Glyoxal and methylglyoxal are vital carbonyl compounds in the
atmosphere and play substantial roles in radical cycling and ozone
formation. The partitioning process of glyoxal and methylglyoxal between the
gas and particle phases via reversible and irreversible pathways could
efficiently contribute to secondary organic aerosol (SOA) formation.
However, the relative importance of two partitioning pathways still remains
elusive, especially in the real atmosphere. In this study, we launched five
field observations in different seasons and simultaneously measured glyoxal
and methylglyoxal in the gas and particle phases. The field-measured
gas–particle partitioning coefficients were 5–7 magnitudes higher than the
theoretical ones, indicating the significant roles of reversible and
irreversible pathways in the partitioning process. The particulate
concentration of dicarbonyls and product distribution via the two pathways
were further investigated using a box model coupled with the corresponding
kinetic mechanisms. We recommended the irreversible reactive uptake
coefficient γ for glyoxal and methylglyoxal in different seasons in
the real atmosphere, and the average value of 8.0×10-3 for
glyoxal and 2.0×10-3 for methylglyoxal best represented the
loss of gaseous dicarbonyls by irreversible gas–particle partitioning
processes. Compared to the reversible pathways, the irreversible pathways
played a dominant role, with a proportion of more than 90 % in the
gas–particle partitioning process in the real atmosphere, and the proportion
was significantly influenced by relative humidity and inorganic components
in aerosols. However, the reversible pathways were also substantial,
especially in winter, with a proportion of more than 10 %. The
partitioning processes of dicarbonyls in reversible and irreversible
pathways jointly contributed to more than 25 % of SOA formation in the
real atmosphere. To our knowledge, this study is the first to systemically
examine both reversible and irreversible pathways in the ambient atmosphere,
strives to narrow the gap between model simulations and field-measured
gas–particle partitioning coefficients, and reveals the importance of
gas–particle processes for dicarbonyls in SOA formation.
Introduction
Glyoxal and methylglyoxal, the simplest α-dicarbonyls, are
recognized as being of great importance in atmospheric chemistry due to
their unique physico-chemical properties. The α-dicarbonyl
functionality leads to higher water solubility and reactivity of dicarbonyls
than expected as the α-dicarbonyl functionality is hydrophilic and
contributes to hydrate formation. The hydrate form of carbonyls is less
volatile and more water-soluble than the unhydrated form
(Lim et al., 2013) owing to the strong effect of the two
hydrogen bonding groups in the hydrated form (Elrod et al., 2021).
Moreover, hydrates can easily participate in continuous radical reactions
with higher reactivity by H abstraction to form higher-molecular-weight
oligomers (Michailoudi et al., 2021). The traditional opinion is that
methylglyoxal is less reactive compared to glyoxal due to its unreactive
methyl substitution, while a very recent study noted that methylglyoxal
could be more reactive under an atmospheric-relevant concentration (Li et
al., 2021). Overall, both of them play crucial roles in radiation balance,
air quality, brown carbon formation, and secondary organic aerosol (SOA) formation (Laskin et al.,
2015; Qiu et al., 2020). Moreover, as major carcinogenic and genotoxic
compounds, dicarbonyls can cause serious damage to human health. They have
relatively limited primary sources, except for biomass burning and biofuel
combustion (Zarzana et al., 2017, 2018), compared to
secondary formation that occurs with photo-oxidation of both biogenic
volatile organic compounds (VOCs), such as isoprene, and anthropogenic VOCs,
such as aromatic hydrocarbons (Lv et al., 2019). Considering the
atmospheric sink, glyoxal and methylglyoxal can be lost in the gas phase by
self-photolysis, oxidation by active radicals (such as OH radicals, NO3
radicals), and wet/dry deposition; however, there is still a missing sink for
the two dicarbonyls (Volkamer et al., 2007), which is the gas–particle
partitioning process that will be fully discussed in this study.
Gas–particle partitioning was recently found to be the most important
removal pathway for both glyoxal and methylglyoxal, especially in regions
like Beijing with high particulate matter (PM) pollution that provides
sufficient aerosol surface area. Although they have relatively high vapour
pressure, glyoxal and methylglyoxal can efficiently partition into the
particle phase due to their α-dicarbonyl functionality. The
surface-adsorbed dicarbonyls could alter the properties of the particle's
surfaces, and the organic surface films could act as a kinetic barrier to
gas–aerosol mass transport and thereby influence particle equilibration and
water/gas uptake (Donaldson and Vaida, 2006). Upon physical
adsorption, besides desorption or reaction at the surface, dicarbonyls could
undergo solvation and incorporation into the bulk liquid, and then they
could go through diffusion and chemical reactions in the bulk phase. The
product may return onto surfaces and into the gas phase or stay in the bulk phase
(Davidovits et al., 2006). Moreover, chemical reactions occurring at the
surface or in the bulk phase could in turn accelerate the physical
adsorption and greatly contribute to the formation and growth of atmospheric
particulate matter. While it is difficult to distinguish the surface
reactions and bulk reactions in field observations, we regard both of them
as particle-phase reactions in this study. The chemical reactions occurring
in the gas–particle partitioning processes can be divided into reversible
pathways, including reversible hydration and self-oligomerization, and
irreversible pathways, which can be driven by oxidative compounds. These
processes can also efficiently explain observed aerosol properties –
including relatively high oxygenation levels, compositions such as organic
acids and oligomers, and higher light absorption – that cannot be explained
by traditional absorptive models of gas–particle partitioning (Pankow,
1994; Pankow and James, 1994; Odum et al., 1996).
Many laboratory and model studies have made a great effort to investigate
the reversible and irreversible pathways of dicarbonyls to further
understand their gas–particle partitioning mechanisms and reveal their
contribution to SOA formation. Fu et al. (2008) found that the
modelled SOA concentrations were largely increased when accounting for
irreversible uptake of dicarbonyls in the GEOS-Chem model. Considering the
surface-controlled reactive uptake of dicarbonyls into the Community Multiscale Air Quality (CMAQ) model, the
aerosol uptake of dicarbonyls accounted for more than 45 % of total SOA in
the eastern US (Ying et al., 2015); similarly, the
contribution of glyoxal and methylglyoxal to SOA formation in China was
14 % to 25 % and 23 % to 28 %, respectively (Hu et al., 2017).
Although reversible and irreversible pathways of dicarbonyls have been
separately investigated in previous studies, solely incorporating just one
pathway into models could lead to a large discrepancy between model results
and observational data, highlighting the importance of comprehensively
considering both reversible and irreversible pathways when quantifying the
gas–particle partitioning process of dicarbonyls (Li et al., 2014; Hu et
al., 2017; Ling et al., 2020).
