Black carbon (BC) is a dominant absorber in the visible spectrum and a potent
factor in climatic effects. Vertical profiles of BC were measured using a
micro-aethalometer attached to a tethered balloon during the Vertical
Observations of trace Gases and Aerosols (VOGA) field campaign, in summer
2014 at a semirural site in the North China Plain (NCP). The diurnal cycle of
BC vertical distributions following the evolution of the mixing
layer (ML) was investigated for the
first time in the NCP region. Statistical parameters including identified
mixing height (
Black carbon (BC), produced from incomplete combustion processes, is a strongly absorbing constituent of atmospheric aerosols (Moosmüller et al., 2009; Bond et al., 2013). As a major absorber in the visible spectrum, BC heats the atmosphere and largely counterbalances cooling effects of scattering aerosols on climate (Jacobson, 2001; Ramanathan et al., 2005; Stier et al., 2007). Another reason for BC to be of public concern is that inhaled BC poses a huge threat to human health (Janssen et al., 2012; Nichols, 2013).
Despite the significance of evaluating radiative forcing by BC, large uncertainties arise from limitations of current knowledge on emissions, distributions, and physical properties of BC (Andreae, 2001; Streets, et al., 2001; Bond et al., 2006; IPCC, 2013). One critical aspect pertinent to climate response of BC is a high sensitivity of BC radiative impact to its vertical distributions (Zarzycki and Bond, 2010; Ban-Weiss et al., 2012; Samset et al., 2013). The importance of BC vertical distributions to the evolution of planetary boundary layer (PBL) and cloud properties has also been demonstrated by previous studies (Yu et al., 2002; Ramanathan and Carmichael, 2008; Ferrero et al., 2014). Nevertheless, vertical profiles of BC or aerosol absorption have only been scarcely measured in a few field campaigns (Safai et al., 2012 and references therein; Ryerson et al., 2013). Available information on BC vertical distributions is particularly limited in China (Zhang et al., 2012; Li et al., 2015; Zhao et al., 2015), issuing a challenge to reliably estimate regional climatic effects of BC under severe air pollution due to rapid economic growth and urbanization in this region (Menon et al., 2002; Liao and Shang, 2015 and references therein).
Platforms normally utilized to perform BC profiling are tethered balloons, aircraft, and unmanned aerial vehicles. BC vertical profiles obtained from in situ measurements using tethered balloons are highly vertically resolved, revealing details within about 1 km above the ground, especially in the thin surface layer that is vital for human beings and where various sources are located (Ferrero et al., 2011a; Babu et al., 2011a; Li et al., 2015). Comparatively, aircraft (Tripathi et al., 2005, 2007; Metcalf et al., 2012; Zhao et al., 2015) and unmanned aerial vehicles (Corrigan et al., 2008; Höpner et al., 2016) are more expensive, although they have advantages of reaching higher altitudes and for aircraft, more on-board instruments of the size and weight unable to be carried by tethered balloons and unmanned aerial vehicles. Fast flight speeds of these two platforms also compromise their spatial resolutions. In addition, a high-altitude balloon has been employed to measure BC vertical profiles at high altitudes in the free troposphere. Two BC polluted layers found at about 4.5 and 8 km altitudes were reported by Babu et al. (2011b). Indirect methods such as recently proposed lidar remote sensing might be able to conduct continuous measurements (Miffre et al., 2015). They are, however, less accurate than in situ measurements.
To advance understanding in impacts of atmospheric components including
trace gases and aerosols on atmospheric environment and climate, an
intensive field campaign, Vertical Observations of trace Gases and Aerosols
(VOGA), was carried out in summer 2014 at a semirural site in the North
China Plain (NCP), one of the most overcrowded and polluted regions in the
world (Shao et al., 2006; Xu et al., 2011; Ma et al., 2011; Chen et al.,
2012). A tethered balloon system equipped with instruments was employed for
high vertical resolution measurements within 1 km above the ground. In this
study, we present results from in situ measurements of BC vertical profiles
using a lightweight (about 280 g) and small-sized (117 mm
The VOGA 2014 field campaign was carried out in the period from 21 June to
14 July at a semirural site, Raoyang (38
The spatial distribution of averaged MODIS aerosol optical depth
(AOD) at 550 nm in
The spatial distribution of average aerosol optical depth (AOD) at 550 nm
acquired from the level 2 version of the Moderate Resolution Imaging
Spectroradiometer (MODIS) data (Levy and Hsu, 2015) is also displayed in
Fig. 1. Features of severe regional aerosol pollution with clearly
defined pollution centers could be recognized from the AOD distributions in
different months of the year 2014. The level of AOD in Raoyang could
represent regional aerosol conditions in NCP in July 2014 well, the month when
most tethered balloon flights took place (Fig. 1c). The AOD distributions
could also cast some light on the seasonal variation of ambient aerosols in
the area where launches of the tethered balloon were carried out, since
measurements of absorbing aerosols are unavailable to obtain the temporal
variation in that area. However, it should be noted that the seasonal cycle
of ambient aerosols depends on both the aerosol loading and the relative
humidity. As a consequence, it is complicated to draw a definite conclusion
only from the AOD dataset about to what extent reported BC concentrations in
this study represent the spatiotemporal variability in the area. A further
examination on BC emissions (0.25
Moreover, it was found that another semirural site Gucheng, about 90 km
north of Raoyang, shared a similarity with Raoyang in the AOD level as well
as BC emissions. Seasonal and diurnal variations of surface BC mass
concentrations in Gucheng were analyzed on the basis of about 6-year
measurements (from 2006 January to July 2012, with data completeness of
77.5 %) using an aethalometer (Model AE-31, Magee Scientific, USA) with a
temporal resolution of 5 min (Zhang et al., 2015). BC mass concentrations
averaged about 9.6
A micro-aethalometer to measure aerosol absorption at 880 nm and a
