Large uncertainties exist when estimating radiative effects of ambient black carbon (BC) aerosol. Previous studies about the BC aerosol radiative forcing mainly focus on the BC aerosols' mass concentrations and mixing states, while the effects of BC mass size distribution (BCMSD) were not well considered. In this paper, we developed a method of measuring the BCMSD by using a differential mobility analyzer in tandem with an Aethalometer. A comprehensive method of multiple charging corrections was proposed and implemented in measuring the BCMSD. Good agreement was obtained between the BC mass concentration integrated from this system and that measured in the bulk phase, demonstrating the reliability of our proposed method. Characteristics of the BCMSD and corresponding radiative effects were studied based on a field measurement campaign conducted in the North China Plain by using our own measurement system. Results showed that the BCMSD had two modes and the mean peak diameters of the modes were 150 and 503 nm. The BCMSD of the coarser mode varied significantly under different pollution conditions with peak diameter varying between 430 and 580 nm, which gave rise to significant variation in aerosol bulk optical properties. The direct aerosol radiative forcing was estimated to vary by 8.45 % for different measured BCMSDs of the coarser mode, which shared the same magnitude with the variation associated with assuming different aerosol mixing states (10.5 %). Our study reveals that the BCMSD as well as its mixing state in estimating the direct aerosol radiative forcing matters. Knowledge of the BCMSD should be fully considered in climate models.
Atmospheric black carbon (BC) is the second strongest absorbing component in the atmosphere (Bond et al., 2013) but the magnitudes of the warming effects are poorly quantified. When emitted to the surroundings, BC particles transform their morphology from fractal to spherical and then grow as fully compact particles with other components depositing on the BC aerosol (Peng et al., 2016). The variation in the shapes of BC aerosols, together with the variation in the mixing states, can lead to substantial changes in aerosol optical properties (Liu et al., 2017; China et al., 2013; Wu et al., 2016, 2018). BC aerosols also have a significant influence on the climate by interacting with clouds (Koch and Del Genio, 2010; Roberts et al., 2008; Stevens and Feingold, 2009), ice and snow (Bond et al., 2013). A recent study shows that the solar absorption of BC can suppress the turbulence in the atmospheric boundary layer (Wilcox et al., 2016). It is found that BC emissions may be responsible for the increase in droughts and floods in China and India (Menon et al., 2002). In addition, BC can pose a serve threat to human health through inhalation (Nichols et al., 2013; Janssen et al., 2011).
Comprehensive studies have been carried out to evaluate the climatic effect
of BC based on the measurement of BC mass concentrations (
Many methods have been proposed to measure BCMSD. For instance, the
BCMSD was measured by sampling the aerosol in the size range from about 50 nm to several micrometers on quartz fiber filter substrates using a
micro-orifice uniform deposit impactor (MOUDI) (Venkataraman and
Friedlander, 1994; Guo, 2016). Cheng et al. (2014) developed a method to
measure the BCMSD by employing two Aethalometers in parallel, with one to
measure total
Recently, Ning et al. (2013) and Stabile et al. (2012) proposed a new method to measure the BCMSD by using a differential mobility analyzer (DMA) in tandem with an Aethalometer (AE). This method has the potential of measuring the BCMSD from 20 to 584 nm with high time resolution. We develop and validate the BCMSD measurement system based on the works of Ning et al. (2013). The developed measurement system was employed in a field campaign in the North China Plain. The characteristics of the measured BCMSD were studied based on the field measurements. Furthermore, the effects of BCMSD variations on the aerosol optical properties and corresponding direct aerosol radiative properties were evaluated. The aerosol optical properties were calculated by using the Mie scattering theory. The direct aerosol radiative forcing (DARF) was estimated by using the Santa Barbara DISORT (discrete ordinates radiative transfer) Atmospheric Radiative Transfer (SBDART) model.
The structure of this paper is organized as follows. Section 2 gives the information about the instrument setup and field measurements. Section 3 gives the detailed method used in this study, which contains (1) conducting multiple charging corrections when deriving the aerosol BCMSD and (2) evaluating the aerosol optical and radiative properties for different BCMSDs. Results and discussions are shown in Sect. 4. The conclusion is drawn in the last part.
