastings et al., 2005; Loeffler et al., 2006; Gomez et al.,
2015; Kua et al., 2008; Avzianova and Brooks, 2013). Previous theoretical
studies have suggested that the direct hydration of GL is thermodynamically
and kinetically unfeasible (Shi et al., 2020; Ji et al., 2020). The rapid
growth of SOA was observed in sulfuric acid nanoparticles (Jang et al.,
2002; Huang et al., 2016; Surratt et al., 2007; Liggio et al., 2005b), while
several other studies have revealed little effect of acids (such as sulfuric
acid) on the formation of GL oligomers (Loeffler et al., 2006; Kroll et
al., 2005). Hence, the aqueous-phase chemical reaction mechanism of GL and
its role in SOA formation are still unclear.
On the other hand, brown carbon (BrC) is also generated by the aqueous-phase
reaction of GL in the presence of amines or ammonium in the troposphere (Y. Li et al., 2021; Shapiro et al., 2009; Galloway et al., 2009; Lee et
al., 2013; Maxut et al., 2015; Marrero-Ortiz et al., 2019; Tuguldurova et
al., 2019; Kua et al., 2011). Zhang and co-workers have revealed a slight
browning of glyoxal-amine mixtures (Marrero-Ortiz et al., 2019; Y. Li et
al., 2021). They have also identified oligomers and N-heterocycles
in NH4HSO4 and (NH4)2SO4, whereas only oligomers
were detected in NaCl (Y. Li et al., 2021). Shapiro et al. have detected
light-absorbing products, slowly formed from GL in a mildly acidic salt
solution of (NH4)2SO4, using ultraviolet–visible (UV-vis) spectrophotometry, and
matrix-assisted laser desorption ionization mass spectrometry (Shapiro et al., 2009). A chamber study of GL uptake
to (NH4)2SO4 solution was performed using a
high-resolution time-of-flight aerosol mass spectrometer and found that
carbon–nitrogen (C-N) compounds are irreversibly produced in the solution (Galloway et al., 2009). However, De Haan et al. (2020)
have described a rapid but reversible BrC formation in
(NH4)2SO4 droplets in dry condition (RH <5 %) using
cavity-attenuated phase shift single-scattering albedo spectrometry (De Haan et al., 2020). In addition, Powelson et al. (2014) found that the reaction of GL with methylamine is more effective than that
with (NH4)2SO4 by using UV-vis and fluorescence spectroscopy (Powelson et al., 2014), while Lian et al. (2020) have proven that the synergistic effect of ammonium and amines contributes to the
formation of imidazole in cloud processing (Lian et al.,
2020). Most previous studies have shown that imidazole and
high-molecular-weight light-absorbing C-N compounds are produced by
different atmospheric chemical reactions of GL with amines / ammonium, but the
reaction mechanism is yet to be clarified.
In this work, the aqueous-phase chemistry of GL in the absence and presence
of methylamine / ammonia (MA / AM) was systematically investigated using quantum
chemical calculations. The fundamental chemical mechanisms of the formation
of oligomers and N-heterocycles were investigated. The chemical composition
and the product distribution were also estimated and characterized by
conventional transition state theory considering solvent cage and
diffusion-limited effects (Methods). The aerosol growth rate for the
heterogeneous chemistry of GL was also evaluated under different atmospheric
conditions. We paid special attention to the key factors in the
formation of SOA and BrC from GL to provide insight into the important role
of the aqueous-phase chemistry of GL.
Methods
All quantum chemical calculations were performed by means of the Gaussian 09
program (Frisch
et al., 2009). The solvent effect of water in the aqueous phase was
considered with a continuum solvation model (SMD) (Marenich et
al., 2009). The solvation free energy includes two components: the bulk
electrostatic contribution and the cavity-dispersion-solvent-structure
contribution arising from short-range interactions between the solute and
solvent molecules. Geometry optimization of all stationary points (SPs), such
as reactants, transition states (TSs), intermediates, and products, was
calculated using the M06-2X functional (Zhao and Truhlar, 2008) with
the 6-311G(d,p) basis set (Ji et al., 2017, 2020), i.e., the
M06-2X/6-311G(d,p) level, which has shown good performance in describing the
geometrical optimization of the heterogeneous reactions of small α-dicarbonyls (Ji et al., 2020; Shi et al., 2020). Thermodynamic
contributions and harmonic vibrational frequencies were calculated at the
same level as that for geometry optimization to identify all SPs as either a
TS (exactly with only one imaginary frequency) or the minima (zero imaginary
frequency). Intrinsic reaction coordinate (IRC) calculations were
implemented to construct the minimum energy pathway (MEP), verifying that
each TS accurately connected the corresponding reactants and products. At
the same level, TS was searched by examining the SP using the TS keyword in
geometry optimization, while the absence of a TS was confirmed if no energy
exceeded the bond dissociation energy along the reaction coordinate (Ji
et al., 2020). The TSs for four deprotonation pathways in GL + MA / AM
reaction systems were identified at the M06-2X/6-311G(d) level because none
of them was searched at the M06-2X/6-311G(d,p) level (detailed discussion
in the Supplement). Pointwise potential curve (PPC) scanning was
performed to further confirm a barrierless process at the M06-2X/6-311G(d,p)
level (Hazra and Sinha, 2011). For this method, all other
geometric parameters were fully optimized, except for fixing the internal
breaking or forming bond length (detailed in the Supplement).
