Introduction
Black carbon (BC) is the most efficient light-absorbing component of
atmospheric aerosols (Jacobson, 2001; Moffet and Prather, 2009; Cappa et al.,
2012) and plays an important role in the global climate system (Ramanathan
and Carmichael, 2008; Bond et al., 2013). However, accurately constraining
the direct radiative forcing (DRF) of BC is a challenge owing to the
discrepancy between observed and modelled estimates of BC light absorption
(Gustafsson and Ramanathan, 2016). For example, a recent study has shown that
the improved model-estimated DRF of BC (+0.21 Wm-1) by including BC
absorption enhancement and separately treating the aging and physical properties
of fossil fuel and biomass burning BC was about 3 times lower than the values
reported in the Intergovernmental Panel on Climate Change (IPCC) 5th
assessment report (+0.6 Wm-2), which suggested an overestimation of
BC lifetime and an incorrect absorption attribution of light-absorbing
organic compounds (brown carbon, BrC) (X. Wang et al., 2014).
BC particles are produced from incomplete combustion of fossil fuels,
biofuels, and residual biomass (Novakov et al., 2003; Bond et al., 2004,
2007). Freshly emitted BC is mainly externally mixed and occurs in
fractal-like agglomerates. Atmospheric BC particles undergo several aging
processes, including coagulation with other particles, condensation of
vapours onto surfaces, and chemical oxidation (Slowik et al., 2004; Zhang et
al., 2008; Petzold et al., 2013). Individual BC particles become coated
(i.e., internally mixed) with sulfate, ammonium, organics, nitrate, and water
(Bond and Bergstrom, 2006; Cheng et al., 2006; Schwarz et al., 2008; Ervens
et al., 2010; Zaveri et al., 2010). Aging processes dramatically change the
morphology, hygroscopicity, and mixing state of BC-containing particles,
thereby altering their optical properties and the magnitude of their
contribution to climate forcing (Jacobson, 2001; Bond and Bergstrom, 2006;
Schwarz et al., 2008; Zhang et al., 2008).
The light absorption enhancement of BC particles caused by coating is
quantified by Eabs, the ratio of the absorption coefficients of
coated and bare BC. Eabs introduces a large uncertainty in the
DRF of BC, which is the second most important contributor to global warming
(Jacobson, 2001; S. Liu et al., 2015). Current models simply adopt a constant
enhancement value (∼ 1.5 or 2) for the calculation of DRF of BC (Cappa
et al., 2012; Bond et al., 2013; X. Wang et al., 2014). In contrast, reported
Eabs values vary widely (Peng et al., 2016; Liu et al., 2017).
Field measurements along the California coast and ground site in Sacramento
(California) (Cappa et al., 2012), Shenzhen (South China) (Lan et al., 2013),
the Nagoya urban area (Japan) (Nakayama et al., 2014), and urban Los Angeles
(USA) (Krasowsky et al., 2016) found negligible absorption enhancement
(Eabs < 1.1) and weak dependence on the extent of
photochemical aging (estimated from the value of
-log([NOx] / [NOy]), where
NOx = NO + NO2 and
NOy includes the sum of NOx and its
oxidation products; Deolal et al., 2012). Biomass burning measurements showed
an absorption enhancement of 1.7 at λ = 532 nm (Lack et al.,
2012). Recent observations in Chinese cities (Peng et al., 2016; X. Cui et
al., 2016; Xu et al., 2016; Chen et al., 2017; Cheng et al., 2017; Q. Wang et
al., 2017) provide evidence for a higher Eabs in polluted
conditions, with values ranging from 2 to 3. The mechanisms responsible for
enhancing BC absorption remain elusive due to the complexity of the aging
process and its varied sources. More studies in receptor locations with a
longer BC aging time are required to better constrain Eabs
(Gustafsson and Ramanathan, 2016; Boucher et al., 2016).
In this work, the influence of photochemical aging on BC mixing state,
Eabs, and aerosol single-scattering albedo (SSA, ω,
defined as the ratio of scattering to extinction coefficient) at a rural site
in east China during the summer was studied by using a volatility tandem
differential mobility analyser (VTDMA) and a thermal denuder (TD) approach
combined with a cavity-enhanced albedometer operating at
λ = 532 nm. In summer, Ox
(Ox = O3 + NO2) exhibits good
correlation with secondary pollutants (Zhou et al., 2014; Cevik et al., 2016;
Ji et al., 2016). The concentration of Ox was used as a
proxy for atmospheric photochemical aging (Hallquist et al., 2016; Q. Wang et
al., 2017). We find that photochemical aging results in the growth of
particle coating and higher fractions of internally mixed BC particles. The
modelling and parameterization of Eabs and SSA capture the
variability of BC coating amount and the particle absorption and provide a
plausible new method to better constrain the contribution of BC to the DRF.
