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PeraoHarnHbie npo6®2Mbi. 2018. T. 21, № 3(1). C. 89-92.
UDK 551.510(520)
ISOTOPIC SOURCE APPORTIONMENT OF CARBONACEOUS AEROSOLS
OBSERVED IN NOTO REGION, JAPAN: IMPACT OF BIOMASS BURNING ON THE EAST ASIAN OUTFLOW
Atsushi Matsuki1*, Reina Yamada2, Kento Kinouchi2, Yoko Iwamoto3, Fumikazu Ikemori4, Masayo Minami5 and Toshio Nakamura5
'Institute of Nature and Environmental Technology, Kanazawa University, Kanazawa, Japan;
2Graduate School of Natural Science and Technology, Kanazawa University, Kanazawa, Japan;
3Graduate School of Biosphere Science, Hiroshima University, Hiroshima, Japan;
4Nagoya City Institute for Environmental Science, Nagoya, Japan;
5Institute for Space-Earth Environmental Research, Nagoya University, Nagoya, Japan;
Email: matsuki@staff.kanazawa-u.ac.jp (*corresponding author)
Recent observation revealed that submicron aerosol particles in Northeast Asia have a variable but relatively high mass fraction (approximately 50%) of carbonaceous matter throughout the year. In order to investigate on their source and transport pathways, the radio carbon isotope (14C) concentration within fine carbonaceous particles collected at NOTO Ground-based Research Observatory (NOTOGRO, 37.45°N, 137.36°E) were analyzed from 26th Jun, 2014 to 17th June, 2015. The results showed that contribution of carbonaceous matter originating from fossil fuel burning is generally small (30 pMC; percent modern carbon), whereas that from modern biological activity and/or biomass burning is large (70 pMC). Concentration of 14C in autumn samples were the highest in all seasons (90pMC), and there were indications that large scale agricultural waste burning in Northeast China was the likely source. Also, sporadic peak of modern carbon was observed which can be attributed to the plume from Siberian forest fire in summer.
Keywords: Aerosols, carbon isotopes, biomass burning, long-range transport.
Introduction
According to a recent report by WHO [9], estimated number of premature deaths attributable to ambient air pollution mounts up to 3.7 mln per annum globally, and fine particulate matters (or aerosols) found in high density in the polluted air are largely responsible for the motility. Also, aerosols and clouds seeded by them both affect the heat budget of the Earth's atmosphere, but their RF (radiative forcing) still involves large uncertainty [4]. Reducing such uncertainty is critical in understanding how sensitive the earth's climate would respond to any changes we make on the particulate density and composition.
One of the least understood characteristics of aerosols affecting above issues is related to the complex source and roles of the carbonaceous aerosols (including organic and soot particles) in the atmosphere. Even the latest generation of numerical models tends to underestimate the observed concentration of organic aerosols and this is particularly true in regions (e.g., East Asia) where anthropogenic and biogenic emissions mix together. The East Asia is also identified as one of the global hotspots of atmospheric aerosols.
The outflow of atmospheric pollutants is increasingly concerned in connection with their impacts not only on public health but also on regional climate.
Carbon trapped in fossil fuels such as coal and oil (which is burried undergraound for millions of years) is deplete in 14C since it is a radioactive isotope of carbon that decay with a half life time of 5730 years. Whereas modern biomass has a characteristic 14C ratios that is basically the same as the CO2 found in the atmosphere, as plants fix carbon through photosynthesis at the primary productive stage. As a result, it is possible to distinguish and apportion the source of carbonaceous aerosols originating from fossil fuel and modern biomass by analyzing 14C concentrations and deriving fossil and modern (non-fossil) carbon ratios.
In order to better characterize the sources of carbonaceous aerosols in the background East Asian outflow region for an extended period, PM25 samples were collected weekly continuously for a year in Noto area (at a remote site on the western coast of main island Japan). In addition to 14C radio carbon analysis, other parameters including biomass burning tracers, S13C stable carbon isotope, BC, PM25 concentrations,
MODIS satellite product and back trajectory analysis were combined to better explain the aerosol carbon sources and their seasonal variation.
Materials and Methods
The Noto peninsula projects north from the western coast of central Honshu, the main island of Japan. It can be regarded as a remote coastal site facing toward the Asian continent, which is isolated from major cities and industrial activities by the surrounding sea. Such location is considered ideal for monitoring background aerosol properties in central Japan without the significant influence from local anthropogenic sources [5, 8], as well as for sensitively detecting slightest changes occurring in the atmospheric constituents carried along by the outflow of continental air-mass. The geographic location of ground-based research station "NOTOGRO" (acronym for NOTO Ground-based Research Observatory) at the tip of Noto peninsula (37.45°N, 137.36°E) is shown in Figure 1.
PM25 samples at NOTOGRO were collected weekly from 26th Jun, 2014 to 17th June, 2015. Two staged high volume air-sampler coupled with a PM2 5 impactor (HV-700R, SIBATA Scientific Technology Ltd.) was installed at the roof of the three-story building approximately 13 m above ground level. The suction pump of the sampler is based on brushless blower, thus free from rotary vanes that can potentially contaminate the carbon analysis. The results presented in this work are based solely on the analysis of fine mode PM25 fraction collected on the backup filter of the 2 staged high volume air-sampler.
Quartz fiber filters (Pall Corporation, 2500QAT-UP) were preheated at 450°C for 2 hours
in an electric oven in order to remove organics in the filter background prior to sampling. After continuously sampling for a week period at a flow rate of 700 L min-1, the recovered filter was kept in a freezer to avoid loss of volatile components until it is extracted and analyzed.