Based on the aforementioned optimized structures, single-point energy (SPE)
calculation was performed to refine potential energy surface (PES) with a
more flexible basis set 6-311 + G(3df,3pd), i.e., at the
M06-2X/6-311 + G(3df,3pd) level. For simplicity, hereinafter they are
denoted as the X//Y, i.e., M06-2X//M06-2X level, where Y is an SPE
calculation at the M06-2X/6-311 + G(3df,3pd) level and X is the geometry
optimized at the M06-2X/6-311G(d,p) level. To further evaluate the results
at the M06-2X//M06-2X level, a higher-level calculation using the CCSD(T)
method with the flexible 6-311 + G(2df,2p) basis set was performed to refine
the PESs. The CCSD(T) method (Lutnaes et al., 2009; Raghavachari and
Trucks, 1989), i.e., coupled cluster approach with single and double
substitutions including a perturbative estimation of connected triple
substitutions, corresponds to a higher electronic correlation method. As
discussed in the Supplement, the M06-2X//M06-2X level is
suitable for predicting energies and kinetics, and also represents a compromise
between computational efficiency and accuracy.
The rate constants (k) of the pathways with TSs were calculated using the
conventional transition state theory (TST) (Gao et al., 2014; Galano and
Alvarez-Idaboy, 2009) based on the above PES information:
k=σkBThexp-ΔG‡RT,
where h and kB are the Planck and Boltzmann constants, respectively, ΔG‡ represents the activation barrier energy with the thermodynamic
contribution corrections and solvent cage effects, and σ is the
reaction path degeneracy. To simulate realistic conditions in the solution,
the rate constants are refined by using solvent cage effects (Okuno, 1997) and diffusion-limited effects (Collins
and Kimball, 1949). For some of the reactions with low free-energy barriers,
the rate constants are found to be close to the diffusion limit, which is
calculated using the Collins–Kimball theory (Gao et al., 2014). The k
values of the pathways without TSs are controlled by the diffusion-limit
effect and thereby are equal to the diffusion-limited rate constants:
kD=4πRDABNA,
where R is the reaction distance, NA denotes the Avogadro number, and
DAB represents the mutual diffusion coefficient of reactants. The
branching ratio (Γ) was determined by the following equation:
Γ=kktotal,
where the k corresponds to the rate constant of each pathway, and the
ktotal is the sum of the k value for each parallel pathway. A detailed
description of the kinetics is provided in the Supplement.
Results and discussionIon-mediated initial reaction of GL
The ion-mediated initial reactions of GL proceed via either proton-mediated
(RH+1) or hydroxyl ion (OH-)-mediated (ROH-1) hydration
(Figs. 1a and S1a in the Supplement), yielding cationic (CIs) or anionic intermediates (AIs).
As discussed in the Supplement, no TS is identified by PPC scanning for all pathways in
ion-mediated reactions (Fig. S2a in the Supplement); that is, all ion-mediated pathways are
barrierless processes unless otherwise stated. Figure 2 presents the natural
population analysis of key species using the natural bond orbital (NBO) method (Glendening et al., 2011), and Fig. S3 in the Supplement lists the optimized
geometries of all SPs. From a geometrical point of view, GL belongs to the
C2h point group, and the positions of two carbonyl groups are
equivalent. The two carbonyl O-atoms and C-atoms exhibit the most negative
and positive natural charges of -0.532 and 0.383 e, respectively,
indicating that the proton- and OH--mediated reactions of GL start from
carbonyl groups.
PES of the GL oligomerization without MA / AM (in kcal mol-1):
(a) the initial reactions of GL and (b) the subsequent oligomerization to
dimers and trimers. The number denotes the total ΔGr for each
reaction.