Despite increasing interest in dicarbonyls and their gas–particle
partitioning processes, the detailed chemical mechanisms of two partitioning
pathways remain poorly understood. First, previous studies have exposed seed
particles to high concentration levels of dicarbonyl vapours, from hundreds of
parts per billion to parts per million levels, or used bulk samples; thus, their applicability to the
real atmosphere requires further validation. Second, prior studies always
used one constant coefficient to present all heterogeneous processes
occurring on the aerosol, which neglects the influencing factors in the real
atmospheric partitioning processes. Further studies have shown that the two
pathways in the gas–particle partitioning process for glyoxal and
methylglyoxal are rather complex, and their relative contribution to the
partitioning process can be influenced by many factors such as relative
humidity (Curry et al., 2018; Shen et al., 2018), particle acidity
(Liggio et al., 2005b; Shi et al., 2020), and particle organic/inorganic
components (Kampf et al., 2013). However, there
persist controversies in the specific partitioning mechanisms of glyoxal and
methylglyoxal, especially conflicting views on their role in SOA formation,
which urgently warrant further investigation.
In this study, five field observations were launched over urban Beijing in
four seasons, and glyoxal and methylglyoxal in the gas and particle phases
were simultaneously measured. Beijing, as the political centre of China, is
the most prosperous city with numerous key environmental issues. Chen et
al. (2021) found that the average concentration of dicarbonyls in Beijing is
lowest among the key regions that have relatively higher PM2.5
concentrations, indicating there is a more efficient partitioning process of
dicarbonyls. Thus, it is more environmentally significant to discuss the
gas–particle partitioning processes in urban Beijing. These processes are
divided into reversible pathways and irreversible pathways, which are based
on the reversibility of the chemical reaction of dicarbonyls occurring in the
condensed phase (Galloway et al., 2009; Ervens and Volkamer, 2010; Kampf
et al., 2013; Ling et al., 2020). On the basis of field-measured data, we
could estimate the product distribution, main influencing factors, and
relative importance of the two gas–particle partitioning pathways for
glyoxal and methylglyoxal in the real atmosphere.
Materials and methodField sampling
We performed field observations on the roof of a six-story teaching building
(26 m above the ground) on the Peking University campus (39.992∘ N, 116.304∘ E) in northwest urban Beijing. The field observations
in this study were launched during four different seasons from 2019 to 2021.
Gaseous carbonyls were collected by adsorption reactions in a
2,4-dinitrophenylhydrazine (DNPH) cartridge (Sep-Pak; Waters Corporation).
The air samples were first passed through an ozone scrubber (Sep-Pak; Waters
Corporation) to eliminate interference by ozone and then trapped in the DNPH
cartridge. To prevent deliquescence of the potassium iodide in the ozone
scrubber, the air samples were mixed with ultrapure nitrogen before being pumped
into the sampling tubing. Air samples were continuously collected every 3 h
in daytime and 9 h in nighttime. The total flow rate was 0.8 Lmin-1.
Particulate carbonyls were collected by a four-channel ambient particle
sampler (TH-16A, Wuhan Tianhong) with a Teflon filter and quartz filters (47 mm,
Whatman). The Teflon filter was used to measure the mass concentration of
collected PM2.5 and water-soluble inorganic compounds (Na+,
NH4+, K+, Mg2+, Ca2+, Cl-, NO3-, and
SO42-). The quartz filters were used for carbonyl analysis. The flow
rate was set at 16.7 Lmin-1, and particle samples were
continuously collected every 12 h daily. Detailed information about field
sampling and analysis was provided in previous studies
(Rao et al., 2016; Qian et al., 2019). To estimate the
positive artefacts by adsorption of gas-phase dicarbonyls onto the filter
(Hart and Pankow, 1994; Mader and Pankow, 2001; Liggio, 2004; Odabasi and
Seyfioglu, 2005), throughout our previous field observations, we placed a
backup quartz filter after the particle sampling quartz filter using an
independent filter holder. The sampling filters would collect the particles
and adsorbed gaseous dicarbonyls, while the backup filter would only collect
gaseous dicarbonyls. The ratio of measured dicarbonyls in the second filter
to that in the first was lower than 20 %, which was equal to the previous
study (Shen et al., 2018). The particulate
concentrations of dicarbonyls used in this study were already corrected by
the possible adsorption artefacts.
The meteorological station was co-located at our sampling site and provided
meteorological parameters. Common trace gases, such as NO/NO2,
SO2, CO, and O3, were detected online by Thermo 42i, 43i, 48i, and
49i analysers, respectively. A TEOM 1400A analyser was applied to measure
the mass concentrations of PM2.5 and PM10, the results of which
were consistent with the PM2.5 weighing results (Fig. S1 in the Supplement). The time
resolution for all of the above data was 1 min. Detailed information about
these five observations is shown in Table S1 in the Supplement.
Sample extraction and analysis
The gaseous carbonyl samples were eluted with acetonitrile (HPLC/GC-MS – high-performance liquid chromatography and gas chromatography–mass spectroscopy – grade) at a flow rate of less than 3 mLmin-1 (higher flow rates can result in reduced recovery), and the particulate carbonyl samples on the quartz filter were
eluted with acidic DNPH solutions in the flask and then were shaken for 3 h
at 4 ∘C with a rotation rate of 180 rpm in an oscillator
(Shanghai Zhicheng ZWY 103D). The derived solutions were placed in darkness
for 12–24 h to ensure complete derivatization, and then they were analysed
by HPLC ultraviolet (HPLC-UV) for
separation and detection. Carbonyls were separated effectively from each
other (Fig. S2 in the Supplement) in this method. They were calibrated using a mixing standard
solution with a concentration range of 0.1–10 µM, and the linearity
was indicated by a correlation of determination (r2) of at least 0.999.
The detailed analysis method was presented in the previous study (Wang
et al., 2009).
The Teflon samples were also extracted by deionized water using an
ultrasonic bath for 30 min at room temperature. The extracted solutions were
analysed by ion chromatography (IC Integrion and Dionex ICS-2000, USA) to
measure the water-soluble inorganic compounds (Na+, NH4+,
K+, Mg2+, Ca2+, Cl-, NO3-, and
SO42-) and low-molecular-weight organic acids (formate, acetate,
and oxalate) in aerosols.
