Light-absorbing organic aerosol, or brown carbon (BrC), has significant but
poorly constrained effects on climate; for example, oxidation in the
atmosphere may alter its optical properties, leading to absorption
enhancement or bleaching. Here, we investigate for the first time the effects
of heterogeneous OH oxidation on the optical properties of a laboratory
surrogate of aqueous, secondary BrC in a series of photo-oxidation chamber
experiments. The BrC surrogate was generated from aqueous resorcinol, or
1,3-dihydroxybenzene, and
Among all atmospheric constituents, aerosols have the most uncertain radiative forcing, partly due to an incomplete understanding of carbonaceous aerosols (Chung et al., 2012). In particular, the climate effects of light-absorbing organic aerosol, or brown carbon (BrC) (Bond, 2001; Kirchstetter et al., 2004), are poorly constrained, compared to those of elemental black carbon (BC) (Ramanathan and Carmichael, 2008).
One source of this uncertainty is the wide range of sources of BrC
(Laskin et al., 2015). Low-temperature biomass burning results
in the formation of primary BrC
(Bahadur
et al., 2012; Chen and Bond, 2010; Lewis et al., 2008; Radney et al., 2017).
Many classes of compounds, including nitroaromatics, polyphenols, and
substituted polycyclic aromatic hydrocarbons
(Lin et al., 2016), have been identified in
primary BrC, but their concentrations and absorptivities vary significantly.
At a site strongly influenced by biomass burning in Germany, the
contribution of nitroaromatics to the absorption of 370
BrC may also form from secondary processes. For example, the ozonolysis of catechol and guaiacol, abundant emissions from biomass burning (Schauer et al., 2001), in the gas phase leads to the formation of lower-volatility products that partition into the condensed phase to form secondary organic aerosol (SOA) that is light-absorbing (Ofner et al., 2011). Light-absorbing SOA has also been observed to form from the gas-phase photo-oxidation of other precursors, e.g. naphthalene (Lambe et al., 2013; Lee et al., 2014). In a recent field campaign, reactions in the condensed phase of cloud droplets or aqueous aerosols have also been shown to result in the formation of light-absorbing SOA (Gilardoni et al., 2016). In the laboratory, the heterogeneous oxidation of catechol has been studied at air–solid (Pillar et al., 2015) and air–water (Pillar et al., 2014; Pillar and Guzman, 2017) interfaces; at the air–water interface, functionalization, which leads to polyphenols and hydroxylated quinones that are expected to be highly absorptive, followed by fragmentation was observed (Pillar and Guzman, 2017). Heterogeneous reactions are sensitive to particle diameter, so they may in part be responsible for the observation of higher concentrations of BrC in particles in the accumulation mode compared to those in the coarse mode (Liu et al., 2013). Reactions of nitrogen-containing species (e.g. ammonium sulfate and methylamine) with aldehydes (e.g. glyoxal and methylglyoxal) have also been shown to result in BrC (De Haan et al., 2009, 2011; Lee et al., 2013; Yu et al., 2011). Furthermore, the formation of intra- or inter-molecular charge transfer complexes, similar to what has been proposed to occur in natural waters (Del Vecchio and Blough, 2004), may enhance the absorption of BrC (Phillips and Smith, 2014, 2015; Rincón et al., 2009).
In addition to the wide range of classes of BrC, the evolution of BrC upon
atmospheric aging contributes to the uncertainty in its climate forcing.
During the lifetime of the particles or cloud droplets, BrC constituents are
photolyzed or react with oxidants; the resulting chemical evolution leads to
the evolution of the optical properties of the aerosol. Field measurements have
demonstrated that the absorption by BrC may decay drastically during
transport, although a small fraction may be recalcitrant
(Forrister et al., 2015). On the other
hand, in a recent field campaign, absorption at 365
In the laboratory, the evolution of absorption induced by photolysis and
oxidation has been observed for a variety of BrC surrogates in the solution
phase, including extracts of biomass burning BrC (Lin et al., 2016; Wong et
al., 2017), extracts of SOA derived from naphthalene under
high-
Here, we consider the fate of secondary BrC constituents from cloud
processing upon droplet evaporation and subsequent exposure to OH radicals.
