Brown carbon (BrC) contributes significantly to aerosol light absorption and
thus can affect the Earth's radiation balance and atmospheric photochemical
processes. In this study, we examined the light absorption properties and
molecular compositions of water-soluble (WS-BrC) and water-insoluble
(WI-BrC) BrC in PM2.5 collected from a rural site in the Guanzhong
Basin – a highly polluted region in northwest China. Both WS-BrC and WI-BrC
showed elevated light absorption coefficients (Abs) in winter (4–7 times those in summer) mainly attributed to enhanced emissions from residential
biomass burning (BB) for heating of homes. While the average mass absorption
coefficients (MACs) at 365 nm (MAC365) of WS-BrC were similar between daytime
and nighttime in summer (0.99±0.17 and 1.01±0.18 m2 g-1, respectively), the average MAC365 of WI-BrC was more than a
factor of 2 higher during daytime (2.45±1.14 m2 g-1)
than at night (1.18±0.36 m2 g-1). This difference was
partly attributed to enhanced photochemical formation of WI-BrC species,
such as oxygenated polycyclic aromatic hydrocarbons (OPAHs). In contrast,
the MACs of WS-BrC and WI-BrC were generally similar in winter and both
showed few diel differences. The Abs of wintertime WS-BrC correlated
strongly with relative humidity, sulfate and NO2, suggesting that
aqueous-phase reaction is an important pathway for secondary BrC formation
during the winter season in northwest China. Nitrophenols on average
contributed 2.44±1.78 % of the Abs of WS-BrC in winter but only
0.12±0.03 % in summer due to faster photodegradation reactions.
WS-BrC and WI-BrC were estimated to account for 0.83±0.23 % and
0.53±0.33 %, respectively, of the total down-welling solar
radiation in the ultraviolet (UV) range in summer, and 1.67±0.72 % and
2.07±1.24 %, respectively, in winter. The total absorption by BrC
in the UV region was about 55 %–79 % relative to the elemental carbon (EC)
absorption.
Introduction
Light-absorbing organic matter, termed brown carbon (BrC), has been
recognized as an important climate forcer due to its ability to directly
interact with both incoming solar radiation and outgoing terrestrial
radiation (Andreae and Gelencser, 2006; Laskin et al., 2015). BrC is a
complex mixture of organic compounds, which collectively show a light
absorption profile increasing exponentially from the visible (Vis) to the
ultraviolet (UV) range. Due to the high abundance of organic aerosol in
continental regions, especially in places with intensive anthropogenic
pollution, the contribution of BrC to aerosol absorption in the near-UV
range is potentially significant (Kirillova et al., 2014b; Huang et al.,
2018; Yan et al., 2015a). For example, a model study showed that BrC
contributes up to +0.25 W m-2 of radiative forcing on a planetary
scale, which is approximately 19 % of the absorption by anthropogenic
aerosols (Feng et al., 2013). Moreover, the strong
absorption of BrC in the UV spectral region can reduce the solar actinic
flux and subsequently affect atmospheric photochemistry and tropospheric
ozone production (Jacobson, 1998; Mohr et al., 2013).
A thorough understanding of the sources and transformation processes of BrC
in the atmosphere is important, but it is still lacking. Biomass and biofuel
combustion, including forest fires and burning of wood and agricultural
wastes for residential cooking and heating, has been shown to be a particularly
important source of BrC (Washenfelder et al., 2015; Desyaterik et al.,
2013; Lin et al., 2017). BrC can also be emitted directly from coal burning
(Yan et al., 2017) and biogenic release of fungi,
plant debris and humic matter (Rizzo et al., 2011, 2013).
In addition, recent studies suggested that secondary BrC can be formed
through various reaction pathways, including photooxidation of aromatic
volatile organic compounds (VOCs) (Lin et al., 2015; Liu et al., 2016),
reactive uptake of isoprene epoxydiols onto preexisting sulfate aerosols
(Lin et al., 2014), aqueous oxidation of phenolic compounds
and α-dicarbonyls (Chang and Thompson, 2010; Nozière and
Esteve, 2005; Smith et al., 2016; Yu et al., 2014; Xu et al., 2018), and
reactions of ammonia or amines with carbonyl compounds in particles or cloud
droplets (Nozière et al., 2007; Laskin et al., 2010; Updyke et al.,
2012; Nguyen et al., 2012; De Haan et al., 2018; Powelson et al., 2014).
However, atmospheric oxidation processes may also cause “photobleach” –
photodegradation of BrC into less-light-absorbing compounds (Lee et al.,
2014; Romonosky et al., 2015; Sumlin et al., 2017), which may complicate the
understanding of BrC in the atmosphere.
A common way to quantify the absorption properties of BrC is to measure the
absorbance of aerosol extracts over a wide wavelength range using
spectrophotometers. This approach can differentiate the interference of
black carbon (BC) or mineral dust (Hecobian et al., 2010).
Most of the studies use ultrapure water to extract organic substance in the
aerosol and thus measure the optical properties of water-soluble BrC
(WS-BrC) (Wu et al., 2019; Hecobian et al., 2010; Kirillova et al., 2014b).
In addition, some studies analyzed the light absorption of BrC extracted
using polar organic solvents such as methanol or acetone (Liu et al.,
2013; Huang et al., 2018; Kim et al., 2016). Since such extracts contain both
water-soluble and water-insoluble chromophores, little information is
available regarding the contribution and formation of water-insoluble BrC
(WI-BrC). However, it is important to understand WI-BrC given the fact that
some water-insoluble organic compounds, such as polycyclic aromatic
hydrocarbons and their derivatives, are effective light absorbers and that
the mass absorption of WI-BrC could be even greater than that of the
water-soluble fraction (Chen and Bond, 2010; Huang et al., 2018; Sengupta
et al., 2018). Thus, it is necessary to extract water-soluble and
water-insoluble organic components separately, e.g., using solvents with
different polarity in sequence. Combining with measurements of BrC molecular
compositions, the UV–vis absorption properties of the water-soluble and
water-insoluble extracts may help us better understand the sources and
formation mechanisms of light-absorbing compounds in the atmosphere.