The PESs of possible pathways in the ion-mediated reaction of GL are
presented in Figs. 1a and S1. For proton-meditated pathways (RH+1),
protonation of carbonyl O-atom (RH+11) is largely exothermic with a
reaction energy (ΔGr) of -96.9 kcal mol-1, to form the
CI11. Alternatively, protonation of GL can also be initiated by the
hydronium ion (H3O+) with a barrierless process. As shown in Fig. S1 in the Supplement, water protonation can provide the corresponding ΔGr value of
-111.0 kcal mol-1, and thus for the convenience of discussion, the
following proton-mediated pathways refer specially to the hydrogen ion
(H+)-initiated reactions. The nucleophilic attack of GL by OH-
(ROH-11) is also exothermic with the ΔGr value of -13.4 kcal mol-1, to yield the AI11. The small exothermicity of ROH-11 implies thermodynamically unfavorable formation of AI11. Table S1 in the Supplement lists the
k values of RH+11 and ROH-11 pathways as well as the ktotal of R1. Herein, the ktotal value of the ion-initiated reactions (the sum of
the k values of RH+11 and ROH-11 pathways) is 6.02 × 109 M-1 s-1, and the k of the ROH-11 pathway contributes
30 % to the ktotal. Further, considering the moderately acidic
condition inside fine particles (Liu et al., 2017; Guo et al., 2012),
proton-mediated reaction of GL is of major significance in the troposphere,
and, thus, the subsequent reactions of CIs are mainly considered in the
following parts.
As shown in Fig. S3, the length of the C-O bond in CI11 is elongated to
1.24 Å, facilitating the subsequent hydration reaction (RH+12).
CI11 reacts via hydration and dehydration to yield diol (DL) with successive
increasing in ΔGr values ranging from -8.0 to -1.0 kcal mol-1. Subsequently, DL protonation occurs at the carbonyl
(RH+21-1) or hydroxyl (RH+22-1) O-atom, leading to the formation
of tetrol (TL) and the first-generation carbenium ion (1st-CB1) (Figs. 1a and S1b). The RH+21-1 and RH+22-1 pathways to form CI21-1 and
CI22-1 are also strongly exothermic with the ΔGr values of
-103.0 and -104.4 kcal mol-1, respectively. The pathway for CI21-1 to
TL proceeds via hydration and deprotonation with successive increasing in
ΔGr values, and the CI22-1 to 1st-CB1 reaction via
deprotonation corresponds to a slightly increasing ΔGr value,
suggesting that both TL and 1st-CB1 are the dominant intermediates.
As shown in Table S1, the k values of RH+21-1 and RH+22-1 pathways
are all 4.14 × 109 M-1 s-1 and their half-lives (t1/2)
are lower than ∼10-4 s. The t1/2 was calculated
using the formula, t1/2=1/(k× [H+]), where k is the rate
constant of the RH+21-1 or RH+22-1 pathway and [H+] is the
concentration of the hydrogen ion (10-6 M) in the weakly acidic
solution. It implies the rapid conversion from DL to TL and 1st-CB1, in
line with the experimental results showing that the abundance of GL monohydrate is
lower than 2 % in acidic conditions (Malik and Joens, 2000).
As discussed in the Supplement, it further confirms that the direct hydration
(RH2O1 and RH2O2) and OH--mediated hydration (ROH-1 and
ROH-2) are kinetically and thermodynamically hindered. Hence, the
cation-mediated initial reaction of GL, as the dominant route in the aqueous
phase, is mainly focused on and explored in the following study.
Oligomerization mechanisms without methylamine / ammoniaDimerization
The dominant intermediates, TL and 1st-CB1, can subsequently conduct
electrostatic attraction with each other. As shown in Fig. S3, the C-O(H)
bond of 1st-CB1 after protonation is elongated by 0.05 Å relative
to the C=O bond of GL, attributable to the presence of a carbenium ion
center. The natural charge of the carbenium ion center in 1st-CB1 is
0.547 e (Fig. 2), implying that it is liable to nucleophilic addition with
a negative natural charge center of TL via electrostatic attraction. All
possible pathways involved in nucleophilic addition between 1st-CB1 and
TL are constructed and depicted in Figs. 1b and S4a.
The driving force for oligomerization
without and with MA / AM: the natural bond orbitals of DL, TL, MA, AM, and CBs
(in e).
The nucleophilic addition of 1st-CB1 with TL (RTL41) is an
exothermic process with the ΔGr value of -6.7 kcal mol-1.
