1. Influence of Asian Continental Outflow on the Regional Background Ozone Level in Northern South China Sea Chang-Feng Ou-Yang 1, Hsin-Cheng Hsieh 1, Sheng-Hsiang Wang 1,2, Jia-Lin Wang 1, Neng-Huei Lin 1, and Guey-Rong Sheu 1 1 National Central University, Taiwan; 2 Goddard Flight Space Center, NASA. 1. Introduction 2. Methodology 3. Results 4. Conclusions Acknowledgments This research was supported by the Taiwan Environmental Protection Administration under Contract EPA-99-FA11-03-A097, and the Taiwan National Science Council under Grants NSC-98-2745-M-008-001, NSC-98-2811-M- 008-073, NSC-99-2811-M-008-081, NSC-99-2111-M-008-011, and NSC-98-2611-M-019-016-MY3. References: 1.Streets, D.G., Bond, T.C., Carmichael, G.R., Fernandes, S.D., Fu, Q., He, D., Klimont, Z., Nelson, S.M., Tsai, N.Y., Wang, M.Q., Woo, J.H., Yarber, K.F., (2003). An inventory of gaseous and primary aerosol emissions in Asia in the year 2000. Journal of Geophysical Research D108, doi:10.1029/2002jd003093. 7-SEAS/2010 Dongsha Experiment 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. In addition to gases, increased levels of other airborne particles have also been observed in the western Pacific. During springtime, winter monsoon predominates over East Asia coinciding with the strongest continental outflow of the year. Asian anthropogenic species are transported by the prevailing westerly winds in spring, increasing the concentration of air pollutants in downwind areas. Numerous studies have examined the impacts of continental outflow on air quality, mainly in the northwest Pacific. However, little is known about impacts at lower latitudes over the northern South China Sea (SCS) and limited knowledge exists on how much continental outflow driven by springtime cold fronts from East Asia affect air quality in the northern SCS. This is the first time that in-situ continuous measurements of gaseous pollutants were collected in the northern SCS area. To understand the downwind distribution of the air pollutants transported and its influences on the air quality in this area, simulations were also carried out in this study. These results could strengthen our understanding of how this relatively pristine region receives pollutants from higher latitudes. The geophysical locations of the stations selected in this study are shown in Fig. 1. Dongsha (DS) (20.70°N, 116.73°E) is located in the northern part of the SCS, approximately 340 km southeast of Hong Kong. Ozone mixing ratios were measured using a UV photometer (EC9810, Ecotech, Australia). The ozone analyzer was calibrated using a traveling secondary standard calibrated to a standard UV ozone meter maintained by the Taiwan EPA. A built-in ozone source was used to make biweekly multi-point calibration checks in the range of 0 to 400 ppbv. Scheduled quality control procedures included daily zero and span checks, biweekly precision checks, quarterly multiple-point calibration, and data validation. Taiwan Air Quality Model (TAQM) was used to investigate the spatial distributions of surface ozone in East Asia. There are 15 non-uniform sigma levels in the vertical direction of TAQM, from the surface up to 100 hPa. The nested domain is downscaled two times from domain 1 to domain 3 with a final horizontal grid size of 9 km x 9 km, which is used to cover the complete Taiwan and Dongsha Island area, including all boundaries situated in the ocean. The emission inventory applied is based on Streets et al. (2003) for East Asia and on the Taiwan Emission Data system, version 6.03 (TEDS 6.03) (Taiwan EPA) for Taiwan. Fig. 3 TAQM simulations of (a) ozone and (b) Taiwan’s NOx during frontal passages. (a) Ozone(b) NOx Dongsha Island In Fig. 2, time series data for ozone and wind observations at DS are shown, with 24-hour moving averages plotted as solid red lines. The mean mixing ratio of ozone at DS was 40.9±16.1 ppb between March 10 and June 26, 2010. Winds from a specific direction (~50°) always accompanied elevated ozone levels (e.g., March 26 - March 31 and April 24 - May 3). Ozone levels occasionally exceeded ~70 ppb during northeasterly winds with high wind speeds averaging 5.3 m s -1 in contrast to lower ozone levels (~30 ppb) observed during mild southerly breezes. Our observations suggest a significant increase in ozone during the frontal passage events at DS. Six frontal passage events were selected for evaluating the impact of air masses transported by the winter monsoon. The mean ozone mixing ratio for the second day after encountering frontal passages was 60.1±10.3 ppb, which is approximately two-times higher (29.3±6.9 ppb) than the day before frontal passages arrived. Based on the backward trajectories (not shown here) and TAQM simulations (Fig. 3), the elevated ozone caused by the Asian continental outflow driven was simulated. Strong northeasterly winds arising from the winter Asian monsoon transported polluted air masses from the northern continent to as far south as Dongsha (latitude 20.70°N), as indicated by elevated ozone levels of approximately 60 ppbv. In contrast, during the calm periods, when the monsoon subsided, low ozone levels of about 30 ppb were detected, which is typical for marine air masses. A case study on April 20-24, 2010 is shown in Fig. 4. These findings reveal a significant impact of Asian continental outflow on the regional air quality of the northern SCS and close-by Taiwan area. NOAA flask data of CO, CO2, and CH4 show seasonal variations with winter/spring maximum and summer minimum during 2010 - 2011 (Fig. 5). Asian continental outflow is likely to be the cause for such seasonality. Fig. 1 TAQM simulation domains Fig. 2 Ozone and wind time-series data observed at DS. Wind data on the upper diagram is shown at 12-hour intervals. Ozone data on the bottom diagram is presented as a black solid line for hourly averages and a red solid line for the 24-hour moving average. Six pairs of events were identified based on the backward trajectories and filled with blue and cyan for the periods when frontal passages were encountered and normal days, respectively. Fig. 4 Continuous hourly data for ozone, CO, and wind data at DSI for April 20-24, 2010. Fig. 5 Measurement of (a) CO, (b) CO 2, and (c) CH 4 at DSI since March 2010. Open squares represent the NOAA flask samples which are preliminary. (b) (a) (c) In this research, we present the first continuous measurements of ozone at DS. This ground-based site satisfactorily represents the background characteristics of gases in the northern SCS region. The simulation results also support measurements, indicating that the continental outflow brought about by the winter monsoon could be immense and intense enough to affect regions as far south as tropical areas with latitudes similar to DS.