Seasonal variation of enhancement ratios of trace gases observed over 10 years in the western North Pacific
Research highlights
► Enhancement ratios of trace gases in the Asian pollution events were examined. ► We found distinct seasonal variations of CO/CO2, CH4/CO and O3/CO ratios. ► They showed large influences of seasonal flux changes in the Asia source regions.
Introduction
East Asian continent is one of the large sources of the anthropogenic pollutants, and their emissions have been rapidly increasing due to the recent economic growth in Asian countries. The gaseous pollutants emitted from the Asian continent affect not only the regional environment over the western North Pacific but also the air quality of other continents as a result of the inter-continental transport (Akimoto, 2003). Based on the observed and simulated ozone measurements in the spring, Tanimoto et al. (2005) reported that the Asian continental outflow from the east coast of China had a significant impact on the downwind regions over the western North Pacific. Other studies have provided observational evidences for various influences on the air quality over certain regions of North America due to the long-range transport of polluted air masses from Asia (e.g. Andreae et al., 1988, Parrish et al., 1992, Husar et al., 2001).
In order to understand the nature and mechanism of the impact of polluted air masses on the atmospheric air quality over the western North Pacific regions, several intensive observational campaigns were carried out to determine the chemical composition of Asian continental outflows. These measurement programs included the Pacific Exploratory Mission (PEM) -West A and B (Hoell et al., 1996, Hoell et al., 1997), the Asian Pacific Regional Aerosol Characterization Experiment (ACE-Asia) (Huebert et al., 2003), and the Transport and Chemical Evolution Over the Pacific (TRACE-P) (Jacob et al., 2003). These intensive observations demonstrated that the anthropogenic emissions from fossil fuel combustion and biomass burnings have a pervasive influence on the chemical composition of the atmosphere over this region. Furthermore, studies of the export mechanism of Asian continental outflow have shown the important role of deep convection activities for vertical transport of the pollutant gases (Folkins et al., 1997), as well as of the widespread pollution caused by the cold front associated with middle latitude cyclones (e.g. Bey et al., 2001, Liu et al., 2003, Sawa et al., 2007), and of the orographic lifting (Liu et al., 2003). However, many of these intensive observations were conducted mainly in winter to spring seasons because of the increased frequency of the Asian continental outflow over the western North Pacific during this time of the year (e.g. Sawa et al., 2004, Suntharalingam et al., 2004, Tohjima et al., 2010). This temporal limitation imposes an important neglect of a fuller understanding of the chemical characterization of the Asian continental air masses over the western North Pacific region during other parts of the year. Wada et al. (2007), for example, reported large influence of Asian continental outflow associated with extremely low CO2 concentration during the summer season at a remote site in western North Pacific.
Japan Meteorological Agency (JMA) has been continuously measuring atmospheric concentrations of carbon monoxide (CO), methane (CH4), carbon dioxide (CO2), and ozone (O3) at three remote sites in western North Pacific for more than a decade (Watanabe et al., 2000, Tsutsumi et al., 2006, Wada et al., 2007). These long-term monitoring programs have been carried out to obtain data to examine transport mechanism of Asian continental outflow and to validate chemical transport models (e.g. Liang et al., 2004, Liang et al., 2005, Yurganov et al., 2005, Wada et al., 2007). In this study, we used these multi-species monitoring data obtained by JMA to examine the variation in the seasonality of the chemical composition of these enhanced trace gases in the polluted air masses exported from the East Asian continent to the western North Pacific region.
Section snippets
Observation sites
JMA is operating three atmospheric monitoring stations at Minamitorishima (24°17′N, 153°59′E, 8 m above sea level), Yonagunijima (24°28′N, 123°01′E, 30 m above sea level), and Ryori (39°02′N, 141°49′E, 260 m above sea level) under the Global Atmosphere Watch programme of the World Meteorological Organization (WMO/GAW) (Fig. 1). Only brief descriptions of the station information are given here because details have already been previously reported elsewhere (Watanabe et al., 2000, Wada et al.,
Enhanced CO event
Since this study focuses on trace gases compositions of polluted air masses in Asian continental outflow, we first examined CO as a tracer for anthropogenic emissions from fossil fuel combustions and/or biomass burnings. Fig. 2 shows, as an example, the hourly mean CO observed at MNM, YON, and RYO in 2008. These 1-year data sets at all the stations reveal a distinct seasonal cycle as well as high-frequency synoptic variation, although the annual mean CO levels depend on the station. An average
Emission ratio of source region
Fig. 8 shows clear seasonal variations of the enhancement ratios of polluted air masses transported from various Asian sources, a result previously limited due to lack of sufficiently long observational data throughout the year. Since the enhancement ratios associated with the ACP events reflect primarily the emission ratios of the source regions, we examined the seasonal emission variations of the trace gases sources in the Asian continent in terms of the measurements taken at YON, MNM and RYO.
Conclusions
In this study, we showed the long-term continuous measurements of CH4, CO2, CO and O3 at three monitoring stations operated by the Japan Meteorological Agency: Minamitorishima (MNM), Yonagunijima (YON), and Ryori (RYO). The data showed synoptic-scale CO enhancements that were defined as Asian continental pollution (ACP) events over the western North Pacific. The frequency of the ACP events with enhanced CO peaks showed strong seasonal variation with a minimum in the summer and a maximum in the
Acknowledgments
We would like to acknowledge all members of the Japan Meteorological Agency for operating the trace gas measuring systems at Minamitorishima, Yonagunijima, and Ryori stations. We wish to thank Dr. Higuchi for useful comments and discussion. We also thank anonymous reviewers for constructive comments.
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