The first question to raise is whether the optical properties of secondary
BrC aerosol evolve at atmospherically relevant OH exposures; another broad
question is whether this evolution is dependent on relative humidity (RH).
To address these questions, we investigate for the first time the
heterogeneous OH oxidation of a secondary BrC surrogate aerosol, generated
from the product mixture of aqueous OH oxidation of resorcinol, or
1,3-dihydroxybenzene. Experiments were conducted in a 1
Experimental setup during aqueous and heterogeneous OH oxidation. CPC represents the condensational particle counter, DMA the differential mobility analyzer, and PASS the photoacoustic spectrometer.
The BrC surrogate was prepared by aqueous OH oxidation of resorcinol. An
aqueous solution of 10
The heterogeneous OH oxidation of a yellow azo dye, Cibacron Brilliant Yellow
3G-P (CBY; Sigma), was also investigated. The dye has a high molecular mass,
831.02
The experimental setup during heterogeneous OH oxidation is illustrated in
Fig. 1. The aqueous solutions of BrC surrogate were aerosolized using a
constant output atomizer (TSI, 3076). To obtain appreciable aerosol
absorption, it was necessary to atomize for roughly 3
The aerosol was then injected into the Mobile Oxidative Chamber for Aging (MOCA), which has been described
in the past (Wong et al., 2015). Briefly, the chamber consists of a
1
During and after particle injection, aerosol size distributions were
monitored using a scanning mobility particle sizer (SMPS), consisting of a
differential mobility analyzer (DMA; TSI, 3081) and a condensational
particle counter (CPC; TSI, 3772). The sample flow rate of the CPC is 1
Aerosol absorption and scattering were measured using a Photoacoustic
Soot Spectrometer (PASS, Droplet Measurement Technologies), equipped with 405 and 781
Following aerosol injection, the size distribution and optical properties of
the BrC aerosol were monitored for about 1
One potential source of bias in photoacoustic measurements is evaporation of water from particles (Baker, 1976; Raspet et al., 2003; Langridge et al., 2013); through evaporation, some of the energy of the absorbed photons may be lost and may not contribute to the detected pressure wave. On this basis, the higher RH was selected to be lower than the maximum operating RH of the PASS (70 %). To verify that evaporation of water did not influence the measurements of absorption coefficients, we alternately sampled with and without a diffusion dryer downstream of the chamber in one experiment.
In a separate set of high-RH experiments,
Representative
To verify that changes in SSA during heterogeneous OH oxidation were induced
by chemical changes to the aerosol rather than small changes in the size
distributions, we compared the observed trends to modelled trends, based on
the observed size distributions and an assumed, constant complex refractive
index of
Schematic of the multi-layer kinetics model of heterogeneous OH oxidation.
To better understand the effects of RH on the evolution of the optical
properties of the BrC surrogate, we constructed a multi-layer kinetics model
based on the Pöschl–Rudich–Ammann (PRA) framework
(Ammann
and Pöschl, 2007; Pöschl et al., 2007). The model particles consist
of a surface layer, two near-surface bulk layers, and the remaining bulk
phase, as illustrated in Fig. 3. The near-surface bulk layers are included
to account for concentration gradients in the bulk phase
(Shiraiwa et al., 2010); each
is 2
The rate constant for diffusion between bulk layers,
The above considerations allow us to calculate concentrations of A, B, and C
in the surface and bulk layers. However, to compare the modelled and
experimental results, we must calculate the relative absorption of the model
particles. We derive the relative absorption of the particles solely from the
concentrations and molar absorptivities, as in a bulk solution; i.e. the
absorption that would be measured upon particle-into-liquid sampling. This
modelled relative absorption is compared to the experimental absorption
coefficient normalized to that which would be expected if the initial complex
refractive index (
Processes other than directly OH-initiated oxidation also occur. For
example, radical products of the initial OH reaction likely form
To produce a laboratory surrogate of secondary BrC, we generated a mixture
of light-absorbing products by the aqueous photo-oxidation of resorcinol and
More generally, light-absorbing products of aqueous photo-oxidation have
been observed for a wide range of phenolic species, including those with
methyl, methoxy, and carbonyl substituents
(Chang
and Thompson, 2010; Gelencsér et al., 2003; Smith et al., 2016). In
detailed mechanistic studies, products of both hydroxylation and
oligomerization have been identified
(Hoffer
et al., 2004; Li et al., 2014; Sun et al., 2010; Yu et al., 2014, 2016).
