Introduction
Organic components of atmospheric aerosols are usually treated as solely
light scattering in global climate models over the near-ultraviolet and
visible range (UV/Vis) of the solar spectrum and therefore only make a
negative contribution to radiative forcing (Feng et al., 2013). However, the
existence of light-absorbing organic carbon (OC) has become more and more
evident in the past decade and can be a regionally important phenomenon (see
reviews of Andreae and Gelencser, 2006, and, more recently, Laskin et al.,
2015, and references therein and Ulevicius et al., 2010). In contrast to
black carbon (BC), which absorbs light efficiently over the whole visible
and UV region, light absorbtion by OC exhibits a distinct wavelength
dependence. The light absorption sharply increases with decreasing
wavelength, making it an efficient absorber in the UV/Vis range. Due to its
characteristic yellowish to brownish color, light-absorbing OC is also often
addressed as brown carbon (BrC).
The light absorption by BrC over the whole solar spectrum are found to be
relatively weak compared to BC (J. Liu et al., 2013). Nevertheless, at near-UV/Vis wavelengths (300–500 nm) BrC has a non-negligible effect on
radiative forcing and the regional and global climate (e.g., Bahadur et al.,
2012; Feng et al., 2013; Jo et al., 2016; Park et al., 2010). For instance,
modeling studies showed that the radiative forcing of BrC relative to BC is
up to 25 % (Feng et al., 2013). Furthermore, BrC light absorption in the
UV range of the spectrum may alter the concentrations of atmospheric oxidants
due to reduced photolysis rates (Jacobson, 1999).
The light absorption of ambient particles is generally quantified by the
determination of the particulate light absorption coefficient
babs, (in Mm-1), which can be normalized by the sample
mass to give the mass absorption efficiency (MAE; in m2 g-1). MAE
can then further be used to estimate the radiative forcing of particulate
BrC, which makes it an important parameter for modeling studies. These
models start with modeled emitted mass, which is converted into
concentrations using a dispersion model. The absorption of aerosols in the
atmosphere is then determined using the appropriate MAE (Feng et al., 2013).
The wavelength dependence of light absorption of a particle sample is
described by the absorption Ångström exponent (AAE) based on the
power-law dependence of babs:
babs,λ1babs,λ2=λ1λ2-AAE.
BC has a weak wavelength dependence and often an AAE of 1.0 is assumed,
which is the Mie theoretical value, though measured values range between
0.8 and 1.1 (Gyawali et al., 2009). Values greater than 1.1 indicate a stronger
wavelength dependence and give evidence for the presence of absorbers at
lower wavelengths, such as BrC and mineral dust (Wang et al., 2013).
Several methods exist to quantify the light absorption properties of BrC.
Filter-based and multi-wavelength measurement techniques are commonly used
to assess the contribution of BrC to the total aerosol light absorption
(Moosmüller et al., 2009). One of these instruments is the
seven-wavelength aethalometer, which operates from 370 to 940 nm. Due to
the main light absorption of BrC at short wavelengths, the 370 nm channel is
of particular interest. To estimate the contributions of BrC and BC to
aerosol light absorption, a simple approach based on wavelength pairs and
assumed AAEs to distinguish between the light absorption of BC and BrC has
often been used (Lack and Langridge, 2013; Sandradewi et al., 2008).
Another method is to extract aerosol particles into solvents, mostly water or
methanol (e.g., Bosch et al., 2014; Chen and Bond, 2010; Cheng et al., 2011;
J. Liu et al., 2013). The advantage of this approach is that BC is insoluble
in these solvents and therefore removed by filtration. The extracts are then
further analyzed by UV/Vis spectrophotometry. Commonly, light absorption at
365 nm is used to characterize water-soluble BrC (e.g., Bosch et al., 2014;
Cheng et al., 2011; Hecobian et al., 2010). This wavelength is in a range
where the light absorption of inorganics does not interfere, whereas the
light absorption of organics shows a sufficiently high intensity (Hecobian et
al., 2010). Observed MAE values (related to the water-soluble OC, WSOC) for
water-soluble BrC typically range between 0.41 and 1.80 m2 g-1
(Cheng et al., 2016; Hecobian et al., 2010), where higher values are usually
found in winter.
Until now, field measurements of the light absorption properties of
water-soluble BrC have been performed mostly in the US and Asia, and
measurements in Europe are scarce. Duarte et al. (2005) investigated the
optical properties of WSOC from aerosol samples collected at a rural site in
Portugal. They found a seasonal variation with a higher absorptivity in
autumn than in summer. Some studies have reported the optical properties of
humic-like substances (HULIS), which are a part of WSOC (e.g., Baduel et
al., 2010; Utry et al., 2013).
Sources of BrC are very diverse. Known primary sources are biomass burning
(BB; Kirchstetter and Thatcher, 2012; Lack et al., 2013; Mohr et al., 2013)
and fossil fuel and residential coal combustion (Bond, 2001; Olson et al.,
2015). Moreover, BrC can be produced by secondary reactions of anthropogenic
(Lin et al., 2015a; Nakayama et al., 2010) or biogenic precursors (Flores et
al., 2014; Kampf et al., 2012; Lin et al., 2014).
The molecular composition of BrC still remains largely unknown due to its
complex nature. Several attempts have been made to characterize BrC at a
molecular level (Lin et al., 2015a, b). Among the identified compound
groups are large macromolecules, like HULIS (Hoffer et al., 2006), aldol
condensation products (Noziere and Esteve, 2005, 2007) and
nitrogen-containing compounds formed by the reaction of atmospheric
aldehydes with ammonia or amines, e.g., imidazoles (Galloway et al., 2009;
Lin et al., 2015b; Yu et al., 2011). Nitrated aromatic compounds (NACs),
which are the focus of the present study, comprise another group of major
contributors to BrC (Jacobson, 1999; Mohr et al., 2013). Nitrophenols (NPs)
can be either emitted directly into the atmosphere, e.g., by traffic exhaust
(Nojima et al., 1983; Tremp et al., 1993) and wood burning (Hoffmann et al.,
2007), or secondarily formed by the nitration of precursor compounds like
phenol either in the gas phase or liquid phase (Bolzacchini et al., 2001;
Vione et al., 2002). NPs have been quantified at many different locations,
especially in Europe (Cecinato et al., 2005; Iinuma et al., 2010; Kahnt et
al., 2013; Zhang et al., 2010), but there is surprisingly little information
on NP concentrations in Asia (Chow et al., 2015). Nitrated salicylic acids
(NSAs) have also been recently detected in atmospheric aerosol particles
(Kitanovski et al., 2012; van Pinxteren and Herrmann, 2007). A strong
correlation with nitrate was found by Kitanovski et al. (2012) in samples
from Ljubljana, Slovenia, suggesting secondary formation from precursor
compounds such as salicylic acid, which has been found in biomass-burning
aerosols as a lignin degradation product (Iinuma et al., 2007).
Studying BrC at a molecular level is considered important, since even trace
levels of a compound could have a significant impact on the light absorption
properties of particles (Kampf et al., 2012). However, little is known about
the contributions of specific light-absorbing compounds to the light
absorption of either ambient aerosols or aqueous extracts from ambient
particles. Mohr et al. (2013) estimated the relative contribution of NACs to
particulate BrC light absorption to be about 4 % at 370 nm at a
measurement site in the United Kingdom with high influence of BB aerosols.