Experimental
The field site
Measurements were performed at Shouxian National Climatological Observatory
(32∘25′47.8′′ N, 116∘47′38.4′′ E) in Anhui Province
from 16 June to 23 July 2016. Shouxian County is located in China's
north–south climate transition zone and is affected by the east Asian
monsoon. The new observatory is situated about 15 km south of the previous,
historically important observation site (Fan et al., 2010; Li et al., 2011;
Deng et al., 2012); it is a rural background site surrounded by basic
farmland protection areas and has no significant industrial pollution sources
or tall buildings nearby.
Instruments were installed in a temperature-controlled room with two sample
inlets about 1 m above the roof (Fig. S1 in the Supplement). Each inlet
consisted of one PM2.5 cyclone (BGISCC2.654) with a 50 % cut point
at 2.5 µm, and was first dried below 40 % relative
humidity (RH) using a diffusion drier. The sampling rates at both inlets were
controlled with mass flow controllers (MFCs) and set at 10 L min-1. One
of the inlets was used for the volatility measurements; the other inlet
stream was used for the optical measurements. Trace gas pollutants such as
CO, NOx, SO2, and O3 were, respectively, measured by Thermo 48i, 42i, 43i, and 49i analyser.
Volatility measurement
Size-resolved mixing state of BC was measured with a VTDMA developed
in-house. The VTDMA was structurally
similar to other systems described in the literature (Cheng et al., 2009,
2012; Wehner et al., 2009; Cheung et al., 2016) and comprised the following:
an electrostatic classifier (DMA, differential mobility analyser, TSI 3080) for the initial selection of
monodisperse particles;
a custom-built stainless steel heating tube (inner
diameter of 0.77 cm, 80 cm long, and heated to 300 ± 5 ∘C)
for removing nonrefractory particulate matter; and
a
scanning mobility particle sizer (SMPS, TSI 3936) comprising a DMA (TSI 3080)
and a condensation particle counter (CPC, TSI 3776) for measuring the size
distribution of the heated sample in the range of 15 to 661 nm.
Diffusion losses and the effect of multicharged particles were corrected by
the instrument software. The residence time of the sample in the heating tube
was about 1.2 s and is comparable with other VTDMA systems (0.3–1 s)
(Brooks et al., 2002; Philippin et al., 2004; Villani et al., 2007).
Optical measurement
The optical properties of dry PM2.5 particles were measured with a
cavity-enhanced albedometer operating at λ = 532 nm (Zhao et
al., 2014; Xu et al., 2016). The albedometer combined broadband cavity-enhanced spectroscopy (BBCES) with an integrating sphere (IS) for direct, in
situ, and simultaneous measurement of extinction (bext) and scattering
(bscat) coefficients, thus allowing the calculation of the absorption
(babs) coefficient and SSA. Compared with our previously reported
470 nm system (Zhao et al., 2014; Xu et al., 2016), the new 532 nm
albedometer was modified by inserting a quartz tube within the IS to prevent
the degradation of the IS reflectivity and to reduce the sample's residence
time (Dial et al., 2010; Onasch et al., 2015). The sample volume of the
albedometer was about 0.3 L and the flow rate was 1.5 L min-1 at
atmospheric pressure.
The details of the evaluation of the instrument have been described in our
previously published paper (Zhao et al., 2014; Xu et al., 2016; Fang et al.,
2017). Detection limits of each parameter were determined by using an Allan
variance analysis. With a 30 s integration time (an average of 300
individual spectra, each with 100 ms exposure time), the detection limits
under ambient aerosol loading condition for the scattering and extinction
measurements were better than 0.15 and 0.12 Mm-1, respectively. The
accuracy of the instrument was evaluated with laboratory-generated, NIST
traceable monodispersed polystyrene latex (PSL) spheres. During field
observations, the optical system was calibrated with N2, CO2, and
PSL every 2 weeks.
The total uncertainties (summed in quadrature of each error source) in
extinction, scattering, absorption coefficients, and SSA measurements were
estimated to be less than 4 %, 3 %, 5 %, and 4 %,
respectively. Uncertainty in extinction mainly arose from the uncertainties
in mirror reflectivity (1 – R, ∼ 1 %), the ratio of cavity
length to the cell length containing the air sample when the cavity mirrors
were purged (RL, ∼ 3 %), and particle losses in the
cavity (∼ 2 %). Uncertainty in the scattering measurement was
mainly caused by uncertainties in the experimentally determined scattering
calibration coefficient (K', ∼ 2 %), particle losses in the
cavity (∼ 2 %), and the truncated fraction of total scattering.
(Since most particles in the observation were smaller than 1 µm,
the uncertainty associated with truncation angle was < 1 %, as
discussed in Sect. S2 in the Supplement.) Since measurements of the
extinction and scattering coefficients were of the identical sample, particle
losses do not affect the SSA measurement (Zhao et al., 2014; Xu et al.,
2016).