Quality assurance and quality control
As carbonyl compounds are ubiquitous in environmental media, the following
measurements were conducted during the sample collection, pretreatment, and
analysis to ensure the accuracy of results: (1) before sampling, flow
calibration and airtightness tests of sampling devices were conducted, and
flow differences were less than 10 %; (2) after sampling, the gas-phase
samples were resealed by their end cap and plug and stored in the provided
pouch in a cool environment (<4∘C); the particle-phase
samples were stored in the sealed boxes wrapped by pre-baked aluminium foils
in a freezing environment (<-18∘C), and both gas-phase and
particle-phase samples were extracted and analysed within a week; (3) the
extraction processes were conducted in fume hood with glassware, which was
rinsed with acetonitrile at least three times; and (4) a calibration run was
performed each day to determine the response factor of the detector and
recalibration was performed if the relative deviation of the response factor was beyond 5 %.
Blank samples were collected every 3 d and then were stored and
extracted by the same procedure as that for ambient samples. The blank
gas-phase samples were collected by placing blank DNPH cartridges near the
gas inlet for the same duration without artificial pumping, and the blank
particle-phase samples were collected by placing a blank quartz filter on the
PM2.5 inlet with a flow rate of 0 Lmin-1. All data used in this study were calibrated by blanks.
The limit of detection (LOD) of two methods was 50 pptv (parts per trillion by volume) for gaseous
carbonyls and 1 ngm-3 for particulate carbonyls, which is
similar to the previous literature (Shen et al.,
2018). Sample amount to limit of detection ratios were significantly higher
than 1.0 for both gas- and particle-phase samples, indicating that the
sensitivity of the methods was sufficient to analyse the samples.
Additional field samplings were launched to estimate the sampling efficiency
during the collection. Two blank DNPH cartridges were connected in tandem to
assess the sampling efficiency of gas-phase carbonyls. The sampling
efficiency of the cartridges was the ratios of dicarbonyl concentrations in
the first cartridge to the total concentrations in the two cartridges, and
the results were more than 95 % for both glyoxal and methylglyoxal.
Similarly, a backup Teflon filter was placed after the particle sampling
Teflon filter using an independent filter holder to estimate the particle
collection efficiency. Both Teflon filters were weighed by a semi-micro
balance (Sartorius, Germany) to obtain the mass concentration of collected
particles. The mass concentrations of particles collected on the backup
filter were close to zero, indicating that the sampling efficiency of
particle was more than 99 %.
Moreover, recovery tests were also conducted using two methods – adding
standard solution and repeated extraction. We added the standard solution at
three spiked levels of 0.025, 0.25, and 2.5 µg (namely 50 µL of 0.5, 5, and 50 µgmL-1 analytical standards) into blank DNPH cartridges and on a blank
quartz filter to determine the carbonyl lost during the extraction and
derivation. Then the cartridges and filters were extracted in the same
way as the ambient samples. Each group was set with five parallel cartridges/filters. The recoveries were ranged from 88 % to 96 % for the gas-phase method and ranged from 85 % to 96 % for the particle-phase method. Moreover, we also estimated
the recovery efficiencies by repeated extraction, and the recoveries were the
ratios of dicarbonyl concentrations in the first extraction to the total
concentrations in the two extractions. The results ranged from 92.8 % to
99.9 % for the gas-phase method and ranged from 90 % to 99.9 % for the
particle-phase method.
Estimation of effective partitioning coefficient
To estimate the effective partitioning process of gas-phase carbonyls to the
particle phase, we could use Pankow's absorptive partitioning theory for the
gas–organic phase (Eqs. 1 and 2) (Odum et al., 1996) and Henry's law
for the gas–liquid phase (Eq. 3):
1Kpf=CpCg×TSP,2Kpt=RTfom106MWOMζpL0,3effKH=103cpcg×M×ALWC/ρwater.
In Eq. (1), Kpf (m3µg-1) is the field-measured gas–particle partitioning coefficient;
Cp (µgm-3) is the concentration of dicarbonyls
in the particle phase, which is derived from the analysis of extracts,
including monomers and their reversibly formed products (the product
distribution is discussed in Sect. 3.2); Cg (µgm-3) is the concentration of dicarbonyls in the gas phase; and TSP
(µgm-3) is the mass concentration of suspended
particles (mass concentrations of PM2.5 were used in this study). In
Eq. (2), Kpt (m3µg-1) is the theoretical gas–particle partitioning coefficients
determined by Pankow's absorptive model, fom is the absorbing fraction
of total particulate mass, MWOM (gmol-1) is the mean
molecular weight of the organic phase, and ζ is the activity
coefficient of target compounds. In the estimation of
Kpt in this study, fom and ζ
are unity and MWOM=200gmol-1, as used in previous
studies (Healy et al., 2008; Williams et al., 2010; Xie et al., 2014;
Shen et al., 2018), and pL0 (Pa) is the
supercooled vapour pressure of compounds as a pure liquid at temperature T,
which is calculated by the extended aerosol inorganic model (E-AIM;
http://www.aim.env.uea.ac.uk/aim/ddbst/pcalc_main.php, last access: 23 May 2021) (Clegg et al., 1998). The possible uncertainty in Kpt calculation is fully discussed in the Supplement. In Eq. (3), effKH (Matm-1) is the field-derived effective Henry's law coefficient; cp (µgm-3) and cg (atm) are particle- and gas-phase concentrations of carbonyls, respectively; and
ALWC (µgm-3) is the aerosol liquid water content
calculated by the thermodynamic model ISORROPIA-II (forward model, metastable
state), the results of which are comparable to the actual measured contents
confirmed by previous studies (Guo et al., 2015).
The irreversible reactive uptake coefficient γ could efficiently
describe the irreversible pathways of the gas–particle partitioning process
of dicarbonyls driven by OH radicals. We could estimate the reactive uptake
coefficient γ based on the effective Henry's constant via theory
calculation (Hanson et al., 1994; Curry et al., 2018) and then
calculate the effective uptake rate, keff,uptake, following Eqs. (4)–(7):
41γ=1α+v4RTeffKHklDaq×1[cothq-1/q],5v=8RTπMX,
6q=Rpl=RpDaqkl,7keff,uptake=14v×γ×Asurf,
where γ is the dimensionless uptake coefficient, v (ms-1) is the gas-phase thermal velocity of glyoxal/methylglyoxal,
Daq (m2s-1) is the diffusion coefficient in the
liquid phase, α is dimensionless mass accommodation coefficient,
effKH (Matm-1)
is the effective Henry's law constant calculated by field-measured data in
Table 2, R is the universal gas constant, kl (s-1) is the
first-order aqueous loss rate, MX (kgmol-1) is the
average molar mass of gas-phase dicarbonyls, q is the parameter for
measuring in-particle diffusion limitations, Rp (m) is the particle
radius, l (m) is the diffusion reactive length, keff,uptake
(s-1) is the effective uptake rate, and Asurf (m2m-3) is the aerosol surface area density. This
formulation is based on the effective Henry's law constant under high relative humidity (RH)
conditions (RH>40 %). Moreover, the formulation describes the
reactive uptake due to irreversible multiple-phase loss processes in the
presence of OH. The uncertainty in the γ calculation is mainly
attributed to the uncertainty in OH concentration, which was 3×10-12 M on average and varied from 5.5×10-14 to
8×10-12 M (Herrmann et al., 2010).