Zhang et al. (2013) calculated a contribution of NACs to aqueous extract
light absorption of 4 % from the Los Angeles Basin (USA).
The present study aims to expand the understanding of BrC and water-soluble
BrC by investigating its spatial and temporal variation in different, very
diverse environments (urban, rural, biogenic, high BB influence).
Furthermore, the contribution of individual light-absorbing organic
compounds to the BrC light absorption is included.
Experimental
Measurement campaigns
Measurements took place at five sampling sites (three German and two
Chinese), in six different atmospheric conditions. An overview over the
sites, sampling periods, geographical coordinates and the type of
atmospheric environment is given in Table 1. Average temperatures and wind
speeds are given as campaign averages.
The Leibniz Institute for Tropospheric Research (TROPOS (winter) campaign,
2014) is located in Leipzig in eastern Germany and can be regarded as a
moderately polluted urban background site impacted by a mixture of various
sources (van Pinxteren et al., 2016). During the measurement period this
site was strongly influenced by BB aerosols, as indicated by high
levoglucosan concentrations. The first half of the campaign (24 to 31
January) was characterized by low 12 h mean temperatures ≤ 0 ∘C. Afterwards, the temperature gradually increased towards the end of the
campaign (up to 10 ∘C as 12 h mean). The periods of lower
temperatures often coincided with increased emissions from residential
heating. The average wind speed was 2.2 m s-1.
The measurement site Waldstein in the Bavarian Fichtel Mountains (Waldstein
(summer) campaign, 2014; Plewka et al., 2006) is located about 180 km southwest of Leipzig in a low mountain range. The site is surrounded by forest,
where spruce is the dominant species. Measurements took place on a tower at
a height of 21 m (about 780 m above sea level). The measurement period was
characterized by low wind speeds (< 1.3 m s-1) and
predominantly sunny weather (average temperature of 19 ∘C).
Because of the surrounding forest, the influence of freshly emitted biogenic
organics is expected to be high.
Overview over sampling sites, used terminology, sampling
period, sampling time given in local time (LT) and according atmospheric conditions during the
sampling period.
Sampling site
Designation
Geographic coordinates
Sampling period (sampling duration per filter)
Comments
Germany
Roof of the Leibniz Institute for Tropospheric Research, Leipzig
TROPOS (winter)
51.35∘ N, 12.43∘ E
24 January to 8 February 2014 (12 h, day,06:00–18:00 LT, and night, 18:00–06:00 LT)
Urban background, strong influence of biomass-burning aerosol during measurement period (van Pinxteren et al., 2016)
Melpitz
Melpitz(winter)
51.53∘ N, 12.93∘ E
24 January to 4 February 2014 (24 h, 00:00–24:00 LT)
Rural background (Spindler et al., 2010)
Melpitz(summer)
16 to 25 July 2014 (24 h, 00:00–24:00 LT)
Bavarian Fichtel Mountains
Waldstein (summer)
50.14∘ N, 11.87∘ E
16 July to 25 July2014 (11 h, day, 07:00–18:00 LT, and night, 19:00–06:00 LT)
BayCEER Waldstein observatory, F-BEACH campaign, surrounded by forest, high local biogenic emissions (Plewka et al., 2006)
China
Xianghe
Xianghe(summer)
39.75∘ N, 116.96∘ E
9 to 14 July and 21July 2013 to 1 August 2013 (12 h, day, 06:00–18:00 LT and night,18:00–06:00 LT)
CAREBeijing–NCP 2013 campaign (Kecorius et al., 2015), rural back-ground, urban outflow
Wangdu
Wangdu (summer)
38.71∘ N, 115.13∘ E
4 to 24 June 2014(12 h, day, 06:00–18:00 LT, and night, 18:00–06:00 LT)
CAREBeijing-NCP 2014 campaign,rural background, biomass burning episode occurred during the measurement period (Kecorius et al., 2016)
Measurements at Melpitz, a rural background site, located about 50 km northeast of Leipzig, were carried out in winter (Melpitz (winter) campaign,
2014) and summer (Melpitz (summer) campaign, 2014). The Melpitz (winter)
campaign was characterized by an average temperature of -3 ∘C and
an average wind speed of 2.8 m s-1. The Melpitz (summer) campaign was
characterized by an average temperature of 22 ∘C and a mean wind
speed of 2.0 m s-1. The dates in winter and summer correspond to the
measurement periods of the TROPOS (winter) and Waldstein (summer) campaigns,
respectively. Due to the proximity of TROPOS and Melpitz, the two sites are
influenced by similar regional air masses.
The Chinese measurement sites Xianghe (Xianghe (summer) campaign, 2013) and
Wangdu (Wangdu (summer) campaign, 2014) are both located in the Hebei
Province in the North China Plain (NCP). The campaigns were part of the
CAREBeijing-NCP campaigns in 2013 and 2014. Xianghe is situated between the
two megacities of Beijing and Tianjin, and Wangdu is located 170 km southwest of Beijing. The average temperature and wind speed at the Xianghe
(summer) campaign were 26 ∘C and 0.8 m s-1, respectively.
For the Wangdu (summer) campaign a mean temperature of 26 ∘C and
a mean wind speed of 4 m s-1 were measured. Both sites can be regarded
as regional background stations for the NCP. In comparison to the summer
campaigns in Germany, the average PM10 (particulate matter with an
aerodynamic diameter ≤ 10 µm) concentrations were about 3 to
7 times higher at the Chinese sites.
Sampling, chemical analysis and back trajectories
PM10 was collected on quartz fiber filters with a Digitel DHA-80 high
volume filter sampler (MK 360, Munktell, Falun, Sweden; flow rate: 0.5 m3 min-1). To minimize blank content, the filters were
pre-baked for 24 h at 105 ∘C. Day and night samples (11 h or 12 h; see Table 1) were taken during each campaign except for Melpitz (winter)
and Melpitz (summer), where particles were collected for 24 h. After
sampling, filters were stored in clean aluminium tins at -20 ∘C
in the dark until extraction (extraction was done within a year after
sampling). It is assumed that storage at -20 ∘C prevents
chemical degradation of the sample. Analysis of WSOC, levoglucosan, NACs and
UV/Vis spectrophotometry measurements was carried out using aqueous filter
extracts of different portions of filter in ultrapure water. Details on the
different methods are given below. The aqueous filter extract was always
filtered through a pre-cleaned syringe filter (0.45 µm, Acrodisc 13,
Pall, Dreieich, Germany).
NACs and UV/Vis spectra were determined from the aqueous extract by
extracting 11–28 pieces of the filter (1.54 cm2 each) into 10 mL of
ultrapure water. NPs were analyzed according to the method described in
Teich et al. (2014) based on hollow fiber liquid-phase micro extraction for
analyte enrichment and capillary electrophoresis electrospray ionization
mass spectrometry (CE-ESI-MS, Agilent 3DCE instrument, Bruker Esquire
3000+ ion trap mass spectrometer). Samples from the Waldstein (summer)
campaign were not analyzed for NPs due to the limited availability of
filter material.
NSAs were enriched by evaporating an alkalinized aliquot of the aqueous
filter extract (1.8 mL plus 200 µL 10 mM NaOH) to dryness in a vacuum
concentrator (miVac, Genevac Ltd., UK) and redissolving the residue into 40 µL
ultrapure water. Subsequent analysis was carried out by CE-MS as
described in van Pinxteren et al. (2012). Standard addition was carried out
for quantifying the NSA compounds by co-injecting a standard solution plug
at different concentration levels into the CE capillary after the sample
plug.