The sampled ambient air was divided into two channels: the first channel was
directly pumped into the albedometer to measure the ambient absorption
coefficient (babs,ambient); another channel was installed with a
TD (Dekati Ltd., Finland) operating at 300 ∘C to evaporate
semi-volatile particulate components for measuring the absorption coefficient
of refractory BC (rBC; babs,TD) (Olson et al., 2015). These two
channels were switched automatically every 5 min with an electric ball
valve. The flow rate of the TD was 10 L min-1. Particle losses inside
the TD are discussed in detail in Sect. S3 in the Supplement and are
generally caused by diffusional and thermophoretic processes (Wehner et al.,
2002; Fierz et al., 2007). The optical loss of the TD of ambient aerosol was
estimated to be ∼ 32 %. The measured babs,TD was
corrected with the particle losses for further calculation of the absorption
enhancement
(Eabs = babs,ambient / babs,TD).
The total uncertainty in Eabs measurement was about 9 %
(mainly contributed by uncertainties in the measurement of
babs,ambient (5 %), babs,TD (5 %), and
particle losses inside TD (6 %)).
Time series of CO, NO2, O3, and
Ox (O3 + NO2) concentrations, as
well as the concentrations of PM2.5 and PM1.0 during the
measurement period.
Results and discussion
The concentrations of PM1.0, PM2.5, and trace pollutants (CO,
NO2 and O3) measured at the station over the measurement
period are shown in Fig. 1. For assessing the effect of photochemical
oxidation on the aerosol optical properties, the time series of the
Ox concentration is also shown in the figure. Both
Ox, PM1 and PM2.5 concentrations have clear
diurnal cycles. The corresponding meteorological conditions are shown in the
Supplement Fig. S8. The average ambient temperature (T), relative humidity (RH),
and wind direction (WD) were 26.0 ± 3.3 ∘C,
90 ± 11 %, and 2.0 ± 1.1 m s-1, respectively. The prevailing winds were
southerly. Generally, the low wind speed favoured the accumulation of pollutants,
and the RH was also quite high. The average concentrations of PM2.5 and
PM1.0 were 28 ± 14 and 25 ± 13 µg m-3,
respectively. The 48 h backward trajectories ending at 500 m above ground
level at the Shouxian site are shown in Fig. S9 in the Supplement. The trajectories were
aggregated into five groups after taking into account the wind direction, speed,
and the geometric distance between individual trajectories (S. Wang et al.,
2017). The air masses in clusters 1, 3, 4, and 5 originated from long-range
transport with high speeds for over 40 h. Air masses in cluster 2 originated
from the vicinity of Anhui province and moved slowly. The long aging time and
residence time of the air masses led to well-aged particles before arriving
at the observation site. All air masses were at relatively low altitudes
(< 1500 m) and remained within the boundary layer over this 2-day
period.
Examples of the particle number size distributions of ambient
aerosol (black points), as well as the VTDMA measured room temperature bypass
sample (∼ 25 ∘C, blue open circles), and the sample passed
through a custom-built heating tube at 300 ∘C
(Dp,300∘C, red points) for the initial selected
diameter of 150 nm (Dp). The corresponding Gaussian fit of the
size distributions are shown as black, blue, and red lines. The
size distribution obtained after heating was divided into three size ranges
according to previously reported empirical cutting diameters. (1) Particles
with diameters Dp,300∘C / Dp < 45 % were
denoted as “high-volatility” (HV) and were not considered to be BC.
(2) Particles with diameters 45 % < Dp,300∘C / Dp < 82 %
were considered to be internally mixed BC particles (a non-volatile core coated
with a volatile shell) and were denoted as “medium-volatility” (MV). (3)
Particles with diameters 82 % < Dp,300∘C / Dp < 120 %
were denoted as “low-volatility” (LV) and were considered to be externally
mixed BC.
Size-resolved mixing state of BC
Following the approach of Philippin et al. (2004), Cheng et al. (2009), and
Wehner et al. (2009), most compounds were assumed to be volatilized at
300 ∘C and the residual non-volatile particles were regarded as rBC. An example of the measured size distribution is shown in
Fig. 2. The heated size distribution was divided into three size ranges –
“high-volatility” (HV), “medium-volatility” (MV),
and “low-volatility” (LV) – to calculate the number fraction of internally mixed BC particles:
Fin=NMV/(NMV+NLV),
where NMV is the number concentration of MV particles and is considered
to be internally mixed BC. NLV is the number concentration of LV and is
considered to be externally mixed BC (Cheng et al., 2009, 2012; Wehner et al., 2009;
Cheung et al., 2016).
Correlation between the hourly-averaged number fractions of
internally mixed BC (Fin) and the photochemical oxidant
(Ox) mixing ratios for different size bins (50, 100, 150,
200, 250, and 300 nm). Data points are colour-coded with respect to the
concentrations of CO (an indicator of primary BC emissions). Low
Fin values generally appear at high CO concentrations, and vice
versa. For 150, 200, 250 and 300 nm diameters, Fin values
increased with oxidant concentration. The slope of the linear regression (red
line) is representative of the oxidation rate of Fin (the fit
standard error is shown in brackets).