The subsequent hydration (RTL42) and deprotonation (RTL43) exhibit
a small ΔGr value of -5.2 kcal mol-1, to yield a
ring-opening dimer (ROD1) that has been identified under acidic condition
using thermal desorption-ion drift-chemical ionization mass spectrometry
(TD-ID-CIMS) (Y. Li et al., 2021). On the other hand, 1st-CB2 is
produced via protonation (RH+31) and dehydration (RH+32) of TL,
similar to the formation of 1st-CB1 (Fig. S1c). 1st-CB2 is also
attacked by TL (RTL51) to form CITL51, which then proceeds via
hydration (RTL52) and deprotonation (RTL53) to yield ROD3 (Fig. S4b in the Supplement). An intermolecular isomerization pathway exists from ROD1 to ROD3 (R6,
Fig. S4c); that is, ROD3 can be formed from ROD1 via protonation
(RH+61), hydration (RH+62 and RH+63), and deprotonation
(RH+64), with the total ΔGr value of -113.5 kcal mol-1. As shown in Fig. 2, the natural charge of C-atom in 1st-CB2
is more positive than that in 1st-CB1, implying a stronger
electrostatic attraction between 1st-CB2 and TL. However, the ΔGr value of RTL51 is -3.1 kcal mol-1, which is higher than
that of RTL41 (Fig. S4a–b). It indicates that the reactivity of the
positive charge centers in carbenium ions is affected by both electrostatic
attraction and steric effect.
The nucleophilic addition reaction of 1st-CBs with DL is also
illuminated and presented in Figs. 1b and S4a–b, although DL is not the most
dominant product in the aqueous-phase reaction of GL. Both ROD2 and ROD1
are generated from DL, respectively, through analogous pathways to
RTL4 and RTL5. As shown in Fig. 2, the hydroxyl O-atom in TL has a
larger negative natural charge than that in DL, implying that there is a
stronger electrostatic attraction between 1st-CBs and TL. In addition,
the ΔGr values of association reactions of 1st-CBs with DL
are -5.3 (for 1st-CB1) and -1.8 (for 1st-CB2) kcal mol-1,
respectively, which are less negative than those with TL. This indicates that
the most abundant dimers correspond to the oligomeric pathways arising from
1st-CBs with TL in weakly acidic condition.
Trimerization and oligomerization
Similar to the formation of 1st-CBs, as shown in Fig. 1b, the
ring-opening dimers then repeat protonation and dehydration to form six
second-generation carbenium ions (2nd-CBs) (Fig. S5 in the Supplement), which further
engage in the formation of cyclic dimers (CDs) or ring-opening trimers
(ROTs). Figures S6–S7 in the Supplement present the schematic energy diagram for the formation
of CDs and ROTs, and Fig. S8 in the Supplement depicts the optimized geometries of SPs
involved in the aforementioned pathways. In total, three CDs and nine ROTs
are produced and identified via R8–R20 routes, and the overall schematic
diagram is shown in Fig. 1b. For example, protonation (RH+81-1) and
dehydration (RH+81-2) of ROD1 yield 2nd-CB1, which undergoes
intramolecular isomerization to produce CI111 (R111). CI111 is
further hydrolyzed (R112) and deprotonated (R113) to yield CD1.
Alternatively, the association of 2nd-CB1 with TL and DL yields CITL151
and CIDL151, respectively, and their subsequent pathways are similar to
the pathways of the formation of RODs, resulting in the formation of ROT1 and ROT2 (Fig. S7a). Current results reveal that cyclic oligomers are
difficult to be formed from the CBs with the positive charge center close to the
O(H) atom, such as 2nd-CB2 and 2nd-CB4 in Fig. 1b. Moreover, the
isomeric conversion can also occur among the nine ROTs via protonation,
hydration, and deprotonation processes (R21–R27 in Fig. S9 in the Supplement).
The formation of 3rd-CBs from the
ring-opening trimers (in kcal mol-1). The value
represents the ΔGr of each
step reaction.
Similar to dimerization pathways, the nine ROTs further engage in
protonation and dehydration reactions to produce 25 third-generation carbenium ions (3rd-CBs) (R28–R36 in Fig. 3). In this
study, the configurations of oligomers with the lowest energies are applied
because many isomers of dimers and trimers can be yielded. Subsequently,
25 3rd-CBs undergo intramolecular isomerization, hydration,
and deprotonation to further yield 12 cyclic trimers (CTs) (Fig. S10 in the Supplement and
Table S2 in the Supplement). Based on kinetic rate calculations, the k values of dimer and
trimer formation are ∼109 M-1 s-1 in the
aqueous phase, limited by liquid-phase diffusion. It should be pointed out
that the rate constants of dimer and trimer formation obtained from our
theoretical calculations are distinct from those previously investigated by
Ervens and Volkamer (Ervens and Volkamer, 2010). The rate constants obtained in
this previous study are ∼10-2 and
∼100 M-1 s-1 for dimer and trimer formation based
on the direct nucleophilic addition between GL and GL or GL and GL hydrates.