The post sample treatment procedure is basically identical to what is described elsewhere [2]. The graphite samples prepared by the post sampling treatment were loaded in an aluminum target, and the carbon isotopic composition were analyzed by the accelerator mass spectrometry 14C system (High Voltage Engineering Europe, Model 4130-AMS) at the Institute for Space-Earth Environmental Research, Nagoya University. In addition, the other parameters including concentrations of specific organic compounds, S13C, black carbon, PM25 and back trajectory analysis were combined to better constrain the carbon sources.
Results and Discussion
The sampling and radiocarbon analysis by this study provided almost a year-round data set of weekly 14C concentrations found in the fine-mode carbonaceous aerosols in the remote coastal region in central Japan. The seasonal variation of the 14C concentration is shown in Figure 2 as the time series of obtained pMC (percent modern carbon) values. The plots are shown for total 27 samples in which we successfully recovered >90% carbon as graphite during the sample processing.
Despite the remoteness of the sampling site from major industrial and urban activities, we have seen nevertheless, significant seasonal variation in 14C
Figure 1. Geographical setting of NOTOGRO station
concentrations ranging over 57.4-89.6 pMC throughout the year (Figure 2). The minimum and maximum 14C concentrations differed by about 30 pMC. When compared against other reported pMC values from urban areas relevant to East Asia, for example 33-48 pMC in Beijing [10], 31-54 pMC in Tokyo [7] and 25-65 pMC in Nagoya [3], our values lie in the upper end, highlighting larger contribution from modern (non fossil) sources. The contrast with Nagoya may be particularly relevant because the city lies almost in the same longitudinal zone (35.15°N, 136.97°E) as our remote measurement site but it is one of the major cities along the other side of the main island Japan (i.e. facing the Pacific Ocean).
The strong seasonal variability in the modern and fossil carbon fractions should reflect changes in both the monsoonal wind patterns and emission profiles of multiple sources. In winter, fine aerosol particles surviving the wet and cold winter monsoon showed minimum 14C concentrations which dropped down to 57.4 pMC, indicating largest influence of fossil fuel combustion due to increased demand for domestic heating in the continental cities. In summer in the contrary, long-range transport from the continent became less active. Instead, local to regional sources within Japan gained relative importance. The active photochemistry and resulting SOA (secondary organic aerosols) formation may partly be responsible for the larger modern carbon fractions found around 70 pMC. In spring, the 14C concentration showed a general increasing trend most likely reflecting the end of heating season and mixing of carbon from more diverse sources and air-mass origins.
The maximum 14C concentrations were observed in a sample collected in late July (89.6 pMC), and all of the samples collected in October (83.7-89.5
pMC). These values indicated that the increased carbonaceous aerosols (soot, organics) observed during these events were predominantly of modern origin. Additional chemical analysis of biomass burning tracers, as well as stable carbon isotope strongly linked the former event with a sporadic plume from Siberian forest fire, and latter with the systematic outflow from post-harvest open field burning involving C4 plants (e.g. maize straws) in northeastern China. To the best of our knowledge, this is the first report to point out the link between the growing maize straw burning practice in the region and its impact on the downwind aerosol composition based on the carbon isotopes.
Wheat and maize straws are the major agricultural residues in China and field burning of such biomass is still a common practice [6]. Although such practice has been banned since the late 90th in relation to deteriorating regional air quality, major changes in the energy structure turned down the demands of the residues as attractive energy sources and facilitated field burning. Such biomass burning is expected to peak with crop harvests. Huang et al. [1] constructed a multi-annual, emission inventory of crop burning in different parts of China based on MODIS fire products. They demonstrated a strong seasonality in the agricultural fire counts in the north eastern China (which coincides with the region in concern), with peaks in spring (March, April, May) and in autumn (October) as well. Interestingly, the inter-annual analysis between 2003-2010 revealed that the second peak in October was hardly visible in the early years, but gradually became prominent in the later years [1], which suggests that the second peak of post-harvest burn in October in this region is a rather new and growing practice which emerged merely over the course of the last decade or so.
Figure 2. Seasonal variations of 14C concentration found in weekly PM2 5 samples collected at Noto peninsula (western coast of central Japan)
Conclusion
This study provided an almost year-round data set of weekly 14C concentrations in the fine-mode carbonaceous aerosols representing remote coastal region in central Japan, where it is often subject to outflow from different parts of the Asian continent. The results revealed relatively high contribution of modern carbon, 70 pMC on yearly average, which can be attributed to primary and secondary emissions from natural biological activity and/or biomass burning (including naturally or human lit fire). This in turn indicated that the fraction of carbon originating from anthropogenic fossil fuel combustion (e.g. coal, oil) accounts for approximately 30% at the background site downwind of East Asia.
The result of this study revealed dynamic sea-sonality in the relative contribution from modern and fossil carbon sources, and helped narrow down the source of carbonaceous aerosols in the East Asian outflow region. The use of such data can be further extended to constrain complex organic aerosol emission and evolution in East Asia where both anthropogenic and natural sources intricately affect each other. Especially, the geographical setting of our station makes our results more representative of a remote, background area directly under the influence of the East Asian outflow. Comparison with other urban sites will be meaningful to isolate local impacts from what is being transported externally.
Finally, there was a clear increase in organic mass fractions and BC concentrations within fine aerosols during biomass burning episodes despite long distances from the identified sources. It has many important implications in terms of the regional air quality, climate and water cycles. In particular, how much impact these biomass burning plumes have on the cloud nucleating activities of atmospheric aerosols in the downwind regions remains to be resolved.
Acknowledgement
We acknowledge Ms. Mika Sawano, Ms. Haru-ka Naya and all staff members of the Noto branch office of Kanazawa University for their continued support in the atmospheric monitoring. This study was supported by the Japan Society for Promotion of Science (JSPS) KAKENHI Grant Number JP 26701001.
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