Oligomers form by C–C or C–O radical coupling
(Kobayashi and Higashimura, 2003). C–C
coupling might be expected to lead to greater absorption enhancement, since
the resulting biphenyls and larger oligomers may have some degree of
delocalization across the rings (Zhang et
al., 2010). Indeed, based on density functional theory calculations,
Magalhães et al. (2017) have shown that the absorptivity of bi- and
terphenyls is greater in water than in the gas phase, because the planarity
and delocalization increase. Interestingly, 5 of the 10 most abundant
products of the aqueous photo-oxidation of 0.1
Time series of
This observed size dependence of SSA can be compared to Mie theory
calculations, based on the measured size distributions and an assumed
complex refractive index. The complex refractive index should not change
without photolysis or heterogeneous OH oxidation, so we manually scanned
values of
In one experiment, BrC particles were sampled from the chamber, alternately,
with and without a diffusion dryer upstream of the DMA. The conditioning was alternated every 15
Having constrained the size dependence of the optical properties, based on
the closure between measured and calculated SSA values, we now consider the
effects of photolysis on the optical properties of the BrC surrogate. The
emission of the UV-B black lights in the chamber, with a peak at
310
Time series of
Results from a photolysis experiment at 60 % RH are illustrated in Fig. 5.
In each experiment, the value of
In the past, photolysis has been
observed to lead to the absorption enhancement of other BrC surrogates (Saleh
et al., 2013; Wong et al., 2017; Zhao et al., 2015; Zhong and Jang, 2014).
For example, the absorbance at 400
Time series of relative SSA during
In past studies, the period of absorption enhancement due to photolysis has
been shown to be followed by bleaching
(Wong
et al., 2017; Zhao et al., 2015; Zhong and Jang, 2014). In the present
experiments, the irradiation time is limited by the particle losses; after
about 3
Time series of
The evolution of the optical properties of the BrC surrogate during a
photo-oxidation experiment is affected by deposition, photolysis, and
heterogeneous OH oxidation. Results of a representative photo-oxidation
experiment at 60 % RH are shown in Fig. 7; replicate experiments were
performed at each RH. The initial geometric mean surface diameter was
slightly higher than in the photolysis experiment described above (about
196
As described above, the evaporation of water from particles can result in a negative bias in photoacoustic measurements (Langridge et al., 2013). If heterogeneous OH oxidation significantly increased the hygroscopicity of the particles, the water content of the aerosol would increase during the experiment. The resulting increase in the magnitude of the bias could contribute to apparent bleaching. In addition to this instrumental artifact, progressively more water uptake could lead to genuinely greater scattering coefficients, which would also result in apparent bleaching. We investigated these potential effects by alternately sampling with and without a diffusion dryer downstream of the chamber at 60 % RH. As shown in Fig. S7, the absorption coefficient does not depend on the conditioning. The scattering coefficient is about 10 % lower for the dried particles (see Fig. S8), but this difference is roughly steady during photo-oxidation. In other words, the particles do not become significantly more hygroscopic, and the changes in absorption and scattering coefficients are indeed due to the chemical evolution of the particles.
To ensure that the sequential absorption enhancement and bleaching described above are distinctive features of the BrC surrogate, we also investigated the heterogeneous OH oxidation of a yellow dye aerosol at 60 % RH. In this case, we observed uniform bleaching, as shown in Fig. S9, consistent with bulk aqueous studies of similar azo dyes (Georgiou et al., 2002). There was no decrease in the geometric mean surface diameter, so there was little or no volatilization. It is likely that OH attacks the azo nitrogen–nitrogen bond (Hisaindee et al., 2013). The product fragments would be large enough to remain in the particle phase; for example, the smaller fragment would still contain one sulfonate group and two aromatic rings.