To assess the influence of atmospheric photochemical aging on the mixing
state of BC, scatter plots of Fin at different diameters and
Ox concentrations are shown in Fig. 3. Data points are
colour-coded with respect to the concentrations of CO, which is related to
primary BC emission. The value of Fin reflects the competition
between fresh emission and atmospheric aging (Cheng et al., 2012). The
freshly emitted particles have lower Fin values, while the aging
process converts externally mixed particles into internally mixed and then
increases the value of Fin. In this work, low Fin
values tended to appear at high CO concentrations, consistent with freshly
emitted BC. The observed decreasing trends in Fin with
Ox for particle diameters of 50 and 100 nm were probably
due to the increment of the relative contribution of fresh emissions in the
small particle size range.
A positive correlation between Fin and Ox was
observed for particle diameters of 150, 200, 250, and 300 nm, respectively.
The corresponding oxidation rates of Fin were
0.07 and 0.12 % ppb-1 and 0.16 %, and
0.17 % ppb-1. Particles of these sizes have greater
internal mixing and may be more susceptible to photochemical oxidation
processes. Very recently, Q. Wang et al. (2017) reported a similar
correlation between the number fraction of thickly coated rBC
(FrBC, the mixing state of individual rBC was measured with
single particle soot photometer, SP2) and Ox concentration
in highly polluted megacities. The reported oxidation rates of
FrBC were 0.58 % ppb-1 for Beijing and
0.84 % ppb-1 for Xi'an, respectively. Photochemical aging resulted
in higher amounts of internally mixed BC and a larger fraction of
thickly coated BC under more oxidizing conditions.
Time series of the optical parameters and absorption enhancement
(Eabs) at λ = 532 nm at a time resolution of
10 min. Properties shown are the extinction (bext), scattering
(bscat), and absorption coefficients (babs), the SSA
(ω) of ambient particles (bambient), and particles passed
though the thermodenuder (bTD) at 300 ∘C (after
correcting for particle losses).
Temporal and diurnal variations in optical properties
Time series of the measured optical properties are shown in Fig. 4 and
include the extinction (bext), scattering (bscat),
and absorption (babs) coefficients, the SSA for ambient particles
(bambient) and for particles passed through TD (bTD),
and the corresponding Eabs. The mean (and standard deviation) of
bext,ambient, bscat,ambient, and bext,TD,
bscat,TD were 92 ± 64, 81 ± 55, 12 ± 7, and
6.5 ± 4.1 Mm-1, respectively. The scattering fraction remaining
(bscat,TD / bscat,ambient) was about
0.09 ± 0.05 and indicated that most of the coating species evaporated
in the TD at 300 ∘C. Our value is comparable to the value of 0.08 ± 0.02 reported by Nakayama et al. (2014). The change in the
morphology during heating was negligible.
The observed diurnal variation in aerosol optical parameters
(extinction (bext), scattering (bscatt), absorption
(babs) coefficients, and SSA (ω)) of ambient particles
(bambient, a–c, g) and particles passed
through the thermodenuder at 300 ∘C after correcting for particle
losses (bTD, d–f, h). The absorption
enhancement (Eabs, i) was calculated as the ratio
between babs,ambient / babs,TD. The mass
concentrations of PM2.5 (j) and the mixing ratios of
CO (k) and Ox (l) are also shown for
assessing the effect of photochemical oxidation. The optical measurement at
λ = 532 nm covered the period 16 June to 23 July 2016. The box
and whisker plots show the mean (dots), median (centre solid line), lower and
upper quartile (boxes), and 5th and 95th percentile (whiskers).
The observed diurnal variation in optical parameters
(bext,ambient, bscat,ambient,
ωambient, babs,ambient,bext,TD,bscat,TD, babs,TD, ωTD,
Eabs), mass concentrations of PM2.5, as well as the mixing
ratio of CO and the photochemical oxidant (Ox) are shown
in Fig. 5. Broadly similar diurnal patterns were observed for the extensive
optical properties (bext,ambient, bscat,ambient,
babs,ambient, bext,TD, bscat,TD,
babs,TD) of ambient particles and particles passed through the
thermodenuder, and the mass concentrations of PM2.5. A strong diurnal
variation and similar diurnal patterns in ωambient, ωTD, Eabs, and Ox were observed.
Patterns of the extensive optical properties and PM2.5 indicate some
local particle emissions from early morning anthropogenic activities. While
these changes are radiatively significant, changes in PM2.5 during the
early daylight period are weak, suggesting that emitted particles are small
and contribute little to the overall particle mass concentration. The SSA
shows that particles tend to be more strongly absorbing in early morning than
later in the day; however, measured SSA values are not especially low (mean
ωambient ≥ 0.85), consistent with the background
nature of the Shouxian site. Thus, freshly emitted particles are therefore
relatively unimportant at this site. CO concentrations show minor diurnal
variation, consistent with the regional nature of air masses at this site.
Daytime increases in the boundary layer into the mid-afternoon are especially
evident in the PM2.5 concentration profile. In contrast, ambient
scattering and extinction profiles are broadly flat over the same period,
indicating more intense photochemical processing and extensive secondary
aerosol generation. The same effect is responsible for the mid-afternoon
maximum in the intensive optical property ωambient.