Overall, eight NACs were determined as part of BrC: 3-nitrosalicylic acid
(3NSA) and 5-nitrosalicylic acid (5NSA), 4-nitrophenol (4NP),
2-methyl-4-nitrophenol (2M4NP), 3-methyl-4-nitrophenol (3M4NP),
2,6-dimethyl-4-nitrophenol (2,6DM4NP), 2,4-dinitrophenol (2,4DNP) and
3,4-dinitrophenol (3,4DNP). The standard compounds were purchased in high
purity (≥ 98 %) from either Fluka or Sigma-Aldrich (Munich,
Germany). As a typical BB tracer, levoglucosan can help to investigate the
influence of BB aerosols on the observed concentrations (Iinuma et al.,
2009; Simoneit, 2002). Levoglucosan was analyzed with a Dionex ICS-3000
system coupled with a pulsed amperometric detector (Thermo Fisher
Scientific, Sunnyvale, CA, USA). A portion of sampled filter (9.42 cm2)
was extracted in 20 mL of ultrapure water by shaking with a laboratory
orbital shaker for 120 min. Detailed chromatographic conditions and the
merits of analysis can be found elsewhere (Iinuma et al., 2009).
Levoglucosan data are available for the campaigns Xianghe (summer), Wangdu
(summer) and TROPOS (winter). No data are available for the Melpitz (winter),
Melpitz (summer) and Waldstein (summer) campaigns due to limited amount of
filter material.
OC and elemental carbon (EC) were determined from the filter by a
thermal–optical method using the Sunset Laboratory Dual-Optical Carbonaceous
Analyzer (Sunset Laboratory Inc., Tigard, OR, USA) following the EUSAAR 2
temperature protocol and applying a charring correction using light
transmission (Cavalli et al., 2010).
The aqueous extract for determining WSOC was prepared by extracting 21.5 cm2 of the filter samples from the Xianghe (summer), Wangdu (summer),
Melpitz (winter) and Melpitz (summer) campaign and 38.5 cm2 for samples
from the campaigns Waldstein (summer) and TROPOS (winter) into 25 and 30 mL of ultrapure water, respectively, followed by 20 min of ultrasonication.
After filtration, the extract was injected into a TOC-VCPH analyzer
(Shimadzu, Japan) operating in the NPOC (nonpurgeable organic carbon) mode.
More details on the method can be found in van Pinxteren et al. (2009).
The air mass origin was estimated by 96 h back trajectories calculated using
the HYSPLIT model (Stein et al., 2015).
Instrumentation for aerosol light absorption measurements
Particulate light absorption was measured by a seven-wavelength
aethalometer, model AE33 (Aerosol d.o.o., Slovenia), during the TROPOS
(winter) and Waldstein (summer) campaigns. A detailed description of the
instrument is given in Drinovec et al. (2015). Briefly, aerosol particles
are collected on a filter tape. Light attenuation is measured continuously
through this aerosol laden filter (time resolution of 1 s). When a fixed
attenuation threshold is reached, the tape advances to a new filter spot.
Filter-based measurements feature nonlinear loading effects, caused by the
increasing deposition of the sample in the filter loading during the
measurement, and filter matrix light scattering effects (Weingartner et al.,
2003). The aethalometer AE33 measures the loading effect by using a
dual-spot approach, where attenuation measurements are carried out
simultaneously on two differently loaded spots. Multiple scattering effects
were compensated by normalizing the particulate light absorption coefficient
babs against data from a multi-angle absorption photometer
(MAAP; Petzold and Schonlinner, 2004) which was also located at the
measurement sites. The determined normalization factor (for light scattering
effects) C was 1.69 for the TROPOS (winter) campaign and 2.06 for
the Waldstein (summer) campaign. More details are given in the Supplement
Sect. S1.
The relative contribution of BrC and BC to babs,370 was
determined following the method used by Kirchstetter and Thatcher (2012).
For the calculations, it was assumed that BC was the only absorbing species
at 940 nm, that OC and BC were externally mixed and that the AAE of BC was 1.0.
These assumptions are consistent with recently published studies (Martinsson
et al., 2015; Mohr et al., 2013). Using Eq. (1), babs,370,BC
was determined by extrapolating from 940 nm to 370 nm, and the contribution
of BrC was then calculated using Eq. (2):
babs,λ=babs,λ,BrC+babs,λ,BC.
The AAE was fitted over all seven wavelengths and is given as the average of
the wavelength dependencies with the measurement time resolution.
UV/Vis spectrophotometry
Light absorption of aqueous solutions were measured with a Lambda 900
UV/Vis spectrophotometer (Perkin Elmer) using quartz cells (Secomam,
France). The aqueous filter extracts were analyzed at pH≈2
(acidified with H2SO4, in the following also indicated with the
subscript “A”) and at pH≈10 (addition of NaOH, in the following
also indicated with the subscript “B”). Hence, the target compounds were
either in their neutral or deprotonated state to obtain the lower and upper
limit for the contribution of NACs to the BrC light absorption In principle,
it could be possible that introducing acids or bases into the system induces
unforeseen chemical reactions influencing the total light absorption of the
aqueous extract. However, to minimize the risk of potential chemical
modification the solutions where kept in the dark and analyzed as soon as
possible after preparation by UV-Vis spectrophotometry. The influence of the
pH on MAE and the aqueous light absorption coefficient (Abs) is discussed in
Sect. 3.1.1. Spectra were recorded from 300 to 800 nm.
The interpretation of the aqueous extract light absorption follows the
method described by Hecobian et al. (2010). In general, according to the
Beer–Lambert law the absorbance (Aλ) of a solution is
defined as
Aλ=-log10II0=l⋅c⋅ελ=l∑ici⋅ελ,i,
where I0 and I are the intensity of the incident and
transmitted light, respectively, l the absorbing path length,
c the concentration of absorbing species in solution and
ελ is the wavelength-dependent molar
extinction coefficient. The resulting data were then converted to Absλ:
AbsλMm-1=Aλ-A800VlVa⋅l⋅ln10,
where A800 is the reference wavelength to account for any
baseline drift, Vl is the volume of water used for extraction
(10 mL), Va is the volume of air passed through the filter
and ln(10) was used to convert the common logarithm to a natural logarithm.
The path length l was either 2 or 5 cm.
Absλ is the light absorption coefficient of the aqueous extract
solutions and is not to be mistaken for babs, the light
absorption coefficient of ambient particles. This nomenclature was chosen in
accordance with other studies (J. Liu et al., 2013, 2015).
Using the mass concentration of WSOC (in µg m-3), MAE was
calculated by
MAEm2g-1=AbsλWSOC.
The molar extinction coefficient at 370 nm, ε370,
was determined for each target compound by preparing a dilution series of
standard compounds in water. Subsequently, a three point curve was recorded
twice. According to the Beer–Lambert law, the slope of the regression in
an absorbance–concentration plot is proportional to ε. The molar extinction coefficients for each compound under acidic and
alkaline conditions are presented in Table S2.
In other studies dealing with aqueous extracts, Abs values were given for
365 nm (e.g., Bosch et al., 2014; Cheng et al., 2011; Hecobian et al.,
2010). We chose 370 nm to characterize the aqueous extract light absorption
properties to match the 370 nm channel of the aethalometer.