Result and discussionObservation results and partitioning coefficients calculationDicarbonyls in the gas and particle phases
We launched five field observations in different seasons. Table S1 details
the information about the field observations, including observation periods,
sample volume, and meteorological parameters. In total, we collected 387
gas-phase samples and 130 particle-phase samples in four seasons. In these
samples, carbonyls were simultaneously measured in both gas phase and
particle phases. A total of 10 carbonyls were measured in the gas phase, including
formaldehyde, acetaldehyde, acetone, propionaldehyde, methacrolein,
butyraldehyde, methyl vinyl ketone, benzaldehyde, glyoxal, and
methylglyoxal, and six carbonyls were measured in the particle phase,
including formaldehyde, acetaldehyde, acetone, propionaldehyde, glyoxal, and
methylglyoxal. In this study, we mainly discuss the gas–particle
partitioning processes of glyoxal and methylglyoxal because of their
significant roles in atmospheric chemistry.
Time series of meteorological parameters and gas- and
particle-phase glyoxal and methylglyoxal observed in different seasons:
(a) summer, 20 July–4 August 2019;
(b) winter, 5–19 January 2020;
(c) autumn, 24 October–7 November 2020;
(d) winter, 8–26 January 2021; and
(e) spring, 26 March–6 April 2021.
Statistical data for the α-dicarbonyls in gas and particle
phases in different seasons.
SeasonDates of the measurementsGas phase (ppbv) Particle phase (ngm-3) (yyyy.mm.dd)GlyoxalMethylglyoxalGlyoxalMethylglyoxalSummer2019.07.20–08.040.13±0.070.87±0.5410.18±6.639.50±5.62Spring2021.03.26–04.060.02±0.020.12±0.0815.24±17.506.07±2.79Autumn2020.10.24–11.070.07±0.030.15±0.099.33±4.249.15±3.62Winter2020.01.05–01.19, 2021.01.08–01.260.06±0.050.11±0.0928.77±25.3314.61±10.15
Figure 1 and Table 1 show the temporal characteristics of and seasonal
variation in glyoxal and methylglyoxal, respectively. Gaseous dicarbonyls
showed obvious seasonal variation. Concentrations in summer (0.99±0.59 ppbv) were generally much higher than in other seasons, followed by
autumn and spring, and the concentrations in winter were the lowest. This
seasonal variation could be partly attributed to the higher temperature and
more intensive radiation in summer, which could greatly enhance the
secondary formation of gaseous carbonyls via photochemical reactions. The
diurnal variation in the dicarbonyls during summer support this
interpretation of the data; gas-phase dicarbonyls exhibited obviously
diurnal variations in summer, whereas this variation was irregular in other
seasons (Fig. S3 in the Supplement). The concentration levels of gaseous dicarbonyl in summer
rapidly increased after sunrise, remained relatively high during the daytime
(12:00–14:00 LT), and then decreased at dusk. Although methylglyoxal has a
shorter lifetime compared to glyoxal (glyoxal 2.9 h vs. methylglyoxal 1.6 h)
(Fu et al., 2008), its gas-phase concentration levels were generally
higher than those of glyoxal, consistent with previous studies (Rao et
al., 2016; Mitsuishi et al., 2018; Qian et al., 2019) mainly due to the
relatively larger production from isoprene and acetone for methylglyoxal.
The concentrations of particulate dicarbonyls were an order of magnitude
smaller than the gaseous concentrations using the unit of nanogram per cubic
metre of air (ngm-3 air). The average particulate glyoxal and
methylglyoxal were 19.37 and 11.24 ngm-3, respectively, which were
slightly higher than previously reported values (Zhu et al., 2018; Shen
et al., 2018; Qian et al., 2019; Cui et al., 2021). Dicarbonyls measured in
the particle phase also showed obvious seasonal variation. The particulate
concentrations of the two dicarbonyls in winter (43.38±32.42ngm-3 air) were 2–2.3 times higher than those in other seasons,
suggesting that the dicarbonyls were more favoured in the particle phase in
winter. Moreover, particulate dicarbonyls in different seasons exhibited the
same diurnal variation (Fig. S3). The particulate concentrations of
dicarbonyls in daytime were generally higher than those in nighttime,
especially in winter.
Gas–particle partitioning coefficient
Dicarbonyls could partition between gas and aerosol phases or the liquid
phase, following Pankow's absorptive partitioning theory or Henry's law,
respectively, as listed in Table 2. Both gas–particle partitioning
coefficient (Kpf) and effective Henry's
law coefficient (KHf) were calculated on
the basis of field-measured data and were in the range of
10-4–10-2m3µg-1 and
106–108Matm-1, respectively. The partitioning
coefficient values of the two dicarbonyls exhibited the same seasonal
variation, as winter and spring > autumn > summer. A
higher aerosol concentration accompanied by higher aerosol surface area
concentration and lower relative humidity resulted in a higher partitioning
coefficient in winter and spring, when heavy pollution and sandstorms always
occurred. In the case of temperature variation varying from 265.53 to
310.75 K in different seasons, lower temperature promoted the
gas-to-particle partitioning processes as
Kpf values for the dicarbonyls and
temperature showed negative correlation with significant difference (p<0.001) (Fig. S4 in the Supplement). Moreover, The
Kpf and
KHf values of glyoxal were always higher
than those of methylglyoxal, implying the former was more likely to
partition to the particle phase; this could be attributed to their different
structures. Glyoxal were more soluble and reactive because of the adjacent
electron-poor aldehydic carbons, whereas methylglyoxal was more stable due
to the reduced electron-deficient ketone moiety (Kroll et al.,
2005).
Comparison of the field-measured partitioning coefficient Kf
values for the dicarbonyls and their corresponding theoretical Kt
values in different seasons.
∗ Theoretical gas–particle partitioning coefficients were calculated
on the basis of Eq. (3), and theoretical Henry's law coefficients here
referred to the Henry's law constant in pure water, which were calculated on
the basis of Eqs. (S1) and (S2) in the Supplement (Ip et al., 2009; Sander, 2015).