The measurement system setup was based on the works of Stabile
et al. (2012) and Ning et al. (2013) as schematically shown
in Fig. 1. The ambient sample aerosol particles were firstly dried to below
a relative humidity of 30 % through a Nafion drying tube before passing
through to the DMA (Model 3081, TSI, USA). The DMA scanned aerosol particles
with diameter ranges from 12.3 to 697 nm over a period of 285 s and
started another scanning after a pause of 15 s, so one complete cycle
took 5 min. The sheath and sample flow rates of the DMA were 3 and
0.5 L min
The schematic diagram of the instrument setup.
An Aethalometer (AE33, model 33, Magee, USA) was used to measure the
From 21 March to 9 April 2017, an intensive field measurement was
conducted to characterize the ambient dry aerosol BCMSD corresponding to
aerosol mobility diameters at the AERONET BEIJING_PKU station
(39
Five steps were involved in calculating the BCMSD using the raw data from the
measurement system: (1) correcting the “loading effect” and “multiple
scattering effect” of
The Aethalometer (AE51 and AE33) is a well-developed and widely used
instrument to measure the
Corrections to the measured
Time correction was needed because time gaps between voltages implied on the DMA (particle size) and sample particles measured by different instruments were not the same. The time correction procedures were conducted every day during the field measurement to ensure that the time deviations of the CPC and the AE51 were constrained within 2 s.
Figure S3 gives the time series diagram of scanned aerosol diameters by DMA,
measured
In the work of Ning et al. (2013), a lot of effort was made to evaluate the performance of the instrument. They considered the diffusion corrections and particle charging corrections. However, the particle charging corrections were limited to single-particle charge ratio as they mentioned that they simplified the particle charge correction by applying the peak electrical mobility for the calculation of representative particle size for each mobility bin and single-particle charge ratio for each primary mobility. They ignored the fact that the aerosol samples selected by the DMA were quasi-monodisperse with different charges and different diameters.
We propose a new algorithm for the multi-charge corrections of the
size-resolved
When the DMA is charged with a negative voltage, those aerosols with a small
range of electrical mobility (
When the scan diameter is set as
The multiple charging corrections can be expressed as computing the
With
Finally, the
MAC of different size ranges is necessary when transforming the size-resolved
The size-resolved MAC was calculated using the Mie scattering model
(Bohren and Huffman, 2007). Based on the Mie scattering
theory, MAC values vary for different aerosol core diameter and different
total diameter. Results from SP2 measurements show that the size distribution
of the BC core diameter peaked at around 120 nm in Beijing (Zhang et
al., 2017). For each aerosol diameter, the MAC value with core diameter of
120 nm was used to transform the size-resolved absorption coefficient into the BCMSD. MAC values with core
diameter at
Calculated mass absorption coefficient of different aerosol.
An example of the multiple charging corrections of the size-resolved
Case of multiple charging correction processing.
The
Based on the measurement results, the BCMSD had two modes for most of the
conditions. The BCMSDs are assumed to be of two lognormal distributions as
The Mie scattering model was used to study the influence of the BCMSD
variation on the aerosol optical properties. When running the Mie model,
aerosol PNSD and BC data were necessary. In this study, The BCMSD was assumed to
be a lognormal distributed.
The aerosol PNSD and
The measured time series of mass concentrations for
In this study, the SBDART model (Ricchiazzi et al., 1998)
was employed to estimate the DARF. In our study, the instantaneous DARF for
cloud-free conditions at the top of atmosphere was calculated for an irradiance
wavelength range from 0.25 to 4
The aerosol optical properties and the corresponding aerosol optical profiles vary with different BCMSD. Then the DARF should be different for different BCMSDs. By estimating the DARF using different aerosol BCMSDs, the influence of BCMSD on the aerosol radiative properties can be studied.
The time series of measured PM
The relationship between the
Our measurements show that the BCMSD had two modes with the coarser mode ranging between 430 and 580 nm in mobility diameter. Many field measurements have revealed that most of the BC mass locates in the aerodynamic diameter range of 320 and 560 nm using the MOUDI (Hu et al., 2012; Huang and Yu, 2008). When the aerodynamic diameter was transformed into mobility diameter with assumption of an aerosol effective density of 1.3, the measured BC aerodynamic diameter range corresponded to mobility diameter range of 280 and 491 nm. Therefore, the measured size range for coarser mode of BCMSD agreed well with the previous measurement.
The measured aerosol in the field site was representative of the urban aerosol. The BC particles emitted by vehicles contributed significantly to the total aerosol BC mass. These BC particles were rarely coated or thinly coated, and the BC core diameter peaked around 120 nm (Zhang et al., 2017). Therefore, the BCMSD of the finer mode measured in our study corresponded to these uncoated or thinly coated particles.