Calculation of the contribution of nitrated aromatic compounds to light
absorption of aqueous extracts and particulate brown carbon
The contribution of the target compounds to the aqueous extract light
absorption was computed by first calculating the absorbance of the single
compound using Eq. (3) and then determining the percentage on the total
aqueous solution light absorption.
To assess the contribution of the NACs to particulate BrC light absorption
at 370 nm, the molar extinction coefficient of each compound was converted
to the liquid-phase molecular absorption cross section bliq
(Jacobson, 1999):
bliqcm2molecule-1=1000ln(10)ε370NA,
where NA is the Avogadro constant and ln(10) is used to
convert the common logarithm of the molar extinction coefficient (base 10)
to the natural logarithm. The MAEcomp of each compound was then
calculated by dividing bliq by the mass of one molecule. The
subscript “comp” denotes the specific compound. If the MAEcomp and
the mass concentration in air of the compound are known,
babs,comp of the single compound in ambient particles can be
calculated by the following equation:
babs,comp=MAEcom⋅[comp],
where [comp] is the mass concentration of the compound in the aerosol.
By knowing babs,comp and babs,BrC, the relative
contribution of the individual compound on the particulate BrC light
absorption can be calculated.
Optical properties and calculated contribution of NACs to
the light absorption of aqueous extracts and particulate BrC. The values are
given as minimum–maximum (mean ± standard deviation). Acidic
conditions are indicated by the letter “A” and alkaline conditions are
indicated by the letter “B”. Highest and lowest mean values for each
category are marked in bold.
Waldstein (summer)
Melpitz(summer)
TROPOS(winter)
Melpitz (winter)
Xianghe (summer)
Wangdu (summer)
Optical properties
Abs370A (Mm-1)
0.21–0.70 (0.51 ± 0.11)
0.57–1.34 (0.84 ± 0.24)
0.33–8.96 (3.91 ± 2.49)
1.75–10.6 (4.13 ± 2.51)
0.82–4.68 (2.02 ± 0.81)
1.10–21.8 (4.64 ± 4.16)
Abs370B (Mm-1)
0.63–1.15 (0.88 ± 0.13)
0.88–1.78 (1.19 ± 0.30)
0.69–14.5 (6.80 ± 3.94)
3.28–15.2 (6.63 ± 3.50)
1.20–9.07 (3.16 ± 1.50)
1.87–27.2 (6.65 ± 5.55)
MAE370A (m2 g-1)
0.10–0.25 (0.17 ± 0.03)
0.19–0.30 (0.22 ± 0.03)
0.41–1.30 (0.84 ± 0.20)
0.52–1.79 (0.86 ± 0.33)
0.30–0.52 (0.38 ± 0.06)
0.31–1.01 (0.55 ± 0.15)
MAE370B (m2 g-1)
0.24–0.36 (0.29 ± 0.03)
0.27–0.43 (0.31 ± 0.05)
0.86–2.21 (1.45 ± 0.29)
0.84–2.57 (1.37 ± 0.45)
0.34–1.15 (0.59 ± 0.14)
0.49–1.40 (0.81 ± 0.21)
babs370 (Mm-1)
3.72–8.59 (6.23 ± 1.33)
4.13–129.4 (54.2 ± 34.07)
babs370,BrC (Mm-1)
0.62–1.69 (0.94 ± 0.28)
1.25–54.4 (21.8 ± 14.2)
Contribution to light absorption (%)
NACs to Abs370A
0.02–0.33 (0.15 ± 0.10)
0.06–0.22 (0.13 ± 0.05)
0.03–4.22 (1.21 ± 0.82)
0.15–2.03 (0.87 ± 0.57)
0.04–1.32 (0.46 ± 0.24)
0.24–1.83 (0.56 ± 0.31)
NACs to Abs370B
0.02–0.25 (0.13 ± 0.03)
0.10–0.48 (0.24 ± 0.12)
0.97–9.71 (3.35 ± 2.04)
0.47–5.00 (2.48 ± 1.45)
0.21–2.85 (0.1.02 ± 0.0.54)
0.56–4.44 (1.34 ± 0.72)
NACs to babs370,BrC (A)
0.01–0.22 (0.10 ± 0.06)
0.01–1.13 (0.25 ± 0.21)
NACs to babs,370,BrC (B)
0.02–0.31 (0.13 ± 0.09)
0.01–5.42 (1.13 ± 1.03)
Results/discussion
Optical properties of water-soluble and particulate brown carbon
In the following section, the general optical properties of water-soluble
and particulate BrC are presented. Optical properties of water-soluble BrC
(represented by the aqueous extract light absorption) are given as
MAE370 and Abs370.
Particulate BrC was characterized by the particulate light absorption
coefficient (babs,370) and the AAE. The results of the optical
properties are summarized in Table 2.
Influence of acidic and alkaline conditions on the mass absorption
efficiency and the aqueous light absorption coefficient
Values for MAE and Abs have usually been directly derived from filtered
aqueous filter extract without considering the pH of the solutions (e.g.,
Bosch et al., 2014; Cheng et al., 2011; Hecobian et al., 2010). However,
depending on the nature of the absorbing species, pH alters the
absorptivity. Alif et al. (1987, 1990, 1991) showed for NPs that their anionic
form is a much stronger absorber than their neutral form. Moreover, the
absorption maximum shifts towards longer wavelengths under alkaline
conditions. The dissociation in a solution depends on the pKa value of
the compound and at different pH levels different mixtures of
neutral/ionized compounds exist. Generally, ambient particles are estimated
to be more acidic (pH < 4, Scheinhardt et al., 2013), making the
findings under acidic conditions more relevant for ambient conditions.
However, Hinrichs et al. (2016) reported that NPs adsorbed to a particle
surface exhibit a significant red shift similar to the aqueous light
absorption spectrum of their deprotonated form. Hence, under certain
conditions the deprotonated forms can also be important.
Temporal variation of the concentrations of nitrated aromatic
compounds and levoglucosan (Levo) at each measurement site (a–f) and their
optical properties. The aqueous light absorption coefficient (Abs) and mass
absorption efficiency (MAE) are given at 370 nm for acidic conditions
(indicated by the subscript “A”) and for alkaline conditions (indicated by
the subscript “B”).
![]()
Comparing alkaline and acidic conditions, strong correlations have been
found for Abs, MAE and the relative contribution of the individual compounds
to Abs over all campaigns (R2≥0.93, see Fig. S3). On average,
Abs370 increased by a factor of 1.4 and MAE by a factor of 1.6 under
alkaline conditions compared to the acidic conditions. As indicated by the
good correlation, this factor is quite consistent and independent of the
measurement site.
The molar extinction coefficient at 370 nm increases with pH for the
individual compounds (see Table S2). A higher pH leads to deprotonation of
hydroxyl and carboxyl groups, which may lead to a shift of the absorption
maximum towards longer wavelengths. The light absorption of the aqueous
extracts is a sum of many different light-absorbing species. Therefore, the
relative contribution to the aqueous extract light absorption for individual
compounds is affected differently. For example, the relative contribution of
3NSA and 2,6DM4NP to Abs370 decreases at pH 10. No difference in the
contribution to Abs370 was observed for 5NSA. The highest increase in
the contribution to Abs370 was found for 2,4DNP with a factor of 6.8.
