Ground-based measurements of black carbon (BC) were performed near
an industrial source region in the early summer of 2014 and at a remote
island in Japan in the spring of 2015. Here, we report the temporal
variations in the transport, size distributions, and mixing states of the
BC-containing particles. These particles were characterized using a
continuous soot monitoring system, a single particle soot photometer, and an
aerosol chemical speciation monitor. The effects of aging on the growth of
BC-containing particles were examined by comparing the ground-based
observations between the near-source and remote island sites. Secondary
formation of sulfate and organic aerosols strongly affected the increases in
BC coating (i.e., enhancement of cloud condensation nuclei activity) with air
mass aging from the source to the outflow regions. The effects of wet removal
on BC microphysics were elucidated by classifying the continental outflow air
masses depending on the enhancement ratios of BC to CO (
Black carbon (BC)-containing particles in the atmosphere can significantly affect the radiative budget of the Earth through two effects: direct (light absorption and scattering) and indirect (aerosol–cloud interactions) effects (Bond et al., 2013; references therein). The difficulty in the estimation of these effects in the atmosphere results from both the short lifetime relative to other greenhouse gases and the variable physicochemical properties of BC-containing particles. The BC itself is water-insoluble immediately after emission, but it subsequently exhibits increased hygroscopicity (McMeeking et al., 2011) and cloud condensation nuclei (CCN) activity (Kuwata et al., 2007) through atmospheric transport and aging. Only small amounts of water-soluble materials on BC particles are needed to cause their activation to form cloud droplets under moderate supersaturation conditions (Kuwata et al., 2007, 2009). It is considered that BC-containing particles are removed from the atmosphere mainly by wet deposition (Seinfeld and Pandis, 2006).
The horizontal and vertical distributions of aerosols can be substantially altered by their atmospheric lifetimes (e.g., Lawrence et al., 2007). Moreover, Lawrence et al. (2007) suggested that the removal processes of BC such as dry deposition, below-cloud processes (i.e., washout), and in-cloud processes (i.e., rainout) can greatly change the atmospheric lifetimes. The in-cloud processes include nucleation scavenging and scavenging by the preexisting cloud droplets. Precipitation followed by in-cloud processes leads to the irreversible removal of BC-containing particles. Samset et al. (2014), using multiple global model data sets constrained by aircraft observations, suggested that the atmospheric lifetime of BC largely affects its distribution, especially in the Northern Hemisphere, and this results in significant variations in global direct radiative forcing values. The removal of BC has been considered an important issue for the geochemical carbon cycle as well as for climate science. The BC-containing particles deposited onto the ocean surface can affect ocean surface particles, dissolved organic carbon (DOC), and microbial processes by absorbing DOC, stimulating particle aggregation, and changing the size distribution of suspended particles (Mari et al., 2014).
Previous modeling studies have dealt with BC aging processes (condensational growth and coagulation) in box and regional-scale models and parameterized timescales for the conversion of BC-containing particles from water-insoluble to -soluble in global models (Oshima et al., 2009; Liu et al., 2011; Oshima and Koike, 2013). However, quantitative knowledge of the variability in microphysical parameters of BC-containing particles and the timescale of their aging processes is still limited, and thus more investigation is needed for near-source and remote regions (Samset et al., 2014). Moteki et al. (2012) reported the first observational evidence of the size-dependent activation of BC in air masses uplifting from the planetary boundary layer (PBL) to the free troposphere (FT) in east Asia in the spring of 2009, as the part of the Aerosol Radiative Forcing in east Asia (A-FORCE) aircraft campaigns (Oshima et al., 2012). A similar altitude dependence of the BC size distribution and similarity in the BC mixing state were observed in other aircraft measurements conducted in east Asia in winter (Kondo et al., 2016). Selective removal of larger BC-containing particles though cloud processing, which is predicted by Köhler theory, was qualitatively observed in the atmosphere. This observational evidence indicates that the size distributions and mixing states of BC-containing particles have a large impact on the global- and regional-scale spatial distributions of BC through their upward transport from the PBL to the FT associated with cloud processes. Despite the importance of the size distributions and mixing states of BC-containing particles in the PBL, the measurements of their microphysical properties are still limited around the source regions in east Asia.
Kanaya et al. (2016) have conducted long-term measurements of BC for 6 years (2009–2015) at Fukue Island, and they reported the emission and removal of BC in east Asia using these data sets. This study determined that the transport efficiency of BC aerosol particles through the PBL was substantially reduced by wet removal. Here, we examine the effects of aging and wet removal during transport on the changes in BC size distributions and mixing state, as well as concentrations, based on ground-based measurements conducted at the same site in the spring of 2015 using a single particle soot photometer (SP2) and an aerosol chemical speciation monitor (ACSM). We first describe the meteorological characteristics of the east Asian region in the spring of 2015. Then, we discuss the relative importance of the below-cloud (i.e., washout) and in-cloud scavenging (i.e., rainout) processes for the removal of BC as well as the transport patterns of the east Asian outflow air masses in spring. The loss of BC-containing particles for that period is investigated using a similar approach to that used by Kanaya et al. (2016), and this is performed in connection with the associated changes in BC microphysics and their relevance to the transport pathways.