The total
A lognormal distribution was used to fit each mode of the BCMSD by using the
least square method as introduced in Sect. 3.2. For each mode, the
geometric mean diameter (
During the measurement period, both
Variations of aerosol optics properties using the measured mean
aerosol PNSD and
GSD for the coarser mode and the finer mode showed very different properties as shown in Fig. S7b. For the second mode, GSD varied from around 1.49 to 1.68 with a mean value of 1.57. The GSD became decreased with the changing pollution condition, which indicated that BC-containing aerosols tend to accumulate to a small range of diameters during the aging process. This phenomenon was consistent with the fact that larger particles grew relatively slower in diameter because the growth ratio of small aerosol particles is proportional to the negative power of it's diameter. For the first mode, GSD ranged from 1.41 to around 1.86 with a mean value of 1.63. However, the GSD of the finer mode tended to be larger when the surrounding air was cleaner, which might be related to the complex sources of the BC emission. A small amount of freshly-emitted BC particles can have substantial influence on the mass size distribution of the finer mode because the BC concentrations of the finer mode were small, especially under clean conditions. In general, the GSD of the coarser mode was a good indicator of the BC aging process and that of the finer mode could partially reflect the complex sources of the BC fine particles.
Variations in
The relationship between the
Note that the GSD get slightly increased with the increment of
The aerosol optical parameters using the measured mean aerosol PNSD and mean
The estimated DARF values for different GSD and
Comparison of the DARF and heating rate values under different BC mixing states and different BCMSD conditions.
Mixing states of BC play significant roles in calculations of aerosol optical properties and estimations of DARF (Jacobson, 2001). As a comparison, we estimated the DARF under different conditions of BC mixing state: (1) internally mixed, (2) externally mixed and (3) core-shell mixed. Table 1 gives the estimated DARF and mean heating rate within the mixed layer under different mixing state conditions. Results showed that the DARF under different BC mixing state conditions may change by 10.50 %, which shared the same magnitude with 8.45 % variation in DARF caused by BCMSD variations. In addition, the heating rate was estimated to vary by 9.71 %. These results highlighted that the BCMSD plays significant roles in variations of aerosol optical properties and estimations of DARF as well as the air heating rate caused by the existence of BC particles. It was recommended that a real-time measured BCMSD be used when estimating the aerosol DARF instead of a constant one. The BCMSD is as important as that of the BC mixing states.
Knowledge of the BC microphysical properties especially the size-dependent information can help reduce the uncertainties when estimating the aerosol radiative effects. BCMSD is an important quantity in its own right, being directly and indirectly applicable to the determination of the sources, aging processes and mixing states of BC aerosols. In this study, the characteristics of BCMSD were studied from the field measurement results by using our own developed measurement algorithm.
The BCMSD measurement system was developed and validated based on the works
of Ning et al. (2013) by using a differential mobility
analyzer (DMA) in tandem with an Aethalometer (AE). When deriving the BCMSD, a
comprehensive multiple charging correction algorithm was proposed and
implied. This algorithm was validated by closure study between the measured
total
The developed measurement system was employed in a field campaign in the North China Plain from 21 March to 9 April 2017. The BCMSD was found to have two quasi-lognormal modes with peaks at around 150 and 500 nm. These two modes were consistent with the previous measurement results by MOUDI (Wang et al., 2015; Hu et al., 2012). The BC mass concentrations for the coarser-mode peaks were about twice that of the finer mode.
The characteristics of the BCMSD were studied by fitting the shape of BCMSD
with a binormal distribution. The relationships between the fitted
When the BCMSDs changed with the polluting conditions, the corresponding
aerosol optical properties changed significantly. Sensitivity studies found
that the aerosol
The variations in DARF were estimated to be due to the variations in the BCMSD by
using the SBDART model. Results showed that the DARF can vary by about
8.45 % for different BCMSDs and the heating rate for different measured
BCMSD conditions could change from 2.24 to 2.50 K d
The data used in this study are available by requesting them from the authors.
The supplement related to this article is available online at:
GZ, CZ, JT and YK designed and conducted the experiments; CS, YY, CZ and GZ discussed the results.
The authors declare that they have no conflict of interest.
This research has been supported by the National Key R&D Program of China (grant no. 2016YFA0602001) and the National Natural Science Foundation of China (grant no. 41590872).
This paper was edited by Veli-Matti Kerminen and reviewed by two anonymous referees.