The observed differences in the contribution to Abs370 are related to
both the molar extinction coefficient, which alters with the pH, and the
composition of the solution. A decrease in the relative contribution to the
light absorption at higher pH is most likely due to greater contributions of
other, now overlapping light-absorbing compounds at this specific
wavelength.
The increase of Abs370 with higher pH is relatively modest (a factor of
1.4) compared to the increase of the molar extinction coefficient of
individual NACs (an average factor of 4.5). This might indicate that the
absorption properties of the majority of light-absorbing compounds are
barely affected by the pH. These findings are only valid for the
investigated wavelength of 370 nm and are likely to differ over the whole
spectral range.
Scatter plot of the aqueous light absorption coefficient
Abs370 with levoglucosan concentration for the campaigns (a) TROPOS
(winter,), (b) Xianghe (summer), (c) Wangdu (summer, BB episode) and (d)
Wangdu (summer, non-BB episode). Abs370 is given for acidic
conditions (indicated by the subscript “A”, black squares) and for
alkaline conditions (indicated by the subscript “B”, red dots). Note
different axis scales.
![]()
Due to the close relationship between MAE and Abs under alkaline and acidic
conditions, only the values for the acidic conditions are discussed in the
following section (Sect. 3.1.2), while both of them are always displayed in
the figures and tables.
Mass absorption efficiency and aqueous light absorption coefficient
of water-soluble brown carbon
The temporal evolution of Abs370 and MAE370 for each
measurement site is shown in Fig. 1. The highest mean MAE370A values were found for the campaigns Melpitz (winter,
0.86 ± 0.33 m2 g-1) and TROPOS (winter, 0.84 ± 0.20 m2 g-1)
followed by Wangdu (summer, 0.55 ± 0.15 m2 g-1) and Xianghe
(summer, 0.38 ± 0.06 m2 g-1). The lowest values were observed
under summer conditions in Germany, where the values for the Melpitz
(summer) campaign slightly exceeded the MAE370A observed at the
Waldstein (summer) campaign (0.22 ± 0.03 m2 g-1 over
0.17 ± 0.03 m2 g-1). Measured MAE370A values for China
are in the same range or slightly lower than values reported in other
studies. For instance, several studies were conducted in Beijing in summer,
where Cheng et al. (2011) determined an average MAE of 0.7 m2 g-1,
Du et al. (2014) found 0.5 m2 g-1 and Yan et al. (2015) observed
0.7 m2 g-1 (all values determined at 365 nm). The findings for
the TROPOS (winter) and Melpitz (winter) campaigns suggest regional behavior
and are comparable to observed MAEs from urban sites in the US (Hecobian et al. (2010), 0.47–0.87 m2 g-1, annual mean, determined
at 365 nm). Low MAE values, similar to the observed MAEs at the
Melpitz (summer) and Waldstein (summer) campaigns, have been found at urban
and rural sites in the US (0.21 and 0.12 m2 g-1, respectively,
determined at 365 nm) when the concentration of levoglucosan was low
(< 50 ng m-3). Although the average Abs370A is similar
for the campaigns of Wangdu (summer), TROPOS (winter) and Melpitz (winter)
(4.64, 3.92 and 4.13 Mm-1, respectively), the observed mean
MAE370A for the Wangdu (summer) campaign is lower, indicating that
despite higher concentrations of OC and WSOC during the Wangdu (summer)
campaign (see Table S3), fewer light-absorbing compounds were present.
A correlation between levoglucosan and Abs370 was found during the
TROPOS (winter) campaign (R2=0.8) and during the BB episode of the
Wangdu (summer) campaign (R2=0.9), when BB aerosol was abundant
(see Fig. 2). This correlation suggests that BB aerosol is a major
contributor to the aqueous extract light absorption. The connection between
BB and light-absorbing BrC was also observed in a number of other studies
(e.g., Desyaterik et al., 2013; Hoffer et al., 2006; Lack et al., 2013; Mohr
et al., 2013). During non-BB episodes of the Wangdu (summer) campaign and
the Xianghe (summer) campaign, only a weak or no correlation at all was
observed between levoglucosan and Abs370 (see Fig. 2). This indicates
the presence of sources for light-absorbing compounds other than BB
aerosols. A decay of levoglucosan more rapid than the decay of NACs might
also be a cause for this observation (Hoffmann et al., 2010).
Temporal evolution of the particulate light absorption coefficient
babs at 370 nm derived from the aethalometer measurements
during (a) the TROPOS (winter) campaign and (b) the Waldstein (summer)
campaign. The fractions of black carbon (dark grey area) and brown carbon
(light grey area) to the total measured babs were calculated
as described in Eq. (2).
![]()
Regarding seasonal differences, the aqueous extract light absorption was
much lower in summer compared to winter samples. This is a general trend,
observed at many measurement sites (e.g., Du et al., 2014; Hecobian et al.,
2010; Kim et al., 2016). Kim et al. (2016) suggested that this is either
because the amount of emitted BrC is much higher during winter or that
summer BrC is less water soluble. By comparing aqueous extract light
absorption to ambient particle light absorption measurements, this issue will
be further explored and discussed in the following section.
Particulate light absorption coefficient and absorption
Ångström exponent of particulate brown carbon
The temporal evolution of the measured particulate light absorption
coefficient babs at 370 nm as well as the determined
contributions of particulate BrC and BC to the light absorption at 370 nm
are shown in Fig. 3. Normalizing the determined babs for particulate
BrC by the according OC content gives the mass absorption efficiency for BrC
in the particle (MAE370,BrC,particle). The temporal variation of
MAE370,BrC,particle is displayed in Fig. S4. The AAE (as campaign
average) obtained by fitting over all aethalometer wavelengths was
1.5 ± 0.1 and 1.2 ± 0.3 for the campaigns TROPOS (winter) and
Waldstein (summer), respectively. An AAE close to 1 indicates that BC is
the dominant species (Kirchstetter et al., 2004). The increased AAE values
indicate that BrC plays a larger role for the TROPOS (winter) campaign than
for the Waldstein (summer) campaign, where its contribution is minor.
During the BB influenced TROPOS (winter) campaign, the mean total
babs,370 was 54 Mm-1. Particulate BrC contributed about
40 % to the overall light absorption at 370 nm and an average
MAE370,BrC,particle of 1.95 m2 g-1 was calculated. The
influence of BB on the light absorption of ambient particles has also been
observed in other studies. For instance, Kirchstetter and Thatcher (2012)
estimated that the contribution of BrC to the light absorption of wood smoke
is 49 % below 400 nm from samples collected in wintertime California
(USA). Mohr et al. (2013) found a BrC contribution of 46 % to
babs,370 at a UK site that was highly influenced by BB. Our
findings on the contribution of BrC to babs, 370 during the
TROPOS (winter) campaign are consistent with these earlier measurements. The
light absorption was observed to be higher for easterly air masses than
those from the west, a trend also seen in the NAC concentrations of the
TROPOS (winter) campaign (see Sect. 3.2). Easterly winds were often
accompanied by lower temperatures or thermal inversion layers or both.
Therefore, higher concentrations of light-absorbing compounds (and higher
particulate light absorption) might be a result of both increased emissions
(locally and in the source region) and lower dispersion of absorbing
pollutants due to the thermal inversion layer.
