Influence of Asian continental outflow on the regional background ozone level in northern South China Sea Chang-Feng Ou-Yang a, b , Hsin-Cheng Hsieh a , Sheng-Hsiang Wang b, c, d , Neng-Huei Lin b , Chung-Te Lee e , Guey-Rong Sheu b , Jia-Lin Wang a, * a Department of Chemistry, National Central University, Chungli 320, Taiwan b Department of Atmospheric Sciences, National Central University, Chungli 320, Taiwan c Earth System Science Interdisciplinary Center, University of Maryland, College Park, MD, USA d Goddard Space Flight Center, NASA, Greenbelt, MD, USA e Graduate Institute of Environmental Engineering, National Central University, Chungli 320, Taiwan highlights < It is the first continuous measurement of surface ozone in the northern SCS. < The Asian continental outflows can affect latitudes as far south as Dongsha’s. < Dongsha is shown to be ideal for baseline establishment without land emissions. article info Article history: Received 3 January 2012 Received in revised form 10 July 2012 Accepted 16 July 2012 Keywords: Ozone Continental outflow 7SEAS South China Sea abstract Asian continental outflow is known to have very pronounced seasonality, which in turn can alter the trace gas contents of the Northern Hemisphere. In this study, field measurements were conducted during the spring of 2010, Dongsha, a small island situated between Taiwan and the Philippines, served as a remote site for monitoring surface ozone. Ozone was used as an effective indicator to distinguish between continental and marine air masses. Our measurements suggested that strong northeasterly winds arising from the winter Asian monsoon may have transported polluted air masses from the northern continent to locations as far south as Dongsha (latitude 20.70  N), as indicated by elevated ozone levels of approximately 60 ppbv. In contrast, during periodic calm periods when the monsoon subsided low ozone levels of about 30 ppb were detected, which is typical for marine air masses. This outflow of polluted air masses from the Asian continent and Taiwan facilitated by the winter monsoon was also successfully simulated using the Pennsylvania State University/National Center for Atmospheric Research Mesoscale Model, version 5 (PSU/NCAR MM5) and the Taiwan Air Quality Model (TAQM). Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction As a result of rapid economic growth in East Asia over the past few decades, increased emissions of anthropogenic air pollutants have been transported by continental outflow (Wild and Akimoto, 2001; Akimoto, 2003; Streets et al., 2003). Previous studies have reported that Asian continental outflow has impacted global atmospheric chemistry, affecting air quality over the western Pacific, even as far as North America (Jaffe et al., 2003; Chiang et al., 2009; Ambrose et al., 2011; Wang et al., 2011). In addition to gases, increased levels of other airborne particles have been observed in the western Pacific. Asian mineral dust is transported by the pre- vailing westerly winds in spring (Zhang et al., 1997), increasing the concentration of particulate matter in downwind areas (Liu et al., 2006). In addition, Wang et al. (2010) also have measured acidi- fied cloud water with an approximate seven-fold increase in Ca 2þ in Northern Taiwan, resulting from the pollution transported from industrial and urban regions along the coast of eastern China. Chang et al. (2000) reported that the most serious impacts of long- range transport occur during the northeasterly monsoon period for both sulfate (39%) and nitrogen (37%) deposition in Taiwan. Seasonal variations in tropospheric ozone also have been well documented, with a spring maximum observed over East Asia coinciding with the continental outflow (Monks, 2000; Bey et al., 2001; Lin et al., 2010; Ou-Yang et al., 2012). * Corresponding author. Tel.: þ886 3 4227151x65906; fax: þ886 3 4277972. E-mail address: cwang@cc.ncu.edu.tw (J.-L. Wang). Contents lists available at SciVerse ScienceDirect Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv 1352-2310/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.atmosenv.2012.07.040 Atmospheric Environment 78 (2013) 144e153