China has been experiencing serious atmospheric pollution conditions in
recent decades, and both model and field results showed elevated light
absorption of BrC in most regions of China (Huang et al., 2018; Cheng et
al., 2011; Yan et al., 2017; Li et al., 2016b) compared to developed countries
such as the US (Hecobian et al., 2010; Washenfelder et al., 2015) and
European countries (Mohr et al., 2013; Teich et al., 2017). However,
BrC-related data are scarce in the Guanzhong Basin (Shen et al.,
2017; Huang et al., 2018), which is one of the most polluted regions in China
(van Donkelaar et al., 2010). Here we present measurements of
the optical properties of WS-BrC and WI-BrC in PM2.5 collected from a
rural area of the Guanzhong Basin during winter and summer. We also
measured the concentrations of several BrC compounds as well as those of
organic carbon (OC), elemental carbon (EC), water-soluble OC (WSOC) and
inorganic ions. These data were analyzed to examine the effects of source
emissions, daytime photochemical oxidation and aqueous-phase chemistry on
WS-BrC and WI-BrC components in different seasons.
Experimental sectionSample collection
The sampling was conducted at a small village (namely Lincun,
34∘44′ N and 109∘32′ E,
354 m a.s.l.) ∼40 km northeast of Xi'an, the capital of
Shaanxi Province (Fig. S1 in the Supplement). The sampling site is located in the central
part of Guanzhong Basin with no obvious point source of air pollutants in
the surrounding areas. PM2.5 samples were collected twice a day
(∼08:00 to 20:00 and ∼20:00 to 08:00) onto
prebaked (450 ∘C, 6–8 h) quartz fiber filters (Whatman, QM-A, USA)
during 3–23 August 2016 and 20 January–1 February 2017 using a Tisch Environmental
(USA) PM2.5 high-volume (1.13 m3 min-1) sampler. Field blank
samples were also collected by mounting blank filters onto the sampler for
about 15 min without pumping any air. After sampling, the sample filters
were immediately sealed in aluminum foil bags and then stored in a freezer
(-5 ∘C) prior to analysis. Meteorological conditions and
concentrations of O3 and NO2 during this studied period are
presented in Fig. 1.
Temporal variation in meteorological parameters (a, b) and
concentrations of major chemical compositions, Abs365, MAC365 and
AAE of water-soluble and water-insoluble BrC in PM2.5 from the rural
area of northwest China.
Filter extraction and absorption spectra analysis
For each PM2.5 sample, a portion of the filter (∼13.384 cm2) was first extracted in 8 mL of Milli-Q water (18.2 MΩ)
through 30 min of sonication at ∼0∘C. The water extract
was then filtered via vacuum filtration with a 25 mm diameter, 5 µm pore
hydrophobic PTFE membrane filter (Merck Millipore Ltd, Mitex™ membrane
filters, USA). Afterwards, the insoluble PM components collected on the PTFE
membrane filter and remained on the sample filter were rinsed with 2 mL
of Milli-Q water, air-dried and then extracted via sonication in 8 mL of pure
acetonitrile (ACN) (Honeywell Burdick & Jackson, LC/MS grade, USA). The
acetonitrile extract was filtered via a 13 mm diameter, 0.45 µm pore
syringe filter (Pall, Bulk Acrodisc®, PTFE membrane filters,
USA). The light absorption spectra of the water and the acetonitrile
extracts were measured between 190 and 820 nm by a diode-array
spectrophotometer (Hewlett Packard 8452A, USA) using quartz cuvettes with 1 cm length path. Field blank filters were extracted and measured in the same
manner as the samples. Data presented in this study were corrected for the
field blanks (< 10 % relative to field samples).
Chemical analysis
OC and EC were analyzed using a DRI carbon analyzer (model 2001, USA). Another
piece of the filter sample (∼8.6 cm2) was extracted with
Milli-Q water (18.2 MΩ) and filtered through a PTFE syringe filter.
Then the water extract was analyzed for water-soluble inorganic ions
(SO42-, NO3-, NH4+, Cl-, F-,
Ca2+, K+, Na+ and Mg2+) using a Metrohm ion
chromatographer (Metrohm 940, Switzerland) and WSOC using a Shimadzu TOC
analyzer (TOC-L CPH, Japan) and concentrations of individual molecules,
including levoglucosan, parent polycyclic aromatic hydrocarbons (PAHs), oxygenated PAHs (OPAHs), nitrophenols,
isoprene, and α- and β-pinene-derived products, were measured
using a gas chromatographer–electron-impact-ionization mass spectrometer (GC–EIMS, Agilent 7890A-5975C, USA) calibrated by authentic standards.
More details on these measurements can be found in previous publications
(Li et al., 2014).
Data interpretation
In this study, water-insoluble OC (WIOC) was calculated by the difference
between OC and WSOC:
MWIOC=MOC-MWSOC,
where MWIOC, MOC and MWSOC correspond to the mass
concentration (µgC m-3) of WIOC, OC and WSOC, respectively, in
the air.