In contrast, the light absorption of aerosol particles was much lower during
the Waldstein (summer) campaign with a mean total babs,370 of
6 Mm-1, a MAE370,BrC,particle of 0.21 m2 g-1
and a BrC contribution to the particulate light absorption of about 15 %
at 370 nm. The measurements at Waldstein (which is surrounded by a mixed
forest) were carried out in a sunny summer period when high biogenic
emissions would strongly influence the results. Several laboratory studies
have shown that the formation of light-absorbing compounds from biogenic
precursors is negligible (e.g., P. F. Liu et al., 2013; Nakayama et al.,
2010; Song et al., 2013). However, a few studies have suggested that BrC
might be formed in the presence of high ammonia concentrations (Flores et
al., 2014; Updyke et al., 2012) or from isoprene epoxydiols (IEPOX; Lin et
al., 2014). Findings from field measurements in the Amazon Basin confirmed
that the light absorption by biogenic aerosol is much lower than the light
absorption of BB aerosols or BC (Rizzo et al., 2011). Compared to the TROPOS
(winter) campaign, babs for BrC is very small for the Waldstein
(summer) campaign, with an average of 0.94 Mm-1 compared to
21.8 Mm-1. Our findings corroborate those of Rizzo et al. (2011) and
laboratory studies stating that biogenic aerosols alone produce only small
amounts of light-absorbing compounds.
The relative contribution of light-absorbing WSOC to particulate light
absorption can be estimated by applying a conversion factor. Based on Mie
theory calculations, J. Liu et al. (2013) suggested a factor of 2 to
convert the aqueous extract light absorption coefficient (Abs) into the
particulate light absorption coefficient (babs). The temporal
variation of the fraction of the converted Abs370 to the particulate BrC
light absorption is given in Fig. S4. For our data, a good agreement between
the Waldstein (summer) average Abs370A (0.51 Mm-1×2) and the Waldstein (summer) average babs,370,BrC
(0.94 Mm-1) was achieved, suggesting that the observed particulate
light absorption is mainly associated with water-soluble compounds during
this campaign. For the TROPOS (winter) campaign, doubling the average
Abs370A gives a value of 7.8 Mm-1, which is still much
lower than the average babs,370,BrC (21.8 Mm-1). Thus, a
large fraction of non-water-soluble OC likely exists as well, which is highly
light absorbing at 370 nm.
Concentrations of nitrated aromatic compounds under different
atmospheric conditions
This section presents an overview and a comparison of the NAC concentrations
for all campaigns. Table 3 summarizes the measured concentrations in
comparison with other studies at similar locations. The results of the
temporal variation of the target compounds for each campaign are displayed
in Fig. 1.
Measured concentrations of nitrophenols and nitrated
salicylic acids for each campaign in comparison to other studies. The
concentrations are given as minimum–maximum (mean ± standard
deviation).
Location
Concentration in ng m-3
Reference
4NP
2M4NP
3M4NP
2,6DM4NP
2,4DNP
3,4DNP
3NSA
5NSA
Europe
TROPOS (winter), Germany,Jan–Feb 2014
1.08–27.2 (7.09 ± 7.08)
0.75–12.3 (3.64 ± 3.05)
0.50–8.58 (2.60 ± 2.22)
0.11–2.29 (0.65 ± 0.58)
n.d.
n.d.
0.01–3.72 (1.36 ± 1.02)
0.02–2.95 (0.94 ± 0.75)
This study
Melpitz (winter), Germany, Jan–Feb 2014
0.51–10.3 (4.09 ± 3.27)
0.29–8.90 (3.64 ± 3.06)
0.15–6.40 (2.44 ± 2.20)
0.04–2.74 (0.91 ± 0.90)
0.003–0.01 (0.01 ± 0.00)
n.d.
0.09–2.30 (0.66 ± 0.69)
0.06–0.77 (0.32 ± 0.24)
This study
Detling, UK, Jan–Feb 2012
(0.02)b
(5.0)b
–
(3.0)b
(3.0)b
Mohr etal. (2013)
Ljubljana, Slovenia, Dec 2010 to Jan 2011
0.5–3.7 (1.8)
0.31–1.5 (0.75)
0.25–1.2 (0.61)
–
0.02–0.05 (0.02)
–
0.1–3.9 (1.3)
0.2–3.4 (1.4)
Kitanovski etal. (2012)
Mainz, Germany, Winter2006/2007
(5.5)a
–
–
–
–
–
–
–
Zhang etal. (2010)
Rome, Italy, Feb to Apr 2003
(17.8)
–
(7.8)
(5.9)
–
–
–
–
Cecinato et al. (2005)
Hamme, Belgium, winter2010/2011
0.92–3.0 (1.19)b
–
–
–
–
–
–
–
Kahnt etal. (2013)
Melpitz (summer), Germany,Jul 2014
0.01–0.12 (0.06 ± 0.03)
0.03–0.04 (0.04 ± 0.00)
0.02–0.03 (0.03 ± 0.00)
n.d.
n.d.
n.d.
0.07–0.42 (0.17 ± 0.15)
0.02–0.24 (0.09 ± 0.09)
This study
Waldstein (summer), Germany, Jul 2014
–
–
–
–
–
–
0.01–0.35 (0.17 ± 0.11)
0.06–0.51 (0.23 ± 0.12)
This study
Ljubljana, Slovenia, Aug 2010
0.12–0.17 (0.15)
< 0.03–0.05 (0.05)
< 0.03
–
< 0.01
–
0.06–0.12 (0.09)
0.14–0.24 (0.09)
Kitanovski etal. (2012)
Mainz, Germany, summer 2006
(2.8)a
–
–
–
–
–
–
–
Zhang et al. (2010)
Hamme, Belgium, summer2010/2011
0.17–0.62 (0.29)b
–
–
–
–
–
–
–
Kahnt etal. (2013)
China
Xianghe (summer), China,Jul–Aug 2013
0.13–4.49 (0.98 ± 0.78)
0.01–0.85 (0.32 ± 0.21)
0.004–0.46 (0.09 ± 0.07)
0.01–0.23 (0.06 ± 0.05)
2.00 × 10-4–0.10 (0.02 ± 0.03)
0.004–0.17 (0.03 ± 0.04)
0.13–8.99 (1.21 ± 1.45)
0.11–2.70 (0.88 ± 0.64)
This study
Wangdu (summer), China,Jun 2014
0.55–15.0 (2.63 ± 2.66)
0.01–4.35 (0.68 ± 0.78)
0.01–2.03 (0.21 ± 0.35)
3.54 × 10-4–0.47 (0.06 ± 0.09)
0.002–0.41 (0.08 ± 0.10)
0.003–0.02 (0.01 ± 0.01)
0.37–11.3 (3.14 ± 3.05)
0.43–3.27 (1.63 ± 0.78)
This study
Hong Kong, summer 2010–2012c
(0.3)
(0.2)
(0.02)
(0.009)
–
–
–
–
Chow etal. (2015)
a Estimated from graph.b Sum of corresponding m/z ratios, including all isomers.c Concentrations are given as the average of three years.n.d.: not detected
A general concentration trend of 4NP > 3M4NP > 2M4NP
> 2,6DM4NP > 2,4DNP > 3,4DNP and 3NSA
> 5NSA was observed for all campaigns of the present study and is
in good agreement with findings from most other studies. Regarding the sum
of NPs, the mean concentrations were highest during the TROPOS (winter)
campaign (14.0 ng m-3) followed by the Melpitz (winter) campaign
(11.1 ng m-3). These concentrations are a factor of 3 to 10 higher than those
observed during the campaigns Wangdu (summer) and Xianghe (summer) (4.4 and 1.4 ng m-3, respectively). During the Melpitz (summer)
campaign, the NP concentration was 100 times lower (average of 0.1 ng m-3) than during the Melpitz (winter) campaign. For the sum of NSAs,
the highest mean concentrations were found at the Wangdu (summer) campaign
(4.8 ng m-3) followed by the campaigns Xianghe (summer) and TROPOS
(winter) (average of 2.2 ng m-3, each). Average concentrations for NSAs
of ≤ 1 ng m-3 were observed at the campaigns Melpitz (winter,
1.0 ng m-3), Waldstein (summer, 0.4 ng m-3) and Melpitz (summer,
0.2 ng m-3).