Continuous measurements of PM
Map of the investigated region with two observation sites (Yokosuka – open triangle; Fukue Island – closed circle) and five defined areas (1 – Northeast China; 2 – Korea; 3 – Central North China; 4 – Central South China; 5 – Japan). The bimonthly mean BC emission rate (March–April) in 2008 is overlaid on the map (REAS ver. 2.1, Kurokawa et al., 2013).
We deployed an SP2 (Droplet Measurement Technologies, Inc., USA) for the
analysis of microphysical parameters of refractory BC (rBC; Petzold et al.,
2013) from 26 March 2015 to 14 April 2015. The SP2 was calibrated before
starting the ambient measurements. The calibration protocol for our SP2 is
described in Miyakawa et al. (2016). Fullerene soot (FS; stock 40971, lot L20W054, Alfa Aesar, USA) particles were used as a calibration standard for
the SP2. A differential mobility analyzer (DMA; Model 3081, TSI Inc., USA) was
used for preparing the monodisperse FS particles. The analysis of the
calibration results suggests that the full width at half maximum (FWHM) was
typically 30 % of the modal incandescence signal intensity (
Equivalent BC (EBC; Petzold et al., 2013) mass concentrations are continuously measured at Fukue Island using two instruments: a continuous soot-monitoring system (COSMOS; model 3130, Kanomax, Japan) and a multi-angle absorption photometer (MAAP; MAAP5012, Thermo Scientific, Inc., USA). The details of the air sampling and intercomparisons for EBC measurements at Fukue Island have been described elsewhere (Kanaya et al., 2013, 2016). In this study, mass concentrations of EBC measured using the COSMOS were evaluated by comparison with those of SP2-derived rBC. The intercomparison between SP2 and COSMOS will be briefly discussed below.
Figure 2 depicts the correlation between COSMOS–EBC and SP2–rBC hourly mass
concentrations. The unmeasured fraction of the rBC mass was corrected by
the extrapolation of the lognormal fit for the measured mass size distributions
outside the measurable
Correlation plot of SP2–rBC and COSMOS–EBC mass concentrations (at
standard temperature and pressure). The shaded region corresponds to within
The chemical composition of non-refractory submicron aerosols was measured
using an Aerodyne ACSM (Aerodyne, Inc.,
USA.) placed in an observatory container at Fukue Island during the
observation period. The details of the ACSM at Fukue Island have been
described in Irei et al. (2014). The collection efficiency (CE) of the ACSM
was assumed to be 0.5 for this period (Yoshino et al., 2016). We considered
sulfate (SO
Two high-volume air samplers (HV500F, Sibata Scientific Technology Ltd.,
Japan) were deployed on the rooftop of the observatory container. The
sampling flow rate for both samplers was 500 liters per minute (L min
The carbon monoxide (CO) mixing ratio was also continuously measured using a nondispersive infrared (NDIR) CO monitor (model 48C, Thermo Scientific, Inc., USA). Details of the CO measurements including the long-term variations in sensitivity and zero level are discussed elsewhere (Kanaya et al., 2016).
In order to quantify the extent of the removal of BC, we calculated the
hourly enhancement ratio of BC mass concentrations to CO mixing ratios
(
Relative changes in SO
We used the 6-hourly meteorological data, with a resolution of 1
Meteorological fields in east Asia during the observation period
(11 March–14 April 2015) based on NCEP FNL data.
We calculated backward trajectories from the observation site to elucidate the impact of the Asian outflow. Three-day backward trajectories from the observation site (the starting altitude was 0.5 km) were calculated every hour using the National Oceanic and Atmospheric Administration (NOAA) Hybrid Single-Particle Lagrangian Integrated Trajectory model (Draxler and Hess, 1997; Draxler, 1999; Stein et al., 2015) with the meteorological data sets (NCEP's Global Data Assimilation system, GDAS). In this study, the residence time over specific source regions was used as an indicator of their impacts on the observed air masses. We defined five domains for assessing the impact over the Asian continent: Northeast China (NE), Korea (KR), Central North China (CN), Central South China (CS), and Japan (JP) (Fig. 1). The period when air masses passed over the domains NE, KR, CN, and CS at least for 1 h is defined as that of “continental outflow”. The impacts of precipitation on the observed air masses were assessed by a parameter referred to as the “accumulated precipitation along trajectory” (APT; Oshima et al., 2012). In this study, we calculated the APT values by integrating the amount of hourly precipitation in the Lagrangian sense along each 3-day back trajectory of the sampled air masses. The hourly variations in APT were merged into the observed gas and aerosol data sets.