Relationship between (a) ω,
(b) ωTD, and (c) Eabs with
Ox concentrations. The gray circles are the measurement
data with 1 h time resolution. The hourly averaged ω,
ωTD, and Eabs were then binned in
Ox with the same data points (80 points) in each bin. The
corresponding mean (solid dot), median (centre solid line), lower and upper
quartile (boxes), and 10th and 90th percentile (whisker) are shown as the box
and whisker plots. The corresponding frequency distribution of each
parameters is shown in (d)–(g).
Influence of photochemical aging on Eabs and SSA
SSA is one of the most relevant intensive optical properties (Jo et al.,
2017) because it describes the relative strength of the aerosol scattering
and absorption capacity and is a key input parameter in climate models.
Changes in particle size, morphology, chemical composition, and mixing state
caused by atmospheric chemical aging processes will alter SSA. The
relationships between hourly-averaged ω, ωTD, and
Eabs with hourly-averaged Ox, as well as the
scattering plot of hourly averaged ω, ωTD, and
Eabs that binned in Ox (with the same data points
in each bin) and the frequency distributions of the hourly averaged data are
shown in Fig. 6. Approximately 90 % of the values of ω,
ωTD, and Eabs during the measurement were within
the range of 0.80–0.91, 0.43–0.65, and 1.0–3.5, respectively.
During the summer, O3 has a central role in the generation of
secondary aerosol. Positive correlations between ω,
ωTD, and Ox concentrations were observed
in our measurements (Fig. 6a and b), which suggests that higher
Ox actually increases the mass fraction of secondary
aerosol particles and the overall ensemble of particle material and SSA. Our
result is consistent with Beijing summer observations, where SSA was linearly
correlated with the mass fractions of secondary aerosols (Han et al., 2017).
The increase in ωTD resulted from incomplete vaporization of
non-volatile constituents in the heating tube (Cheung et al., 2016), the
generation of low-volatility oxygenated organic aerosol during photochemical
aging (Paciga et al., 2016), and the changes in BC morphology (Radney et al.,
2014). Summertime volatility measurement of organic aerosol in the megacity
Paris shown that about 10 % mass fraction remained with a TD operating at
180 ∘C (Paciga et al., 2016). However, recent research demonstrated
that the remaining non- and low-volatile coating has a minor impact on the
absorption measurement of heated particles using TD operating at
250 ∘C (S. Liu et al., 2015). Theoretical and experiment results
show that aging causes the dramatic changes in BC particle morphology (China
et al., 2015; He et al., 2015, 2016; Scarnato et al., 2013; Y. Wang et al.,
2017) and leads to more compact black carbon with higher scattering cross
sections (Peng et al., 2016; Y. Wu et al., 2018), which in turn results in an
increase in ωTD (Radney et al., 2014; Forestieri et al.,
2018). In this regard, the rise in ωTD with increasing
Ox concentration can be used as an indicator of the
changes in BC morphology. The values of ωTD remained stable
for Ox mixing ratios larger than 45 ppbv, which possibly
indicate that the change in the proportion of non-volatile constituents or BC
morphology was negligible.
Eabs also rose with higher Ox mixing ratios
(Fig. 6c and Fig. S10 in the Supplement), but with a different pattern
compared to ω and ωTD. From the scattering plot of
the hourly averaged Eabs that was binned in Ox, a monotonic growth of
Eabs with increasing Ox can be observed below
45 ppbv Ox (from 2.1 to 2.6). For Ox
mixing ratios between 45 and 57 ppbv, Eabs values remained
constant (∼ 2.4). The small drop in Eabs value for the
Ox bin larger than 57 ppbv was probably caused by the
limited data numbers for Ox larger than 75 ppbv and was
statistically insignificance. The most frequently occurring value of
Eabs for the whole measurement was ∼ 1.7.
Four selected case studies of the variations in hourly-averaged
ω, ωTD, and Eabs as a function of
hourly-averaged Ox concentrations. (a, b) Case 1:
on 16 June 2016; winds were typically from the north. (c, d) Case 2:
25 to 26 June 2016, wind direction varied from north to south. (e, f) Case 3: 7 to 8 July 2016; winds were mainly from the southeast.
(g, h) Case 4: 9 to 12 July 2016; winds were mainly from the
northeast. The daytime (from 06:30 to 18:30 local time) and nighttime (from
18:30 to 06:30 local time) data have been marked in different colours and
symbols. The slope of the linear regression (red and blue lines, only for
daytime data) is representative of the oxidation rate of each parameter (the
fit standard error is shown in brackets).
Since the emission sources, weather conditions, and aging degrees of BC
particles varied from day to day, the relationship between Eabs
and atmospheric chemistry is rather complex. Four selected cases with
different wind directions were used to demonstrate this complexity (as shown
in Fig. 7, the day- and nighttime data were separated). The corresponding
wind directions and speeds, RH values, and CO concentrations are shown in
Fig. S10 in the Supplement. The patterns of Eabs with
Ox were different for air masses from different
directions. For cases 1 and 2, the mean values of Eabs were
comparable (1.9 ± 0.2 for case 1 and 1.8 ± 0.6 for case 2), and
the hourly-averaged ω and ωTD grew with increasing
Ox in both cases. In case 1, Eabs ranged from
1.5 to 2.3, with a growth rate of ∼ 0.01 ppbv-1 in the daytime.