A clear seasonality can be seen in the data from the German sites. The
observed concentrations of the different species are generally lower in
summer than in winter because the sources of NACs are stronger during winter
and generally connected to polluted air, as discussed in the Sect. 1.
The campaigns Melpitz (winter) and TROPOS (winter) were carried out in the
same period and the two sites are directly comparable due to their
geographic proximity. Concentrations of the NACs are in the same range at
both sites and show a common pattern. Two periods with concentration maxima
were found. The first period occurred around 25 January and the second peak
was around 30 January 2014 (see Fig. 1a and b). Both periods are
characterized by prevailing easterly winds and shorter mean lengths of
96 h back trajectories. In contrast, air masses during other periods
originated from the southwest and had longer trajectories and increased
residence times over the Atlantic Ocean. Higher concentrations of PM and
many of its constituents with easterly winds are often observed at Melpitz
(Spindler et al., 2010). The long residence time above the continent results
in increased concentrations of pollutants in those air masses. Furthermore,
as described in Sect. 3.1.3, the temperature was generally lower when the
air mass originated from the east, which likely led to higher emissions from
residential heating both locally and in the air mass source regions.
The campaigns Melpitz (summer) and Waldstein (summer) were also conducted in
the same time period. The concentrations of NSAs are comparable at both
sites (0.01–0.51 ng m-3) and are very low compared to the winter
measurements. NPs were not analyzed from the Waldstein (summer) campaign as
explained above.
Day and night samples were collected during the campaigns TROPOS (winter),
Waldstein (summer), Xianghe (summer) and Wangdu (summer). Nighttime
concentrations of the sum of NACs were found to be slightly higher than
during the day (not statistically significant at 95 % confidence level).
The lower daytime concentrations are probably caused by the higher boundary
layer heights during the day, i.e., dilution effects, and/or lower
emission/formations rates during daytime or photolysis processes.
The sum of all target compounds at the TROPOS (winter) campaign was only
weakly correlated with levoglucosan (R2=0.47, Fig. S5), indicating
that BB was not the dominant source of NACs during the campaign. As
mentioned above, the formation or decay processes of levoglucosan and NACs
might be very different, which resulted in no correlation of those species.
It is also known that the sources of NACs are very diverse (see Sect. 1).
At the Wangdu (summer) campaign, a BB episode was observed between 11 June
and 19 June (see Fig. 1f), indicated by a peak in measured levoglucosan
concentrations. During that period, increased concentrations of NACs were
found. Although the NAC concentration generally increased with increasing
levoglucosan concentration, no clear connection between NACs and BB aerosols
was found. For the Xianghe (summer) campaign, levoglucosan concentrations
(average of 109 ng m-3) were lower than for the Wangdu (summer) or
TROPOS (winter) campaign, suggesting that BB played a minor role at this
site during the time of measurements. No correlation was found between the
sum of NACs and levoglucosan. This implies that BB was neither the major
source of NACs in the Wangdu (summer) nor in the Xianghe (summer) campaign.
It was found that NACs can be a product of the photochemical processing of
anthropogenic volatile organic compounds (Jaoui et al., 2008), which might be
a possible source for NACs at the Chinese sites besides BB.
NACs seem to be ubiquitous and are found at each site in measurable amounts
although they are highly variable at the different measurement sites. The
sum of NAC concentrations at the different campaigns followed the order
TROPOS (winter) > Melpitz (winter) > Wangdu (summer)
> Xianghe (summer) > Waldstein (summer)
>
Melpitz (summer). This trend might be considered surprising, since the
average PM10 and OC values were higher at the Chinese sites than at the
German sites (see Table S3). However, the seasonality and influence of BB
aerosol seem to play a large role in the NAC concentration resulting in
lower concentrations at the Chinese summer sites than at the German winter
sites.
Contribution of nitrated aromatic compounds to water-soluble and
particulate brown carbon
In this section, the contribution of NACs to the aqueous extract light
absorption and then the contribution of NACs to the particulate BrC light
absorption are discussed. The relative contribution of NACs to Abs370
is shown in Fig. 4 together with the contribution of NACs to the WSOC
concentration. Table 2 summarizes the contribution of NSAs and NPs to
aqueous extract light absorption (Abs) and particulate BrC light absorption
(babs). The focus is on the relative contribution to the
aqueous extract light absorption at the wavelength of 370 nm to match the
370 nm channel of the aethalometer. To give additional information, the
relative contribution of NACs to Abs (acidic conditions) over a spectral
range of 300 to 500 nm is presented in Fig. 5 as their campaign averages.
The relative NAC contribution to Abs370 varied greatly across the
different sites, ranging from 0.02 to 4.41 % for acidic conditions and
from 0.02 to 9.86 % for alkaline conditions. This result likely reflects
the dependence of NAC source strength on location and seasonality. The
maximum contribution of NACs to Abs370 was found for the TROPOS
(winter) campaign. The contribution of NACs to Abs370 was low for the
campaigns Waldstein (summer) and Melpitz (summer). For the Waldstein
(summer) campaign, a mean contribution of 0.15 % (acidic) or 0.13 %
(alkaline) was observed. Average values of 0.13 % (acidic) and 0.24 %
(alkaline) were obtained from the Melpitz (summer) campaign. This result is
not surprising due to the low influence of BB aerosols or traffic and thus
low NAC concentrations. A recent study by Nguyen et al. (2013) suggested the
formation of BrC from ketoaldehydes derived from biogenic monoterpenes in
the presence of ammonium ions. Thus, this reaction may play a role in
regions with higher influence of biogenic emissions and might be one
explanation for the observed absorption at the Melpitz (summer) and
Waldstein (summer) campaigns. The formed species were suggested to consist
of conjugated aldol condensates, secondary imines and nitrogen containing
heterocycles, The results for the Xianghe (summer) and Wangdu (summer)
campaigns are about half the values of the TROPOS (winter) or Melpitz
(winter) campaigns. This shows that NACs in the present study had a much
higher impact on the light absorption properties during the German winter
campaigns than during the Chinese summer campaigns and indicates that other
compounds play a larger role in particle light absorption at the Chinese
sites. More studies are needed to resolve the molecular identity and
abundance of these light-absorbing compounds.
Top and center: contribution of the sum of NACs to the aqueous
extract light absorption at each site under acidic (a) and alkaline (b)
conditions. Bottom: mass contributions of the sum of NACs to the WSOC (c).