The absorption coefficient of WS-BrC (Absλ,WS-BrC, Mm-1)
or WI-BrC (Absλ,WI-BrC, Mm-1) at a given wavelength
(λ) is determined from the UV–vis spectrum of the water extract
(Hecobian et al., 2010; Laskin et al., 2015):
Absλ=Aλ-A700×VsolventVa×l×ln10×100,
where Aλ is the absorbance of the water (Aλ,WS-BrC)
or ACN (Aλ,WI-BrC) extract at λ, which is corrected for
the field blank. Vsolvent (mL) is the volume of solvent (water or ACN)
used to extract the filter (8 mL) and Va (m3) is the air volume passed
through the filter punch. l (cm) is the optical length of the quartz cuvettes
used for UV–vis measurement, and ln(10) is used to convert the logbase-10
(provided by the spectrophotometer) to natural logarithm. The value 100 is for unit
conversion. A700 (absorbance at the wavelength of 700 nm) is
subtracted to minimize the interference of baseline shift. The mass
absorption coefficient of WS-BrC (MACλ,WS-BrC, m2 g-1) or WI-BrC (MACλ,WI-BrC, m2 g-1) at
the wavelength of λ is calculated using Eq. (3):
MACλ=AbsλM,
where M is the mass concentration of WSOC or WIOC. Note that since it is
possible that not all the WI-BrC was extracted into ACN, the Absλ,WI-BrC (estimated uncertainty is 32 %) and MACλ,WI-BrC
(estimated uncertainty is 33 %) reported in this study are likely the
lower-bound values. Nevertheless, the underestimation is probably
insignificant since Chen and Bond (2010) reported that > 92 % of BrC was extractable by organic
solvents (methanol or acetone).
The wavelength dependence for BrC absorption is fit with a power-law
equation:
Absλ=K×λ-AAE,
where K is a constant and AAE stands for absorption
Ångström exponent. In this study, the AAE for a given
sample is calculated through the linear regression of log(Absλ)
against log λ between 300 and 450 nm. This wavelength range is chosen
because the fits of all the samples in this study are better than
r2=0.99. Note that slightly higher AAE values (by up to 10 %) are
obtained using a wider wavelength range (e.g., 300–550 nm; Fig. S2).
The fraction of solar irradiance absorbed by particulate BrC at a given
wavelength λ is estimated following the Beer–Lambert law:
I0-II0λ=1-e-bap,λ,x×hABL,
where x denotes WS-BrC or WI-BrC, and hABL is the atmospheric boundary
layer height (assuming 1200 m in summer and 600 m in winter) according to
the assumption that the ground measurement results are representative of the
average values in the whole planetary boundary layer (PBL) (Kirchstetter
et al., 2004; Kirillova et al., 2014a). I0 denotes the incident solar
radiance in the form of either actinic flux (quanta s-1 cm-2 nm-1) or irradiance (W m-2 nm-1), which were obtained
using the Quick TUV Calculator (http://cprm.acom.ucar.edu/Models/TUV/Interactive_TUV/, last access: 19 March 2019).
(I0-I) denotes the direct absorption of solar actinic flux or
irradiance by BrC. bap,λ,x corresponds to the absorption
coefficient (bap, m-1) of WS-BrC or WI-BrC at a wavelength of
λ. The absorption properties of BrC extracted by bulk solution may
not entirely reflect the light absorption by ambient aerosols. However, an
estimated conversion factor can be calculated from the light absorption of
size-resolved samples using the Mie theory. Assuming that particles are of
spherical morphology and externally mixed with other light-absorbing
components, an imaginary refractive index (k, responsible for absorption)
could be obtained from MAC using the following equation (Laskin
et al., 2015):
k(λ)=ρ×λ×Absλ4π×MWSOC=ρ×λ×MACλ4π,
where ρ (g cm-3) was particle density and assigned as 1.5, and more
details about Mie theory calculations can be found in the study by
Liu et al. (2013). Previous studies showed that the light
absorption coefficient of particulate BrC (bap,λ,BrC) is around
0.7–2.0 times that from bulk solution (Absλ,WS-BrC or
WI-BrC ) (Liu et al., 2013; Sun et al., 2007). Here, a conversion
factor of 1.3 is applied based on a Mie theory calculation of aerosols in
Xi'an (∼40 km away from the sampling site) (Wu,
2018).
Average spectra of absorption coefficient (Absλ) (a, b)
and mass absorption coefficient (MACλ) (c, d) of water-soluble
(WS-BrC) and water-insoluble (WI-BrC) BrC during daytime and nighttime of
summer and winter. Absorption Ångström exponent (AAE)
is calculated by a linear regression of log Absλ versus log
λ in the wavelength range of 300–450 nm.
Results and discussionOptical absorption characteristics of WS-BrC and WI-BrC
The average absorption spectra of WS-BrC and WI-BrC (λ=300–700 nm) during daytime and nighttime in different seasons are shown in Fig. 2a, b. The absorption Ångström exponents for both
WS-BrC (AAEWS-BrC) and WI-BrC (AAEWI-BrC) are generally higher
than 5, verifying the contribution of BrC to aerosol absorptivity in the
region. The average AAEWS-BrC values are similar between summer (5.43±0.41) and winter (5.11±0.53). Huang et al. (2014) and
Shen et al. (2017) reported comparable AAEWS-BrC values
(5.3–5.7) with no significant seasonal change at urban sites in Xi'an,
suggesting common characteristics of BrC on a regional scale in the
Guanzhong Basin of China. These results are comparable with the data
reported in Guangzhou (5.3) (Liu et al., 2018), but much lower than those
in Beijing (5.3–7.3) (Cheng et al., 2011; Yan et al., 2015b; Du et al.,
2014) and Nanjing (6.7–7.3) (Chen et al., 2018). Moreover,
comparable AAE values were reported for WS-BrC in Switzerland (3.8–5.1)
(Moschos et al., 2018) and Nepal (4.2–5.6) (Wu et al.,
2019; Kirillova et al., 2016), but higher AAEWS-BrC values were found in
the southeastern US (7±1) (Hecobian et al., 2010), Los
Angeles Basin (7.6±0.5) (Zhang et al., 2013) and
South Korea (5.84–9.17) (Kim et al., 2016).