Our protonation-initiated cationic oligomerization involves nucleophilic
addition of DL / TL to CBs, which is fast and barrierless.
Hence, the formation of various ring-opening or cyclic dimers and trimers is
initiated by protonation and subsequently propagated via the electrostatic
attraction, with the rate constants of ∼109 M-1 s-1, ultimately contributing to SOA formation.
Oligomerization mechanisms with methylamine / ammonia
As shown in Fig. 2, a strong electrostatic attraction exists between
1st-CBs and AM or MA because N-atoms exhibit large negative
natural charges (-0.875 e for MA and -1.078 e for AM). Hence, the carbenium
ion-mediated oligomerization of GL with MA / AM to form N-oligomers
is simulated and presented in Fig. 4. Figures S11–S12 depict the optimized
geometries of key SPs. Also, the involved cation-mediated pathways default
to barrierless processes unless stated (Fig. S2b).
The PES for formation of N-heterocycles starting from (a–b) 1st-CBs with MA / AM to dimers, (c) dimers to
N-1st-CBs, and (d–e) N-1st-CBs
with MA / AM to trimers (in kcal mol-1). The number
denotes the ΔGr or ΔG‡ for each reaction step, and
all energies are relative to the corresponding reactants.
The nucleophilic addition of 1st-CBs with MA
Attack of 1st-CBs by MA results in four N-containing ring-opening dimer
(N-ROD) formation. For example, such multistep processes of 1st-CB1
with MA are shown as the following (Fig. 4a–b):
RMA60-RMA61:1st-CB1⟶RMA601:+MACIMA601⟶RMA602:+SO42-TSMA601⟶-HSO4-N-RODMA1⟶RMA611:+H+CIMA611⟶RMA612:-H2OCIMA612⟶RMA613:+SO42-TSMA611⟶-HSO4-N-RODMA2.
For the association pathway of 1st-CB1 with MA (RMA601), the
ΔGr value is -43.2 kcal mol-1. In CIMA601, the length
of the formed C-N bond is 1.50 Å (Fig. S12a in the Supplement). Unlike the
oligomerization without MA, the subsequent deprotonation of CIMA601 is
difficult to be initiated by H2O (Fig. S13 in the Supplement, as discussed in the Supplement). It
implies that deprotonation from N-containing CIs needs to be initiated by
species with larger electronegativity than H2O. Taking into account the
real atmospheric conditions, in this study, deprotonation of the amino group
in CIMA601 is initiated by SO42- (RMA602). As shown in
Fig. S14 in the Supplement, the natural charge center of CIMA601 is located at the amino
H-atom, which is readily abstracted by SO42- to form
N-RODMA1. The RMA602 pathway proceeds via a TS with the small
activation energy (ΔG‡) of -1.7 kcal mol-1. A
pre-reactive complex (COMMA601) is identified prior to the TS and the
corresponding ΔGr value is -1.0 kcal mol-1, which is lower
than that of the corresponding reactants. As illustrated in the Supplement, the
structure of COMMA601 is similar to that of the reactants, except for
the broken bonds. As shown in Fig. 4b, protonation of N-RODMA1 at the
hydroxyl group (RMA611) yields CIMA611 with the ΔGr
value of -122.8 kcal mol-1. The subsequent dehydration of CIMA611
(RMA612) possesses small exothermicity with the Gr value of
-5.7 kcal mol-1, to form CIMA612. Similar to the subsequent
reaction of CIMA601, deprotonation of CIMA612 is also initiated by
SO42- (RMA613), to form the other N-ROD (N-RODMA2), with
the ΔG‡ value of -7.3 kcal mol-1. In addition, there
also exists an intermolecular isomerization pathway from N-RODMA2 to
N-RODMA3 via protonation, hydration, and deprotonation (Fig. 4c):
RMA62:N-RODMA2⟶RMA621:+H+CIMA621⟶RMA622:+H2OCIMA622⟶RMA623:+H2OCIMA623⟶RMA624:-H3O+N-RODMA3.