Both Kpf and
KHf values were relatively close to those found
in previous field-measured studies (Shen et al., 2018; Qian et al., 2019;
Cui et al., 2021). However, compared with the theoretical partitioning
coefficient Kpt calculated by Pankow's
absorptive theory, Kpf values were
approximately 5–7 orders of magnitudes higher than the corresponding
Kpt values. Similarly,
KHf values were approximately 2–5 orders
of magnitudes higher than the theoretical Henry's law coefficient
KHt calculated in pure water, which could
be attributed to salting effects in wet aerosol (Fig. S5 in the Supplement).
Kpf and Kpt
values in this study were close to but slightly higher than the values
published in previous literature (Table S2 in the Supplement), and the discrepancy between
field-measured partitioning coefficients and the theoretical ones is fully
discussed in the Supplement.
To narrow the large discrepancy between field-measured partitioning
coefficients and theoretical ones, we needed to further investigate the
mechanism and product distribution of chemical reactions occurring in the
aerosols during the partitioning processes. The products of the reversible
and irreversible pathways mostly have lower saturated vapour pressure,
thus leading to higher partitioning coefficients compared to monomer
dicarbonyls. Take glyoxal for example; the effective saturation vapour
pressures of the product set in reversible pathways are ∼10-5 Torr in the real atmosphere (Shen et al.,
2018), and the products of the irreversible pathways had much lower vapour
pressure values than those of reversible pathways; for example, the vapour
pressure of oxalic acids and ammonium oxalates are ∼10-5 Torr (Saxena and Hildemann, 1996) and 5.18×10-8 Torr
(Lim et al., 2013), respectively, and those of glyoxal trimer dihydrates
are ∼10-11 Torr at 20 ∘C (Hilal et al., 1994),
indicating the irreversible pathways make larger contributions to the
underestimation of partitioning processes of dicarbonyls. The following
sections further discuss the mechanism and product distribution of
reversible and irreversible pathways to explain the partitioning process of
dicarbonyls.
Reversible pathways
Gas–particle partitioning of dicarbonyls via reversible pathways mainly
consists of hydration and self-oligomerization. Since glyoxal and
methylglyoxal had high water solubility and reactivity, they could easily
dissolve into aerosol liquid water and then form hydrates and oligomers.
Hemiacetal/acetal formation (Loeffler et al., 2006) and aldol
condensation (Haan et al., 2009) are the most
thermodynamically favoured oligomer reactions for glyoxal hydrates and
methylglyoxal hydrates, respectively. The proposed mechanism for the
reversible formation of glyoxal and methylglyoxal in aerosols is shown in
Fig. S6 in the Supplement. By adding excess derivatization agent (like
2,4-dinitrophenylhydrazone in this study), dicarbonyls, as well as their
reversibly formed products, are efficiently transformed into
dicarbonyl-bis-2,4-dinitrophenylhydrazone, which are quantified as monomers
by means of analysis techniques (Kampf et al.,
2013). Moreover, Healy et al. (2008) have confirmed that
derivatization agent was found to efficiently dissolve a trimeric glyoxal
standard and convert the resulting monomers to oxime derivatives, and
oligomers could not be detected in the extracts of filter samples by GC-MS
analysis, also indicating the use of excess derivatization agent could
efficiently convert the hydrates and oligomers back to the monomeric species
by removing dicarbonyl monomers from the extract as soon as they are formed.
Both dissolved dicarbonyl monomers and reversibly formed products are
efficiently transformed into carbonyl-bis-2,4-dinitrophenylhydrazone, which
was quantified by means of HPLC-UV in this study. The concentrations of
dissolved dicarbonyl monomers were estimated using Henry's law coefficients,
which is are used to determine the physical solubility of carbonyls (e.g.
KH=5Matm-1 for glyoxal) (Schweitzer et al.,
1998). The results were negligible compared to the concentrations of
carbonyls in hydrate and oligomer forms. Thus, the concentrations of
particle-phase dicarbonyl in reversible partitioning pathways were close to
the measured concentration of carbonyls by HPLC-UV.
Gas–particle partitioning of dicarbonyls via reversible pathways.
(a) The RH dependence of particulate concentrations of dicarbonyl via
reversible pathways. (b) The product distribution for (i) glyoxal and (ii)
methylglyoxal under different RH conditions. (c) The gas–particle
partitioning coefficients for (i) glyoxal and (ii) methylglyoxal. The black,
red, and blue circles refer to field-measured values, estimated values by
the proposed mechanism, and theoretical values calculated by Pankow's
absorptive model, respectively.
As glyoxal and methylglyoxal have a similar trend under different conditions,
we focused on the total concentration of the two dicarbonyls in the
following discussion. As shown in Fig. 2a, the particulate concentration of
dicarbonyls via a reversible pathway was strongly dependent on RH. It
increased significantly when RH increased from <10 % to 60 %
as dicarbonyls were more favourable to dissolve into hygroscopic aerosols
during their growth (Mitsuishi et al., 2018; Xu et al., 2020). However,
from 60 % to 80 % RH, it exhibited the opposite trend and decreased with
increasing RH as higher water concentrations at elevated RH levels may
dilute the monomer concentration in the particle phase and hinder
oligomerization reactions (Healy et al., 2009), and the product
distribution of the reversible formation could also explain this
phenomenon well. The results exhibited a similar pattern to a previous study, in
which the partitioning of glyoxal and methylglyoxal gradually increased as
RH increased to 40 %, peaked sharply around 50 %, and subsequently
decreased as RH increased towards 80 % (Healy et al., 2009).
Moreover, ionic strength could also influence the reversible partitioning
process as it is closely related to aerosol liquid water and RH conditions.
The presence of inorganic ions could catalyse and participate in
oligomerization reactions via salting effects (Sareen et al., 2010;
Mcneill, 2015). Whereas increasing viscosity of particles with increasing
ionic strength could slow down all particle-phase reactions, the
reversible nucleophilic addition of inorganic ions (e.g. sulfate ions) at
carbonyl carbons deactivates the molecule for further oligomerization
(Kampf et al., 2013).