The AAEWI-BrC shows more obvious seasonal variations with a higher
average value in winter (6.04±0.22) than in summer (5.01±0.58). This difference suggests that the chemical composition of WI-BrC
might be more different in different seasons, due to variations in the
sources and atmospheric formation and aging processes of light-absorbing
hydrophobic compounds.
The light absorption properties of WS-BrC and WI-BrC present obvious
seasonal variations (Fig. 2). The average (±1σ) Abs and MAC
values of BrC at 365 nm (i.e., Abs365,WS-BrC, Abs365,WI-BrC,
MAC365,WS-BrC and MAC365,WI-BrC) during daytime and nighttime in
winter and summer are summarized in Table 1. The value 365 nm was chosen to avoid
interferences from inorganic compounds (e.g., nitrate and nitrite) and to be
consistent with previous studies (Hecobian et al., 2010; Huang et al.,
2018). On average, Abs365,WS-BrC is significantly higher than
Abs365,WI-BrC in summer (5.00±1.28 Mm-1 vs. 2.95±1.94 Mm-1), but the values vary slightly in winter (19.6±8.3 Mm-1 vs. 21.9±13.5 Mm-1). The substantially higher BrC
absorptions in winter correspond to a much higher organic aerosol
concentration – WSOC and WIOC concentrations in winter are on average 4.2
and 14 times the concentrations in summer (Table 1). Elevated OA (organic
aerosol) concentration during winter is due to a combination of lower PBL
height and enhanced primary emissions (e.g., from residential heating) in
the cold season. It is worth noting that the wavelength-dependent Abs of
WS-BrC shows a minor tip at about 360 nm in both seasons (Fig. 2), which
may be related to the contribution of some specific chromophores. For
example, Lin et al. (2015) reported that some
nitrogen-containing organic compounds (such as picric acid or nitrophenol)
have a maximum absorption at a wavelength of ∼360 nm. The tip
possibly caused an overestimation of average Abs and MAC at a wavelength of
365 nm in this study. However, the influence seems insignificant based on a
comparison of average Abs and MAC at wavelengths of 340, 350, 360,
370 and 380 nm (Table S1 in the Supplement).
Average (±1σ) values Abs365, MAC365 and
AAE of WS-BrC and WI-BrC, as well as concentrations of OC, WSOC, WIOC and
measured organic species in the PM2.5 aerosols from the rural site in the
Guanzhong Basin.
a SOAi: tracers of SOA formed from isoprene (SOAi) oxidation,
i.e., the sum of 2-methylglyceric acid, 2-methylthreitol and
2-methylerythritol.
b SOAi: tracers of SOA formed from α- and β-pinene
(SOAp) oxidation, i.e., the sum of pinonic acid, pinic acid and
3-methyl-1,2,3-butanetricarboxylic acid.
c BDL: below detection limit (< 0.17 ng m-3).
The MACs of WS-BrC are comparable between the two seasons (Fig. 2c,
d), with the average MAC365,WS-BrC being 1.00 (±0.18) m2 g-1 in summer and 0.93 (±0.25) m2 g-1 in winter
(Table 1). As summarized in Table 2, the MAC365,WS-BrC measured in this
study, i.e., at a rural site in the Guanzhong Basin of China, is comparable
to or lower than the values observed in Asian cities such Xi'an
(Huang et al., 2018), Beijing
(Cheng et al., 2011), Seoul
(Kim et al., 2016) and New Delhi (Kirillova
et al., 2014b) but obviously higher than those at the regional sites in the
North China Plain (Teich et al., 2017) and the background site on the
Tibetan Plateau (Xu et al., 2020). Moreover, significantly lower
MAC365,WS-BrC values were observed in the US, including in the Los Angeles
Basin (Zhang et al., 2013), the southeastern US
(Hecobian et al., 2010) and Atlanta (Liu et
al., 2013).
In winter, the average MAC365,WI-BrC (0.95±0.32 m2 g-1) is comparable to MAC365,WS-BrC (0.93±0.25 m2 g-1; Table 1). However, in summer the MAC365,WI-BrC is much higher
than MAC365,WS-BrC (1.82±1.06 vs. 1.00±0.18 m2 g-1), indicating a relatively stronger light absorption capability of
hydrophobic chromophores than hydrophilic chromophores. Further, the fact
that the summertime MAC365,WI-BrC is nearly double the wintertime
MAC365,WI-BrC suggests that more light-absorbing molecules are formed
in the warm season.
Figure 2 compares the wavelength-dependent light absorptivity (i.e.,
Absλ and MACλ) of WS-BrC and WI-BrC between day and
night in summer and winter. Higher Absλ,WS-BrC and
Absλ,WI-BrC occurred during daytime in summer but during
nighttime in winter. The MACλ values of WS-BrC are overall similar
between daytime and nighttime in both seasons. However, the MACλ
values of WI-BrC show a significant daytime increase in summer over the whole
wavelength range of 300–700 nm (Fig. 2c). The day–night change of BrC
light absorptivity can be viewed more obviously in Fig. 1e and f, where
the temporal variations in the Abs365 and MAC365 of WS-BrC and
WI-BrC during summer 2016 (3–23 August) and winter 2017 (20 January–1 February) are
presented. The highest day / night ratio of MAC365,WIOC reached 3.8 in
summer, and the average daytime MAC365,WI-BrC in summer (2.45±1.14 m2 g-1) is more than twice the value during nighttime
(1.18±0.36 m2 g-1; Table 1). A possible reason for this
observation is that there are additional sources of WI-BrC during summer
daytime in this rural region, such as secondary formation of hydrophobic
light-absorbing compounds.