During this period, winds were typically from the north, which corresponding
to a short transported pathway of air masses (Fig. S9 in the Supplement). The
low degree of aging led to a small Eabs value. In case 2,
Eabs ranged from 1.1 to 3.7, and the corresponding growth rates
was ∼ 0.05 ppbv-1 in the daytime. It is worth noting that the low
Eabs values in this period corresponded to low ω values
(Fig. 7c), which indicated the influence on the local emissions on
Eabs. For cases 3 and 4, ω and ωTD
increased slowly with Ox in comparison with cases 1 and 2.
Monotonic relationships were found here, with growth rates of ∼ 0.01
and 0.05 ppbv-1. The large Eabs values in the
daytime than nighttime suggested that photochemistry plays a positive effect
on the increment of absorption enhancement. These results demonstrated the
complex influence of emission and aging degree of BC particles in modifying
the light absorption of BC-containing particles. For air masses from
different directions, the relationship between Eabs and
Ox may be different.
A survey of some field measured Eabs
values.
Method
Location
Eabs
Reference
Description
AFD
Yuncheng, China (rural)
2.25 ± 0.55 (678 nm)
X. Cui et al. (2016)
June–July 2014; ECOC (elemental carbon and organic carbon)
analyser. (Eabs ranged from 1.4 for fresh combustion
emissions to 3 for aged ambient aerosols.)
Jinan, China (urban)
2.07 ± 0.72 (678 nm)
Chen et al. (2017)
February 2014; ECOC analyser. Eabs ∼ 1.3–1.5 for fresh
urban aerosols and ∼ 2–2.5 for aged aerosols.
MAE
California, USA (rural)
∼ 1.16 (532 nm)
Cappa et al. (2012)
June 2010; absorption coefficients at 405 and 532 nm were
measured by PAS; rBC mass concentration was measured with
SP2; SP2 measured rBC core diameter ∼ 174 nm.
Shenzhen, China (urban)
1.07 (532 nm)
Lan et al. (2013)
August–September 2011; absorption coefficients at 405, 532, 781 nm
measured were measured with PAS; rBC mass concentration
was with SP2; MAE532nm = 6.5 ± 0.5 m2 g-1 (with
lowest value of 6.08 m2 g-1 and highest value of 8.5 m2 g-1,
respectively, treated as totally, externally mixed, and internally mixed);
SP2 measured BC, core diameter ∼ 180 nm.
Xi'an, China (urban)
1.8 (870 nm)
Q. Y. Wang et al., 2014
December 2012–January 2013; light absorption was measured
with PAS; rBC concentration was measured with SP2.
London, UK (rural)
1.8 (405 nm) 1.4 (781 nm)
S. Liu et al. (2015)
February 2012; absorption coefficients at 405 and 781 nm were
measured with PAS; rBC mass concentration was measured
with SP2; rBC core diameters ranged from 100 to 200 nm.
Nanjing, China (suburban)
1.6 (532 nm)
F. Cui et al. (2016)
November 2012; absorption coefficients at 405, 532, and 781 nm
were measured with PAS; EC mass concentration was determined
by ECOC analyser.
Beijing, China (suburban)
2.6–4.0 (470 nm)
X. Xu et al. (2016)
November 2014–January 2015, for PM1.0 particles; absorption
coefficient at 470 nm by using a cavity-enhanced EC albedometer;
mass concentration was determined by ECOC analyser.
Beijing, China (urban)
2.4 (405, 532 nm)
Peng et al. (2016)
May–June 2009 in Houston, August–October 2013 in
chamber study; Houston, USA (urban)
Beijing; absorption coefficients at 405, 532, and 870 nm were
measured with PAS.
Manchester, UK (urban)
1.0–1.3 (532 nm)
D. Liu et al. (2017)
October–November 2014; chamber study and open wood
fire measurement; absorption coefficients at 405, 532,
and 781 nm were measured with PAS.
Kanpur, India (urban)
1.8 (781 nm)
Thamban et al. (2017)
January–February 2015; absorption was measured with
PAS; rBC concentration was measured with SP2.
Beijing, China (urban)
3.2–5.3 (365 nm)
Cheng et al. (2017)
Comparison of water-soluble and methanol-soluble
organic carbon; theoretical investigation of Eabs.
Beijing and Xi'an, China (urban)
1.9 (532 nm)
Q. Y. Wang et al. (2017)
February 2013, Xi'an, and February 2014, Beijing;
absorption was measured with PAS; rBC concentration was
measured with SP2.
Guangzhou, China (Suburban)
1.5 ± 0.5 (550 nm)
C. Wu et al. (2018)
February 2012–January 2013; light absorption was measured
with an aethalometer; EC mass concentration was determined
by ECOC analyser.
TD
Toronto, Canada (suburban)
1.6–1.9 (550 nm)
Knox et al. (2009)
December 2006 to January 2007; TD operating at 340 ∘C; optical
properties were measured with PAS and aethalometer.