To roughly estimate the product distribution of the reversible pathway in
the real atmosphere, we simplified reaction mechanisms and calculated the
product distribution on the basis of the kinetic mechanisms listed in Table S3 in the Supplement using a 0-D box model with a steady-state approach. Generally, more
dicarbonyls existed in oligomer forms than in hydrate forms in the
reversible formation. Moreover, their distribution exhibited obvious
seasonal variations. Summer had the highest proportion of hydrate forms,
while winter had the highest proportion of oligomer forms. Detailed
information is shown in Table S4 in the Supplement. The seasonal variation could be attributed
to the RH in different seasons – relatively high in summer and low in
winter. As shown in Fig. 2b, the product distribution of the reversible
formation has a strong dependence on RH. The proportion of dicarbonyls in
hydrate forms increased with increasing RH and could reach more than 75 %
in high RH, while the proportion of dicarbonyls in oligomer forms exhibited
the opposite trend. Hydrates play a dominant role in dilute solutions under
high RH conditions with a relatively high aerosol liquid water
concentration, which might hinder oligomer formation, and large quantities
of oligomers, including dimers and trimers, would form until the aerosol
liquid concentration became greater than 1 M (Liggio et al., 2005b)
when RH decreased. However, the product distribution here was simulated
based on the bulk-phase mechanisms, and higher ionic strength in aerosol
phase would influence reaction equilibria and rate constants (Ervens and
Volkamer, 2010; Mcneill, 2015). The lack of quantitative reaction rate in
aerosol phase could contribute more uncertainties to the simulation,
whereas the RH dependence of product distribution and the order of
magnitude of estimated Kp values were close to those in aerosol-phase,
and the rough simulation could help us to understand the reversible
partitioning pathways of dicarbonyls.
Combined with the vapour pressure of dominant products published in previous
studies (Hastings et al., 2005; Axson et al., 2010), their gas–particle
partitioning coefficient can be roughly estimated and can effectively fit
the field-measured values, as shown in Fig. 2c. The estimated gas–particle
partitioning coefficients in this study are 5 orders of magnitude higher
than the theoretical ones but still 1–2 orders of magnitude lower than the
field-measured coefficients, especially in winter. The difference between
the estimated partitioning coefficients and the field-measured ones suggests
that the current understanding of the equilibrium in reversible formations
cannot reasonably explain the gas–particle partitioning processes of
dicarbonyls. There still exist extra pathways of reversible formation.
Cross-oligomerization of glyoxal and methylglyoxal is non-negligible and
could form similar molecular structure products and contribute to SOA yield
(Schwier et al., 2010). Esterification and amination of diols also
occur in aerosol liquid water but are negligible compared to hydration and
polymerization (Zhao et al., 2006). However, these reactions are
not further discussed here. The hydrates and oligomers mentioned above are
the dominant forms of glyoxal/methylglyoxal in the particle phase, while the
higher molecular oligomers up to nonamer could also exist with a relatively
smaller but still significant fraction at equilibrium. Although the
reactions are thermodynamically reversible, upon evaporation of the aerosol
liquid water, the oligomer formation is faster than the evaporation of
dehydrated dicarbonyls, and the dicarbonyl evaporation is limited
(Liggio et al., 2005b; Loeffler et al., 2006). This results in
relatively stable oligomers and yields SOA. Moreover, other nucleophilic
species may also form oligomers with glyoxal and methylglyoxal and
effectively prevent their evaporation. Besides reversible pathways, higher
carbon number products with lower volatility were mainly formed through
irreversible pathways, such as radical reactions (e.g. OH radicals), which
are fully discussed in the next section.
Irreversible pathwaysIrreversible pathways driven by hydroxyl radicals
Reactive uptake driven by hydroxyl radicals (OH) is the dominant process for
glyoxal and methylglyoxal in their irreversible gas–particle partitioning
pathways. Compared to other irreversible pathways, such as imidazole
formation, glyoxal/methylglyoxal + OH chemistry occurs on much shorter
timescales (Teich et al., 2016). The reaction is the initial step
for most radical-based chemistry of glyoxal/methylglyoxal and has been
proven to be an important source of SOA in both cloud/fog droplets and wet
aerosols (Tan et al., 2012; Lim et al., 2013), producing
low-volatility products such as organic acids, large multifunctional
humic-like substances, and oligomers. The proposed mechanism for the
irreversible pathway of glyoxal and methylglyoxal driven by hydroxyl
radicals in aerosols is shown in Fig. S7 in the Supplement. The OH radicals in aerosol liquid
water are mainly from the direct uptake of gas-phase OH radicals with a
Henry's law constant of 30 Matm-1 (Faust and Allen, 1993), and Fenton reactions are closely related to hydrogen peroxide,
iron ions, and manganese ions in the particle phase. The sources of OH
radicals are one of the major uncertainties in SOA formation
(Ervens et al., 2014).
Summary of calculated uptake coefficient γ and effective
uptake rate coefficient keff,uptake in different seasons for glyoxal
and methylglyoxal.
∗ General is the average value of all the samples observed in the five field observations.
The calculated γ and keff,uptake values for different
seasons are listed in Table 3. The reactive uptake coefficients of glyoxal
were in the range 10-4–10-2, and the average value of
8.0×10-3 in this study was close to the ones representing the
loss of glyoxal by surface uptake during the KORUS-AQ campaign in a very
recent study (Kim et al., 2022). The value slightly
exceeded the one commonly used in model simulations (γ=2.9×10-3), which was based on an experimental study for
(NH4)2SO4 aerosols at 55 % RH (Liggio et al., 2005a),
and also far outweighs the uptake coefficients of glyoxal on clean and
acidic gas-aged mineral particles (γ=10-6–10-4)
(Shen et al., 2016), implying that a real atmospheric aerosol
provides a far more reactive interface for physiochemical processes than
that of mineral particles. Moreover, uptake coefficients for methylglyoxal
were with an average value of 2.0×10-3 and were higher than
those reported in other experimental studies, which varied from 10-6 to
10-3 (Curry et al., 2018; De Haan et al., 2018). On the one
hand, conflicting with previous experimental results (Waxman et al.,
2015), methylglyoxal exhibited an unexpected salting-in effect in the real
atmosphere due to much more complex compositions and higher ionic strength
in ambient particles, which was also reported in other observational studies
(Shen et al., 2018; Cui et al., 2021). The higher Henry's law
coefficient values in Eq. (4) could lead to higher uptake coefficient values.