Figures 3 and 4 present the cross correlations of Abs365,WS-BrC and
Abs365,WI-BrC with major chemical components (e.g., WSOC, WIOC and
sulfate) and molecular tracer species in summer and winter. In
winter, Abs365,WS-BrC correlates strongly with WSOC concentration
(r2=0.80), as does Abs365,WI-BrC with WIOC (r2=0.76).
However, their relationships in summer are much weaker, especially for the
correlation between Abs365,WI-BrC and WIOC (r2=0.50).
Considering that secondary OA (SOA) is mainly comprised of water-soluble
compounds, such as polyalcohols or polyacids and phenols
(Kondo et al., 2007), the much higher WSOC / OC ratio in
summer (0.75±0.07) compared to winter (0.50±0.09) confirms
more prevalent SOA formation in summer associated with higher temperature
and stronger solar radiation. Formation of secondary organic chromophores
may lead to a more complex composition of BrC in summer. More evidences on
secondary BrC formation is provided in the subsequent sections.
Numerous studies reported that biomass burning is a dominant source of BrC
in the atmosphere (Desyaterik et al., 2013; Washenfelder et al., 2015). In
the current study, levoglucosan – a key tracer for biomass burning
emissions (Simoneit, 2002) – was determined. As shown in Figs. 3 and
4, levoglucosan correlates well with WSOC and WIOC in both summer and winter
(r2=0.45–0.77), suggesting that biomass burning is an important
source of OA in the rural region of Guanzhong Basin. For most of the periods
in this study, the MAC365,WS-BrC and MAC365,WI-BrC values are
within the range of MAC of biomass burning aerosols (e.g., 1.3–1.8 for
corn stalk (Li et al., 2016a), ∼1.37 for rice straw
(Park and Yu, 2016) and ∼1.9 for biomass burning (BB) smoke particles (Lin
et al., 2017). Also, Abs365,WI-BrC values in both summer and winter correlate
well with levoglucosan (r2=0.74 and 0.62, respectively),
demonstrating an important contribution of biomass burning to WI-BrC despite
the fact that levoglucosan itself is water soluble. The relationships
between the Abs365,WS-BrC and levoglucosan are much weaker
(r2=0.40 and 0.45 in summer and winter, respectively), suggesting
more complex sources of WS-BrC in the region.
Comparison of MAC365,WS-BrC values in the present study and those
reported in earlier studies in China, India and the United States (US).
Sampling siteSampling timeSeasonMAC365,WS-BrCReference(m2 g-1)Lincun, Shaanxi, China3–23 Aug 2016Summer1.00 ± 0.18This study20 Jan–1 Feb 2017Winter0.93 ± 0.25Xi'an, China1 Jun–31 Aug 2009Summer0.98 ± 0.21Huang et al. (2018)15 Nov 2008–14 Mar 2009Winter1.65 ± 0.36Beijing, China20 Jun–20 Jul 2009Summer1.8 ± 0.2Cheng et al. (2011)9 Jan–12 Feb 2009Winter0.7 ± 0.2Xianghe, Hebei, China9–14 Jul and 21 Jul–1 Aug 2013Summer0.38 ± 0.52*Teich et al. (2017)Wangdu, Hebei, China4–24 Jun 2014Summer0.55 ± 0.15*Mt. Waliguan, Qinghai, China1–31 Jul 2017Summer0.48Xu et al. (2020)Seoul, South Korea13 Aug–9 Sep 2013Summer0.28Kim et al. (2016)9 Jan–8 Feb 2013Winter1.02New Delhi, India24 Oct 2010–25 Mar 2011Winter1.6 ± 0.5Kirillova et al. (2014b)Los Angeles Basin, USmid-May–mid-June 2010Summer0.71Zhang et al. (2013)Southeastern US2007Annually0.3–0.7Hecobian et al. (2010)Atlanta, US17 May–29 Sep 2012Summer and Fall0.14–0.53Liu et al. (2013)
* Data at Xianghe and Wangdu were the averaged MAC of WSOC at
a wavelength of 370 nm (i.e., MAC370,WS-BrC).
Molecular characterization of BrC aerosols
Five categories of molecular tracer compounds, i.e., parent-polycyclic
aromatic hydrocarbons (parent PAHs), oxygenated PAHs (OPAHs), nitrophenols,
isoprene-derived products (SOAi) and α- and β-pinene-derived products (SOAp), were determined by the GC–EIMS
technique to investigate the formation pathways of BrC in this study. Their
average concentrations as well as daytime and nighttime differences are
summarized in Table 1, and the temporal variation profiles of the sum
concentrations of each category, together with levoglucosan time series, are
presented in Fig. S3.
Cross correlations between Abs365,WS-BrC, Abs365,WI-BrC, selected chemical compositions and RH in
summer. The numbers at the upper right denote the linear correlation
coefficients (r2) of the corresponding scatter plots.
PAHs and their oxidized products are important BrC chromophores, since the
large conjugated polycyclic structures are strongly light-absorbing in the
near-UV range (Samburova et al., 2016; Huang et al., 2018). A total
of 14 parent PAHs and 5 OPAHs (Table S2) were determined in this study.