California, USA (rural)
1.06 (532 nm)
Cappa et al. (2012)
June 2010; TD operating at 250 ∘C; absorption coefficients
at 405 and 532 nm were measured by PAS; SP2 measured
rBC core diameter ∼ 174 nm.
Boulder, USA (forest fire)
2.5 (404 nm) 1.4 (532 nm)
Lack et al. (2012)
September 2010; TD operating at 200 ∘C; absorption coefficients
at 404, 532, and 658 nm were measured with PAS; SP2 measured
rBC core diameter: 140 ± 10 nm.
Nagoya, Japan (urban)
781 nm, TD 300 ∘C
Nakayama et al. (2014)
August 2011, January 2012; TD operating at 100, 300, and 400 ∘C;
1.10 ± 0.09 (August)
absorption coefficients at 405 and 781 nm were measured with PAS.
1.02 ± 0.11 (January)
London, UK (rural)
1.3 (405 nm) 1.4 (781 nm)
S. Liu et al. (2015)
February 2012; TD operating at 250 ∘C; absorption
coefficients at 405 and 781 nm were measured with PAS; rBC
core diameters ranged from 100 to 200 nm.
Noto Peninsula, Japan (rural)
1.22 (781 nm, ranged
Ueda et al. (2016)
April–May 2013; TD operating at 300 or 400 ∘C; absorption
coefficients at 405,
532, and 781 nm were measured with PAS.
from 1.07 to 1.38)
California, USA (urban)
1.03 ± 0.05 (870 nm)
Krasowsky et al. (2016)
February–March 2015; TD operating at 230 ∘C; absorption
at 870 nm was measured with PAS.
Shouxian, China (rural)
2.3 ± 0.9 (532 nm)
This work
June–July 2016; TD operating at 300 ∘C; absorption at
532 nm was measured with a cavity-enhanced albedometer.
A list of recently reported Eabs values is shown in Table 1. The
averaged and standard deviation of Eabs value at
λ = 532 nm for this work was 2.3 ± 0.9, which agreed well
with values from Boulder using the same TD method combined with a photoacoustic
spectrometer (PAS) (Lack et al., 2012) from Yuncheng (X. Cui et al., 2016)
and Jinan (Chen et al., 2017) using an aerosol filtration–dissolution (AFD)
method and from Beijing (Peng et al., 2016; Xu et al., 2016; Cheng et al.,
2017) based on the mass absorption efficiency (MAE) method. Our result is
also comparable to that reported in laboratory studies of thickly coated BC
particles where Eabs ranged from 1.8 to 2.4 (Bond et al., 2013).
A chamber study by Peng et al. (2016) suggested that the primary BC was in a chain-like structure with low particle density and then collapsed to
a semi-spherical particle. During this stage, there is no significant absorption
enhancement (Eabs ranged from 1.0 to 1.4). With continued coating
growth with several hours' aging in the chamber, the semi-spherical particle was
further collapsed and finally transformed to fully compact spherical, internally mixed BC particles (Eabs increased to
∼ 2.3–2.4) (Gustafsson and Ramanathan, 2016). Recent morphologically
constrained modelling developed by Y. Wu et al. (2018) demonstrated that
after full aging, the BC particles became a more compact aggregation, which
leads to a stable range of Eabs (averaged value ∼ 2.5, with
a minimum value of ∼ 2 and a maximum value of ∼ 3.5).
Photochemical aging processes lead to internal mixing and a larger coating
fraction that enhances the light-absorbing capacity of BC particles (Lack and
Cappa, 2010). Our finding of the growth of Eabs associated with
the increasing Ox concentration suggests that secondary
organic aerosol (SOA) includes light-absorbing organic compounds (BrC) (Xu et
al., 2016), and that BrC's overall contribution to particle absorption grows
under more oxidizing conditions. As discussed in the next section, we find an
increase in the imaginary part of the complex refractive index (CRI) of the coated shell.
Scatter plot of Eabs and ω for different
photochemical oxidant concentrations. Variation in the observed diurnally
averaged absorption enhancement and SSA (solid points, colour-coded with
respect to the concentrations of Ox) is used for the
modelling constraint. Both Eabs and ω increase with
Ox mixing ratio. The open circles are the single-particle
Mie theory calculation results with an optimized BC core size of 160 nm. The
CRI of BC was fixed at 1.85 + i 0.71. The real part of the CRI of the
coating material was fixed at 1.55. The changes in the imaginary part of the
CRI and the thickness of the coating material were colour-coded and shown as
the different dimensions (open circle).