For the RMA62 pathway, the total ΔGr value for protonation,
hydration, and deprotonation of N-RODMA2 is -108.1 kcal mol-1,
yielding N-RODMA3. N-RODMA3 then repeats protonation and
dehydration to yield N-containing CBs (N-1st-CBs), which subsequently
engage in the nucleophilic addition with MA to form in sequence N-containing
ring-opening trimers, N-ROTMA1 and N-ROTMA2, (Fig. 4c–e):
RMA63-RMA65:N-RODMA3⟶RMA631:+H+CIMA631⟶RMA632:-H2ON-1st-CBMA1⟶RMA641:+MACIMA641⟶RMA642:+SO42-TSMA641⟶-HSO4-N-ROTMA1⟶RMA651:+H+CIMA651⟶RMA652:-H2OCIMA652⟶RMA653:+SO42-TSMA651⟶-HSO4-N-ROTMA2.
Subsequently, N-ROTMA2 undergoes the nucleophilic addition with
1st-CBs rather than protonation, attributing to geometric
characteristics, i.e., no hydroxyl groups in N-ROTMA2 to protonation
(Fig. S11 in the Supplement). For example, the reaction of N-ROTMA2 with 1st-CB1 to
form N-heterocycles involves the following stepwise pathways (Fig. S15 in the Supplement):
RMA66-RMA67:N-ROTMA2⟶RMA661:+1st-CB1CIMA661⟶RMA662:+TSanti→synCIMA662⟶RMA663:+SO42-TSMA662⟶-HSO4-N-IMMA1⟶RMA671:+H+CIMA671⟶RMA672:-H2OTSMA671→N-CTMA1.
The nucleophilic addition of N-ROTMA2 with 1st-CB1 (RMA66) is
largely exothermic with the ΔGr value of -33.8 kcal mol-1
to overcome the barrier of subsequent intramolecular torsion and
H-abstraction pathways. The intramolecular torsion from CIMA661 to
CIMA662 proceeds via a TS, with the small ΔG‡ value of
5.1 kcal mol-1. The ΔG‡ value of the H-abstraction
pathway (RMA663) is 18.4 kcal mol-1. Subsequently, protonation of
N-IMMA1 occurs at the hydroxyl group to form CIMA671, which undergoes
dehydration via a TS, yielding an N-containing cyclic tetramer (N-CTMA1,
in Fig. S15). The ΔG‡ and ΔGr values of
RMA672 are 17.3 and -89.4 kcal mol-1, respectively. The
association reactions of 1st-CB2 with MA are also investigated and
discussed in the Supplement, via the similar stepwise pathways of 1st-CB1 with MA
(Figs. S16–S17), yielding an N-containing cyclic tetramer (N-CTMA2). The
intermolecular isomerization reaction from N-CTMA1 to N-CTMA2 is
also observed (Fig. S18a in the Supplement). All N-containing dimers, trimers, and tetramers
subsequently contribute to N-heterocycles, which are the important
precursors of BrC aerosols. Because all deprotonation reactions proceed via
the corresponding TS in the presence of MA and their rate constants fall in
the range of (1.17–1.30) × 109 M-1 s-1 (Table S3),
deprotonation is the rate-limiting step to propagate N-heterocycles. It
implies that N-heterocycle formation is more dependent on the content of
inorganic compounds or inorganic salts in aerosol rather than on particle
acidity.
The nucleophilic addition of 1st-CBs with AM
The carbenium ion-mediated reactions to N-heterocycles in the presence of AM
involve the stepwise processes (Figs. 4 and S15–S17), similar to the
nucleophilic addition of 1st-CBs with MA; that is, the formation of
N-heterocycles from GL with AM involves three vital steps (Fig. S19 in the Supplement): (1) the nucleophilic reaction of AM with CBs to form N-RODs, (2) protonation and
dehydration of N-RODs to yield N-containing CBs, and (3) the formation and
propagation of N-heterocycles by the association reactions of N-containing
CBs with AM. Alternatively, GL can be attacked by ammonium ion
(NH4+) to produce CIAM601 due to the equilibrium reaction
between AM and NH4+ in solution, with the ΔGr value of
-0.9 kcal mol-1 (Fig. 4a). CIAM601 engages in vital steps (2) and (3) to finally form N-heterocycles (N-CTAM1 and N-CTAM2).