Parent PAHs are unsubstituted PAHs mainly emitted directly from incomplete
combustion of coal, biofuel, gasoline or other materials, whereas OPAHs can
be emitted directly from combustion sources or formed from photochemical
oxidation of the parent PAHs. The time trends of parent PAHs and OPAHs are
highly similar in both seasons (r2=0.90 and 0.98 in summer and
winter, respectively; Figs. 3 and 4), suggesting that they have common
combustion sources. In addition, both parent PAHs and OPAHs presented good
correlations with levoglucosan, particularly in winter (r2=0.69
and 0.73, respectively; Fig. 4), indicating that biomass burning is an
important contributor to ambient particulate PAHs in the region. PAHs, as
well as levoglucosan, are elevated during nighttime in winter, corresponding
to enhanced biomass burning emissions from heating-related activities as
well as reduced boundary layer height at night. In contrast, the average
daytime concentrations of parent PAHs (11.6±5.7 ng m-3) and
levoglucosan (142±89 ng m-3) in summer are respectively about 1.95 and 2.58
times the values at night (Table 1). The daytime
enhancement of OPAH concentrations in summer is even more pronounced with
an average day / night ratio of ∼4.6 and as high as 9.8 for
individual OPAH species (e.g., 6H-henzo(cd)pyrene-6-one; Fig. S4). Both
parent PAHs and OPAHs, which are hydrophobic and thus mainly exist as WIOC,
demonstrate a good linear relationship with Abs365,WI-BrC in both
winter and summer (r2=0.49–0.83; Figs. 3 and 4). However, the
good correlation between OPAHs and Abs365,WI-BrC in summer appears to
be mainly driven by daytime production, as the correlation coefficient
(r2) is 0.72 for the daytime data but is < 0.1 for the
nighttime data (Fig. S5a). These results suggest that photochemical
formation of light absorption compounds is an important source of BrC during
summer in the Guanzhong Basin.
Cross correlations between Abs365,WS-BrC, Abs365,WI-BrC,
selected chemical compositions and RH in winter. The numbers at the upper
right denote the linear correlation coefficients (r2) of the
corresponding scatter plots.
We estimated the potential contribution of parent PAHs and OPAHs to the
light absorption of WI-BrC using a method reported in
Samburova et al. (2016). Details on the method are
presented in the Supplement. Table S3 summarizes the
solar-spectrum-weighed mass absorption coefficients for PAHs
(MACPAH,av) used in the calculation. As shown in Fig. 5, the
contribution of parent PAHs to solar-spectrum-weighed absorption coefficient
of WI-BrC varies between 0.55 % and 0.66 % with slight diurnal or season
variations (Table S3). However, the contribution of OPAHs clearly shows
higher daytime values, especially in summer. The average contribution of
OPAHs to the solar-spectrum-weighed absorption coefficient of WI-BrC in
summer is 0.51±0.28 % during daytime and 0.34±0.19 %
during nighttime. These results indicate that more secondary water-insoluble
aromatic chromophores were produced via photochemical oxidation during
summertime in the rural region.
Average contribution of parent PAHs and OPAHs to the bulk light
absorption of WI-BrC (300–700 nm) during daytime and nighttime of summer
and winter.
Nitrophenols were identified as one of the most important light-absorbing
compounds in particles and cloud water influenced by BB emission in China
(Desyaterik et al., 2013). These compounds can be
either directly emitted from burning of biomass (Xie et al., 2019)
or formed in the atmosphere through gas-phase and aqueous-phase reactions of
aromatic precursors including benz[a]pyrene (Lu et al., 2011),
naphthalene (Kitanovski et al., 2014), catechol and
guaiacol (Ofner et al., 2011), and toluene
(Liu et al., 2015) in the presence of NOx. In
this study, only a few nitrophenol compounds were detected in PM (Table S2)
and their average (±1σ) concentration is 0.94 (±0.26) ng m-3 in summer and 72.6 (±63.7) ng m-3 in winter. The
wintertime concentrations of nitrophenols measured in the current study are
comparable to those detected in Shanghai (Li et
al., 2016b), at Mt. Tai in Shandong Province of China
(Desyaterik et al., 2013) and Ljubljana in Slovenia
(Kitanovski et al., 2012), but the summertime concentrations
observed are more comparable to those detected in the Los Angeles Basin in
the US (Zhang et al., 2013). The substantially lower
concentration of nitrophenols in summer may be related to rapid
photodegradation in the atmosphere. Indeed, according to a laboratory study
conducted by Zhao et al. (2015), the timescale for
photobleaching of nitrophenols can be an hour or less. Furthermore, as
shown in Fig. S5b, during wintertime, when low temperature and weak solar
irradiation suppress the photodegradation process, nitrophenols' concentration
anticorrelates with the O3 mixing ratio in a nonlinear manner
(r2=0.60). On average, concentration of nitrophenols in winter is 2.5 times higher during nighttime than during daytime, whereas the nighttime
concentrations of levoglucosan and PAHs are only slightly higher than the
daytime concentrations (by 11 % and 33 %, respectively; Table 1).
Levoglucosan and PAHs are less photochemically reactive than nitrophenols.
These results confirm that nitrophenols, and other photoreactive BrC
compounds, may undergo significant atmospheric degradation during
summertime.
Both summertime and wintertime Abs365,WS-BrC correlated well with the
concentrations of nitrophenols (r2=0.51–0.72, Fig. S5c, d),
suggesting an important contribution of nitrated aromatic compounds to light
absorption of WS-BrC in the study area. Using the MAC of individual
nitrophenol reported in Zhang et al. (2013), we calculated
that the contributions of nitrophenols to aerosol light absorption are
6.5–27 times higher than their mass contributions to WSOC and that the
fractions are much higher in winter (2.44±1.78 %) than in summer
(0.12±0.03 %; Table S3). In addition, due to a significantly higher
abundance of nitrophenols during nighttime in winter, their fractional
contribution to aerosol absorption is on average 2.5 times higher than
during the day (3.47±2.03 % vs. 1.41±0.29 %).
Average direct solar absorption of water-soluble and water-insoluble
BrC during summer and winter.