![]()
The relative contribution of NACs to Abs is wavelength dependent. Under
acidic conditions, the contribution of NACs to the aqueous extract light
absorption increases towards lower wavelength, reaching a maximum that was
found to generally lie in the range of 330–350 nm (see Fig. 5). Moreover,
it can be seen that maximum values differ for individual compounds. Under
alkaline conditions, this maximum shifts towards the range of 400–420 nm
(see Fig. S6). As stated above, the results for acidic conditions are likely
to be more atmospherically relevant. Therefore, a higher influence of NACs
towards shorter wavelengths suggests that they can be more important in
terms of their influence on atmospheric photochemistry, e.g., O3
photolysis (Jacobson, 1999).
Relative contribution of NACs to Absλ over the
spectral range of 300 to 500 nm for each measurement campaign (a–f) under
acidic conditions. The data are presented as campaign averages. Due to
instrumental issues, data for lower wavelengths are not always available.
![]()
The relative mass contribution of NACs to the WSOC concentration at the
different campaigns follows the same pattern as the contribution of NACs to
the aqueous extract light absorption and also the same pattern as the NAC
concentrations (see Sect. 3.2 and Fig. 4). The highest mass contributions of
NACs were found during the TROPOS (winter) campaign followed by the Melpitz
(winter) campaign, and lowest values were observed for the campaigns
Waldstein (summer) and Melpitz (summer). The average mass contribution of
NACs to WSOC is less than 0.2 % at each site. In contrast, NACs have a
mean contribution to aqueous extract light absorption of 1 %, which is a
factor of 5 larger than their mass contribution to WSOC. Our results
corroborate the findings of other studies, which show that even small
amounts of light-absorbing compounds can significantly impact the light
absorption properties of particles. As a result, a detailed investigation of
the molecular composition of BrC is important (Kampf et al., 2012; Laskin et
al., 2015).
The correlation between NAC concentrations and Abs370 for each campaign
is displayed in Fig. 6. The correlation is good for the TROPOS (winter) and
Melpitz (winter) campaign (R2=0.7, each) and weaker for the Wangdu
(summer) campaign (R2=0.6). No correlation was found for the
campaigns Xianghe (summer), Melpitz (summer) and Waldstein (summer). In
general, the presence of high NAC concentrations also indicates higher
values for Abs370 than at sites with lower NAC concentrations. However,
due to the diverse sources and sinks (e.g., photobleaching, Zhao et al.,
2015) of NACs and BrC, NACs cannot be considered as the only tracers for
BrC for very different atmospheric conditions.
The average contribution of NACs to particulate BrC light absorption was
found to be 0.10 % for the Waldstein (summer) campaign and 0.25 % for
the TROPOS (winter) campaign for the protonated forms and 0.13 and
1.18 %, respectively, for their deprotonated forms. The contribution
during the Waldstein (summer) campaign is similar for the aqueous extract
and aerosol light absorption, which is consistent with the assumption
discussed above that the observed light absorption is mostly derived from
WSOC and no further water-insoluble BrC was present at this site.
Scatter plots of the sum of NACs concentrations with the aqueous
light absorption coefficient (Abs370) for each campaign: (a) Melpitz
(winter), (b) TROPOS (winter,), (c) Melpitz (summer), (d) Waldstein
(summer), (e) Xianghe (summer) and (f) Wangdu (summer). The aqueous light
absorption coefficient is given at 370 nm for acidic conditions (indicated
by the subscript “A”, black squares) and for alkaline conditions
(indicated by the subscript “B”, red dots).
![]()
To the best of the author's knowledge, up to now only two other field
studies have determined the contribution of individual compounds to BrC
light absorption. Zhang et al. (2013) estimated the contribution of eight
NACs to aqueous extract light absorption in Los Angeles to be about 4 %
at 365 nm, while Mohr et al. (2013) estimated a NAC contribution of 4 %
to particulate BrC light absorption at 370 nm at a BB influenced site in the
UK. The findings from Zhang et al. (2013) are in the same range as our
findings for the TROPOS (winter) campaign (for Abs370), whereas the
estimated contribution of NACs to particulate BrC light absorption by Mohr
et al. (2013) is a factor of 16 larger than our observed contributions. One
possible reason for this discrepancy is that the mean
babs,BrC, measured in the present study is about twice as
high as observed by Mohr et al. (2013) (10 Mm-1) while the
concentrations of NACs are in the same range in both studies. Moreover, Mohr
et al. (2013) considered a different suite of NACs than the present study.
Another important point is that the calculation by Mohr et al. (2013) used
values based on the absorption maxima of the individual species, which
usually lie below 370 nm (for their protonated form). In contrast, in the
present study the molar extinction coefficient of each compound was directly
determined at 370 nm. When calculations were made based on the absorption
maxima of protonated NACs, an average contribution to the particulate BrC
light absorption of 1 % was found for the present study, i.e., a factor of 4 higher than the calculation at 370 nm.
Conclusions
In the present study, the contributions of eight NACs to the light
absorption of aqueous particle extracts and particulate BrC were determined
in Germany and China at five measurement sites under six different
atmospheric conditions.
The effect of pH on the aqueous extracts light absorption was investigated.
Absorption of extracts at 370 nm, Abs370, increased by a factor of 1.4
and MAE by a factor of 1.6 when the pH was increased from 2 to 10. A general
seasonal trend of MAE370 and Abs370 being higher in winter than in
summer was observed, which is in agreement with other studies. The MAE
values during winter times impacted by BB in Germany exceeded those of the
Chinese background stations during summer. Very low MAE and
babs,370 were observed at the forest site Waldstein in summer,
indicating that freshly emitted biogenic aerosols are only weakly absorbing.
In contrast, a strong relationship was found between the light absorption
properties and BB aerosol concentrations, corroborating findings from other
studies. An average contribution of particulate BrC to
babs,370 of 46 % was observed during the TROPOS (winter)
campaign and 15 % during the Waldstein (summer) campaign. The AAE of
particulate BrC was found to be 1.5 and 1.2 for the campaigns TROPOS
(winter) and Waldstein (summer), respectively.
The relative contribution of NACs to the aqueous extract light absorption
was highly variable depending on the measurement site, ranging from 0.02 to 4.41 % under acidic conditions and 0.02 to 9.86 % under
alkaline conditions. This indicates that the emission strength of light-absorbing compounds and the composition of BrC differed between the sites.
In addition, the formation and decay processes might be very different in
the respective environments. The mean contribution of NACs to the
particulate BrC light absorption was 0.10 % (acidic conditions) and 0.13 %
(alkaline conditions) during the Waldstein (summer) campaign and 0.23 % (acidic conditions) and 1.15 % (alkaline conditions) during the
TROPOS (winter) campaign. A correlation between the NAC concentration and
Abs370 was observed during the campaigns TROPOS (winter) and Melpitz
(winter), at other sites the correlation was weak or nonexistent.
The mass contribution of NACs to WSOC was 5 times lower than their
contribution to the aqueous extract light absorption. This corroborates
conclusions of other studies that even small amounts of light-absorbing
compounds can have a disproportionately high impact on the aerosol light
absorption properties (Kampf et al., 2012).
Field studies on the molecular composition of BrC are still scarce,
especially for sites that have no or little influence of BB aerosols.
Therefore, more efforts are needed to assess BrC on a molecular level, since
a deeper knowledge of the present light-absorbing compounds can improve the
understanding and prediction of BrC aging processes and its radiative
forcing (Laskin et al., 2015). A further investigation of BrC compounds may
also result in finding tracer compounds for specific BrC sources.