On the other hand, a recent study also provided direct experimental
evidence to confirm that methylglyoxal is more reactive and has larger
uptake coefficients on seed particles in atmospherically relevant
concentrations (Li et al., 2021). The γ values for both glyoxal
and methylglyoxal exhibited similar seasonal variations, which were lowest
in summer and reached their highest in winter. This seasonal variation could
be attributed to RH variation and particle composition. Moreover, the
effective uptake rate (keff,uptake), which is regarded as a pseudo-first-order reaction rate, is a net result of competition between
reversible and irreversible processes, and it varied from 10-4
to 10-5s-1 in the real atmosphere in this study. As shown in
Fig. 3a, the negative dependence of keff,uptake on RH also
confirmed that the irreversible uptake of dicarbonyls could be inhibited in
high RH conditions. What is more, as we can see in Fig. 3b, the
irreversible uptake increased exponentially with increasing SNA (SNA:
sulfate, nitrate, and ammonia) concentrations mainly because higher SNA
concentrations always occurred in lower RH conditions with lower aerosol
liquid water content (Fig. S8 in the Supplement), and the irreversible uptake was promoted by
combined efforts of RH effects and ion effects, whereas, for a given RH,
the uptake coefficient γ for both glyoxal and methylglyoxal showed a
weak dependence on the ratio of SNA with significant scatter (Fig. S9 in the Supplement).
Gas–particle partitioning of dicarbonyls via irreversible
pathways. (a) The RH dependence of irreversible uptake rate for glyoxal and
methylglyoxal. (b) The SNA dependence of uptake coefficients for (i) glyoxal
and (ii) methylglyoxal; SNA refers to the concentration of sulfate, nitrate,
and ammonia in wet aerosols. (c) The RH dependence of particulate
concentrations of dicarbonyl via irreversible pathways. (d) The
corresponding modelled product distribution in wet aerosols under different
RH conditions.
Moreover, it is worth noting that under extremely low RH (<40 %), the aerosol was not completely deliquescent, and the uptake
coefficients based on Henry's law could not explain the irreversible
pathways. Previous research indicated that the irreversible uptake of
dicarbonyls could still occur under low RH conditions (Liggio et al.,
2005a; De Haan et al., 2018) and that these uptake values were generally
lower due to the inefficient reactive uptake process onto the crystallized
aerosols.
Reactive uptake of dicarbonyl compounds
We could not directly measure the particulate concentration of dicarbonyls
via an irreversible pathway as the dicarbonyls irreversibly reacted with
oxidative radicals on aerosols. To quantitatively evaluate the contribution of
the irreversible pathway of dicarbonyls, we calculated their average
concentration based on Eqs. (S3)–(S7) in the Supplement with the calculated
γ values in this study. The samples estimated here were collected
under high RH conditions (RH>40 %) because of the calculation
limitation of irreversible uptake coefficients. Although the products of
irreversible pathways could not be directly detected in particle phase and
did not directly contribute to the increase in particulate dicarbonyls, the
irreversible pathways could contribute to the decrease in gaseous dicarbonyls
and explain well the overestimation of modelled dicarbonyl mixing ratios,
which are about 3–6 times higher than the observed ones (Volkamer et al.,
2007; Ling et al., 2020).
The total particulate concentration of glyoxal and methylglyoxal via
irreversible pathways varied from several to more than 100 nanograms per
microgram PM2.5 (ngµg-1 PM2.5), and it was strongly dependent
on RH, as shown in Fig. 3c, which generally decreased with increasing RH.
Concentrated inorganic solutions and relatively higher ionic strength in
aerosol water under low RH conditions could jointly contribute to the
hydration of dicarbonyls, the products of which could easily participate
into the following irreversible radical reactions via H abstraction.
To further discuss the product distribution of the reaction of
glyoxal/methylglyoxal with hydroxyl radicals, we used the kinetic mechanisms
of glyoxal/methylglyoxal + OH chemistry proposed by Lim et
al. (2013) on the basis of a 0-D box model with a steady-state approach. The
average OH radical concentration setting in the modelling was 3.2×10-12 M, which is based on the hypothesis of the Henry equilibrium of
OH radicals between the gas and particle phases (Sander, 2015; Shen et
al., 2018). Oxalate can be considered as a tracer for this aqueous
chemistry since it does not have any other significant chemical sources.
Oxalate was detected in the particle-phase samples by ion chromatography.
The modelling results of oxalate concentration agreed well with the measured
values, and their deviations were in the considered range (Fig. S10 in the Supplement).
Meanwhile, we can estimate the distribution of major products in
irreversible glyoxal/methylglyoxal + OH radical chemistry under different RH
conditions, as illustrated in Fig. 3d. Generally, oxalate is the major
product in wet aerosols, contributing ∼60 %, and its
proportion increases with increasing RH. Besides oxalate, oligomers also
play significant roles in glyoxal/methylglyoxal + OH radical chemistry with a contribution of ∼30 %, and their proportion is maximum
under relatively low RH conditions. The RH dependence of the product
distribution could mainly be attributed to the particulate concentration of
glyoxal/methylglyoxal, which significantly affects the OH radical chemistry.
With relatively high carbonyl concentrations (0.1–10 M) in aerosol liquid
water, self-reactions of organic molecules become more favourable, resulting
in new carbon–carbon bonds and high molecular weight oligomers via
radical–radical chemistry (Lim et al., 2013). Moreover, besides
OH radical chemistry, reaction with sulfate and ammonium also contributes to
the oligomer formation and irreversible uptake of gaseous dicarbonyls
(Ortiz-Montalvo et al., 2014; Lin et al., 2015; Lim et al., 2016). The
oligomer proportion could be more than 30 % in concentrated carbonyl
solutions (∼0.1 M) and only account for 1 % in diluted
solutions (∼0.01 M).
Relative importance of two partitioning pathways
Table 4 summarizes the particulate concentration of glyoxal and
methylglyoxal via reversible and irreversible pathways in different seasons.
The average particulate concentrations of glyoxal (0.43 ngµg-1 in the
reversible pathway and 24.26 ngµg-1 in the irreversible pathway) were
generally higher than those of methylglyoxal (0.25 ngµg-1 in the
reversible pathway and 16.53 ngµg-1 in the irreversible pathway), mainly
due to the relatively higher water solubility and reactivity of glyoxal.
Comparing two gas–particle partitioning processes, the irreversible pathway
played extremely dominant roles and generally accounted for 96.7 % and
95.0 % for glyoxal and methylglyoxal, respectively. The proportion of the
irreversible pathway varied from 90 % to 99.9 % and reached its highest
in summer for glyoxal (98.8 %) and in autumn for methylglyoxal (99.2 %),
while it was minimum in winter (92.9 % for glyoxal and 92.8 % for
methylglyoxal). Overall, the irreversible pathway played a dominant role in
the gas–particle partitioning process for both glyoxal and methylglyoxal in
the real atmosphere, while the reversible pathway was also substantial and
non-negligible, especially in winter, with an proportion of ∼10 %. Furthermore, as discussed above, the particulate concentrations of
dicarbonyls and their relative importance were influenced by environmental
factors such as relative humidity and particle composition, which could
jointly influence both the reversible and irreversible pathways of
dicarbonyls. As shown in Fig. 4, the proportion of irreversible pathways
in the gas–particle partitioning process for dicarbonyls increased with
aqueous SNA concentrations and reached maximum when SNA concentrations were
more than 100 M under low RH conditions. Moreover, higher organic
concentrations in aerosol may lead to an OH-limit environment, hindering the
irreversible pathways driven by radicals and influencing the relative
importance of the two pathways (Waxman et al., 2013; Ervens et al.,
2014). But the OH limitations are still inclusive due to the uncertainties
in the sources of OH in aerosol particles (Herrmann et
al., 2010).