On a global scale, biogenic VOCs, mostly consisting of isoprene and
monoterpenes, are nearly an order of magnitude more abundant than
anthropogenic VOCs (Guenther et al., 2006), and their secondary products
are estimated to be a predominant contributor to global SOA burden
(Heald et al., 2008). Recent studies (Lin et
al., 2014; Nakayama et al., 2012, 2015) showed that a large
number of biogenic SOA compounds are light absorptive. Some tracers of SOA
formed from isoprene (SOAi) and α- and β-pinene (SOAp)
oxidation were measured in the summertime samples (Table S2), and their
temporal variations are shown in Fig. S3. No biogenic SOA tracer species
were detectable in the winter samples in this study. Similar results were
obtained in our previous study at Mt. Hua in the Guanzhong Basin
(Li, 2011). These findings are consistent with low emissions
of biogenic VOCs and low oxidation rates in this region during cold seasons.
The average concentrations of SOAi and SOAp tracers in summer are
18.6±9.7 and 22.0±6.7 ng m-3, respectively. Neither
SOAi tracers nor SOAp tracers showed significant correlations with
the absorption coefficient of WSOC or WIOC, suggesting a low contribution of
biogenic SOA to aerosol light absorption in the region. In addition,
compared to the MAC values observed in this study, the MACs of biogenic SOA
reported in literature are much lower, on average, by nearly an order of
magnitude (Laskin et al., 2015), which further supports an
insignificant contribution of biogenic sources to BrC in this region. This
finding is consistent with the fact that the Guanzhong Basin is a highly
polluted region, where the major emission sources of organic aerosols are
anthropogenic.
Variation in BrC during extreme haze events in winter
In recent years, extremely severe haze events with very high PM2.5
concentrations (up to 500–600 µg m-3) and low visibility (lower
than 1 km) occurred frequently during wintertime in China (Huang
et al., 2014). In this study, a heavy haze event occurred during 21–26 January
when PM2.5 concentration at the rural site increased continuously from
∼100 to 430 µg m-3 and visibility
decreased from > 10 to ∼1.4 km (Fig. 1b,
d). Similar to most haze events occurring in northeast China, this event was
associated with stagnant meteorological conditions with low wind speed
(< 1 km s-1), which promote the accumulation of pollutants. In
addition, secondary inorganic aerosol species, e.g., SO42-,
NO3- and NH4+, increased sharply (Fig. 1d), which
indicates secondary aerosol formation was enhanced during the haze event
despite the low solar irradiance and low O3 concentration (e.g., 2--40µg m-3; Fig. 1c). Recent studies
by Wang et al. (2016) and Cheng et al. (2016) reported
dramatic increases in secondary inorganic components, mainly sulfate,
nitrate and ammonium (SNA), during haze periods in China and attributed the
increases to enhanced aqueous reactions under high-relative-humidity (RH)
conditions with NO2 being an important oxidant. Moreover,
Huang et al. (2014) observed that SOA also increased noticeably
during haze periods in winter. Indeed, as shown in Fig. 4, SO42- correlates well with RH (r2=0.64) and NO2 (r2=0.56) in
winter. In addition, Abs365,WS-BrC, which increases continuously
during the haze period with a peak value at 43.3 Mm-1 (Fig. 1e),
correlates well with RH (r2=0.65), sulfate (r2=0.84) and
NO2 (r2=0.70) (Fig. 4). In contrast,
Abs365,WI-BrC presents obvious diurnal variation during the haze
period, and the correlation of RH (r2=0.40), sulfate (r2=0.46)
and NO2 (r2=0.41) with Abs365,WI-BrC is also much
weaker than that with Abs365,WS-BrC. These results suggest that
aqueous oxidation played a role in the formation of WS-BrC
(Laskin et al., 2015) during the haze period, although
stagnant meteorological conditions with low wind speed can also promote its
accumulation. This finding is consistent with our previous study conducted
in Xi'an (Wu et al., 2020), which also found a secondary formation
of BrC in winter by using stable carbon isotope composition analysis. In
contrast, a slowly decreasing trend of MAC365,WIOC was observed during
the haze period, suggesting that some of the water-insoluble BrC species
were oxidized to form water-soluble chromophores, possibly through
aqueous-phase reactions.
It is worthwhile to mention that 27 January 2017 was the Chinese New Year's
Eve and a large number of fireworks were set off for celebration. During
this night, the concentrations of PM2.5, OC, EC, WSOC and WIOC as well
as SNA were 25 %–51 % lower than their wintertime average concentrations
due to the higher wind speed favoring atmospheric dispersion (Fig. 1).
However, the MAC365,WS-BrC (1.81) increased to about 2 times its
average value in winter, and Abs365,WS-BrC (20.5 Mm-1) also showed
a slight increase. Meanwhile, metal ions, which are abundant in fireworks
(Wu et al., 2018; Jiang et al., 2015), such as K+, Mg2+ and
Ca2+, increased substantially during the night as well (Fig. S6). These
results indicate that the increase in MAC365,WSOC during the Chinese
New Year's Eve is likely mainly contributed by metal-containing
light-absorbing compounds emitted from fireworks (Laskin et al.,
2015; Tran et al., 2017).
Estimation of direct absorption of solar radiation by BrC
Since the light absorption of BrC is mainly in the UV spectral region, an
important concern is that BrC can reduce the solar actinic flux and thus
affect atmospheric photochemistry and tropospheric ozone production
(Jacobson, 1998; Mohr et al., 2013). In this study, the direct absorptions
of solar radiation by both WS-BrC and WI-BrC were estimated by using Eq. (7).
Figure S7 presents the incident solar irradiance and actinic flux spectra
determined for the region under midday summer (10 August 2016 at 13:00
Beijing (BJ) time, UTC+8) and winter (25 January 2017 at 13:00 BJ time) conditions.
Note that the local time at Guanzhong is ∼1 h later than BJ time.