Unlike photolysis, heterogeneous OH oxidation is strongly dependent on RH,
as shown in Fig. 6a. In contrast to the rapid absorption enhancement and
bleaching observed at 60 % RH, the period of absorption enhancement is
prolonged at 15 % RH, on a timescale similar to that of the photolysis
experiments. The peak absorption enhancement at 15 % RH results in a lower
relative SSA than at 60 % RH. To better understand the effects of RH on
heterogeneous OH oxidation, we compare experimental results with those of
the multi-layer kinetics model described above. In all, the model has five
adjustable parameters; these are two uptake coefficients,
Time series of
At 60 % RH, the experimental features can be reproduced (see Fig. 8) by
setting
We find that the observed trend in relative absorption at 60 % RH – in
particular, the significant bleaching – can be reproduced only if the aging
particles are taken as well-mixed, so the diffusion of A from the bulk to the
surface is not restricted. As shown
in Fig. 8b, the concentrations of A, B, and C are the same in all four
layers. At 60 % RH, secondary organic material derived from the gas-phase
photo-oxidation of toluene has a diffusion coefficient on the order of
10
Finally, the experimental features are reproduced by setting the molar
absorptivities of B and C (
At 15 % RH, the experimental features are reproduced (see Fig. 8) by
setting
In general, we emphasize that the set of molecular properties presented above, which best fit the experimental data, were selected after multiple trials with different input parameter values and different model scenarios as well. Although the mechanism and final parameter set fit the data remarkably well, we acknowledge that the complex nature of the inherent chemistry, with changes in optical properties occurring alongside concentration, suggests that this parameter set should only be viewed semi-quantitatively; i.e. the solution is most useful for substantiating the mechanism leading to the changes in absorption.
In this study, we have demonstrated for the first time that secondary BrC
aerosol derived from a phenolic precursor is susceptible to further
photochemical aging after cloud processing and droplet evaporation.
Specifically, at 60 % RH, OH exposure induced a rapid absorption
enhancement followed by the relatively slow bleaching of the surrogate BrC
aerosol; at 15 % RH, OH exposure induced only slow absorption
enhancement. Moreover, we have constructed a multi-layer kinetics model that
captures the general features of the evolution of the optical properties of
the particles. The candidate parameters suggest that the oxidation is very
efficient, possibly even involving free-radical chain reactions, and the
surrogate BrC aerosol is very
viscous at 15 % RH. Free-radical chain reactions may be more important in the
atmosphere, where a lower concentration of OH results in a lower
concentration of
Using our measurements and those of others, we can now speculate on the
photochemical behaviour of BrC in the atmosphere. Recently, Sumlin et al.
(2017) observed bleaching due to the heterogeneous OH oxidation of primary
BrC derived from the smoldering of Alaskan peat, which lost almost 50 % of its absorption at
375 and 405
The aerosol studied here is a reasonable proxy for the secondary BrC that
may form in the atmosphere upon evaporation of cloud droplets. Biomass
burning emits BC, primary BrC and non-absorbing organic compounds, in both
the gas and particle phases. These organic compounds include phenolic
species, derived from the decomposition of lignin during combustion
(Simoneit, 2002). Resorcinol is a
representative phenolic emission of biomass burning
(Simoneit,
2002; Veres et al., 2010; Wang et al., 2009); for example, Schauer et al. (2001) observed about 50
All data presented in the figures in the main text and the
supplement are available at the University of Toronto Dataverse
(
The supplement related to this article is available online at:
EGS and JPDA designed the experiments. EGS performed the experiments and wrote the manuscript with contributions from JPDA.
The authors declare that they have no conflict of interest.
This research was funded by the Natural Sciences and Engineering Research Council of Canada (NSERC). Elijah G. Schnitzler gratefully acknowledges a postdoctoral fellowship from NSERC. Edited by: Rainer Volkamer Reviewed by: two anonymous referees