Calculated relative importance of reversible and irreversible
pathways in the gas–particle partitioning processes and their contribution
to the particulate matter.
a[X]P,rever is the concentration of particle-phase carbonyl
via reversible pathway (ngµg-1) and its proportion
(%).
b[X]P,irrever is the concentration of particle-phase carbonyl
via irreversible pathways (ngµg-1) and its proportion
(%).
c General is the average value of all the samples observed in the
five field observations.
Correlation between the proportion of the irreversible pathway in the
gas–particle partitioning process for dicarbonyls and aqueous sulfate,
nitrate, and ammonia (SNA) concentrations in ambient aerosols under different
relative humidity conditions.
Comprehensively considering the contribution of both reversible pathways and
irreversible pathways occurring in gas–particle partitioning processes could
benefit the ambient dicarbonyl simulations. Ling et al. (2020) found
that the observation and simulation of the gas-phase concentration level of
dicarbonyls could reach reasonable agreement when the irreversible uptake
and reversible partitioning were incorporated into the model as these
jointly contribute ∼62 % to the sink of dicarbonyls.
Moreover, the contribution of gas–particle partitioning processes of
dicarbonyls to SOA formation was higher as the two partitioning pathways
were jointly considered. In this study, gas–particle partitioning processes
of dicarbonyls accounted for a relatively large proportion of total particle
mass (PM2.5), on average ∼5 % considering both
reversible and irreversible gas–particle partitioning pathways. Since a
large fraction of PM2.5 mass in Beijing consists of SOAs
(∼30 %) (Huang et al., 2014), we could roughly estimate
the contribution of gas–particle partitioning processes of dicarbonyls to
SOA yields (by mass). There were approximately 25 % SOAs formed from
glyoxal and methylglyoxal in this study. However, the particulate
dicarbonyls calculated here only contained simple reversible pathways and
irreversible pathways driven by OH radicals. More complicated chemical
processes, such as NO3 radical chemistry, were not considered, which
still resulted in the underestimation of their contribution to SOA
formation.
Conclusions
We simultaneously measured glyoxal and methylglyoxal concentrations in the
gas and particle phases in different seasons over urban Beijing. Based on
field-measured data, the field-derived gas–particle partitioning
coefficients were calculated and found to be 5–7 magnitudes higher than the
theoretical values. Such a large discrepancy provides field evidence that
the gas–particle partitioning process does not occur by physical absorption
alone but also results from the combined and simultaneous effects of
reversible and irreversible pathways. Hydration and oligomerization occurred
in the reversible pathway, producing compounds with lower volatility in the
condensed phase, and the irreversible pathway could accelerate the uptake of
gaseous dicarbonyls. The two pathways jointly contributed to the
underestimation of gas–particle partitioning of dicarbonyls.
This study systemically considers both reversible and irreversible pathways
in the ambient atmosphere for the first time. Compared to the reversible
pathways, the irreversible pathways play a dominant role in the gas–particle
partitioning process for dicarbonyls, accounting for ∼90 %
of this process. We recommend the irreversible reactive uptake coefficient
for glyoxal and methylglyoxal in different seasons in the real atmosphere.
The values we calculated here are higher than those used in model
simulations to date, especially for methylglyoxal which exhibits an
unexpected salting-in effect under an atmospheric-relevant concentration. We
expect the application of these parameterizations will increase the
calculated contribution of irreversible uptake of dicarbonyls to SOA
formation and narrow the gap between model predictions and field
measurements of ambient dicarbonyl concentrations. Moreover, relative
humidity and inorganic particle compositions are defined as the most
important factor influencing particulate concentration and product
distribution of dicarbonyls via both reversible and irreversible pathways,
implying the significance of considering different RH conditions in
dicarbonyl SOA simulations.
Furthermore, we note that there may be other potential explanations for the
increase in particle mass caused by dicarbonyls and the uncertainty in the
gas–particle partitioning process, including physical adsorption, reversible
pathways, and irreversible pathways. Physical adsorption of dicarbonyls could
be enhanced by water-soluble organics and mineral dust. Other reversible
pathways, such as adducts formed from glyoxal with inorganic species, could
also promote the gas–particle partitioning process. Irreversible pathways
driven by other oxidants, such as NO3 radicals, can also perform a
substantial role. Shen et al. (2016) found that glyoxal could
irreversibly produce formic acid, glycolic acid, and oligomers on particles
without illumination or extra oxidants. Besides gas–particle partitioning,
particulate dicarbonyls formed via the heterogeneous reaction of VOCs could
contribute to the uncertainty in partitioning research.
Dong et al. (2021) recently revealed that aqueous
photo-oxidation of toluene could yield glyoxal and methylglyoxal via a
ring-cleavage process. Overall, the real gas–particle partitioning process
of glyoxal and methylglyoxal is more complicated, and their contribution to
SOA formation is still indistinct; thus, more laboratory experiments and
field measurements are urgently needed to improve our understanding of the
gas–particle partitioning process for glyoxal and methylglyoxal.
Data availability
The data are accessible by contacting the corresponding author (zmchen@pku.edu.cn).
The supplement related to this article is available online at: https://doi.org/10.5194/acp-22-6971-2022-supplement.
Author contributions
In the framework of the five field measurements in different seasons, ZC
and JH designed the study, and JH performed all carbonyl measurements used
in this study, analysed the data, and wrote the paper. ZC helped interpret
the results, guided the writing, and modified the manuscript. XQ and PD
contributed to the methods of sampling and analysing gas- and particle-phase
carbonyls. All authors discussed the results and contributed to the final
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.
Acknowledgements
We thank Shiyi Chen at Peking University for
providing the data for the meteorological parameters, trace gases, and
PM2.5 mass concentrations.
Financial support
This research has been supported by the National Natural Science Foundation of China (grant no. 41975163).
Review statement
This paper was edited by Ivan Kourtchev and reviewed by four anonymous referees.
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