radiosonde to measure meteorological parameters (pressure, temperature,
relative humidity, wind speed, and wind direction) were attached to a
helium-filled tethered balloon, with a volume of 30 m
AE-51 operated with a similar principle to the aethalometer as described in
Hansen et al. (1984). The intensity of transmitted light through a 3 mm
diameter sample spot (
At a high temporal resolution of 1 s, a steady increase of ATN with sampling
time was not usually found due to instrumental noise. Acquired data consisted of many large values with positive or negative signs. A
post-processing method, the Optimized Noise-reduction Averaging (ONA) algorithm,
has been developed by Hagler et al. (2011), where adaptive time-averaging of
the BC data was conducted with the time window of averaging optimally chosen
by For each data point Find the largest value of the matrix Replace Replace any Repeat step 2–4 until there are no positive numbers in the matrix. Repeat step 1–5 for
The smoothing window
Measured
The
AE-31 suffered instrumental artifacts in the same way as AE-51. Details of
the correction scheme developed for tackling AE-31 artifacts were
described in Ran et al. (2016). Briefly, the correction scheme combined the
modified Virkkula method (Virkkula et al., 2007) to treat the shadowing
effect and the Schmid method (Schmid et al., 2006) to treat filter multiple
scattering and aerosol scattering effects. The modified Virkkula method
assumed a linear relationship of BC mass concentrations and time across the
filter change, particularly, a quadratic relationship for special cases
where ambient BC experienced a peak-shaped variation, instead of constant BC
mass concentrations as in Virkkula et al. (2007). The wavelength-dependent
correction factor (
MAAP continuously measured aerosol absorption coefficients at the actual
wavelength of 637 nm (
Three steps were taken to obtain the
Reduced major axis regression of attenuation coefficients
Instead of measured temperature (
A set of 67 vertical profiles of BC and meteorological parameters were selected. The mixing layer (ML) could be clearly discerned for profiles measured in the daytime. Exceptions were those measured around noon, when the top of the ML, often higher than 1 km in summer, was beyond reach of the tethered balloon. In addition, several launches took place in the evening to observe BC vertical distributions shaped by the nocturnal boundary layer (NBL). A statistical summary of the field campaign and the meteorology is given in Table 1. Most profiles were sampled under mild winds in the morning and evening, when relatively stable conditions suitable for launches of the tethered balloon were more easily encountered. Air masses were mainly carried, either by southeasterly or southwesterly winds, from areas densely populated and heavily industrialized, also ramified with railways and highways.
A statistical summary of the field campaign and the meteorology.
Vertical profiles of BC mass concentrations (
Figure 4 illustrates an example of vertical distributions of BC,
The mixing height could be determined by applying the gradient method to the
entire dataset (Seibert et al., 2000; Kim et al., 2007). Generally, the
mixing height determined from profiles of
Statistical parameters for BC vertical profiles measured during different periods. Notations for the parameters have been given in the text.
Table 2 summarizes calculated parameters for BC vertical profiles during
different periods. Average
Statistically, vertical profiles of BC were categorized into two types,
according to their shapes along the normalized height (
Vertical distributions of
An investigation on diurnal variations of BC vertical profiles was able to be undertaken on 1, 8, and 13 July, when measurements generally covered the period from early morning to afternoon or evening. On other observational days, the dataset was incomplete due to aborted launches under strong winds or precipitation. For clarity, only the vertical profile during the descent of each launch is plotted in Fig. 6. Additionally, profiles not included in the selected dataset are also shown.
Vertical distributions of BC separately displayed for 1, 8,
and 13 July. Profiles collected at different time are shown in different colors.
As in Fig. 4, dots represent 20 m averaged
A distinct diurnal variation was found both in the shape of BC vertical
profiles and the level of
Measurements of BC vertical distributions in different regions.
It was interestingly noticed that a polluted layer with a thickness of about
0.3 km, extending from the height of 0.2 to 0.5 km, lay right above the
ML in the early morning on 1 July. The polluted layer could also be
accordingly recognized from meteorological features of more moisture and
wind shear across its boundaries. Wind direction was southeast in the ML and
the FT, while it was southwest in the polluted layer. Average
Figure 7 displays
In situ measurements of black carbon (BC) vertical profiles were carried out at a semirural site during the VOGA 2014 summer field campaign, using a micro-aethalometer attached to a tethered balloon system. A set of 67 BC vertical profiles was reliably collected and diurnal variations of BC vertical profiles were examined.
Vertical distributions of BC and meteorological parameters, as well as the
mixing height (
All vertical profiles presented in this paper and the code for the FMS algorithm can be accessed by contacting corresponding authors, Liang Ran (shirleyrl@mail.iap.ac.cn) and Zhaoze Deng (dengzz@mail.iap.ac.cn).
This research was funded by the National Natural Science Foundation of China
(NSFC) under grant no. 41305114, 41205098, 41330442, and 41127901. This work
was also supported by China Special Fund for Meteorological Research in the
Public Interest (no. GYHY201206015) and National Science and Technology
Major Project (2016YFC0200403). We are grateful to Yong Wang, Chuncheng Ji,
Qihua Du, Hanze Yu, and Xinpan Li for their cooperation in launching the
tethered balloon. We also thank Raoyang Meteorological Bureau for providing
the location for our measurements. The Terra and Aqua/MODIS Aerosol Level 2
products were acquired from the Level 1 & Atmosphere Archive and
Distribution System (LAADS) Distributed Active Archive Center (DAAC),
located in the Goddard Space Flight Center in Greenbelt, Maryland
(