Different from the case in the presence of MA, N-CTAM1 and N-CTAM2
can subsequently proceed with deprotonation to form N-CTAM3 and N-CTAM4
(Fig. S18b), respectively, because of the presence of H-atoms in amino
groups of N-CTAM1 and N-CTAM2 and the absence in N-CTMA1 and
N-CTMA2. This explains that N-CTAM3 and N-CTAM4 are
identified in the presence of ammonium salts (such as
(NH4)2SO4) (Lee et al., 2013; Yu et al., 2011), while
N-CTMA1 is observed in the presence of MA (De Haan et al.,
2009). Hence, the carbenium ion-mediated mechanism also provides a key
pathway for the formation of BrC from GL in the presence of ammonium salts.
Estimation of the rates for the oligomerization without and
with MA / AM and the growth rates to SOA and BrC under different atmospheric
conditions.
a Numbers are the typical measured values of GL from Cerqueira et al. (2003), Lawson et al. (2015), Qian et al. (2019), and Munger
et al. (1995).b The values are the rates of the oligomerization without and with
MA / AM (in s-1).ckrate(total) is the sum of the krate values without and
with MA / AM. The values in parentheses are from Liggio et al. (2005a).d The values without and with MA / AM are assumed as the growth rates
to SOA and to BrC, respectively. The values in parentheses are from Liggio et al. (2005a).
Schematic diagram of formation and propagation of
oligomers in the absence of MA / AM. The inside circle ring
represents the ion-mediated initial reaction of GL to yield DL, TL, and
1st-CBs; the middle circle ring corresponds to the
formation of RODs and 2nd-CBs; the outer circle ring
denotes the formation and propagation of ROTs from the association reactions
of 2nd-CBs with DL / TL.
Estimation of the heterogeneous GL reaction rates and growth rates of
SOA and BrC formation
To evaluate the atmospheric regions where the heterogeneous reaction of GL
will have significance, the heterogeneous GL reaction rates and the growth
rates of SOA and BrC formation (GRSOA and GRBrC) are estimated under
rural, remote, and urban conditions using the predicted carbenium
ion-mediated reaction mechanism of GL mentioned earlier. First, the
heterogeneous GL reaction rates without or with MA / AM were driven by the
expression as follows:
krate=k×Cg×γGL,
where Cg is the gas phase concentration under rural, remote, and urban
conditions, k is the calculated rate constant of protonation reaction (4.20 × 109 M-1 s-1) in the absence of MA / AM or
deprotonation (1.17/1.32× 109 M-1 s-1) in the
presence of MA / AM, and γGL is the uptake coefficient of GL under the
urban condition.
The heterogeneous GL reaction rate (krate(total)) is the sum of the
rates without and with MA / AM. Table 1 lists the krate without and with
MA / AM and krate(total) under rural, remote, and urban conditions as well
as the available experimental data (Liggio et al., 2005a). The
krate(total) values are 4.62 × 10-3, 9.25 × 10-4, and 1.85 × 10-3 s-1 under the three aforementioned conditions. The krate value under the urban condition almost agrees with
that of the experimental data and is slightly larger than the values of the
experimental data under other conditions (Liggio et al., 2005a).
The lower values under remote and rural conditions are explained by the
γGL used here, which is more suitable for the urban condition (Liggio et al., 2005a). Second, the growth rate (GRSOA) (Ji
et al., 2020) is expressed as
GRSOA=dGLprotondt×AWC,
where dGLprotondt represents the rate of GL
protonation given by krate× [H+] ×HGL,
where krate is the rate of heterogeneous GL reaction without MA / AM (in
Table 1), and [H+] is the concentration of the hydrogen ion (10-6 M) in the weakly acidic solution, and AWC is the aerosol water content (1.1 × 10-10) (Ji et al., 2020). The GRBrC is also calculated
using Eq. (5), where dGLprotondt is
replaced by dGLdeprotondt=krate× [SO42-] ×HGL, i.e.,
GRBrC=krate×[SO42-]×HGL×AWC,
where krate is the rate of heterogeneous GL reaction with MA / AM, and
[SO42-] is the concentration of SO42- (5.0 × 10-7 M) in the weakly acidic solution. Considering that the
deprotonation occurs readily by acidic anions (e.g., SO42-), the
GRBrC values obtained in this study may be underestimated. The
GRSOA and GRBrC values under the different atmospheric conditions are
also presented in Table 1. The GRSOA is 1.41 µg m-3 h-1
under the urban condition, which is larger than the GRBrC (0.44 µg m-3 h-1 for MA and 0.45 µg m-3 h-1 for AM).
The total growth rate (the sum of the GRSOA and GRBrC values) is also
consistent with the experimental data (1.44 µg m-3 h-1) (Liggio et al., 2005a). Hence, it is reasonable to expect the
occurrence of a carbenium ion-mediated mechanism in ambient aerosols as a
means of SOA and BrC formation.