Table 3 presents a summary of the calculated direct solar absorptions of
BrC. In summer, the direct attenuation of actinic flux by WS-BrC and WI-BrC
is estimated at 1.55×1014±0.43×1014
and 1.03×1014±0.64×1014 quanta s-1 cm-2, respectively, in the UV range (300–400 nm), which accounts for
0.83±0.23 % and 0.53±0.33 %, respectively, of the total
down-welling radiation. In winter, the direct absorptions by BrC are higher,
with WS-BrC and WI-BrC on average accounting for 1.67±0.72 % and
2.07±1.24 %, respectively, of the total down-welling radiation in
the UV range. These results suggest that BrC may have a significant
influence on atmospheric photochemistry in the UV range. In the visible
spectral region (400–700 nm), the contributions of WS-BrC and WI-BrC to
the total down-welling radiation are negligible: 0.10±0.03 % and
0.07±0.05 % in summer and 0.15±0.06 % and 0.15±0.08 % in winter, respectively.
Another concern of BrC is that it can absorb solar irradiance to influence
tropospheric temperature in a similar way as black carbon (BC) or elemental
carbon (EC) (Feng et al., 2013; Laskin et al., 2015). In our study, the
direct absorption of solar irradiance by WS-BrC and WI-BrC is estimated at
0.51±0.14 and 0.34±0.21 W m-2 in summer and 0.57±0.25 and 0.68±0.41 W m-2 in winter in the UV range. To
evaluate the contribution of BrC to total aerosol absorption, we also
estimated the direct absorption of EC based on the carbon analyzer data
according to the method described by Kirillova et al. (2014b) and Kirchstetter and Thatcher (2012) (see Supplement).
The estimated contributions of light absorption of BrC relative to EC are
shown in Table 3. In the visible region, the contribution is estimated at
10.0±3.52 % in summer and 4.99±1.23 % in winter for
WS-BrC and 6.19±2.42 % and 4.51±1.44 %, respectively, for
WI-BrC. However, in the UV range, the fractions increase to 49.3±14.5 % in summer and 25.9±5.47 % in winter for WS-BrC and
29.4±11.0 % and 29.0±10.4 % for WI-BrC, which are within
the range of the values reported in other regions in China (Huang
et al., 2018), India (Kirillova et al., 2014b) and South Korea
(Kirillova et al., 2014a). On the other hand, the
direct light absorption of WI-BrC represents a substantive contribution to
that of total BrC in this study, which is about 40 % in summer and more
than 50 % in winter in both the UV and visible ranges, emphasizing the
important role that WI-BrC likely plays in atmospheric chemistry and the
Earth's climate system, especially in China.
Summary and conclusion
Both WS-BrC and WI-BrC showed elevated Abs in winter (4–7 times higher than
those in summer), corresponding to much higher concentrations of WSOC and
WIOC due to a combination of lower PBL height and enhanced primary emissions
(e.g., from residential heating) in the cold season. No significant
differences were found for the daytime and nighttime MACs of WS-BrC in
summer or for the MACs of WS-BrC and WI-BrC in winter. However, the average
daytime MAC365,WI-BrC was more than twice the nighttime value in
summer. We found that the average daytime concentrations of both parent PAHs
and levoglucosan in summer were around 2 times the values at night, and
the daytime OPAH concentration was more than 4 times the nighttime
value. Moreover, OPAHs correlated well with Abs365,WI-BrC in summer
during daytime (r2=0.72) but not during nighttime (r2<0.1). These results demonstrated that photochemical formation of BrC and
enhanced BB emissions (e.g., from cooking) contributed to the higher daytime
MACs in summer. In winter, the Abs of WS-BrC correlated strongly with
relative humidity, sulfate and NO2, suggesting that aqueous-phase
reactions played an important role in the formation of secondary BrC.
Abs365,WS-BrC correlated well with the concentrations of nitrophenols
in both seasons, suggesting an important contribution of nitrated aromatic
compounds to light absorption of WS-BrC. However, this contribution is much
lower in summer due to faster photodegradation reactions of these compounds.
WS-BrC and WI-BrC were estimated to account for 0.83±0.23 % and
0.53±0.33 %, respectively, of the total down-welling solar
radiation in the UV range in summer and 1.67±0.72 % and
2.07±1.24 %, respectively, in winter. The substantive contribution
of WI-BrC to total BrC absorption (∼40 % in summer and
> 50 % in winter) emphasizes the important role that WI-BrC
likely plays in atmospheric chemistry and the Earth's climate system.
Data availability
Data are available by contacting the corresponding authors.
The supplement related to this article is available online at: https://doi.org/10.5194/acp-20-4889-2020-supplement.
Author contributions
JiaL, QZ, GW, KFH and JC designed the experiment.
JiaL, GW and KFH arranged the sample collection. JinL, LaL and CW collected the samples. JiaL, JinL, JW, WJ
and LiL analyzed the samples. JiaL, QZ and GW performed
the data interpretation. JiaL, QZ and GW wrote the paper.
Competing interests
The authors declare that they have no conflict of interest.
Special issue statement
This article is part of the special issue “Multiphase chemistry of secondary aerosol formation under severe haze”. It is not associated with a conference.
Acknowledgements
This work was financially supported by the National Nature
Science Foundation of China (nos. 41773117, 91644102, 41977332, 91543116).
Jianjun Li also acknowledged the support of the Youth Innovation Promotion
Association CAS (no. 2020407). The authors gratefully acknowledge the National
Center for Atmospheric Research for the provision of the solar actinic flux
and irradiance data (Quick TUV Calculator, http://cprm.acom.ucar.edu/Models/TUV/Interactive_TUV/, last access: 19 March 2019) used
in this publication.
Financial support
This research has been supported by the National Nature Science Foundation of China (grant nos. 41773117, 91644102, 41977332, 91543116), and the Youth Innovation Promotion Association CAS (no. 2020407).
Review statement
This paper was edited by Jingkun Jiang and reviewed by three anonymous referees.
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