Coating absorption and light absorption enhancement
Mie theory, which was treated as the basis of the IPCC 5th assessment report
due to its computational efficiency and applicability to radiative transfer
models (Jo et al., 2017), is a powerful tool for optical data interpretation
(Lack et al., 2012) and the reliability of the core–shell model has been
verified in many optical closure studies (Lack et al., 2012; Ma et al., 2012;
S. Liu et al., 2015; C. Wu et al., 2018). According to Peng et al.'s (2016)
chamber study results, BC particles change to a fully compact spherical
morphology in less than 1 day. Volatility measurements and analysis of the
air masses indicated that the atmospheric aerosol observed in summer at the
rural site was well aged. In this work, the particle size distribution
information was not available. A method based on single-particle core–shell
Mie theory (Bohren and Huffman, 1983; Saleh et al., 2015) was developed to
interpret the observed changes in Eabs associated with
Ox in this work. The sensitivity of this assumption is
discussed in Sect. S7 in the Supplement. The modelling was based on
simultaneous constraining of Eabs and SSA to retrieve the
fraction contribution of BC absorption (fBC), lensing-driven
enhancement (fLens), coating absorption (fShell), as
well as the coated shell diameter (DShell) and the imaginary part
of the CRI of the shell (kShell).
A scatter plot of measured diurnally averaged Eabs and SSA for
different photochemical oxidant concentrations is shown in Fig. 8. The solid
points are the observed results and colour-coded with respect to the
concentrations of Ox. The open circles are the
single-particle Mie core–shell modelled results with an optimized BC core size
of 160 nm and colour- and size-coded with respect to the imaginary part of
the CRI of coating material (kshell) and the diameter of coating
material (Dshell), respectively. The colour-coded plot shows the
connection between Eabs, SSA, and atmospheric photochemistry. The
modelled results are consistent with the observed results. Both SSA and
Eabs values rise with increasing Dshell and
kshell, indicating that the coating thickness and absorption play
key roles in determining SSA and Eabs. Both Eabs and
SSA increased under more oxidizing conditions. This can be explained by the
photochemical production of coating species: with more intense photochemical
aging, the fraction of internally mixed BC particles and coating thickness
increased. Thickly coated BC was also observed by Q. Wang et al. (2017) under
higher Ox mixing ratios.
The retrieved diurnal variations in the (b) coating
thickness (DShell) and (c) imaginary part of the CRI
(kShell) of the coated materials, and (d) the relative
contribution of the absorption of BC (fBC), lensing effect
(fLens), and absorption of the shell (fShell).
Broadly similar patterns were observed for DShell,
kShell, fShell, and
(a) Ox concentrations.
The corresponding Mie theory calculation results are shown as open circles in
Fig. 8 (with further details in Sect. S7 in the Supplement). Comparisons of
modelling and observation Eabs and SSA are shown as a scatter
plot in Supplement Fig. S12. By fixing the BC core diameter, we can retrieve
information on the coating shell (DShell, kShell) and
each contribution to light absorption (fBC, fLens,
fShell) under different oxidant conditions (Lack and Cappa,
2010), as shown in Fig. 9. The retrieved Dshell ranged from 386
to 440 nm. The corresponding Dshell / Dcore
ratio ranged from 2.41 to 2.75, within the range of values (2–4) reported by
C. Wu et al. (2018). The plot of measured and modelled Eabs with
different Dshell / Dcore is shown in the
Supplement Fig. S13. The values of kshell ranged from 0.004 to
0.008 with a diurnal average value of 0.006 (±0.001). A comparison of the
retrieved kshell with previously reported k values of fresh and
aged organic materials is shown in Supplement Fig. S14, which includes BC,
BrC aerosol production from biomass burning (BB), atmospheric humic-like
substances (HULISs), Suwannee River fulvic acid aerosol (SRFA), and secondary
organic material (SOM) produced by photo-oxidation of anthropogenic and
biogenic organic precursors. The value of kshell reported here is
comparable with those of BB aerosols (Chakrabarty et al., 2010) and SRFA
(Bluvshtein et al., 2016), and is larger than those of SOM (Liu et al., 2013;
P. Liu et al., 2015), HULIS (P. Liu et al., 2015) and urban BrC (Cappa et
al., 2012).
The retrieved diurnal variations in coating shell (DShell,
kShell) and the fractional contribution to light absorption
(fBC, fLens, fShell) are shown in Fig. 9.
Broadly similar patterns were observed for DShell,
kShell, and fShell with Ox.
DShell and Eabs tended to be lower in early morning
and evening, which was in accordance with anthropogenic activity (with a high CO
concentration, as shown in Fig. 5). Peak values of DShell,
kShell, and fShell appeared in the mid-afternoon,
which corresponded to a more intense photochemical processing and extensive
secondary aerosol generation and resulted in a thicker and more absorbing
coating shell. The fractional contribution of fBC,
fLens, and fShell ranged from 35 % to 49 %,
35 % to 42 %, and 11 % to 30 %, respectively, with a mean
value of 43 % ± 4 %, 39 % ± 2 %, and
18 % ± 5 %. A ternary plot is shown in the Supplement Fig. S15.
Our results suggest that the contribution of the lensing effect to absorption
enhancement is limited (Bond and Bergstrom, 2006). The lensing effect is reduced due
to the greater absorption of the shell (Lack and Cappa, 2010). The change in
optical properties at higher oxidant conditions implies a non-negligible
contribution of absorbing secondary aerosol material to photochemistry and
should receive more attention in climate modelling (Jo et al., 2016).