The mean meteorological field during the observation period (11 March–13 April 2015) is discussed for the purpose of characterizing the general
features of the wind flow and precipitation in this region. The migrating
anticyclone and cyclone passed alternately over east Asia during this
period; this pattern is typically dominant in spring over east Asia (Asai et al.,
1988). Figure 3a shows the mean sea level pressure (SLP) and mean horizontal
winds at the 850 hPa level in east Asia during the observation period. The
mean equivalent potential temperature (
Figure 3c shows the temporal variations in surface pressure and precipitable water at the observation site. The surface pressure is anticorrelated well with the precipitable water. During the observation period, migratory cyclones and anticyclones occurred occasionally (3 times each). The occurrence of migratory cyclones advected moist air, which could have contributed to the wet removal of BC during transport in the PBL. In contrast, the occurrence of anticyclones advected dry air, which could have contributed to the efficient transport of BC from the source regions.
Figure 4a depicts the mean precipitation over east Asia during the
observation period. Mean precipitation showed a latitudinal gradient over
eastern China and the Yellow Sea and East China Sea region (i.e., increasing
precipitation from south to north), and these results suggest that transport
pathways can greatly affect the wet removal of aerosols. The APT was
compared with the averaged latitude of each trajectory for 48 h backwards from the time of
In this study, the removal processes including dry deposition and below-cloud
scavenging were considered to be minor. The dry deposition in this region has
already been evaluated by Kanaya et al. (2016), who found a minimal decrease in
Temporal variations in air mass origin and concentration of trace
species. Top panel: fractional residence time of air masses passing over
selected area (red – Central South China; orange – Central North China; blue – Northeast China; green – Korea; pink – Japan; black – other regions such as the ocean). Middle panel: mass concentrations of BC measured using COSMOS
(black markers) and SP2 (red markers). Bottom panel: concentrations of CO
(black markers), SO
Mean chemical composition of fine aerosols during the observation period.
Temporal variations in the concentrations of BC (measured using COSMOS and
SP2), SO
Figure 6a and b show scatterplots of CO with BC and SO
Correlation between aerosol mass concentrations and CO mixing ratio
colored according to the APT.
The cloud processes of aerosol particles not associated with precipitation
can also reduce the slope of their correlation. However, no decreasing
tendency of BC–CO and SO
Chemical compositions of fine aerosols were investigated in terms of the APT
and RH
Summaries of BC microphysical parameters measured at Yokosuka and Fukue Island.
Number and mass size (
The
Figure 8 depicts the probability density of the
Probability density function of the estimated
Not only in-cloud scavenging of BC-containing particles but also subsequent
precipitation (i.e., the rainout process) can account for the changes in the
microphysical parameters of BC detected in this study. Our results show a
decrease in both the peak diameters,
Note that the magnitude of the change in the peak
The transport pathways of the continental outflow air masses are
horizontally and vertically variable in spring in east Asia because of the
frequent alternate cyclone–anticyclone activities in spring (Asai et al.,
1988). Oshima et al. (2013) examined the three-dimensional transport
pathways of BC over east Asia in spring and showed that the PBL outflow
through which BC originating from China was advected by the low-level
westerlies without uplifting out of the PBL was one of the major pathways
for BC export from continental east Asia to the Pacific, thus supporting the
general features of microphysical properties of BC in continental outflow
obtained by this study. Mori et al. (2014) measured the seasonal variations
in BC wet deposition fluxes at another remote island in Japan (Okinawa,
Ground-based measurements of BC were performed near an industrial source
region and at a remote island in Japan. We have reported the temporal
variations in the transport and the microphysics of the BC-containing
particles, measured using COSMOS, SP2, and ACSM. The impacts of air mass
aging upon the growth of BC-containing particles were examined by comparing
the ground-based observations from the near-source and remote island
sites.
The data given in this study will be available on request to the corresponding author (miyakawat@jamstec.go.jp).
The authors declare that they have no conflict of interest.
This study was supported by the Environment Research and Technology Development Fund (S7, S12, and 2-1403) of the Ministry of Environment, Japan, and the Japan Society for the Promotion of Science (JSPS), KAKENHI Grant numbers JP26550021, JP26701004, JP26241003, JP16H01772, and JP16H01770, and was partially carried out in the context of the Arctic Challenge for Sustainability (ArCS) Project. The authors would like to thank N. Moteki at the University of Tokyo for assistance with the SP2 calibrations. M. Kubo, T. Takamura, and H. Irie (Chiba University) are also acknowledged for their support at the Fukue Island Atmospheric Environment Monitoring Station. The authors are grateful to Gavin McMeeking and an anonymous reviewer for constructive comments to improve this paper. Edited by: R. Sullivan Reviewed by: G. R. McMeeking and one anonymous referee