Conclusions
This study provides valuable insight into the aqueous chemistry of GL and
also reveals the rate-limiting steps in the absence and presence of
MA / AM. In the absence of MA / AM (Fig. 5), the
cation-mediated oligomerization is characterized by barrierless pathways and
strong electrostatic attraction as follows: (I) protonation, hydration, and
deprotonation of GL to yield DL and TL, (II) protonation and dehydration to
yield CBs, and (III) the formation of dimers from the association reactions
of CBs with DL and TL. Each dimer repeats steps (II) and (III) to propagate
the oligomerization. In the presence of MA / AM, step (III)
starts from nucleophilic addition of CBs with MA / AM rather than
with DL / TL due to the stronger electrostatic attraction between CBs and
MA / AM (Fig. 2). However, the key mechanistic step in the
propagation of N-heterocycles is deprotonation of N-containing cationic
intermediates. Our results of two distinct mechanisms indicate that BrC
formation is more dependent on the aerosol content of inorganic compounds or
inorganic salts rather than particle acidity, compared with the formation of
SOA.
On the other hand, there exist the competing pathways with the initial
protonation pathway for the cationic oligomerization of GL without and with
MA / AM, (i.e., the association reactions of CIs with OH-).
These competing pathways lead to CIs returning the corresponding
reactants and affect the fate of GL. Figure S20 and Table S1 present the
ΔGr and k values of the association reactions for some key CIs
with OH-. Generally, the fate of CIs at each step is dominantly
determined by the initial protonation rather than the reaction with
OH-. For example, the ΔGr value of the association pathway
of CI11 with OH- is -66.8 kcal mol-1, and its k value is 1.47 × 109 M-1 s-1. Compared with the protonation of GL
to CI11, this reaction is thermodynamically and kinetically unfavorable. The
branching ratios show that 70 % of GL proceeds via the protonation pathway to
finally form DL. Because protonation is favorable in the acidic aerosol, the
cation-mediated oligomerization of GL without and with MA / AM can
efficiently proceed to contribute to SOA and BrC under the atmospheric
conditions.
Using our predicted heterogeneous GL reaction rates, the aqueous
heterogeneous lifetime (τ) of GL is estimated to be 3.60 min under
the urban conditions, somewhat shorter than that of experimental data (5.0 min)
(Liggio et al., 2005a). However, the τ values are 89 and 61 min under
rural and remote conditions due to low GL levels, respectively (Liggio et
al., 2005a). It indicates a more important role of the aqueous heterogeneous
reaction of GL in urban air quality relative to other conditions. On the
other hand, the τ values determined here for rural, remote, and urban
conditions are all lower than those of the photolysis (211 min) and
photooxidation of GL (Liggio et al., 2005a). Especially, the
τ value under the urban condition is significantly shorter than the total
gas-phase loss (125 min) (Liggio et al., 2005a). The results
indicate that even under relatively clean conditions, the heterogeneous GL
loss rates are faster than the loss rates due to other gas-phase processes
and are significantly rapid in polluted regions. Given that GL
contributes to 6.9 % of the total radical production at midday (Aiello, 2003), the heterogeneous GL loss to particles implies the
reduction of HOx and demands further study. Our work reveals the
fundamental chemical mechanism of SOA and BrC formation from small α-dicarbonyls and also provides the kinetic and mechanistic data for
atmospheric modeling to assess the budget of SOA and BrC formation on urban,
regional, and global scales.
Data availability
All raw data can be provided by the corresponding authors
upon request.
The supplement related to this article is available online at: https://doi.org/10.5194/acp-22-7259-2022-supplement.
Author contributions
YJ and QS designed the research; YJ, QS, XM, LG, JW,
and YL performed the research; YJ, QS, XM, LG, JW, YL, YG, GL, RZ, and
TA analyzed the data; YJ and QS wrote the paper. YJ, XM, RZ, and TA
reviewed and edited the paper.
Competing interests
The contact author has declared that neither they nor their co-authors have any competing interests.
Disclaimer
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Financial support
This work was financially supported by National
Natural Science Foundation of China (grant nos. 42077189 and 4201001008),
Natural Science Foundation of Guangdong Province, China (grant nos.
2019B151502064), Local Innovative and Research Teams Project of Guangdong
Pearl River Talents Program (grant nos. 2017BT01Z032), and Science and
Technology Key Project of Guangdong Province, China (grant nos.
2019B110206002).
Review statement
This paper was edited by Yafang Cheng and reviewed by Deming Xia and two anonymous referees.
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