Influence of Asian continental outflow on the regional background ozone level in northern South China Sea

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).